SPELEOTHEM RECORD OF SOUTHERN ARIZONA PALEOCLIMATE,
54 TO 3.5 KA
By
Jennifer Diane Miller Wagner
________________________
A Dissertation Submitted to the Faculty of the
DEPARTMENT OF GEOSCIENCES
In Partial Fulfillment of the Requirements
For the Degree of
DOCTOR OF PHILOSOPHY
In the Graduate College
THE UNIVERSITY OF ARIZONA
2006
2
THE UNIVERSITY OF ARIZONA
GRADUATE COLLEGE
As members of the Dissertation Committee, we certify that we have read the dissertation
prepared by Jennifer Diane Miller Wagner
Entitled:
SPELEOTHEM RECORD OF SOUTHERN ARIZONA
PALEOCLIMATE, 54 TO 3.5 KA
and recommend that it be accepted as fulfilling the dissertation requirement for the
Degree of Doctor of Philosophy
____________________________________________________________Date: 11/10/06
Julia E. Cole
____________________________________________________________Date: 11/10/06
P. Jonathan Patchett
____________________________________________________________Date: 11/10/06
J. Warren Beck
____________________________________________________________Date: 11/10/06
Jay Quade
Final approval and acceptance of this dissertation is contingent upon the candidate's
submission of the final copies of the dissertation to the Graduate College.
I hereby certify that I have read this dissertation prepared under my direction and
recommend that it be accepted as fulfilling the dissertation requirement.
____________________________________________________________Date: 11/10/06
Dissertation Director: Julia E. Cole
3
STATEMENT BY AUTHOR
This dissertation has been submitted in partial fulfillment of requirements for an
advanced degree at the University of Arizona and is deposited in the University Library
to be made available to borrowers under rules of the Library.
Brief quotations from this dissertation are allowable without special permission,
provided that accurate acknowledgment of source is made. Requests for permission for
extended quotation from or reproduction of this manuscript in whole or in part may be
granted by the head of the major department or the Dean of the Graduate College when in
his or her judgment the proposed use of the material is in the interests of scholarship. In
all other instances, however, permission must be obtained from the author.
SIGNED: Jennifer Diane Miller Wagner
4
ACKNOWLEDGEMENTS
I would like to thank my committee members for all their advice and
encouragement: Julie Cole, Jon Patchett, Warren Beck, Jay Quade, and Jon Overpeck. I
thank Jerry Trout of the US Forest Service, Dennis Hoburg, and Bill Peachy for
assistance in obtaining stalagmite and water samples from Cave of the Bells. I appreciate
the extensive assistance I have received while conducting lab work and data analysis
from: Toby Ault, Heidi Barnett, Wes Bilodeau, David Dettman, Mihai Ducea, Chris
Eastoe, Tim Fischer, Gideon Henderson, Clark Isachsen, Austin Long, Christa Placzek,
and Rick Toomey. NSF Earth System History 03-18480, UA small Faculty Grant, GSA
student grant, and University of Arizona Department of Geosciences student grants
provided funding for this research.
5
DEDICATION
For my husband, Trey Wagner, who keeps me on track.
6
TABLE OF CONTENTS
ABSTRACT ....................................................................................................8
INTRODUCTION ........................................................................................ 10
REFERENCES ............................................................................................. 15
APPENDIX A: ABRUPT MILLENNIAL CLIMATE CHANGE DURING
THE LAST GLACIAL IN SOUTHERN ARIZONA INFERRED FROM A
SPELEOTHEM ISOTOPIC RECORD........................................................ 17
Abstract ..................................................................................................... 17
Introduction ............................................................................................... 18
Setting........................................................................................................ 19
Methods ..................................................................................................... 20
Results ....................................................................................................... 21
Speleothem Morphology ....................................................................... 21
Chronology ............................................................................................ 22
Cave Temperature and Hydrology......................................................... 23
Oxygen Isotopes .................................................................................... 24
Discussion ................................................................................................. 25
Evaluation of Potential Disequilibria .................................................... 25
Climate Influences on Speleothem 18O Values ................................... 26
Glacial Maximum to Early Holocene.................................................... 27
Millennial Variability in MIS3 .............................................................. 29
Potential Mechanisms of Climate Variability ....................................... 31
Spectral Analysis ................................................................................... 34
Conclusions ............................................................................................... 36
References ................................................................................................. 37
Figure captions .......................................................................................... 43
APPENDIX B: MID-HOLOCENE CLIMATE IN SOUTHERN ARIZONA
INFERRED FROM SPELEOTHEM STABLE ISOTOPES ....................... 60
Abstract ..................................................................................................... 60
Introduction ............................................................................................... 61
Setting........................................................................................................ 63
Methods ..................................................................................................... 65
Chronology................................................................................................ 68
7
TABLE OF CONTENTS - Continued
Results ....................................................................................................... 69
Discussion ................................................................................................. 71
Oxygen Isotopes .................................................................................... 71
Comparisons to Other Paleoclimatic Records From the Southwest...... 75
Spectral Analysis ................................................................................... 78
Potential Mechanisms for Mid-Holocene Climate Observations.......... 79
Conclusions ............................................................................................... 81
Acknowledgements ................................................................................... 82
References ................................................................................................. 82
Figure captions .......................................................................................... 87
APPENDIX C: USING LONG-TERM RECORDS OF ISOTOPES IN
PRECIPITATION FROM TUCSON, ARIZONA TO CALIBRATE CAVE
WATER ISOTOPIC RESPONSE TO CLIMATE IN CAVE OF THE
BELLS, ARIZONA ...................................................................................... 98
Abstract ..................................................................................................... 98
Introduction ............................................................................................... 99
Setting...................................................................................................... 101
Methods ................................................................................................... 102
Results ..................................................................................................... 105
Discussion ............................................................................................... 107
COB Waters......................................................................................... 107
Tucson Precipitation ............................................................................ 111
Conclusions ............................................................................................. 113
Acknowledgements ................................................................................. 114
References ............................................................................................... 115
Figure captions ........................................................................................ 117
8
ABSTRACT
In the semi-arid southwestern US, the lack of continuous records of climate over
the last glacial cycle has precluded a complete understanding of the rates and timing of
past regional changes in climate. Speleothems can provide high-resolution, continuous
record of moisture, temperature, and, potentially, vegetation variations and can be
precisely dated by uranium-series disequilibrium. We have produced two U-series dated
speleothem
18
O records from Cave of the Bells (COB). COB is located in Santa Cruz
County, Arizona on the east side of the Santa Rita Mountains (31°45'N, 110°45'W; 1700
m).
The glacial speleothem
18
O record (53 to 8.5 ka) confirms that deglaciation in
the Southwest proceeded via a stepwise shift, mirroring the Bølling-Allerød warming and
Younger Dryas cooling, beginning around 15 ka. There is no evidence of early warming
before the decline of the large ice sheets. In Marine Isotope Stage 3 (MIS3; 53 to 30 ka),
we observe millennial variations similar to Dansgaard-Oeschger (DO) events first seen in
Greenland ice core
18
O records with wet/cold conditions indicated by our cave record
during glacial stadials (cold periods) and dry/warm during glacial interstadials (warmer
periods). High-resolution U-series dating allows for refinement of the timing of DO
events in MIS3, and spectral analysis confirms the presence of a 1515-year climate cycle
during this time.
The
18
O data from a Holocene stalagmite (~6.9 to 3.5 ka) average ~3‰ higher
than modern and exhibit substantial multidecadal to multicentury variation. We propose
that in addition to drier/warmer conditions in the winter, a stronger summer monsoon and
9
perhaps warmer summer temperatures supplied waters with higher
cave during the mid-Holocene. Spectral analysis of early part of the
18
O values to the
18
O record reveals
variability at periods of 233 years and at 142 and 52. After ~4.9 ka a prominent shift from
centennial to multidecadal periods of variability (a 70 to 50-year cycle) is observed and
there is a slight decrease in average
18
O values. This shift is coincident with a
hypothesized increase in El Niño activity, which is correlated to wet winters in the
modern southwest, in the tropical Pacific at ~5 ka.
10
INTRODUCTION
Population in the western United States increased 20 to 60% during the 1990s
(http://www.census.gov/population/cen2000/phc-t2/tab03.pdf), a trend that is predicted to
continue. This growth and periodic drought, such as the recent ~2000-2005 drought, are
already straining the limited groundwater resources and surface water reservoirs in the
region. Recognition of the demographic reality and the west’s vulnerability to drought is
spurring cooperative planning between federal, state, local, and tribal governments
(http://www.doi.gov/water2025/Water2025-Exec.htm). This planning requires a complete
understanding of the range of climate variability possible in the western states, as well as
the ocean/atmospheric conditions (both long and short term) under which droughts or wet
periods are likely to occur.
The past 100-200 years of instrumental and historical climate data are inadequate to
understand the full range of climate variability in the southwest USA. By developing
longer records of regional climate fluctuations, we can determine how frequently such
events as megadroughts (or wet, warm, or cold periods) occurred and whether they were
more frequent and/or intense during certain intervals, e.g. different global background
climate, such as ice ages or during times of changed radiative forcing, such as the midHolocene.
Continuous, high-resolution (subdecadal to century-scale) paleoclimate records
with well-constrained chronologies that extend further back than a few thousand years are
relatively rare from the southwest USA. Tree-ring records provide annual records of
climate from forested regions in the southwest, but typically extend only a few centuries,
11
with the longest chronologies reaching ~2000 years (e.g. Grissino-Mayer, 1996; Hughes
and Graumlich, 1996; LaMarche, 1974; Salzer and Kipfmueller, 2005). Packrat middens
have traditionally been one of the main sources of paleoclimate information in the semiarid southwest. They offer radiocarbon-dated “snapshots” of vegetation at a particular
place and time, and suites of middens can be interpreted in climatic terms (e.g.
Betancourt et al., 1990). Continuous records from lakes are few (e.g. Anderson, 1993;
Benson et al., 2002; Castiglia and Fawcett, 2006; Hasbargen, 1994; Hevly, 1985;
Menking and Anderson, 2003; Metcalfe et al., 2000), low-resolution (e.g. ~1000 years for
Lake Estancia), and, because they are dated mostly by radiocarbon, do not extend back
much farther than ~45 ka.
Speleothems can provide high-resolution, continuous record of moisture,
temperature, and, potentially, vegetation variations and can be precisely dated by
uranium-series disequilibrium. We have produced two U-series dated speleothem
18
O
records from Cave of the Bells (COB). Cave of the Bells is located in Santa Cruz County,
Arizona on the east side of the Santa Rita Mountains (31°45'N, 110°45'W; 1700 m),
about 75 km to the southeast of Tucson, Arizona. The modern vegetation above the cave
is best characterized as oak-juniper woodland with an under story of C4 grasses and CAM
succulents (Stable isotope composition of speleothem calcite and associated cave and soil
CO2, Cave of the Bells, Arizona, hereinafter referred to as Fischer et al. in preparation,
2006). The cave is situated in the Permian Colina Limestone below an isolated hill at
shallow depths.
12
The first stalagmite
18
O record spans ~53 to 10 ka (Abrupt millennial climate
change during the last glacial in southern Arizona inferred from a speleothem isotopic
record, Appendix A), the second is from the mid-Holocene (Mid-Holocene climate in
southern Arizona inferred from speleothem stable isotopes, Appendix B). Speleothem
18
O values are determined by the temperature in the cave and the
18
O values of the
dripwaters, which are determined by the amount, temperature and seasonality of
precipitation that supplies them. All caves are unique, and paleoclimate interpretation of
speleothem calcite
18
O and
13
C data is aided by an understanding of the modern
processes that impact the stable isotopes of water infiltrating the cave. In this study we
also present the results of three years of COB dripwater and precipitation monitoring and
analyze ~25 years of Tucson, Arizona precipitation
18
O and D data for relationships to
temperature, amount, and seasonality of rainfall (Using long-term records of isotopes in
precipitation from Tucson, Arizona to calibrate cave water isotopic response to climate in
Cave of the Bells, Arizona, Appendix C).
The glacial speleothem
18
O record (53 to 8.5 ka), detailed in Appendix A,
demonstrates, for the first time, the rapidity and frequency of Late Quaternary
hydroclimatic variations in the semi-arid southwestern USA. In the older part of the
record (53 to 30 ka), we also observe millennial variations similar to DansgaardOeschger (DO) events first seen in Greenland ice core
18
O records (Dansgaard et al.,
1993) with wet/cold conditions indicated by our cave record during glacial stadials (cold
periods) and dry/warm during glacial interstadials (warmer periods). These climate
oscillations in the southwest are likely related to shifts in the long-term average position
13
of the westerly storm tracks- wetter and cooler when in a southerly position- and could
also be influenced by the state of the Pacific- wet/cool during times of a dominant El
Niño-like and/or positive PDO-like pattern. An increase in the ratio of summer to winter
precipitation infiltrating the cave is also likely during some interstadials, possibly because
of increased summer insolation at the latitude of the cave due to precessional changes in
the Earth’s orbit. High-resolution U-series dating allows for refinement of the timing of
DO events in Marine Isotope Stage 3, and spectral analysis confirms the presence of a
1515-year climate cycle during this time, which has been found in some North Atlantic
climate records. The COB
18
O record also shows that deglaciation in the southwestern
USA proceeded via a stepwise shift, with dramatic changes occurring in less than 300
years, mirroring the Bølling-Allerød warming, Younger Dryas cooling, and the early
Holocene warming seen in Greenland ice core
18
O records.
In Appendix B, we present high-resolution (<10 years)
stalagmite (~6.9 to 3.5 ka). The stalagmite has
18
18
O data from a Holocene
O values that are on average 3‰ higher
than modern. We propose that this increase is due to drier/warmer conditions in the
winter and a stronger summer monsoon (and perhaps warmer summer temperatures),
driven by increased summer insolation. These conditions would have supplied waters
with higher
18
18
O values to the cave. Spectral analysis of early part of the mid-Holocene
O record reveals variability at periods of 233 years and at 142 and 52 years, within the
Suess and Gleissberg bands as identified by Ogurtsov et al. (2002) in various proxies of
solar variability. After ~4.9 ka a dramatic shift from centennial to multidecadal periods of
variability (a 70 to 50-year cycle) is observed and there is a slight decrease in average
14
18
O values. This shift is coincident with an increase in El Niño activity in the tropical
Pacific ~5 ka; in the modern climate El Niños are correlated to wet winters.
The data presented in Appendix C underpins the climate interpretations of the
speleothem
18
O data presented in Appendixes A and B. We collected dripwaters and
precipitation at Cave of the Bells for ~3 years. Dripwater
18
O and D values are
relatively stable over the monitoring period and between locations in the cave. The
18
O
values average –9.6‰ ±0.2‰ and D values -67‰ ±1.2‰ (VSMOW). Comparisons to
average seasonal values of precipitation indicates that the dripwaters originate mostly
from local winter precipitation, with summer monsoon rains contributing at most 15 to
45% to the total. An analysis of the variations in the
18
O values of ~ 25 years Tucson
precipitation reveals that, in keeping with global patterns (Rozanski et al., 1993),
18
O
values are higher when temperatures are warmer (although this relationship is weak to
nonsignificant during the winter season) and rainfall amounts are less, and that average
summer monsoon (Jul-Sept) precipitation
18
O values are ~3.6‰ greater than average
winter (Oct-Mar) values.
The relationships between Tucson precipitation
18
O values and temperature,
amount, and seasonality of precipitation help to constrain the possible climatic causes of
past variations in speleothem
18
determine that when speleothem
O values. From this study we have been able to
18
O values are higher than modern (~-10.6‰ VPDB)
the climate was likely drier and/or summer precipitation may have increased relative to
winter, such that it was able to comprise a larger portion of shallow groundwater
recharge. When speleothem
18
O values are less than modern, conditions were wetter
15
with perhaps less summer relative to winter moisture. The effect of changing
temperatures is less clear. If winter precipitation is the dominant source of dripwaters, as
in the modern system, then the calcite fractionation effect of decreasing
18
O values with
increasing temperatures could cancel out the slight trend of increasing precipitation
18
O
values with increasing temperatures. However, if summer precipitation dominates
recharge, then the overall effect of increasing temperatures will be to increase calcite
18
O values.
REFERENCES
Anderson, R. S. (1993). A 35,000 Year Vegetation and Climate History from Potato
Lake, Mogollon Rim, Arizona. Quaternary Research 40, 351-359.
Benson, L., Kashgarian, M., Rye, R., Lund, S., Paillet, F., Smoot, J., Kester, C., Mensing,
S., Meko, D., and Lindstrom, S. (2002). Holocene multidecadal and
multicentennial droughts affecting Northern California and Nevada. Quaternary
Science Reviews 21, 659-682.
Betancourt, J. L., Van Devender, T. R., and Martin, P. S. (1990). Packrat Middens: The
last 40,000 years of biotic change, pp. 467. The University of Arizona Press,
Tucson.
Castiglia, P. J., and Fawcett, P. J. (2006). Large Holocene lakes and climate change in the
Chihuahuan Desert. Geology 34, 113-116.
Dansgaard, W., Johnsen, S. J., Clausen, H. B., Dahljensen, D., Gundestrup, N. S.,
Hammer, C. U., Hvidberg, C. S., Steffensen, J. P., Sveinbjornsdottir, A. E.,
Jouzel, J., and Bond, G. (1993). Evidence for General Instability of Past Climate
from a 250-Kyr Ice-Core Record. Nature 364, 218-220.
Grissino-Mayer, H. D. (1996). A 2129-year reconstruction of precipitation for
northwestern New Mexico, USA. In "Tree Rings, Environment, and Humanity."
(J. S. Dean, D. M. Meko, and T. W. Swetnam, Eds.), pp. 191-204. Radiocarbon.
Hasbargen, J. (1994). A Holocene Paleoclimatic and Environmental Record from
Stoneman Lake, Arizona. Quaternary Research 42, 188-196.
16
Hevly, R. H. (1985). A 50,000 year history of Quaternary environment; Walker Lake,
Coconino Co., Arizona. In "Late Quaternary Vegetation and Climates of the
American Southwest." (B. F. Jacobs, P. L. Fall, and O. K. Davis, Eds.), pp. 141154. Contributions Series-American Association of Statigraphic Palynologists.
American Association of Statigraphic Palynologists, Houston.
Hughes, M. K., and Graumlich, L. J. (1996). Multimillennial dendroclimatic records from
western North America. In "Climatic Variations and Forcing Mechanisms of the
Last 2000 Years." (P. D. Jones, R. S. Bradley, and J. Jouzel, Eds.), pp. 109-124.
Springer Verlag, Berlin.
LaMarche, V. C. (1974). Paleoclimatic Inferences from Long Tree-Ring Records.
Science 183, 1043-1048.
Menking, K. M., and Anderson, R. Y. (2003). Contributions of La Nina and El Nino to
middle holocene drought and late Holocene moisture in the American Southwest.
Geology 31, 937-940.
Metcalfe, S. E., O'Hara, S. L., Caballero, M., and Davies, S. J. (2000). Records of Late
Pleistocene-Holocene climatic change in Mexico - a review. Quaternary Science
Reviews 19, 699-721.
Rozanski, K., Aruguas-Araguas, L., and Gonfiantini, R. (1993). Isotopic patterns in
modern global precipiation. In "Continental Indicators of Climate." (P. Swart, J.
A. McKenzie, and K. C. Lohman, Eds.), pp. 1-36. American Geophysical Union
Monograph 78.
Salzer, M. W., and Kipfmueller, K. F. (2005). Reconstructed temperature and
precipitation on a millennial timescale from tree-rings in the Southern Colorado
Plateau, USA. Climatic Change 70, 465-487.
17
APPENDIX A: ABRUPT MILLENNIAL CLIMATE CHANGE DURING
THE LAST GLACIAL IN SOUTHERN ARIZONA INFERRED FROM A
SPELEOTHEM ISOTOPIC RECORD
Jennifer D. M. Wagner1*, Julia E. Cole1, 2, J. Warren Beck3, P. Jonathan Patchett1, and
Gideon M. Henderson4
1
Department of Geosciences, University of Arizona, Tucson, Arizona 85721
Department of Atmospheric Sciences, University of Arizona, Tucson, Arizona 85721
3
Accelerator Mass Spectrometry Facility, Department of Physics, University of Arizona, Tucson,
Arizona 85721
4
Department of Earth Sciences, Oxford University, Oxford, UK
2
Abstract
Prolonged drought and temperature anomalies in the semi-arid southwestern USA
strain ecosystems as well as water resources upon which the burgeoning population of the
region depends. In the Southwest detailed paleoclimate records spanning the last one to
two thousand years are available (e.g. Salzer and Kipfmueller, 2005), but the region lacks
a well-constrained, continuous climate history over the last glacial cycle. We present the
first such record from a stalagmite from Cave of the Bells, southeast of Tucson, Arizona.
High-resolution (average 30 yr)
18
O data from a sample spanning ~8.5-53 ka (uranium-
thorium chronology) indicate a stepwise deglacial shift with dramatic changes occurring
in less than 100-300 years, mirroring ice core
18
O records of the Bølling-Allerød -
Younger Dryas in Greenland (Dansgaard et al., 1993). In the older part of the record (3053 ka), we observe millennial variations similar to Dansgaard-Oeschger (DO) events first
seen in Greenland ice core
18
O records, with wet/cold conditions occurring during
glacial stadials and dry/warm during glacial interstadials. An increase in the ratio of
summer to winter precipitation is also likely during some interstadials, which could be
18
related to increased summer insolation. High-resolution dating allows for refinement of
the timing of DO events in Marine Isotope Stage 3 (~54-29 ka), and spectral analysis
reveals the presence of a 1515 year cycle in MIS 3 akin to that recognized in North
Atlantic climate records. This record demonstrates, for the first time, the rapidity and
frequency of Late Quaternary hydroclimatic variations in the semi-arid southwestern
USA, and supports a link between these millennial variations and the position of the
westerly jet and Pacific Ocean variability, which control modern winter precipitation at
this site.
Introduction
In the semi-arid southwestern US, the lack of continuous records of climate over
the last glacial cycle has precluded understanding the rates and timing of past regional
changes in climate and moisture availability. Discontinuous records from pack-rat
middens, groundwater, speleothem, spring, and lake deposits (Allen and Anderson, 2000;
Betancourt et al., 1990; Pigati et al., 2004; Polyak et al., 2004; Zhu et al., 1998) clearly
display large climate variations associated with the demise of glacial conditions, and
cooler/wetter glacial conditions have been inferred from these and additional proxies
(Anderson, 1993; Hevly, 1985; Winograd et al., 1992). Millennial-scale variations have
been reconstructed from lake records in the Great Basin (Benson et al., 2003) and from
marine records in the Santa Barbara basin (Hendy and Kennett, 1999), but defining the
timing of these events is limited by the uncertainty of radiocarbon calibration in the late
Quaternary and, for the Great Basin lakes, the reservoir effect. Here we present a
19
continuous isotopic record from a speleothem that documents, for the first time, the
timing and amplitude of climate variations in this region during the most recent glacial
and deglacial periods.
Setting
We collected the stalagmite from Cave of the Bells (COB) located in Santa Cruz
County, Arizona on the east side of the Santa Rita Mountains (31°45'N, 110°45'W; Fig.
1) at an elevation of 1700m. The cave is located in the Permian Colina Limestone below
an isolated hill at shallow depths, indicating that the infiltrating water that forms the
speleothems originates from local precipitation and is not supplied by regional
groundwater. Cave humidity is very high and there is active formation growth. There is
only one small opening to the outside, and the cave temperature is a constant 19.5°C. The
vegetation above the cave is best characterized as oak-juniper woodland with an
understory of C4 grasses and CAM succulents (Stable isotope composition of speleothem
calcite and associated cave and soil CO2, Cave of the Bells, Arizona, hereinafter referred
to as Fischer et al. in preparation, 2006).
In our study region, roughly half the annual precipitation comes during the
summer monsoon. But high temperatures and vegetation demand, combined with
“flashy” distribution, cause most of this water to be lost to runoff or evapotranspiration.
Westerly frontal storms from the Pacific supply the other half of the annual total from
October through March. Dissipating tropical cyclones can also contribute significant
moisture, in certain years, during September and October (Sheppard et al., 2002 and
20
references therein). Cooler temperatures and lower rainfall rates lead to greater recharge
of the groundwater system during the winter months (Baillie, 2005; Eastoe et al., 2004;
Wahi, 2005).
Methods
After removal from the cave, the stalagmite was cored with a 1” drill bit and the
core halved and polished. One side was sectioned into 4-6mm increments with a handheld drill and thin saw blade attachment for low-resolution U-Th dating on TIMS. The
other side was sampled parallel to the growth axis for stable isotope analysis (at 100 µm
increments with a micromill) and for AMS radiocarbon analysis and U-Th (at ~1 and
~1.5-3 mm increments, respectively, with a diamond impregnated wire saw).
U/Th dating was performed using various techniques and laboratories, including
Micromass Sector 54 TIMS and Micromass MC-ICP-MS at the University of Arizona
and a Nu-Plasma MC-ICP-MS at Oxford University (Table 1, Fig. 3). All U-Th
chemistry, modeled after Edwards et al. (1987), was performed at the University of
Arizona (Placzek et al., 2006). TIMS analyses were performed at the University of
Arizona and MC-ICP-MS at Arizona and Oxford University (for details of Oxford
procedure see Robinson et al., 2002).
Stable isotopes are expressed using the delta notation as in the following example:
18
Osample = {((18O/16O)sample/(18O/16O)reference) –1} * 1000‰
All water stable isotope values are referenced to the VSMOW standard and calcite stable
isotopes values to the VPDB standard. Micromilled powders were analyzed for
18
O and
21
13
C isotopes on a Micromass Optima dual inlet stable isotope mass spectrometer with an
automated carbonate preparation system at University of Arizona, with better than 0.08‰
and 0.04‰ analytical precisions, respectively. Water
18
O and D values were measured
on a Finnigan Delta S gas source mass spectrometer in the Stable Isotope Laboratory of
the Department of Geosciences at the University of Arizona. Water samples were
prepared and measured according to methods in Craig (1957), and precisions are ±0.08‰
for
18
O values.
Spectral analysis was performed on the GISP2, GRIP (SFCP2004), Hulu Cave,
and Cave of the Bells
18
O records with the SSA-MTM Toolkit (Ghil et al., 2002). We
subtracted the first reconstructed component (RC) from singular spectrum analysis (SSA)
from the raw data, which decreased very low frequency variability. The Multitaper
Method (MTM) was used to determine statistical significance of spectral peaks and the
Maximum Entropy Method (MEM), which produces more sharply defined spectral peaks,
was used to determine the main frequency of the significant variance (Obrochta and
Crowley, 2005).
Results
Speleothem Morphology
COB-01-02 was approximately 13 cm tall and roughly columnar with a diameter
of 4 to 6 cm. This stalagmite has no visible detrital material incorporated into the calcite
along the growth axis. The calcite is straw colored and semi-transparent with the
exception of two areas that are less transparent (cloudy) and whiter. Mineralogical
22
analysis via X-ray diffraction on portions of the stalagmite indicates that it is composed
of calcite and contains no aragonite.
Chronology
The stalagmite chronology was derived from 62 (including six replicates) TIMS
and MC-ICP-MS U-Th dates taken nearly continuously along the growth axis; the sample
spans the period from 8.5 – 53 ka (all dates relative to 1950; Table 1, Fig. 3). Ages were
corrected for initial Th assuming a value similar to bulk upper continental crust,
230
Th/232Th activity = 0.8 ± 0.4 (Taylor and McLennan, 1995). This speleothem calcite
was especially clean, with negligible amounts of detrital material that could contribute
initial Th; 230Th/232Th activity ratios ranged from ~100 to well over 10,000, so in all cases
any age correction due to initial Th was much less than uncertainty from other sources.
There is a subtle cloudy zone in the otherwise very clear calcite at a depth of ~26 mm that
corresponds to a large increase in ages between two adjacent samples; we interpret this to
be a hiatus in deposition between ~23.5 and 30 ka. There is another smaller cloudy zone
near the very top of the sample at ~3.5 mm. There are no dates completely above this
cloudy zone, but the three dates that include this section appear anomalously young
relative to those that come after. Therefore we assume this upper cloudy interval from
~3.5-4.1 mm is another short hiatus and exclude the three uppermost dates that include
this cloudy zone from the age model for the upper section. Because there are no dates
completely above the hiatus at 3.5 mm, the upper portion of the record is presented as
“floating” in the early Holocene, and no absolute ages are assigned. The age model from
3.5 to 26 mm is based on a third order polynomial fit to the 15 dates between the hiatus at
23
~3.5 and ~26 mm. The age model for the rest of the record was constructed by fitting a
weighted spline with 12 degrees of freedom to the 44 dates (including four replicates)
after this hiatus (Fig. 3). Speleothem growth rates vary from 0.7 to 13.7 mm/ky (average
5.3).
Cave Temperature and Hydrology
The measured cave temperature is slightly warmer than mean annual temperature
at the elevation of the cave site, which should be approximately 16°C. A lake deep in the
cave is even warmer, ~24.4°C (http://www.fs.fed.us/r3/coronado/forest/recreation/
caves/bells.shtml), indicating the cave may be heated geothermally, an effect documented
in the nearby Kartchner Caverns (Buecher, 1999).
At Cave of the Bells, a three-year monitoring study (February 2003 to May 2006)
demonstrates that modern infiltrating waters derive mainly from winter precipitation (Fig.
2), in keeping with the regional pattern (Using long-term records of isotopes in
precipitation from Tucson, Arizona to calibrate cave water isotopic response to climate in
Cave of the Bells, Arizona, hereinafter referred to as Wagner et al., in preparation,
2006a). The
18
O value of the dripwaters from three sites in the cave averages ~-9.6‰
(VSMOW) and has varied less than 1‰ amoung all three sites. The long term (19812005) weighted average of Oct-Mar precipitation in Tucson, Arizona (~75 km to the NW
of the cave site, elevation 810 m) is –9.2‰, whereas summer monsoon (Jul-Sept)
precipitation averages much higher, -5.6‰ (Wagner et al., in preparation, 2006a). As
24
expected, cave drip-water
18
O values are slightly less than the Tucson winter average
because of the elevation effect (Rozanski et al., 1993).
Oxygen Isotopes
We sampled the speleothem core at 100µm resolution to produce a record of
stalagmite
18
O values with a temporal resolution of 10-130 yrs (Fig. 5-6). Carbon
isotope data have also been generated and will be discussed elsewhere. Stalagmite
18
O
values during the LGM were up to 4.5‰ less than those of the early Holocene (Fig. 5)
and ~2‰ less than modern.
We observe a striking similarity between the millennial climate events seen in our
speleothem
18
O record between 30 and 53 ka and Dansgaard-Oeschger events first
identified in Greenland ice core
18
O records of temperature (Grootes and Stuiver, 1997;
Johnsen et al., 2001) and later recognized around the globe in a variety of terrestrial
(Burns et al., 2003; Genty et al., 2003; Spotl and Mangini, 2002; Wang et al., 2001) and
marine (Hendy and Kennett, 1999; Peterson et al., 2000) environments. These events in
the North Atlantic entail a rapid warming followed by a slow cooling to glacial
background, and are sometimes punctuated with rapid cooling that is accompanied by
increased iceberg discharge in the North Atlantic- a Heinrich Event. In the Cave of the
Bells speleothem
18
O record, values are increased during interstadial (warm) times in
the Greenland records and decreased during stadials (Fig. 5-6).
25
Discussion
Evaluation of Potential Disequilibria
Oxygen isotopes from speleothem calcite can potentially be used as a quantitative
indicator of paleoclimate if the relationships of modern calcite to climate are determined
and if the calcite precipitates in equilibrium with the cave dripwaters. Typically
equilibrium precipitation is assessed via a “Hendy test” (Hendy, 1971) along a growth
band showing no increase in
covariation between
18
18
O and
O values moving away from the growth axis and no
13
C values. This was impractical for our samples; the
growth rate was very slow so we could not be sure we were sampling the exact same time
interval while moving off to the side of the main growth axis. Also this test is typically
done across the entire stalagmite with up to a centimeter between samples and we only
removed a ~2.5 cm core. Replication of all or part of the record to ensure the speleothem
is recording climate and not being impacted by processes unique to a particular drip
pathway is also not possible at this time due to a lack of samples that overlap in age.
Analysis of modern calcite deposited on drip plates carried out by Mickler et al.
(2006) suggests that covariation between
on plot of
18
O (x-axis) versus
13
18
O and
13
C values (slope greater than ~0.52
C values) along the growth axis (“temporal test”) can
be a warning sign of nonequilibrium deposition, although in certain locations there are
plausible climate scenarios that could explain such a relationship. A plot of COB-01-02
18
O and
13
C values (Fig. 4) shows a very poor correlation between the two with slopes
less than 0.52 over the record as a whole and the MIS 3 interval, and, for the deglacial
26
time period, there is actually a negative slope. However, the short early Holocene section
does have a slope of ~0.82.
Climate Influences on Speleothem
Speleothem calcite
18
18
O Values
O values are determined by the
18
O value of the water
entering the cave (which is controlled by the temperature, amount, seasonality, and
source of precipitation) and cave temperature (usually the mean annual temperature) and,
on Quaternary time scales, global ice volume changes. Speleothem
18
O values that are
less than modern cannot be due solely to an increase in the ratio of winter/summer
precipitation infiltrating groundwater, because very little summer precipitation infiltrates
under the modern hydrologic regime, even though summer precipitation makes up about
half of the annual total (Wagner et al., in preparation, 2006a). However, speleothem
18
O
values that are greater than modern may reflect a combination of drier/warmer conditions
and increased infiltration of summer precipitation with greater average
winter precipitation. We assume that changes in the
18
18
O values than
O value of precipitation at our site
reflect the influence of the amount and temperature of precipitation, with higher values
recorded during drier/warmer periods, and seasonality, with higher values recorded
during times of increased infiltration of summer relative to winter precipitation (Rozanski
et al., 1993; Wagner et al., in preparation, 2006a).
Speleothem
18
O values were also corrected for the effects of ice volume on
global water vapor during the last glacial period by subtracting estimates of seawater
18
O values (Lea et al., 2002) from our measured
18
O values. During the glacial the
27
global average oceanic
18
O was up to ~1.2‰ more positive because more negative
18
O
water is preferentially removed during evaporation and stored in the large ice sheets.
Thus, as the ice sheets melted the ocean and global water vapor became more negative
while the cave
18
O values became more positive. Correcting for the ice volume effect on
global water vapor shows the full magnitude of the shift in cave
18
O values from the
glacial maximum to early Holocene, and within the glacial itself.
Glacial Maximum to Early Holocene
Low stalagmite
18
O values at the LGM compared to the early Holocene (Fig. 5)
and modern, indicate that conditions were much wetter and cooler, in agreement with
packrat midden (Arundel, 2002; Betancourt et al., 2001; Betancourt et al., 1990; Cole and
Arundel, 2005; Holmgren et al., 2003), paleovegetation (Anderson, 1993; Hevly, 1985;
Thompson et al., 1993), and groundwater (Stute et al., 1995; Zhu et al., 1998) data from
the region. Our record contains a hiatus between ~23.5-30 ka, a time when cooler,
moister conditions are well documented in this region and when our own
18
O data are
near their minima (consistent with wet/cool conditions). This contrasts with findings from
New Mexico caves where faster growth is associated with wetter conditions and slower
growth or cessation of deposition with drier conditions when a suite of speleothems are
evaluated (Polyak and Asmerom, 2001; Polyak et al., 2004).
Increasing speleothem
18
O values indicate a rapid drying, probably accompanied
by some warming and an increase in summer precipitation, coincident with the BøllingAllerød (BA). This was followed by a decrease in
18
O values, signifying wetter and
28
cooler conditions, during the time of the Younger Dryas (YD) before the final transition
into the early Holocene. The mid-point of the drying/warming BA transition occurs at
15.06 ka in our record, about 400 years before that warming in Greenland. This is broadly
consistent with the timing of warming/drying seen in paleohydrologic and packrat
midden records from the southwest USA (Fig. 5). We do not, however, observe the
significantly early warming (> 10 ky) with respect to Greenland at termination I found in
marine cores at the modern southern boundary of the cold south flowing California
current (~32° N). Herbert et al. (2001) have hypothesized that early warming off the coast
of California caused by the collapse of this current could be a regional signal that is
propagated inland and the cause of the early warming before terminations II and III seen
in the Devils Hole record (Winograd et al., 1992) from Nevada (36.5° N, 116.25° E). A
recent extension of the Devils Hole record to 4.5 ka shows that Termination I also
appears to occur at this site 5,000 to 10,000 years earlier than the decline in ice volume
(Winograd et al., 2006).
Lower
18
O values at ~13.0 ka indicate a return to wetter/cooler conditions at our
cave site during the Younger Dryas. The upper cloudy interval in the late YD signals a
brief hiatus. The exact end of the YD is less certain because of this hiatus, but is
estimated to be at 11.48 ka, which is in keeping with global records. The chronology of
the top few millimeters is not well constrained and the higher
18
O values of this section,
indicating drier/warmer conditions and/or more summer relative to winter precipitation,
are displayed “floating” in the earliest Holocene.
29
Millennial Variability in MIS3
The Greenland ice cores, GISP2 and GRIP, have provided the standard for
discussing the pacing and climate patterns of DO events, but uncertainty in the ice core
chronologies increases substantially with age. Although the patterns seen in the
18
O
records of the different Greenland cores are nearly identical, the timing of the events
diverges after 40 ka. The GISP2 age model is based on annual layer counting (up to ~55
ka) and ice flow modeling. Uncertainty in the GISP2 chronology, as estimated by the
authors, increases from 1 to 2% up to ~39 ka, to 5 to 10% from 39 to 45 ka, and then 10
to 20% from 45 to 110 ka (Meese et al., 1997). The original GRIP chronology has
undergone two revisions. First, a refinement of the ice accumulation model-ss09sea
(Johnsen et al., 2001). The ss09sea age model was then further revised through
correlation to a marine core (which had been dated via radiocarbon and calibrated to
calendar years by 230Th dates on corals) with additional tie points from various
speleothem chronologies for the early part of MIS 3 to produce the SFCP 2004 model
(Shackleton et al., 2004). Absolutely dated records such as those found in speleothems
will be necessary to determine the true timing and pacing of DO events in the glacial
(McDermott, 2004). Hulu and Dongge Caves in China have yielded U-Th dated
18
O
records that are interpreted as recording variations in Asian Monsoon strength over the
last 160,000 years (Dykoski et al., 2005; Wang et al., 2001; Wang et al., 2005; Yuan et
al., 2004). Comparisons with ice cores over the deglacial and late MIS3 have shown that
periods of increased summer monsoon precipitation relative to winter precipitation in
Asia are synchronous with warm interstadials in Greenland.
30
Although the Cave of the Bells speleothem
18
O record has a very similar pattern
to those from the two ice cores, the timing of a particular event’s onset can differ by
1000-3000 years amongst the three (Table 2, Fig. 6-7). The discrepancies are also larger
in the older part of the records when uncertainty in the ice chronologies is highest. The
COB record is very similar in timing to the U-Th dated Hulu Cave record (Wang et al.,
2001), but there are a few events where there is a significant difference in event timing
outside of the uncertainties of both chronologies. Most notably, at the COB site DO 8
begins 1,770 years before DO 8 in the Hulu Cave record and DO 12 1,030 years later
than at Hulu Cave. In detail the classic “saw tooth” pattern of rapid warming followed by
slow cooling found in the Greenland ice records is evident in the COB DO 14 but less
evident in DO 12, and almost reversed in DO 8. However, the patterns in the COB
18
O
record closely match those found in records of sea surface temperature from planktonic
forams offshore California in the Santa Barbara Basin (Hendy and Kennett, 1999) (Fig.
8). This implies that while our record is tracking global shifts in climate there remains a
regional imprint on the fine details of the response. Regional processes could also be a
cause of the apparent timing discrepancies between Hulu Cave and Cave of the Bells;
other high-precision absolutely dated records will be required to confirm the chronology
of the DO events that occurred prior to the later part of MIS 3. DO cycles are not present
in the Devils Hole
18
O record from MIS 3 although the resolution should be fine enough
to discern at least the longer DO cycles (Winograd et al., 2006).
31
Potential Mechanisms of Climate Variability
COHMAP (1988) and more recently Bromwich et al. (2004) modeled a split in
the jet stream over North America, largely due to the presence of the Laurentide ice
sheet, during the last glacial maximum with the southern branch dipping down to about
30° N. This would have brought overall cooler temperatures as well as more moisture
from the Pacific to southern Arizona. Our speleothem
18
O values reach a minimum
during the LGM, consistent with wetter/cooler conditions and more winter relative to
summer precipitation infiltrating the cave (Fig. 5).
In the Great Basin, Owens and Pyramid Lakes record millennial oscillations from
24 to 52 14C ka. Although uncertainties in the age models of the lakes makes comparisons
tentative, it appears that North Atlantic DO interstadials were associated with warm and
wet conditions at the lakes and stadials with cool and dry (Benson et al., 2003). Pollen
and macrofossils from Potato (Anderson, 1993) and Walker Lakes (Hevly, 1985) in
Arizona also indicate that the LGM was much colder and wetter than present and MIS3
cooler and wetter with some indication of drier intervals.
We suggest that the jet stream was in a more southerly position during stadials (as
well as the LGM) bringing cooler temperatures to our Cave site as well as the Great
Basin Lakes, but because our cave site was near the boundary of the jet, the location also
received increased moisture from the Pacific. By contrast, the regions farther north in the
Great Basin were drier during stadials because of strong northeasterly flow of cold dry air
off of the Laurentide ice sheet (Benson et al., 2003). Although this circulation pattern at
the LGM is mostly driven by the presence of the large ice sheet, we do not expect that
32
large ice sheet changes were responsible for the millennial variability observed in the
western US. GCM studies (Broccoli et al., 2006; Dahl et al., 2005) indicate that during
times of cooling in the North Atlantic (due to diminished thermohaline circulation forced
in the models with freshwater input) the intertropical convergence zone (ITCZ) shifted
south. The models also suggest that the southwest cooled and experienced wetter winters
during these times of North Atlantic cooling (Vellinga and Wood, 2002; Dahl, pc.;
Broccoli, pc).
Santa Barbara Basin sediments also point to warmer sea surface temperatures
during interstadials due to decreased strength of the south-flowing cold California current
from a northern shift in the position of the North Pacific High pressure system, and,
hence, the jet stream (Hendy and Kennett, 1999). In the modern climate, cool northern
Atlantic waters (negative AMO-Atlantic Multidecadal Oscillation) are associated with
relatively wet winters in the southwest but also the Great Basin (McCabe et al., 2004)
Another possibility is that North Atlantic DO climate events are being propagated
to the southwest U.S. by ocean/atmosphere dynamics in the Pacific Ocean. In the modern
southwest, El Niños (La Niñas) are associated with wetter (drier) winters (Sheppard et al.,
2002) and this teleconnection is accentuated during times when the Pacific Decadal
Oscillation (PDO) is in a positive (negative) phase (Gershunov and Barnett, 1998). A
similar link may have been in effect during the glacial, but the behavior of ENSO in the
glacial is unclear from current studies. Marine records from the Pacific suggest that
conditions were more El Niño-like during the LGM (Koutavas et al., 2002; Palmer and
Pearson, 2003) and stadials (Stott et al., 2002), and millennial speleothem records from
33
Oman and China also reveal weaker summer monsoons, which today are associated with
El Niños, during stadials (Burns et al., 2003; Wang et al., 2001). But other records that
indicate dry conditions in northern Australia (Turney et al., 2004), a northward-displaced
Atlantic ITCZ (Peterson et al., 2000), and, close to Arizona, warm surface waters and
increased “southern-component” intermediate waters in the Santa Barbara Basin (Hendy
and Kennett, 1999; Hendy and Kennett, 2003) suggest the opposite, that the Pacific was
in an El Niño-like state during interstadials. Clement et al. (1999; 2000) proposed a
model of ENSO behavior modulated by the orbital precessional cycle that predicts strong
and/or more frequent El Niño events during times of perihelion in boreal winter-spring,
but coral data presented by Tudhope et al. (2001) suggested that this forcing may be
dampened by overall cool conditions in the glacial. However, other model results suggest
more El Niño-like conditions during the LGM with frequent larger magnitude El Niño
and La Niña events (Otto-Bliesner et al., 2003). Our record is consistent with more El
Niño-like conditions during stadial intervals and the LGM, but background conditions
may have been different enough in the glacial that ENSO teleconnections were weaker
outside the tropics.
The magnitude of the shifts in COB speleothem
18
O values during some of the
DO interstadials requires an explanation beyond changes in precipitation amount and
temperature. During the BA, DO 7, and DO 14 speleothem
18
O values were above
modern values and the peak values of the other glacial DO events. There is no evidence
that these intervals were drier/warmer than present. We propose that increased summer
insolation during these intervals spurred an increase in the infiltration of summer
34
monsoon precipitation relative to winter and that this, in conjunction with the
drier/warmer winters brought about by the jet stream moving north during interstadials,
led to the observed higher than modern
18
O values (Fig. 9). Increased summer
precipitation due to increased summer insolation also helps to explain the very high
18
O
values during the early Holocene from COB-01-02 and the mid-Holocene from COB-0103 (Mid-Holocene climate in Southern Arizona inferred from speleothem stable isotopes,
hereinafter referred to as Wagner et al. in preparation, 2006b). Insolation at 30º N in June
is not, however, a perfect match with the low frequency variablitiy in the COB
18
O
records. The BA and DO 7 roughly correspond with peak insolation, but DO 14 and the
mid-Holocene lag the insolation peaks, although summer insolation during these times is
greater than modern.
Spectral Analysis
A pacing of approximately 1500 years has been identified in climate proxy
records from Greenland ice cores and North Atlantic sediments (Bond et al., 1999;
Mayewski et al., 1997). Clemens (2005) suggests that the precise period of this muchdiscussed variance is an artifact of the GISP2 age model. Clemens analyzed the spectra of
the two Greenland ice cores and compared the results to the absolutely dated Hulu Cave
record (Wang et al., 2001) over the interval 10.5 to 60 ka. He found that while GISP2
displays strong variance at a period of 1470 years, GRIP (SFCP2004 age model) has a
markedly different spectrum with peaks at 1667 (with a shoulder at 1490) and 1190
years. The spectrum of the Hulu Cave speleothem record contains peaks at 1667, 1490,
35
and 1190, closely resembling the spectrum of GRIP (SFCP2004). Clemens suggests that
these three millennial cycles found in the GRIP and Hulu Cave records may derive from
heterodynes (difference tones) of centennial solar cycles (~716, 501, 352, and 285 years)
identified from records of
14
C production in the atmosphere. Braun et al. (2005) also
attempt to link solar multidecadal and centennial variability to millennial oscillations in
the glacial. Using an intermediate complexity model with background glacial conditions
they were able to obtain climate shifts similar to DO events with a spacing of ~1470
years in response to North Atlantic freshwater forcing on a timescale derived by
superimposing ~87 and ~210 year solar cycles.
Our Cave of the Bells speleothem record is absolutely dated with U-Th and thus
provides an independent determination of the dominate periods of climate variability in
the last glacial. The COB record is not continuous over the last glacial, thus, we limited
our spectral analysis for all the records to the period in each that spans DO events 5-14.
The records were each sampled at equal increments at the highest resolution possible.
The COB and GRIP (SFCP 2004) data were sampled at 40-year increments, making it
possible to discern century-scale variability directly; the GISP2 record was sampled at an
interval of 150 years and Hulu Cave at 200 years.
We found significant variance at the ~1500-year period in GISP2, GRIP
(SFPC2004), and our COB record over the interval spanning DO 5-14 (Table 3, Fig. 10).
The Hulu Cave record does not have significant variance around 1500 years; its
millennial variability is skewed towards periods greater than 1600 years. Both ice cores
also contain significant variance at ~1030 years, while the cave records display variance
36
that brackets this period at 1163 (Hulu) and 890 (Hulu and COB). Hulu and COB also
both have multicentury variance at ~650-660 years. Multicentury to multidecadal
variance from these four records shares a few periods of variability with published
estimates of solar variability from
14
C atmospheric production records (e.g. Damon and
Peristykh, 2000). Common spectral peaks are seen at 714 years from GISP2 (similar to
706 from
14
C); 281, 231, and 219 years from COB (similar to 288, 228, and 207 ); and
142, 133, 127,122,105, 96, and 86 from GRIP(SFCP 2004) (similar to 149, 136, 130,123,
104, 97.5, and 88).
These results suggest that despite uncertainties in the ice core age models
(discussed above), the variance at a period of ~1500 years is replicated in the absolutely
dated COB speleothem record. Less clear is why the spectrum of the Hulu Cave
speleothem record is different from the other three. The timing of the DO events is very
similar in the Hulu and COB records, although the shape of the DO events in the Hulu
Cave
18
O record are perhaps not as similar as those in COB to the ice core records,
which could partly be a function of higher resolution in the COB record.
Conclusions
The U-Th dated speleothem
18
O record from Cave of the Bells documents that climate
in southern Arizona moved in step with global climate over the last glacial cycle, with no
sign of early warming at the last termination, and provides precise timing for the
transitions in MIS 3. The warming during Dansgaard-Oeschger events and the BøllingAllerød in the North Atlantic coincided with drying/warming and/or an increase in
37
summer relative to winter precipitation in southern Arizona. Cooling during stadials and
the Younger Dryas corresponded with wet/cool conditions and/or a decrease in summer
relative to winter precipitation. These climate oscillations in the southwest are likely
related to movement in the long-term average position of westerly storm tracks- wetter
and cooler when in a southerly position- and could also be influenced by the state of the
Pacific- wet/cool during times of a dominant El Niño-like and/or positive PDO-like
pattern. Precessional changes in summer insolation at the cave site also may have
enhanced the magnitude of the increase in
18
O values during some of the interstadials by
increasing the ratio of summer relative to winter precipitation. More speleothem records
from a north-south transect through the southwest could help to determine the location of
the westerly jet through time and track the relative strength of the summer monsoon.
Spectral analysis of the COB record confirms the presence of a ~1500 year cycle of
climate variability during the glacial.
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43
Figure captions
Figure 1. Map of locations discussed in text. Tus-Tucson, Arizona, USA. COB- Cave of
the Bells. PL- Pyramid Lake. ML-Mono Lake. OL- Owen’s Lake (Benson et al., 2003).
DH- Devils Hole (Winograd et al., 1992). SBB- Santa Barbara Basin (Hendy and
Kennett, 1999). LE- Lake Estancia (Allen and Anderson, 2000). GMC- Guadalupe
Mountain Caves (Polyak et al., 2004).
Figure 2. Twenty four years (1981-2005) of weighted monthly 18O (VSMOW) values of
Tucson precipitation and the weighted mean over this interval (personal communication,
Austin Long, University of Arizona Geosciences). Note the high degree of variability in
every month regardless of season. The bar shows the range of measured dripwater values,
which falls clearly in the range of winter season values.
Figure 3. U-Th dates versus depth from the top of the stalagmite. Solid curving lines are
polynomial and spline fits, used for the age model of the sections before and after the
long hiatus from ~23 to 30 ka, respectively.
Figure 4. “Temporal test” for speleothem equilibrium deposition (after Mickler et al.,
2006). Gray x are early Holocene values (slope of 0.8). Black squares are MIS 3 values
(slope 0.37). Gray triangles are deglacial values (slope –0.1). The dashed line is the trend
for the entire speleothem record (slope 0.27).
Figure 5. Blow-up of deglacial interval; numbers refer to Dansgaard-Oeschger (DO)
events. BA- Bølling-Allerød, YD- Younger Dryas. (a.) In blue, Cave of the Bells
stalagmite COB-01-02 18O (VPDB) values. Heavy line is corrected for ice volume, light
line is uncorrected 18O values. Higher values indicate drier/warmer/more summer
relative to winter precipitation in southern Arizona. (b.) In green, Greenland ice core
(GISP2) 18O (VSMOW) values; higher values indicate warmer conditions (Grootes and
Stuiver, 1997). (c.) In red, Hulu Cave stalagmite 18O (VPDB) values from samples PD
(10.5 to 19.3 ka) and MSD (18.3 ka on) (Wang et al., 2001). Notice reversed scale, lower
18
O values indicate a higher ratio of summer to winter monsoon precipitation. (d.)
Cessation of spring deposits in San Pedro River valley (just to the east of COB) at 15.4 ka
(Pigati et al., 2004). (e.) Cold and warm periods inferred from packrat middens in the
Grand Canyon (Cole and Arundel, 2005) roughly corresponding to the YD and BA. (f.)
Wet periods in the Guadalupe Mountains, NM inferred from speleothem growth (Polyak
et al., 2004). (g.) Periods with highstands in Lake Estancia, NM (Allen and Anderson,
2000).
Figure 6. Deglacial interval through MIS 3, numbers refer to Dansgaard-Oeschger (DO)
events; BA- Bølling-Allerød, YD- Younger Dryas. (a.) In blue, Cave of the Bells
stalagmite COB-01-02 18O (VPDB) values. Heavy line is corrected for ice volume, light
line is uncorrected 18O values. Higher values indicate drier/warmer/more summer
44
relative to winter precipitation in southern Arizona. (b.) In green, Greenland ice core
(GISP2) 18O (VSMOW) values; higher values indicate warmer conditions(Grootes and
Stuiver, 1997). (c.) Hulu cave stalagmite 18O (VPDB) values from samples PD (10.5 to
19.3 ka) and MSD (18.3 ka on) (Wang et al., 2001). Notice reversed scale, lower 18O
values indicate higher ratio of summer to winter monsoon precipitation.
Figure 7. Mid-point of the start of DO events in COB minus the mid-point of the start of
the DO events in: Hulu Cave (Wang et al., 2001), red circles; GISP2 (Grootes and
Stuiver, 1997), green squares; GRIP ss09sea (Johnsen et al., 2001), solid blue triangles;
and GRIP SFCP2004 (Shackleton et al., 2004), open blue triangles.
Figure 8. Comparison of the pattern of DO events seen in the (a.) planktonic foram G.
bulloides 18O record of sea surface temperature in Santa Barbara Basin (Hendy and
Kennett, 1999), in purple, and (b.) Speleothem 18O values from Cave of the Bells, in
blue. Notice the reversed scale in the foram record, lower values indicate warm SST.
Higher speleothem values indicate drier/warmer/more summer relative to winter
precipitation in southern Arizona.
Figure 9. COB speleothem 18O values from (a.) in light blue, mid-Holocene COB-01-03
and (b.) in dark blue, COB-01-02. COB-01-02 dark blue line is corrected for ice volume
and gray line is raw 18O values. The black bar represents modern calcite 18O value of ~10.6. (c.) Insolation at 30°N during June (Berger and Loutre, 1991).
Figure 10. Spectra of (a.) GISP2 (Grootes and Stuiver, 1997), (b.) GRIP (SFCP2004)
(Shackleton et al., 2004), (c.) Hulu Cave (Wang et al., 2001), and (d.) COB. Thin black
lines are the MTM spectra and heavy black lines the MEM. Gray curves are 95%
confidence from MTM. The vertical gray bar denotes the 1515-1470 year band.
45
Figure 1.
46
2.0
-2.0
-4.0
-6.0
-8.0
-10.0
-12.0
18
Tucson Precipitation (VSMOW)
0.0
Avg. cave waters -9.6
-14.0
-16.0
0
2
4
6
Month
Figure 2.
8
10
12
47
Figure 3.
48
-2
y = 0.80x + 0.21
2
R = 0.19
y = 0.37x - 3.54
13C (VPDB)
-4
2
R = 0.20
-6
y = 0.23x - 4.90
2
R = 0.03
-8
y = -0.96x - 16.78
2
R = 0.11
-10
-13.0
-11.0
-9.0
18O (VPDB)
Figure 4.
-7.0
-5.0
49
Figure 5.
50
Figure 6.
51
2.50
COB minus other reccords (ka)
2.00
1.50
1.00
COB older
0.50
0.00
-0.50
COB younger
Hulu Cave
-1.00
GISP2
-1.50
ss09sea
-2.00
SFCP2004
-2.50
30.00
35.00
40.00
45.00
COB age (ka)
Figure 7.
50.00
55.00
60.00
52
-7.5
5
6
7
8
12
13
14
0.5
1.5
a.
2.5
-8.5
5
-9.5
6
7
8
3.5
13
18
12
-10.5
4.5
b.
5.5
-11.5
18
O COB-01-02 (VPDB)
14
O G. bull. Hole 893A (VPDB)
-6.5
-12.5
30000
35000
40000
45000
Year BP
Figure 8.
50000
6.5
55000
53
600
-7.0 a.
1-BA
14
b.
5 67
8
12 13
30N Jun
550
YD
-11.0
18
O (VPDB) COB
-9.0
500
c.
-13.0
-15.0
450
0
5000 10000 15000 20000 25000 30000 35000 40000 45000 50000
Calendar Age (yr)
Figure 9.
54
1515-1470
a. GISP2
1.0E+07
b. GRIP
Scaled power
1.0E+05
1.0E+03
c. Hulu
1.0E+01
1.0E-01
d. COB
1.0E-03
0
0.0005
0.001
0.0015
Freq (cycle/yr)
Figure 10.
0.002
0.0025
#
$
$
$
&
#
$
&
&
#
&
*
*
$
$
#
$
$
$
#
$
$
#
*
*
*
*
*
1-6
1-6R
3-5
5-7
7-10
7-12
7-12R
10-11.5
12.5-14
12.5-16.5
14-15.5
16-19
19-21.5
19.75-21.75
21.75-23.25
21-25
23.25-25
25-26.75
26.75-28.5
30-34
33-34.5
36-37.25
35-39
40.5-43
47.5-49.5
49.5-51.5
51.5-53.5
53.5-55.5
Depth
mm from top
Table 1
U-Th data
3.5
3.5
4
6
8.5
9.5
9.5
10.75
13.25
14.5
14.75
17.5
20.25
20.75
22.5
23
24.125
25.875
27.625
32
33.75
36.625
37
41.75
48.5
50.5
52.5
54.5
9487
10266
11009
11784
12469
12013
12211
13456
12997
12696
13137
14641
16746
16406
19781
18255
20868
23149
30441
31934
33809
34663
34566
36883
38607
38422
39776
40382
a
Age
Center Depth
mm from top yr (before 1950)
b
138
128
56
48
207
94
135
232
218
113
218
105
120
81
136
123
76
109
101
239
105
109
224
295
173
166
184
165
Error
2
0.20526
0.22216
0.23712
0.25696
0.27294
0.26213
0.26730
0.29358
0.28360
0.27814
0.28585
0.32176
0.37209
0.36518
0.43519
0.40303
0.45925
0.50565
0.60944
0.63977
0.67389
0.69262
0.68410
0.73118
0.77468
0.77813
0.79912
0.80541
238
230
0.20520
0.22198
0.23643
0.25666
0.27248
0.26208
0.26729
0.29351
0.28345
0.27799
0.28575
0.32047
0.37007
0.36400
0.43221
0.40198
0.45888
0.50421
0.60919
0.63974
0.67386
0.69259
0.68407
0.73111
0.77462
0.77761
0.79837
0.80530
238
Th/ U
Th/ U
measured corrected
230
230
Th/
Th
2280
862
248
635
424
4011
17384
3032
1270
1371
1978
174
126
213
97
260
830
226
1491
11565
14191
13336
12067
5678
7543
847
586
3920
232
0.160
0.158
0.153
0.191
0.187
0.188
0.183
0.168
0.218
0.200
0.206
0.247
0.234
0.226
0.227
0.225
0.245
0.217
0.197
0.186
0.183
0.208
0.188
0.188
0.206
0.213
0.205
0.188
U
ppm
238
234
238
2.47
2.48 Hiatus
2.48
2.52
2.54
2.53
2.54
2.55
2.54
2.55
2.54
2.58
2.63
2.64
2.65
2.65
2.68
2.69 Hiatus
2.57
2.59
2.61
2.62
2.60
2.63
2.69
2.71
2.71
2.70
( U/ U)int
55
*
#
#
$
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
#
*
55.5-57.5
55-60
55-60R (85%)
55-60R (15%)
60-62
62-64
64-65.5
65.5-67
67-68.75
68.75-71
73-75
75-76.5
76.5-78.5
80.5-82
82-84
86-88
88-90
90-92
92-94.5
94.5-96.5
96.5-98.5
98.5-100
100-102
102-104.5
104.5-106
106-108
108-109.5
109.5-111
110-114R
114.5-115.5
Depth
mm from top
Table 1 continued
56.5
57.5
57.5
57.5
61
63
64.75
66.25
67.875
69.875
74
75.75
77.5
81.25
83
87
89
91
93.25
95.5
97.5
99.25
101
103.25
105.25
107
108.75
110.25
112
115
40424
39509
39974
40159
41025
40691
41491
41510
41260
42069
43777
44769
44494
45465
45627
46244
46194
46638
46654
46770
47767
47034
47512
47585
48202
50075
50425
50256
50657
52271
a
Age
Center Depth
mm from top yr (before 1950)
b
239
343
190
499
172
177
167
170
215
160
174
250
207
253
254
170
207
193
242
170
233
211
226
220
285
343
379
308
734
249
Error
2
0.80756
0.79094
0.79678
0.80201
0.82267
0.81926
0.82660
0.82687
0.82097
0.83658
0.86890
0.88994
0.89137
0.90318
0.90840
0.90676
0.90053
0.90821
0.90584
0.90434
0.92366
0.90021
0.90630
0.89960
0.90607
0.94105
0.95594
0.96089
0.96309
1.00886
238
230
0.80748
0.79089
0.79658
0.80184
0.82261
0.81920
0.82654
0.82675
0.82090
0.83647
0.86846
0.88933
0.89118
0.90313
0.90831
0.90667
0.90046
0.90813
0.90576
0.90427
0.92356
0.90012
0.90622
0.89952
0.90598
0.93655
0.95183
0.95966
0.96047
1.00868
238
Th/ U
Th/ U
measured corrected
230
230
Th/
Th
5538
8312
2235
2600
8012
7428
7043
3992
6371
4331
1054
784
2428
9919
5050
5599
6768
5778
6183
6306
4654
5149
5786
5904
5044
106
117
392
185
2834
232
0.187
0.188
0.188
0.187
0.208
0.232
0.205
0.198
0.200
0.228
0.236
0.238
0.234
0.219
0.242
0.243
0.209
0.212
0.186
0.217
0.132
0.154
0.162
0.200
0.191
0.195
0.199
0.181
0.176
0.157
U
ppm
238
234
238
2.71
2.70
2.69
2.70
2.73
2.73
2.71
2.71
2.71
2.72
2.74
2.76
2.77
2.77
2.77
2.74
2.72
2.73
2.72
2.71
2.72
2.68
2.68
2.66
2.65
2.67
2.70
2.73
2.71
2.78
( U/ U)int
56
115-121
115-121R
121-126
121-126R
118
118
123.5
123.5
50764
51718
52660
52735
a
Age
Center Depth
mm from top yr (before 1950)
b
712
650
368
351
Error
2
0.98177
0.99514
1.02352
1.02130
238
230
Th/
Th
3112
1582
931
6803
232
Th = 0.8
Th/
232
230
0.156
0.152
0.155
0.166
U
ppm
238
234
Th/
232
Th activity of 0.8 ± 50%
* UA-U on TIMS, Oxford-Th on MC-ICP-MS
# UA-U and Th on TIMS
$ UA- U and Th on MC-ICP-MS
& UA- U on MC-ICP-MS, Oxford- Th on MC-ICP-MS
230
238
2.77
2.77
2.81
2.80
( U/ U)int
Error for all samples includes measurement error, decay constant uncertainty, and initial Th assuming an estimated value
for
b
230
0.98161
0.99483
1.02298
1.02122
238
Th/ U
Th/ U
measured corrected
230
Ages corrected assuming initial Th has a value similar to bulk upper continental crust,
a
#
$
#
#
Depth
mm from top
Table 1 continued
57
3
4
5
6
7
8
9
10
11
12
13
14
COB
32.54
33.54
35.86
39.24
40.63
42.59
43.85
46.62
48.46
52.96
11.48
15.06
11.53
14.70
28.01
29.96
32.89
33.90
35.39
37.47
40.03
41.80
43.85
47.65
49.40
53.00
11.66
14.69
27.84
29.03
32.37
33.59
35.32
38.43
40.25
41.17
42.58
45.46
47.25
52.14
Hulu Cave GISP2
27.40
28.56
32.26
33.58
35.38
38.34
40.34
41.74
43.68
47.36
49.78
55.08
GRIP
ss09sea
Ages, in ka, of the mid-point of the warming transition.
Post YD
BA
Event
Table 2
Comparison of Dansgaard-Oeschger event timing.
Modified from Shackleton et al. (2004).
11.50
14.62
29.00
30.06
33.44
34.64
36.29
39.00
40.83
42.10
43.87
47.24
49.45
54.29
GRIP
SFCP2004
0.17
-0.05
0.54
0.81
0.38
1.42
1.27
1.16
1.21
0.82
Ages differences, in ka.
-0.35
-0.36
0.47
1.77
0.60
0.79
0.00
-1.03
-0.94
-0.04
COB minus…….
Hulu Cave GISP2
0.28
-0.04
0.48
0.90
0.29
0.85
0.17
-0.74
-1.32
-2.12
GRIP
ss09sea
-0.90
-1.10
-0.43
0.24
-0.20
0.49
-0.02
-0.62
-0.99
-1.33
GRIP
SFCP2004
58
383
COB-01-02 (DO 5-14) MTM & MEM (150)
328
142
281
133
2151
266
127
1797
231
122
1515
1515
1477
219
185
178
172
168
86
Multi-century to multidecadal
105
101
96
93
652
661
891
890
759
1163
1023
Millennial to mulitcentury
1031
714
b
Periods (yr)
111
81
600
109
488
MTM = Multitaper Method (resolution 2, tapers 3) and MEM = Maximum Entropy Method (order noted, ~1/4 of N, # of points in time series)
on time series detrended by removing 1st Reconstructed Componet from Singular Spectrum Analysis (SSA);
results from SSA-MTM Toolkit for spectral analysis, Ghil, et al. (2002), http://www.atmos.ucla.edu/tcd/ssa/guide/
b
Significant above 95% from MTM
a
GRIP04 (DO 5-14)
172
MTM & MEM (30)
Hulu Cave (DO 5-14)
MTM & MEM (150)
MTM & MEM (150)
GRIP04 (DO 5-14)
3419
2740
MTM & MEM (40)
GISP 2 (DO 5-14)
COB-01-02 (DO 5-14) MTM & MEM (150)
2793
Spectral Method
Record
a
Table 3
Millennial, multicentury, and multidecadal variability in glacial climate records
450
59
60
APPENDIX B: MID-HOLOCENE CLIMATE IN SOUTHERN ARIZONA
INFERRED FROM SPELEOTHEM STABLE ISOTOPES
Jennifer D. M. Wagner1*, Julia E. Cole1, 2, J. Warren Beck3, P. Jonathan Patchett1, and
Gideon M. Henderson4
1
Department of Geosciences, University of Arizona, Tucson, Arizona 85721
Department of Atmospheric Sciences, University of Arizona, Tucson, Arizona 85721
3
Accelerator Mass Spectrometry Facility, Department of Physics, University of Arizona, Tucson,
Arizona 85721
4
Department of Earth Sciences, Oxford University, Oxford, UK
2
Abstract
We have collected stalagmites from Cave of the Bells (elevation 1700 m) located
~75 km southeast of Tucson, Arizona on the northeast side of the Santa Rita Mountains.
High-resolution (<10 years)
18
O data from a Holocene stalagmite (~6.9-3.5 ka, U-series
chronology) exhibit higher values than modern and substantial multidecadal to
multicentury variation. Speleothem
of temperature of formation and
18
18
O values at this site should reflect a combination
O values of the cave waters, which in turn are
controlled by temperature, amount, and seasonality of precipitation. Studies of modern
cave water and precipitation at this site and regional groundwater studies indicate that
most recharge is from winter moisture despite a summer monsoon that contributes ~50%
of annual rainfall. Modern precipitation amount and temperature relationships with the
18
O values of Tucson precipitation suggest that changes in these parameters alone are
not enough to account for the 3‰ increase (relative to modern) in
18
O values observed
in the mid-Holocene stalagmite. We propose that in addition to drier/warmer conditions
in the winter, a stronger summer monsoon and perhaps warmer summer temperatures
supplied waters with higher
18
O values to the cave. Spectral analysis of early part of the
61
18
O record reveals variability at periods of 233 years and at 142 and 52 years which may
be expressions of solar variability. After ~4.9 ka a prominent shift from centennial to
multidecadal periods of variability (a 70 to 50-year cycle) is observed and there is a slight
decrease in average
18
O values. This shift is coincident with a hypothesized increase in
El Niño activity, which is correlated to wet winters in the modern southwest, in the
tropical Pacific at ~5 ka.
Introduction
Population in the western states increased 20 to 60% in the 1990s
(http://www.census.gov/population/cen2000/phc-t2/tab03.pdf), a trend that is predicted to
continue. This growth and periodic drought, such as the recent ~1998-2005 drought, are
already straining the limited groundwater resources and surface water reservoirs in the
region. Recognition of this demographic reality and the west’s vulnerability to drought is
spurring cooperative planning between federal, state, local, and tribal governments
(http://www.doi.gov/water2025/Water2025-Exec.htm). This planning requires a complete
understanding of the range of climate variability possible in the western states, as well as
the ocean/atmospheric mechanisms (both long and short term) that could result in
extreme droughts or wet periods.
The past 100-200 years of instrumental and historical climate data are inadequate to
understand the full range of climate variability in the southwest (Cook et al., 2004). By
developing longer records of regional climate fluctuations, we can determine how
frequently such events as megadroughts (or wet, warm, or cold periods) occurred and
62
whether they were more frequent and/or intense during times of different global
background climate, such as ice ages or during times of changed radiative forcing, such
as the mid-Holocene. Summer insolation was increased and winter insolation decreased
in the mid-Holocene relative to today (Berger and Loutre, 1991). Increased summer
insolation should strengthen the thermal low-pressure system over southern Arizona,
which is an important precursor to summer monsoon precipitation in the region
(Sheppard et al., 2002). Evidence from paleoclimatic data and model simulations
suggests that ENSO variability was weaker, and the tropical Pacific potentially more La
Niña-like, relative to modern (Clement et al., 2000; Cole, 2001). Today, reduced winter
precipitation accompanies La Niña conditions (Sheppard et al., 2002).
Continuous paleoclimate records from the southwest, with subdecadal resolution
and well-constrained chronologies, are relatively rare, particularly for the full Holocene.
Packrat middens have traditionally been one of the main sources of paleoclimate
information in the semi-arid southwest; they offer radiocarbon-dated “snapshots” of
vegetation at a particular time which can be interpreted in climatic terms (e.g. Betancourt
et al., 1990). A well documented “midden gap” occurs in the mid-Holocene, however,
limiting their applicability between about 4 to 9 ka (Betancourt et al., 1993). Continuous
records from lakes are few (e.g. Anderson, 1993; Benson et al., 2002; Castiglia and
Fawcett, 2006; Hasbargen, 1994; Hevly, 1985; Menking and Anderson, 2003; Metcalfe et
al., 2000) and relatively low-resolution (e.g. ~1000 years for Lake Estancia) . Tree-ring
records provide annual records of climate from forested regions in the southwest, but
typically extend only a few centuries, with the longest chronologies reaching ~2000 years
63
(e.g. Grissino-Mayer, 1996; Hughes and Graumlich, 1996; LaMarche, 1974; Salzer and
Kipfmueller, 2005). Speleothems can provide a high-resolution, continuous record of
moisture/temperature variations and can be precisely dated by uranium-series
disequilibrium, and thus far in the region have only been presented for the late Holocene
and deglacial periods (Polyak and Asmerom, 2001; Polyak et al., 2004; Rasmussen et al.,
2006). Here we present a speleothem record of mid-Holocene climate variability from
southeastern Arizona that displays large, long-term changes in temperature and moisture
unprecedented in the instrumental record.
Setting
We collected the stalagmite COB-01-03 from Cave of the Bells (COB), located in
Santa Cruz County, Arizona on the east side of the Santa Rita Mountains (31°45'N,
110°45'W) at an elevation of 1700m (Fig. 1). The vegetation above the cave is best
characterized as oak-juniper woodland with an understory of C4 grasses and CAM
succulents (Stable isotope composition of speleothem calcite and associated cave and soil
CO2, Cave of the Bells, Arizona, hereinafter referred to as Fischer et al. in preparation,
2006). The cave is situated in the Permian Colina Limestone below an isolated hill at
shallow depths, indicating that the infiltrating water that forms the speleothems is from
rain that falls on the immediate area and is not supplied by regional groundwater. Cave
humidity is very high and formations are actively growing. The cave has only one small
opening, and the cave temperature is a constant 19.5°C (Using long-term records of
isotopes in precipitation from Tucson, Arizona to calibrate cave water isotopic response
64
to climate in Cave of the Bells, Arizona, hereinafter referred to as Wagner et al., in
preparation, 2006a).
In our study region, roughly half the annual precipitation comes during the
summer monsoon from July to September (Fig.2), sourced from the Gulf of California
and the eastern tropical Pacific (Wright et al., 2001). Dissipating tropical cyclones can
also contribute significant moisture, in certain years, during September and October.
Periodic westerly frontal storms from the Pacific supply the other half of the annual total
from October to March (Sheppard et al., 2002 and references therein). High temperatures
and vegetation demand, combined with “flashy” distribution, cause most of the summer
precipitation to be lost to runoff or evapotranspiration, while cooler temperatures and
lower rainfall rates allow winter rainfall to preferentially enter the groundwater system,
accounting for the bulk of recharge (Baillie, 2005; Eastoe et al., 2004; Wahi, 2005).
The long-term (1981-2005) weighted average of Oct-Mar precipitation
18
O
values in Tucson, Arizona (~75 km to the NW of the cave site, elevation 780 m) is
-9.2‰, whereas summer monsoon (Jul-Sept) precipitation averages a much higher -5.6‰
(Wagner et al., in preparation, 2006a). At Cave of the Bells, a three-year monitoring
study (February 2003 to May 2006) demonstrates that infiltrating waters derive mainly
from winter precipitation (Fig. 2), in keeping with the regional pattern. The
18
O values
of the dripwaters from three sites in the cave averages ~-9.6‰ (VSMOW) and has varied
less than 1‰ between all three sites over the monitoring period (Wagner et al., in
preparation, 2006a). The cave dripwater values are likely slightly lower than the Tucson
65
winter average because of the isotopic depletion expected at higher elevation (Rozanski
et al., 1993).
Methods
After removal from the cave, the stalagmite was cored with a 1” drill bit and the
core halved and polished. One side was sectioned into 6-7 mm increments with a handheld drill and thin saw blade attachment for low-resolution U-Th dating on TIMS and
MC-ICP-MS. The other side was sampled for high-resolution U-Th (at ~1-3 mm
increments, with a diamond impregnated wire saw) via TIMS and MC-ICP-MS. A thin
polished slab from the middle of the core was sampled along the growth axis for stable
isotope analysis at 80 µm increments with a computer-controlled micromill.
Chemical processing of samples for U-Th, following Edwards et al. (1987), for 10
of the 15 U-Th dates was performed at University of Arizona. Samples were completely
dissolved in ~2M HNO3 (no detrital material was present) and equilibrated with a mixed
spike of 233U and 229Th. U and Th were co-precipitated with Fe(OH)3 and separated with
two stages of ion exchange columns (for more details see Placzek et al., 2006). One
sample was analyzed on a Micromass Sector 54 TIMS at University of Arizona, one on
Micromass MC-ICP-MS at UA, and the U from the remaining eight were run on TIMS at
UA and the Th splits at Oxford University on a Nu-Plasma MC-ICP-MS. For the other
five dates chemical processing of the samples was performed at Oxford University and
both U and Th analyzed on the Nu-Plasma MC-ICP-MS. For these samples chemical
procedures also followed Edwards et al. (1987), but they were spiked with a mixture of
236
U and 229Th.
66
At University of Arizona TIMS analyses largely follow Goldstein and Stirling
(2003), described in detail by Placzek et al. (2006). Th is loaded with graphite on a single
Re filament and the 229Th, 230Th, and 232Th isotopes are measured dynamically on the
Daly detector. The Daly is equipped with pulse-counting system, a pulse-height
discriminator, and retardation filter. Calibration for peak shape, dark noise counts,
multiplier response, discriminator settings, and dead-time are also routinely maintained.
U is run on a triple Re filament and measured in multi-static mode. At the first magnet
position 234U collected is on the Daly and 233U, 235U, and 238U on the Faraday cups. At the
second magnet position 235U is measured on the Daly and 238U on a Faraday cup and then
this ratio is used to correct for Daly gain and mass fractionation. On the MC-ICP-MS at
UA the chemically separated elements are aspirated into the plasma through an Aridus
nebulizer. Uranium is measured in static mode with 234U collected on the ion counter and
the other isotopes on Faradays. Gain is corrected with bracketing internal lab standards
(composition determined by TIMS analysis) and mass fractionation from bracketing
measurements of natural uranium CRM-112a (formerly known as NBS-960). Thorium is
also run in static mode with 230Th measured on the ion counter and external corrections
for gain and mass fractionation utilizing internal lab standards.
Oxford University U-Th MC-ICP-MS procedures are detailed in Robinson et al.
(2002). Samples are introduced via a Cetac Aridus nebulizer. Uranium is measured
statically with 234U in the ion counter and 235U, 236U, and 238U on the Faradays.
Bracketing CRM-145 U standards are used to correct for gain and drift (first standard)
and to check external reproducibility (second standard). Measured 238U /235U ratios of the
67
standards were compared to its true value of 137.88 to correct for mass fractionation.
Thorium is measured dynamically with 229Th and 230Th alternately in the ion counter and
232
Th on the Faraday.
Stable isotopes are expressed using the delta notation as in the following example:
18
Osample = {((18O/16O)sample/(18O/16O)reference) –1} * 1000‰
All water stable isotope values are referenced to the VSMOW standard and calcite stable
isotopes values to the VPDB standard. Oxygen and carbon isotopic analysis was carried
out on a Micromass Optima dual inlet stable mass spectrometer with an automated
carbonate preparation system at University of Arizona, with better than 0.08‰ and
0.04‰ analytical precisions, respectively.
Spectral analysis was performed on the Cave of the Bells
18
O values and
14
production records from Reimer et al. (2004). The cave record and the
14
C
C production
record were sampled at 5-year increments and spectral analysis was performed with the
SSA-MTM Toolkit (Ghil et al., 2002). First, the entire COB
to 6.85 ka (3.5 to 7 ka for
14
18
O record, spanning 3.55
C), was analyzed, and then it was divided into two parts at
around 4.9 ka. We removed low-frequency variability from the COB
18
O record by
subtracting the first reconstructed component (RC) indicated by singular spectrum
analysis (SSA) from the raw data. For the
14
C production record, we used a version
from which the 1000-year trend was removed by the authors. The Multitaper Method
(MTM) was used to determine statistical significance of spectral peaks and the Maximum
Entropy Method (MEM), which yields sharper spectral peaks, was used to identify the
frequency of the significant variance (Obrochta and Crowley, 2005).
68
Chronology
Initial attempts to date COB-01-03 with conventional TIMS methods revealed
that the sample was formed in the mid-Holocene and contained very low concentrations
of U, ~200 ppb (Table 1, Fig. 2). As a result, the amount of Th present in a sample was
generally too small to obtain precise data from TIMS analysis. Abundance sensitivity in
MC-ICP-MS analyses is up to an order of magnitude greater than to TIMS (Goldstein and
Stirling, 2003), allowing for more precise data with smaller amounts of sample material.
The ten dates obtained from three instruments on samples extracted at University of
Arizona are all consistent with one another, but the five dates processed at Oxford are
systematically younger (Fig. 2). Uncertainty for these five is also greater (average of 160
years (2 ) versus 50 years for the other ten) due to poor Th yield from this particular set
of samples (about four times less than normal). In general, the discrepancy in the ages is
greatest for the youngest samples. This suggests there could be a problem with Th blank
during chemical processing at UA, possibly due to contamination from previously
analyzed glacial-age samples. This contamination would not have been detected in our
normal blank monitoring for the mid-Holocene and glacial age cave samples because
they all contain very small amounts of 232Th, which, in conjunction with 229Th from the
spike, is what is measured in blank analyses. However, Th blank data collected during the
processing of very high 232Th samples (average of >200 ng 232Th) in the UA lab also were
very low, less than 18 pg (Placzek et al., 2006). Any carry-over of 230Th would be too
small to detect on its own, but could make very young samples appear slightly older.
More samples will be run to attempt to resolve this issue.
69
All U-Th ages were corrected for initial Th assuming a value similar to bulk upper
continental crust, 230Th/232Th activity = 0.8 ± 0.4 (Taylor and McLennan, 1995).
However, COB-01-03 calcite is extraordinarily clean with negligible amounts of detrital
material that could contribute initial Th. Activity ratios of 230Th/232Th range from ~100 to
over 1,000, so in all cases any age correction due to initial Th was much less than
uncertainty from other sources (with the exception of sample 0-3 where the uncertainty
due to the detrital Th correction is ~1/3 of the total uncertainty). The age model for
COB-01-03 was derived from a third-order polynomial fit to all of the dates (Fig. 2).
Based on this age model, the stalagmite formed between 6.85 and 3.55 ka. The growth
rate varied between 19 and 39 mm/ky with an average of 32 mm/ky. At our sampling
interval of 80µm, this equates to a stable isotope measurement every 1-14 years, with an
average resolution of 2.5 years.
Results
COB-01-03 was approximately 10 cm tall and roughly columnar with a diameter
of 4 to 6 cm, widening to an apron of ~25 cm in diameter at its base. This stalagmite has
no visible detrital material incorporated into the calcite along the growth axis. The calcite
is straw colored and semi-transparent. Mineralogical analysis via X-ray diffraction on
portions of the stalagmite indicates that it is composed of calcite and contains no
detectable aragonite.
In this mid-Holocene stalagmite from COB,
18
O values range from -6.8 to
-8.8‰. By contrast, modern calcite precipitated in equilibrium with measured cave
70
temperature would have a
18
O value of ~–10.6‰ VPDB (based on average dripwater
values of ~–9.6 VSMOW and cave temperature of 19.5°C). Our mid-Holocene values are
similar to early Holocene
18
O values (-7.5 to –8.5‰) from another COB stalagmite
(Abrupt millennial climate change during the last glacial in southern Arizona inferred
from a speleothem isotopic record, hereinafter referred to as Wagner et al., in preparation,
2006-b). There is also a dramatic shift in the speleothem
18
O record towards higher
frequency variability (discussed in detail below) accompanied by a slight decrease in
average
the
14
18
O values at ~4.9 ka (from –7.6 to –7.9‰).
We analyzed the variance spectrum of COB-01-03 speleothem
18
C production record (Reimer et al., 2004). The cave record and
14
O record and
C production
record were interpolated to 5-year increments and spectral analysis was performed with
the SSA-MTM Toolkit (Ghil et al., 2002). First, the entire record, spanning 3.55 to 6.85
ka (3.5 to 7 ka for
14
C), was analyzed, and then it was divided into two parts at around
4.9 ka because of a conspicuous visual change in the periodicity from longer periods to
shorter and each section examined separately (Table 3, Fig. 6). For each analysis there
was no significant difference in the spectra between the data that had the first
reconstructed component removed and the unfiltered data.
The COB and
14
C production records have no obvious correlation (Fig. 7),
except for a slight suggestion of an anticorrelation when there are large positive peaks in
the
14
C production records. But both records do display an increase in the dominant
frequencies of variation through time. The dominant periods of centennial variability for
the full COB speleothem
18
O record are 227, 177, 142 and 105 years; and decadal 71,
71
62, 52, 44-34, 28, 24, 18, and 11 years. In the
14
C production record from 7 to 3.5 ka we
also found variability at ~222 years. In the early part of the COB
18
O record (6.85 to 4.9
ka) the centennial cycles dominate although several of the decadal periods of variability
are still important, most notably the 52, 40, and 23-year cycles (similar to periods we
found during this time in the
14
C production record). After 4.9 ka multicentury
variability is completely absent from the COB-01-03 record, the higher frequency cycles
of 11 and 24 years become more prominent, and a 70 to 50-year cycle appears. After 4.9
the COB and
14
C records also both contain variability at periods of 28 and 18 years.
Discussion
Oxygen Isotopes
Oxygen isotopes from speleothem calcite can potentially be used as a quantitative
indicator of paleoclimate if the relationships of modern calcite to climate are determined
and if the calcite precipitates in equilibrium with the cave dripwaters. Typically
equilibrium precipitation is assessed via a “Hendy test” (Hendy, 1971) along a growth
band showing no increase in
covariation between
18
O and
18
O values moving away from the growth axis and no
13
C values. This was impractical for our samples; the
growth rate was very slow so we could not be sure we were sampling the exact same time
interval while moving off to the side of the main growth axis. Also this test is typically
done across the entire stalagmite with up to a centimeter between samples and we only
removed a ~2.5 cm core. Replication of all or part of the record to ensure the speleothem
is recording climate and not being impacted by processes unique to a particular drip
72
pathway (e.g. evaporation in the vadose zone) is also not possible at this time due to a
lack of samples that overlap in age.
Analysis of modern calcite deposited on drip plates carried out by Mickler et al.
(2006) suggests that covariation between
on plot of
13
C values (y-axis) versus
18
18
O and
13
C values (slope greater than ~0.52
O values) along, rather than just perpendicular
to, the growth axis can be a warning sign of nonequilibrium deposition, although in
certain locations there are plausible climate scenarios that could explain such a
relationship. A plot of COB-01-03
18
O and
13
C values (Fig. 4) shows a very poor
correlation between the two and actually a very slight negative slope, except for the very
oldest part of the stalagmite where there is a strong positive (slope of ~3) covariation
between
18
O and
13
C values. Based on these results, the COB isotopic record does not
appear to be compromised by disequilibrium calcite precipitation.
Speleothem calcite
18
O values are determined by the
18
O values of the water
entering the cave (which is controlled by the temperature, amount, seasonality, and
source of precipitation) and cave temperature (usually the mean annual temperature). At
19.5°C COB is 4.5°C warmer than the expected mean annual temperature (MAT) of
~15°C at an elevation of ~1700 m in this area (estimated from Tucson MAT of 20.4°C
and a lapse rate of 6.5°C/km and nearby Coronado National Monument MAT of 15.8°C
at ~1600 m eleveation). These increased temperatures indicate the cave may be heated
geothermally, an effect documented in the nearby Kartchner Caverns (Buecher, 1999),
which is 1.7 to 4.0°C warmer than MAT. Although the cave temperature may have
73
always been warmer than MAT at the cave site, it is likely that the difference between
MAT and cave temperature has remained constant.
Comparison of Tucson climate data with weighted averages of Tucson
precipitation
18
O values reveals that overall, in keeping with global studies (Rozanski et
al., 1993), precipitation
18
O values correlate positively with temperature and negatively
with precipitation amount (Table 2) (Wagner et al., in preparation, 2006a). Although
today regional groundwaters (Baillie, 2005; Eastoe et al., 2004; Wahi, 2005) and cave
dripwaters result mostly from winter precipitation, it is possible that summer (monsoon)
rainfall could have contributed to cave dripwaters in the past, so both winter (OctoberMarch) and monsoon (July-September) seasonal averages are considered here.
Mid-Holocene speleothem
18
O values are about 3‰ greater than modern (Fig. 5)
and vary by ~2‰ over the course of the record. If cave waters were derived mostly from
winter precipitation, then most of the observed increase must be due to decreased
precipitation amount because the slope of the relationship between winter precipitation
18
O values and average air temperature is very small and not significant (Table 2)
(Wagner et al., in preparation, 2006a). But the decrease in average winter precipitation
amount required to cause the observed increase in
18
O values is more than two times
larger than the amount of precipitation currently received in the winter season (for an
increase of ~3‰, 3‰ divided by
18
O/10 mm precipitation relationship of –0.08
(p<0.025) equals a decrease of ~380 mm versus 154 mm, which is the average amount
currently received in the winter). Clearly, factors other than temperature and amount of
winter precipitation must have been at work.
74
We propose that a shift in the composition of cave waters from mostly winter
precipitation to a mix containing a large proportion of summer monsoon precipitation, in
conjunction with drier conditions in the winter months and perhaps warmer summers,
could explain the increased speleothem
18
O values in the mid-Holocene. Modern Tucson
summer monsoon (June-August) precipitation
18
O values average 3.6‰ greater than
those in winter (Fig 2). Drier winters and wetter summers in the mid-Holocene would
have tended to decrease the seasonal contrast in precipitation
18
O values, by on average
decreasing summer and increasing winter values, because of the inverse relationship
between precipitation
18
O values and precipitation amount. However, warmer summers
during the mid-Holocene would have also likely increased summer precipitation
18
O
values because of the strong positive effect of temperature on modern summer
precipitation
18
18
O values (0.41‰
18
O/°C, p<0.10). This positive change in precipitation
O values with increasing temperatures would also have likely overcome the negative
effect of increasing temperatures on the fractionation of
formation (
calcite-H2O of
~ -0.22‰
18
18
O values during calcite
O/°C).
Evaporation of waters in the vadose zone before entering the cave could have also
increased the
18
O values of cave waters and thus the
18
O values of the speleothem
calcite in the mid-Holocene. In the modern semi-arid environment, when most of the
cave waters are derived from winter precipitation, this does not seem to be a factor. Cave
water
18
O and D values have been relatively stable over the monitoring period and fall
on or just above the COB winter meteoric water line (winter CMWL) (Wagner et al., in
preparation, 2006a). Rains that fall in the summer seem to contribute very little to cave
75
waters, suggesting that most of the summer rain water is immediately used by the
vegetation and/or lost to evaporation before having a chance to enter the cave. However
if summer precipitation was increased in the mid-Holocene it is possible that the waters
that reached the cave could have had their
18
O and D values increased due to partial
evaporation in the vadose zone. Our data does not allow us to determine if this happened
during the mid-Holocene.
Comparisons to Other Paleoclimatic Records From the Southwest
Most records from the southwestern USA and Great Basin indicate that the midHolocene was the warmest and/or driest period of the Holocene. Others, particularly
those that are sensitive to summer moisture, suggest that the mid-Holocene may have
been wet. In the modern climate southern Arizona and New Mexico are just north of the
core summer monsoon region, and moving north, rainfall in regions such as the Colorado
Plateau and Great Basin is less impacted by the summer monsoon. During the early part
of the mid-Holocene (8-6.5 ka), both Owens and Pyramid Lakes in the Great Basin
exhibit highly variable conditions, but for the rest of the interval (6.5-3 ka) the record is
dominated by drought (Benson et al., 2002); indicating a decrease in winter rains. Potato
Lake in central Arizona also recorded persistent dry conditions throughout the middle
Holocene (Anderson, 1993). Lake Estancia in central New Mexico experienced two
episodes of extreme drought in the mid-Holocene during the interval 7.0-5.4 ka 14C
(~7.8-6.2 ka calendar years) (Menking and Anderson, 2003). Pollen from Walker (Hevly,
1985) and Stoneman Lakes (Hasbargen, 1994) in northern Arizona and spring deposits in
76
the Great Basin (Quade et al., 1998) also record a shift from wet to dry conditions at
about 6.5 ka 14C (~7.3 ka calendar years).
A groundwater record from northern Arizona indicates temperatures were 2-4°C
warmer and recharge rates 50% lower (calculated from
18
O and D relationships and 14C
dated groundwaters and flow modeling, respectively) in the early to mid-Holocene than
today (Zhu et al., 1998). Arroyo formation in several drainage basins in southern Arizona
began between 8 and 5.6 ka 14C (~8.9-6.4 ka calendar years). Waters and Haynes (2001)
suggest this is due to falling water tables, because of increased temperatures and
decreased precipitation, which, coupled with vegetation changes, made the valley floors
more susceptible to erosion.
A general paucity of packrat middens dating to the mid-Holocene has been
observed in the southwest, suggesting that either very dry winters and hot summers led to
reduced production of middens, or that high humidity due to increased summer monsoon
precipitation inhibited the crystallization of rat urine which preserves the middens
(Betancourt et al., 1993). The macrofossil assemblages from those middens that have
been recovered support increased summer moisture during the mid-Holocene. Middens
from the Sonoran Desert show evidence of plants from 6.4-4.5 ka 14C (~7.3-5.2 ka
calendar) that particularly suggest warmer winters and increased summer moisture
(McAuliffe and Van Devender, 1998). Arundel (2002) has used modern climate and
vegetation distribution data to determine which aspects of climate limit the ranges of
plants in Southern Arizona (e.g. maximum and minimum rainfall and/or temperatures
broken down by season). Arundel then applied these relationships to a suite of over 200
77
middens spanning the last 40,000 years collected in southern Arizona. Her results
indicated that monsoon rainfall increased and autumn to winter rainfall decreased
between the late glacial and early Holocene. A pollen record from Montezuma Well in
Arizona also indicates abundant summer precipitation before 8.4 ka 14C (although the
record shows summer precipitation decreasing to a minimum from 5-4 ka 14C) (Davis and
Shafer, 1992).
Foraminifer abundance records from the Gulf of Mexico that have been proposed
as proxy for southwest monsoon strength also suggest a increase through the early
Holocene, with a peak in monsoon strength in the mid-Holocene between 6.5 and 4.5 ka
14
C (7 to 4.7 ka calendar years), and a general decline there after (Poore et al., 2005).
However, Barron et al. (2005) interpret multiproxy marine records from the Gulf of
California as pointing to a later initiation of the southwest monsoon at around 6.2 ka.
Metcalfe et al. (2002; 2000) have suggested that lake records from northern
Mexico generally do not support increased monsoon precipitation in the mid-Holocene.
Castiglia and Fawcett (2006) did find evidence of lake highstands in northern Mexico
during the mid-Holocene, at 6.7 to 6.1 ka 14C (~7.6 to 7.0 ka calendar years) and 4.3 to
3.8 ka 14C (~4.9 to 4.2 ka calendar years). They suggest that, against a backdrop of
increased monsoon precipitation in the mid-Holocene, it was periodic increased El Niño
activity in the tropical Pacific that brought reduced temperatures and increased winter
rainfall to northern Mexico and allowed the formation of large pluvial lakes. A study of
river systems in the southwest USA also identified a period of increased floods, which in
the modern climate are usually associated with heavy winter rains or late season tropical
78
cyclones, from 5.8 to 3.9 ka (Ely, 1997). Our cave record supports the inference that
summer monsoon was increased and winter moisture reduced.
Spectral Analysis
We analyzed the variance spectrum of COB-01-03 speleothem
18
O record to
determine the significant periods of variability and how they changed over the length of
the record. These data can also be compared with the periods of variability recognized in
modern solar observational data and proxy archives such as
14
C and 10Be production
(e.g. Bonev et al., 2004; Damon and Peristykh, 2000; Ogurtsov et al., 2002). The
dominant periods of centennial variability for the full COB speleothem
18
O record are
227, 177, 142 and 105 years; and decadal 71, 62, 52, 44-34, 28, 24, 18, and 11 years. The
11 year cycle may be related to the Schwabe cycle of sunspot activity and the 24 year
cycle is close to the 22 year Hale cycle which is the time it takes for the sun’s magnetic
poles, which also reverse each 11 years, to return to the same state. The 227 and 177-year
cycles are in the 260 to 170-year Suess band identified by Ogurtsov et al. (2002) in a
variety of records of solar activity. In the
14
C production record from 7 to 3.5 ka we
also found a 222-year cycle from the Suess band. The Gleissberg cycle is typically
considered to be in a narrow band from 90 to 80 years, but Ogurtsov et al. (2002) argue
that it is actually a more complex broad region of variability from 140 to 90 and 80 to 50
years. The COB
18
O record exhibits variability in these ranges as well.
In the early part of the COB
18
O record (6.85 to 4.9 ka) the centennial cycles of
233 and 142 years dominate although several of the decadal periods of variability are still
79
important, most notably the 52, 40, and 23-year cycle. The
14
C production record also
contains 142, 39 and 23-year during this time. After 4.9 ka multicentury variability is
18
completely absent from the COB-01-03
O record, the higher frequency cycles of 11
and 24 years become more prominent, and a 70 to 50-year cycle appears. After 4.9 the
COB and
14
C records both contain variability at periods of ~50, 28, and 18 years. The
COB and
14
C production records have a few periods of variability in common, and
display an increase in the frequency of significant variation through time. A clear
correlation between the two records is not evident, although there is some indication that
periods of low solar activity correlate with periods of lower
18
O values in the COB
record (Fig. 7). It has been suggested by van Geel et al. (2003) that small decreases in
solar output may be amplified to have a large impact on climate, causing a southern shift
in the westerly storm tracks and perhaps an increase in cloud formation and precipitation,
which would be consistent with lower
18
O values in our COB record.
Potential Mechanisms for Mid-Holocene Climate Observations
We propose that a shift in the composition of cave waters from mostly winter
precipitation to a mix containing a large proportion of summer monsoon precipitation, in
conjunction with drier conditions in the winter months and perhaps warmer summers,
could explain the overall increased speleothem
18
O values (relative to modern) in the
mid-Holocene. COHMAP (1988) modeling supports an increase in summer monsoon
precipitation and summer temperatures through the early to mid-Holocene. Looking just
at the mid-Holocene with two coupled ocean-atmosphere models, Harrison et al. (2003)
80
modeled an enhancement of the southwest summer monsoon driven by orbital forcing
during the mid-Holocene at 6 ka.
In the modern southwest, El Niños are associated with wet winters (Sheppard et
al., 2002). This relationship is enhanced during positive (“warm”) phases of the Pacific
Decadal Oscillation (PDO) when a deep Aleutian low shifts storm tracks south over the
western US and El Niño provides a source of enhanced moisture (Gershunov and Barnett,
1998). Many records from the Pacific (see summary in Cole, 2001; Koutavas et al., 2002;
Tudhope et al., 2001) and model results (Clement et al., 2000) suggest that conditions in
the tropical Pacific were more La Niña-like in the mid-Holocene. If teleconnections
between the topical Pacific and the southwest were similar during the mid-Holocene, an
increase in La Niña-like conditions would have meant drier winters.
The increasing frequency of variability and slight decrease in COB
18
O values
beginning around 4.9 ka could signal a relative increase in winter moisture around this
time. Other records from the Pacific also show a change in their main frequencies of
variability around 5 ka and, potentially, a shift to more frequent El Niño-like conditions.
A Holocene lake record from Ecuador (Moy et al., 2002) shows increasing variance in
what is the modern “ENSO-band” of 2-8 years around 5 ka (variance in the band began
~7 ka), this suggests an increased frequency of El Niño events after 5 ka. Proxies from a
marine core from the Santa Barbara Basin also offer evidence of a shift to conditions
similar to the modern warm PDO/El Niño during the interval 5.2-3.6 ka, accompanied by
increased decadal to century-scale variability in the spectra of the records (Friddell et al.,
2003). A survey of Asian monsoon records conducted by Morrill et al. (2003) also
81
identified a widespread weakening between 5 and 4.5 ka, which the authors suggest may
be linked to increased frequency of El Niños.
Conclusions
Population growth in the southwestern USA and increasing recognition of
region’s vulnerability to drought necessitates an understanding of full range of climatic
variability and the global background conditions under which it has occurred. A new
record of climate from an Arizona speleothem provides a glimpse of conditions during
the mid-Holocene from 6.9 to 3.5 ka. The observed elevated
18
O values from the
stalagmite may be partially explained by the absence or weak expression of El Niños
during the mid-Holocene, which could have contributed to dry winters, coupled with an
enhanced summer monsoon and warmer temperatures driven by higher than modern
summer insolation. Comparisons of the spectra of COB
18
O values and
14
C production
records reveals some periods of variability in common that may indicate that small
fluctuations in solar output have an influence on climate in the southwest either directly
or indirectly. The shift from multicentury (233 and 142-year cycles) to multidecadal
variability (70 to 50-year cycles) in the COB-01-03 record beginning around 4.9 ka, and
the slight trend to lower
18
O values could reflect an increase in the contribution of
winter precipitation to cave dripwaters as El Niños became more frequent/intense and
summer monsoon intensity declined.
82
Acknowledgements
We thank Jerry Trout, Bill Peachy, and Dennis Hoburg for assistance in obtaining
samples from Cave of the Bells. We also thank Heidi Barnett, Wes Bilodeau, Mihai
Ducea, David Dettman, Chris Eastoe, Clark Isachsen, Jon Overpeck, Christa Placzek, and
Trey Wagner all of the University of Arizona. NSF Earth System History 03-18480, UA
small Faculty Grant, GSA student grant, and University of Arizona Department of
Geosciences student grants provided funding for this research.
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(2002). The climate of the US Southwest. Climate Research 21, 219-238.
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Continental-Crust. Reviews of Geophysics 33, 241-265.
Tudhope, A. W., Chilcott, C. P., McCulloch, M. T., Cook, E. R., Chappell, J., Ellam, R.
M., Lea, D. W., Lough, J. M., and Shimmield, G. B. (2001). Variability in the El
Nino - Southern oscillation through a glacial-interglacial cycle. Science 291,
1511-1517.
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86
Waters, M. R., and Haynes, C. V. (2001). Late Quaternary arroyo formation and climate
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the Black Mesa basin, northeastern Arizona, to paleoclimatic changes during the
late Pleistocene and Holocene. Geology 26, 127-130.
87
Figure captions
Figure 1. Location of Tucson, Arizona and Cave of the Bells (COB).
Figure 2. Twenty-four years (1981-2005) of weighted monthly 18O values (VSMOW) of
Tucson precipitation and the weighted mean over this interval, with standard deviation
(personal communication, Austin Long, University of Arizona Geosciences). Note the
high degree of variability in every month regardless of season. The bar shows the average
of measured dripwater concentrations, ~-9.6, which falls clearly in the range of winter
season values.
Figure 3. Depth from the top of the stalagmite versus U-Th dates. Error bars are 2 for
uncertainty in U-Th dates and represent the size of sample along the growth axis, for
some samples they are smaller than the symbol and omitted for clarity. The solid triangle
represents the date measured on TIMS at University of Arizona. The X represents the
date measured on MC-ICP-MS at UA. Solid circles represent dates obtained by running
U splits on TIMS at UA and Th splits on MC-ICP-MS at Oxford University. Open
squares represent dates obtained with Oxford chemistry and MC-ICP-MS analysis.
Figure 4. COB-01-03 stalagmite 18O versus 13C values from along the growth axis.
Solid gray triangles are from the earliest part of the stalagmite (~6.85 to 6.55 ka) and
exhibit a strong positive trend (slope = 3) that Mickler et al. (2006) suggest can be
indicative of nonequilibrium calcite deposition, however, in the majority of the record,
open squares, there is no significant covariation between 18O and 13C values (slope = –
0.2).
Figure 5. COB-01-03 stalagmite 18O values. The black horizontal bar denotes the
calculated modern calcite 18O value (~-10.6). The vertical black bar marks the shift from
multicentury to decadal periods of variability in the 18O record at ~4.9 ka. The dashed
line is average COB-01-03 18O of the entire record and solid lines average 18O from
6.85 to 4.9 ka and 4.9 to 3.55 ka.
Figure 6. MTM (thin black line) and the 95% confidence band (heavy gray line) and
MEM (smooth, heavy black line) spectra of (a.) the entire COB-01-03 18O record, (b.)
the early mid-Holocene, 6.8 to 4.9 ka, and (c.) the late mid-Holocene, 4.9 to 3.5 ka. Gray
bars denote periods of interest discussed in the text.
Figure 7. COB-01-03 stalagmite (a.) 18O values (blue diamonds) and (b.) 14C
production 1000-yr residuals (black squares) (Reimer et al., 2004) sampled at 5 year
increments. Heavy lines are 5 point moving averages. Intervals where low solar activity
corresponds to low 18O values are highlighted.
88
Figure 1.
89
2.0
-2.0
-4.0
-6.0
-8.0
-10.0
-12.0
18
Tucson Precipitation (VSMOW)
0.0
Avg. cave waters -9.6
-14.0
-16.0
0
2
4
6
Month
Figure 2.
8
10
12
90
7500
7000
6500
Years BP
6000
5500
5000
4500
4000
3
2
y = 0.0021x - 0.2095x + 32.725x + 3547.2
3500
2
R = 0.9758
3000
0
Figure 3.
50
100
Depth along growth axis (mm)
91
y = -0.2067x - 6.0936
R2 = 0.0167
COB-01-03
13
C
-3.0
-5.0
-7.0
y = 3.0324x + 16.203
R2 = 0.8741
-9.0
-10.0
-8.0
-6.0
COB-01-03
Figure 4.
-4.0
18
O
92
Figure 5.
93
Figure 6.
94
Figure 7.
Table 1
b
35
32
122
81
26
25
170
52
38
190
95
77
237
37
46
Error
2ó
238
230
238
Th/ U
Th/ U
measured corrected
0.063291 0.062890
0.063223 0.063175
0.060775 0.060730
0.061386 0.061338
0.081791 0.081696
0.080828 0.080775
0.082489 0.082462
0.091860 0.091644
0.097425 0.097239
0.095897 0.095866
0.098155 0.098037
0.104182 0.104110
0.104803 0.104757
0.111117 0.110987
0.117562 0.117390
230
Th
232
234 238
U
( U/ U)int
ppm
0.195
1.860
0.239
1.866
0.231
1.000
0.209
1.000
0.217
1.901
0.224
1.859
0.215
1.821
0.177
1.861
0.162
1.925
0.178
1.906
0.156
1.920
0.160
1.890
0.177
1.856
0.184
1.879
0.191
1.881
238
Th = 0.8
122
1023
965
879
658
1182
2047
323
397
1774
635
1094
1413
641
512
232
Th/
Th/
230
230
Th/
232
Th activity of 0.8 ± 50%
* UA-U TIMS, Oxford-Th MC-ICP-MS
# UA-U and Th TIMS
$ UA- U and Th MC-ICP-MS
& Oxford chemistry and MC-ICP-MS
230
Error for all samples includes measurement error, decay constant uncertainty, and initial Th assuming an estimated value
for
b
Ages corrected assuming initial Th has a value similar to bulk upper continental crust,
a
*
*
&
&
*
*
&
*
*
&
#
$
&
*
*
a
Age
Depth
Center Depth
mm from top mm from top yr (before 2005)
0-3
1.5
3767
3-5
4
3771
5-7
6
3552
18.5-20
19.25
3818
39-41.5
40.25
4814
41.5-44
42.75
4867
52.5-55
53.75
5026
69-70
69.5
5536
70-73
71.5
5683
76-78
77
5591
76-82
79
5738
83-90
86.5
6209
95-97
96
6294
97-99
98
6681
99-101
100
7071
U-Th data
95
96
Table 2
18
Relationships between the
O of Tucson precipitation and
temperature, amount, and seasonality.
Temperature
Seasonala
18
R2
O /°C
Significancec
N
Oct to Mar
-0.18
0.06
21
Oct to Marb
-0.01
<0.001
20
Jun to Sept
0.41
0.15
p<0.10
21
Amount
Seasonala
18
O /10 mm
Oct to Mar
Oct to Mar
b
Jun to Sept
R2
Significancec
-0.06
0.15
p<0.10
21
-0.08
0.27
p<0.025
20
-0.15
0.54
p<0.01
21
Seasonality
Season
18
O (VSMOW)
Amount (mm)
Oct to Mar
-9.2
154
Jun to Sept
-5.6
162
a
Monthly weighted averages (weighted by amount) for each
parameter were further combined into weighted seasonal averages.
b
minus 1989 which is an outlier
c
N
Significance determined by one-sided Student's t-test
Table 3
321
233
1706
385
227
2564
88
111
142
142
144
135
66
96
104
105
142
146
105
142
222
222
177
177
56
52
52
87
71
88
71
71
46
53
61
74
62
62
39
40
40
41
49
67
52
52
b
Period (yr)
37
36
33
32
28
26
24
28
29
46
25
37
23
37
23
52
28
41
44
56
33
40
44
34
23
23
23
19
23
44
23
18
34
18
Multitaper Method (resolution 2, tapers 3) and Maximum Entropy Method (period noted for each series).Minus RC1- time series
detrended by removing 1st Reconstructed Componet from Singular Spectrum Analysis (SSA); results from SSA-MTM Toolkit for spectral analysis,
Ghil, et al. (2002), http://www.atmos.ucla.edu/tcd/ssa/guide/
b
Significant above 95% from MTM, exact frequency/period determined from Maximum Entropy Method (MEM) peak
a
MEM (90)
minus RC1 MEM (90)
COB-01-03 (4.9 to 6.85 ka)
C (4.9-7 ka)
MEM (90)
COB-01-03 (4.9-6.85)
14
MEM (60)
MEM (60)
C (3.5-4.9 ka)
14
COB-01-03 (3.55-4.9 ka)
MEM (150)
minus RC1 (MEM 150)
COB-01-03 (3.55- 6.85 ka)
C (3.5-7 ka)
MEM (150)
COB-01-03 (3.55-6.85 ka)
14
Spectral Method
Record
a
Centennial and multidecadal variability in COB-01-03
19
17
17
18
18
39
20
28
12
28
12
15
15
11
37
18
11
24
24
11
97
98
APPENDIX C: USING LONG-TERM RECORDS OF ISOTOPES IN
PRECIPITATION FROM TUCSON, ARIZONA TO CALIBRATE CAVE
WATER ISOTOPIC RESPONSE TO CLIMATE IN CAVE OF THE
BELLS, ARIZONA
J.D.M. Wagner, J. E. Cole, C. Eastoe, and A. Long
Abstract
We present the results of a three year precipitation and dripwater monitoring
study at Cave of the Bells (COB) located in Santa Cruz County, Arizona on the east side
of the Santa Rita Mountains (31°45'N, 110°45'W; 1700 m). Dripwater
18
O and D
values are relatively stable over the monitoring period and between locations in the cave.
The
18
O values average –9.6‰ ±0.2‰ and D values -67‰ ±1.2‰ (VSMOW).
Comparisons to local precipitation values indicates that the dripwaters are sourced mostly
from local winter precipitation, with summer rains contributing only slightly to the total.
An analysis of the variations in the
18
O values of Tucson precipitation reveals that
average summer monsoon (Jul-Sept) precipitation
18
O values are ~3.6‰ greater than
average winter (Oct-Mar) values, also, in keeping with global patterns (Rozanski et al.,
1993),
18
O values are higher when temperatures are warmer (although this relationship
is weak during the winter season) and rainfall amounts are less. The relationships
between Tucson precipitation
18
O values and temperature, amount, and seasonality of
precipitation help to constrain the possible climatic causes of past variations in
speleothem
18
O values. From this study we have been able to determine that when
speleothem calcite
18
O values are higher than modern (~-10.6‰ VPDB), the climate
99
was likely drier and/or summer precipitation may have increased relative to winter, such
that it comprised a larger portion of shallow groundwater recharge, and when speleothem
18
O values are less than modern, conditions were wetter with perhaps less summer
relative to winter moisture. The effect of changing temperatures is less clear. If summer
precipitation is the dominant source of cave dripwaters then there should be an increase
in speleothem
18
O values with increasing temperature, but if winter precipitation
dominates there may be no effect or a very slight decrease in speleothem
18
O values
with increasing temperatures. Although the measured cave temperature of 19.5°C is
higher than the surface mean annual temperature of ~15°C this differential should have
been constant over Quaternary time scales and thus should not affect our interpretations
of paleoclimate from COB speleothems which primarily record relative changes in
moisture amount and/or seasonality.
Introduction
Continuous well-dated paleoclimate records are rare in the semi-arid southwest.
Speleothems can provide a high-resolution, continuous record of moisture, temperature,
and, potentially, vegetation variations and can be precisely dated by uranium-series
disequilibrium. We have produced two U-series dated speleothem
18
O and
13
C records
from Cave of the Bells, one from the mid-Holocene (Mid-Holocene climate in southern
Arizona inferred from speleothem stable isotopes, hereinafter referred to as Wagner et al.,
in preparation, 2006-a), and one that spans ~53 to 10 ka (Abrupt millennial climate
change during the last glacial in southern Arizona inferred from a speleothem isotopic
100
record, hereinafter referred to as Wagner et al., in preparation, 2006-b). If speleothem
calcite is precipitated in equilibrium, its
the cave and the
18
18
O values are determined by the temperature in
O values of the dripwaters, which are determined by the amount,
temperature and seasonality of precipitation that supplies them. All caves are unique and
paleoclimate interpretation of speleothem calcite
18
O and
13
C data requires an
understanding of the modern processes that impact the stable isotopes of water infiltrating
the cave. In this study we present the results of three years of COB dripwater and
precipitation monitoring.
We also analyze ~25 years of Tucson precipitation
18
O and D data (which is a
good analog for precipitation at the cave site) for relationships to temperature, amount,
and seasonality of rainfall. In our study region, roughly half the annual precipitation
comes during the summer monsoon from July to September (Fig.2, Table 2), sourced
from the Gulf of California, the eastern tropical Pacific, and a contribution from the Gulf
of Mexico (Sheppard et al., 2002; Wright et al., 2001). Dissipating tropical cyclones can
also contribute significant moisture, in certain years, during September and October.
Periodic westerly frontal storms from the Pacific supply the other half of the annual total
from October to March (Sheppard et al., 2002 and references therein). High temperatures
and vegetation demand, combined with “flashy” distribution, cause most of the summer
precipitation to be lost to runoff or evapotranspiration, while cooler temperatures and
lower rainfall rates allow winter rainfall to preferentially enter the groundwater system,
accounting for the bulk of recharge in the region’s aquifers (Baillie, 2005; Eastoe et al.,
2004; Wahi, 2005).
101
Interannual variability in winter precipitation in the southwest is linked to
ocean/atmosphere dynamics of the Pacific and, to a lesser degree perhaps, Atlantic
Ocean. When the Pacific Decadal Oscillation (PDO) is in a positive phase (cool sea
surface temperature (SST) anomalies in the northern western to central Pacific and warm
SST anomalies along the western coast of North America and the tropical central to
eastern Pacific), the atmospheric Aleutian low is stronger. This causes westerly storms to
be diverted farther south than normal, and, during El Niño years, there is more moisture
available from the tropical Pacific (Gershunov and Barnett, 1998) leading to wetter
winters in the southwest (Sheppard et al., 2002). The opposite case also holds. La Niñas
are associated with drier winters and this teleconnection is also accentuated during times
when the PDO is in a negative phase. Cool anomalies in Northern Atlantic SST (negative
AMO-Atlantic Multidecadal Oscillation) are correlated to increased precipitation in the
southwest and when the PDO is also positive this moisture anomaly is stronger and more
widespread (McCabe et al., 2004). There are, however, no clear links between Pacific
and/or Atlantic anomalies and monsoon variability in the modern climate (Sheppard et
al., 2002).
Setting
Cave of the Bells is located in Santa Cruz County, Arizona on the east side of the
Santa Rita Mountains at an elevation of 1700 m (31°45'N, 110°45'W) (Fig. 1). The
vegetation above the cave is best characterized as oak-juniper woodland with an
understory of C4 grasses and CAM succulents (Stable isotope composition of speleothem
102
calcite and associated cave and soil CO2, Cave of the Bells, Arizona, hereinafter referred
to as Fischer et al. in preparation, 2006). The cave is situated in the Permian Colina
Limestone below an isolated hill at shallow depths, indicating that the infiltrating water
that forms the speleothems originates as rain that falls on the immediate area and is not
supplied by regional groundwater. Tucson is located approximately 75 km to the
northwest of the cave site at an elevation of ~780 m, and lies between the Santa CatalinaRincon Mountains (to the north and east), the Tucson Mountains (to the west), and Santa
Rita Mountains (to the south).
Methods
Precipitation and cave dripwaters were collected from Cave of the Bells from
February 2003 to May 2006. A cylindrical rain gauge was placed above the entrance to
COB, and, because waters were only collected about once per month, a small amount of
oil was placed inside the rain gauge. As the gauge filled with water the oil provided a
barrier between the rainwater and the atmosphere to minimize evaporative loss (which
would cause
18
O and D values to increase) before sample collection. During the
approximately monthly sampling trips the volume of rainwater collected was measured
and the water was transferred from the rain gauge to a high-density polyethylene (HDPE)
bottle small enough that it could be filled to the top with little to no air space. The bottle
top was then wrapped with parafilm and, once back in the lab (less than one day), stored
in a freezer until isotopic analysis was carried out (usually in less than six months). With
103
these collection/storage methods should minimize isotopic exchange between the waters
and air after collection.
Inside Cave of the Bells, dripwater was collected from three locations that
currently have active formation growth. The “D’s Climb” location is located in the next
chamber and above the site from which the mid-Holocene COB-01-03 stalagmite was
collected (Wagner et al., in preparation, 2006-a). The other two locations, “Soda Straw”
and “Popcorn Room” are deep within the cave but near the sampling sites of the COB01-03 or the glacial COB-01-02 (Wagner et al., in preparation, 2006-b). The Soda Straw
location is the closest to the surface of the three.
At each location plastic flexible tubing was run from the end of a soda straw that
was actively dripping to a narrow mouth 1 liter HDPE bottle for collection. The tops of
the bottles were sealed off with parafilm (except for a small hole) to minimize isotopic
exchange between the dripwaters and water vapor in the cave air. Dripwaters were
collected approximately monthly in the same method described above for the rainwater.
After the first ~6 months of this study the 1 liter bottles were switched out for 500 ml
bottles because the accumulation over a month was usually less than 200 ml (Table 1). In
the Popcorn Room the sampling apparatus was moved twice (noted in Table 1) to obtain
more water. Cave temperature was also measured at half hour intervals, with a
downloadable stow-away automated temperature logger between August 11 and October
22, 2003.
Water
18
O and D values were measured on a Finnigan Delta S gas source mass
spectrometer in the Stable Isotope Laboratory of the Department of Geosciences at the
104
University of Arizona. Samples were prepared and measured according to methods in
Craig (1957) and Gehre et al. (1996), and precisions are ±0.08‰ for
18
O values and
±0.9‰ for D values. All water stable isotope values are referenced to the VSMOW
standard and calcite stable isotopes to the VPDB standard.
For long-term analysis of the isotopic-climate relationship we used data on
Tucson precipitation collected under the direction of Austin Long and Chris Eastoe
(Laboratory of Isotope Geochemistry, Department of Geosciences, University of
Arizona). Water from each precipitation event in Tucson has been collected since 1981,
its amount recorded, and
18
O and D values measured. The amount recorded at the
actual rainwater sampling location was used for isotope comparisons, instead of official
Tucson precipitation accumulation data, because precipitation in Tucson is spatially
heterogeneous. Daily temperature data from 1984 to June 2005 at Tucson International
Airport (COOP ID: 028820; WBAN ID: 23160) were obtained from the National
Climatic Data Center. The average daily temperature from this source was matched with
each day in Tucson when precipitation was collected. Tucson precipitation
18
O and D
values and temperature (only for the days when precipitation was collected) were
weighted by precipitation amount monthly. The weighted monthly averages were also
combined into various weighted seasonal averages (Oct-Mar and Jul-Sept), by weighting
each monthly average relative to the total rainfall in that season (see Table 3 for example
of weighting procedure).
105
Results
Dripwaters from all three locations in Cave of the Bells were very similar to each
other and varied little (range of ~1‰) over the ~three years of this study (Fig. 3, Table 1).
Soda Straw
18
O and D values averaged –9.6‰ ±0.22‰ and –66.4‰ ±1.2‰; those
from the Popcorn Room –9.5‰ ±0.14‰ and –66.3‰ ±1.2‰; and those from D’s Climb
–9.8‰ ±0.14‰ and –68.5‰ ±1.2‰. The Soda Straw location recorded the most
variability in
18
O values. The
18
O values of the Popcorn Room and D’s Climb location
dripwaters were more stable through time and Popcorn Room
18
O values were
consistently greater than those from D’s climb by ~0.3 to 0.4‰. There is also a subtle
low-frequency trend in the dripwater data. The
18
O values of waters from all of the sites
generally increase until summer of 2004 and then begin to decrease. Changing the
formation from which waters were collected in the Popcorn Room (as noted in Table 1)
did not lead to a discernible shift in the isotopic values. Cave temperature was a constant
19.5°C (±0.1) over the two and a half months it was measured.
We compared the
18
O and D (not shown) values of rainwater collected at COB
with the weighted average of these values from rain that was collected in Tucson over the
same interval (Fig. 4). The records are positively correlated, R2 = 0.55, when rains have
low
18
O values in Tucson there tend to be low
18
O values measured in rain from the
cave site. The data from Jan-04 was excluded because the rain
18
O value was
exceptionally low at the cave site (-15.4‰) and unusually high in Tucson (-5.6‰).
However, two rain events during Jan-04 collected in the San Pedro Valley to the east of
the cave site had values of –19.4 and –21.2‰ (Baillie, 2005), suggesting that the
106
particular storm(s) that produced this rain did not impact Tucson, but was recorded east
of the cave site. Looking at the data seasonally, correspondence in the winter (Oct-Mar)
was very good, R2 = 0.31, but, there is less covariation for the summer and spring
months. There is also an indication that precipitation collected from COB may have
undergone some evaporation before collection despite our precautions. Most of the COB
values are higher than the rains collected in Tucson over the same interval, although
some of this may be due to different precipitation patterns during these months as
illustrated by the Jan-04 offset.
A plot of
18
O versus D values also supports our contention that Tucson rain
(weighted monthly averages 1981-2005) and COB rain exhibit similar patterns of
variation (Fig. 5). Both Tucson and COB meteoric water lines (TMWL and CMWL) have
slopes (
18
D/
O) less than that of the global meteoric water line (GMWL), 5.9 and
6.9 versus 8. The decreased slope is due to secondary evaporation during rainfall and is
common in arid and semi-arid regions (Clark and Fritz, 1997). Average d-excess values
for precipitation at the two locations are also similar, 9 and 9.9. D-excess values are
defined as D – (8 *
18
O) and are an indication of conditions during primary evaporation
of the water vapor for which rain is condensed and secondary evaporation while
precipitation is falling (Clark and Fritz, 1997). Cave dripwaters plot between the TMWL
and the CMWL.
To evaluate the relationship between climate and isotopic variation we compared
Tucson temperature and amount of precipitation with seasonal and monthly weighted
averages of
18
O values (Fig. 6-7, Table 2). The individual monthly averages are shown
107
for comparison, but seasonal averages are likely more relevant to cave studies because
some of the short-term variations will be buffered by mixing in the shallow groundwater.
We found that Tucson precipitation
18
O values do not have a significant relationship
with temperature in the winter but tend to increase with increasing temperature in the
summer season at a rate of 0.4‰/°C (p < 0.1). There is a negative correlation between
Tucson precipitation
18
O values and amount of precipitation in both seasons. In the
summer seasonal comparison Tucson precipitation
18
O values decrease 0.15‰/10 mm
of precipitation increase (p < 0.01). In the winter seasonal comparison
18
O values
decrease with increasing precipitation amount at a rate about half that of summer,
0.08‰/10 mm of precipitation (p < 0.025). The long-term amount weighted average
18
O
values of Tucson winter (Oct-Mar) and summer monsoon (Jul-Sept) precipitation are
-9.2‰ and -5.6‰.
Discussion
COB Waters
The elevation effect (Rozanski et al., 1993) predicts that the
18
O values of
precipitation in a region should decrease with increasing elevation. Wahi (2005)
attempted to quantify this effect in southern Arizona and found a range gradients of
variation of precipitation
18
O values with elevation: -0.089 to -0.23‰/100 m for winter
precipitation and -0.16 to -0.23‰ for summer precipitation. From these estimates of the
local elevation effect we predict that winter precipitation at the elevation of COB should
108
have long-term average
18
O values of -10 to -11.3‰ and summer precipitation values
should be between -7 to -7.7‰. Over the course of this study, COB dripwater
18
O values
from all three sites ranged from -9.3 to -10.3‰ with an average of -9.6‰ ±0.2‰. This
value suggests that winter precipitation is the dominant component of the cave waters
today, with summer precipitation contributing 45% to 15% (using lower and higher
estimates, respectively, of seasonal
18
O values at the cave site) to the total.
The lack of significant variability and overall low
18
O values of COB dripwaters
contrast with some other caves in semi-arid environments in which dripwater
18
O values
vary substantially within and between years and are always greater than average
precipitation values due to evaporation in the vadose zone (e.g. BarMatthews et al.,
1996). Cave dripwaters plot on or slightly above the winter COB MWL and above the
COB MWL derived from all rains collected (Fig. 5). If the waters were evaporating
before infiltrating the cave they would just plot above the CMWL (Clark and Fritz,
1997). The slope of the line through the dripwaters is less than those of the COB MWLs,
5 versus 6.9 for all months and 7.1 for winter months, but the dripwater values are so
similar that the line through them is less well constrained than that through the
rainwaters.
The cave is located at shallow depths beneath isolated hills so the vadose waters
that feed COB likely have a relatively short residence time. Evidence for this is found in
the Soda Straw location, where dripwater
months after very low
18
18
O values decrease at least 0.3‰ 0 to 1
O values from winter rains are recorded above the cave (Fig.3).
The long record of precipitation
18
O values from Tucson exhibits a high degree of
109
variance (Fig. 2), suggesting that our 3-year record from COB may not accurately reflect
long-term mean conditions at this site. Although we collected rainwaters from the cave
site over this interval there are several gaps in the record and indications that the
18
O and
D values of some of the COB rainwaters may have been increased by evaporation to
some degree before collection. This precludes determining a reliable local weighted
average of the precipitation
18
O values at the cave site. Monitoring is ongoing to further
clarify the relationship between precipitation at the cave site and cave dripwaters.
At 19.5°C COB is 4.5°C warmer than the expected mean annual temperature
(MAT) of ~15°C at an elevation of ~1700 m in this area. This estimate is derived from
Tucson MAT of 20.4°C and nearby Coronado National Monument MAT of 15.8°C (at
~1600 m) and applying a lapse rate of 6.5°C/km. Gering and Kline (1999) found that a
permanent lake deep within the cave had a temperature of ~25°C,
18
O value of -9.6‰
and D values of -66‰ (measured ~1999). Cave temperatures higher than MAT indicate
that the cave may be heated geothermally, an effect documented in the nearby Kartchner
Caverns, which is 1.7 to 4.0°C warmer than MAT (Buecher, 1999). Again, because of the
shallow depth of the cave, it is unlikely that geothermal contribute to cave dripwaters that
we measured in this study or those that formed the speleothems. The similarity between
the cave lake’s stable isotopes and those from cave dripwaters indicate that they both
originate from local meteoric water. Although the cave may always be warmer than
MAT, the difference is not likely to change over Quaternary time scales.
110
Using the measured temperature (19.5°C) and dripwater
and assuming equilibrium, it is possible to calculate the
18
18
O (–9.6‰ VSMOW)
O of modern calcite forming
in the cave. From Kim and O’Neil (1997), where T is in Kelvin,
1,000*ln( ) = 18.03*1,000/T –32.42.
Our measured cave temperature thus gives
calcite-water =
Rwater = (
expected
18
18
calcite-water =
1.02983 and
Rcalcite/Rwater, where
Owater/1000) + 1 * RVSMOW, and RVSMOW = 18O/16O = 0.0020052, yields an
18
O value of cave calcite of ~19.9‰ (VSMOW) which from
OVPDB = 0.97002 *
18
OVSMOW –29.98 (at ~25°C)
converts to approximately -10.6‰ (VPDB) (Clark and Fritz, 1997 and references
therein). We attempted to measure soda straw tips to determine the actual
18
O and
13
C
values of modern calcite, but the results suggest that the particular pieces we collected
may not be modern. Two samples were collected from the Soda Straw location and had
measured
18
13
O values of –7.7 to –7.5‰ and
one soda straw from D’s Climb area had a
6.7‰ (VPDB). The soda straw
10.6‰ from the measured
18
18
18
C values of –7.0 to –6.2‰ (VPDB) and
O value of –9.0‰ and a
13
C value of –
O values are 1 to 3‰ greater than expected value of –
O values of the cave waters and measured temperature.
However, we think these do not point to nonequilibrium calcite deposition in the modern
system because the
13
C values are 1.2 to 2.5‰ less than the -4.5 to –5.0‰ predicted
from modern studies of the cave carbon system (Fischer et al. in preparation, 2006) and if
18
O values are increasing due to fractionation,
13
C values should be as well.
111
Tucson Precipitation
We analyzed the relationship of Tucson precipitation
18
O values to climate
because Tucson precipitation is a good analog for precipitation at the Cave of the Bells.
Although identical rain events may not occur at both sites, long-term variations in climate
that impact the average temperature, amount, and seasonality of rainfall will be reflected
at both locations. We examined the amount-weighted Tucson precipitation
18
O values
for relationships to temperature and amount of rainfall on monthly, seasonal and annual
time-scales. On first examination, temperature would seem to be the primary control of
the variation in the seasonal cycle of Tucson precipitation
18
O values, with
18
O values
increasing as temperature increases in the summer and decreasing in the fall to winter
(Fig. 2). The main deviation from this trend, the very high values in May, reflect the
additional amount effect of secondary evaporation of light rains in the very dry foresummer. The average June
18
O value is lower than that of May, even though the average
amount of precipitation in June is about the same, because much of the June precipitation
occurs the later part of the month during years with early monsoon activity (monsoon
onset is usually in early July). Relative humidity is higher after the onset of monsoon
conditions, and this reduces evaporation during rainfall.
Despite the correspondence of precipitation
18
O values and temperature in the
annual cycle, orthogonal regression of amount-weighted
18
O values versus amount-
weighted temperature reveals that interannual and intraseasonal variations in temperature
in the winter months (Oct-Mar) do not correlate very well with variations in precipitation
18
O values. Only the monthly data exhibit the expected (Rozanski et al., 1993) positive
112
relationship with temperature (although it is very weak, 0.14‰/°C, R2 = 0.03, p < 0.025),
and the seasonal data actually do not have a significant relationship with temperature
(Fig. 6, Table 2). However, summer monsoon (Jul-Sept) precipitation
18
O values do
increase (0.41‰/°C, R2 = 0.15, p < 0.1) with increasing temperature, and the monthly
values exhibit almost the same trend.
The Tucson precipitation data suggest that the amount effect has a larger
influence on the interannual and interseasonal variations in precipitation
18
O values (Fig.
7, Table 2). The trends in the monthly and seasonal data in summer are about the same,
-0.2‰/10 mm precipitation. The winter precipitation also records a decrease in
18
O with
increasing rainfall, but the rate of decrease in the monthly data is over four times greater
than that in the seasonal averages, -0.35 versus –0.08‰/10 mm (with R2 of 0.11, p < 0.01
and R2 of 0.27, p < 0.025, respectively).
These results suggest that variations in cave dripwater
speleothem calcite
18
18
O values (and, thus,
O values) are primarily controlled by changes in the
18
O values of
precipitation due to changing amounts of rainfall and/or the relative contribution of
winter and summer precipitation. The role of changing temperatures is less clear.
Fractionation of oxygen isotopes between the water and calcite during calcite
precipitation is related to temperature such that increasing temperatures in the cave (when
MAT increases) will cause calcite
18
O values to decrease ~0.22 to 0.24‰/°C (Kim and
O'Neil, 1997). This effect could largely conterbalance most of the concurrent increase in
precipitation (and, thus, dripwater)
18
O values with increasing temperature that the long-
term Tucson data reveal. If the dripwaters originate mostly from winter precipitation we
113
would predict that the temperature effect on calcite fractionation would dominate, leading
to slightly increased values of
18
O with decreased temperatures. But if dripwaters
originate mostly from summer precipitation the effect of temperature on the
precipitation would dominate leading to increased values of
18
18
O of
O calcite with increased
temperatures (temperature of precipitation plus temperature of calcite formation
relationship: 0.41‰/°C + -0.22‰/°C
0.2‰/°C).
Conclusions
A three-year monitoring study of precipitation and dripwaters from Cave of the
Bells reveals that
18
O and D values within the cave are relatively stable over the
interval (~-9.6 ±0.2‰ and -67‰ ±1.2‰ VSMOW, respectively) and are likely derived
mainly from winter rains in the immediate area of the cave. Although the measured cave
temperature of 19.5°C is higher than the surface MAT of ~15°C this difference should
have been constant over Quaternary time scales and thus should not affect our
interpretations of paleoclimate from COB speleothems which record relative changes in
moisture amount and/or seasonality, and, perhaps, temperature. Examining the long,
continuous record of Tucson precipitation
18
O values reveals relationships to
temperature, amount, and seasonality of precipitation that help to constrain the possible
climatic causes of variations in speleothem calcite
18
18
O values in the past. Higher calcite
O values than modern (~-10.6‰ VPDB) likely reflect drier conditions and/or times
with increased summer relative to winter precipitation infiltrating to the cave. Lower
calcite
18
O values likely reflect wetter conditions. Because in the modern system most of
114
the recharge comes from winter precipitation, lower speleothem calcite
18
O values
cannot be due to a large shift in winter relative to summer precipitation infiltrating into
18
the cave. The effect of changing temperatures on speleothem
O values is less clear. If
winter precipitation is the dominant source of dripwaters, as in the modern system, then
the calcite fractionation effect of decreasing
18
O values with increasing temperatures
could cancel out the slight trend of increasing precipitation
18
O values with increasing
temperatures. However, if summer precipitation dominates recharge then the overall
effect of increasing temperatures will be to increase calcite
18
O values.
Acknowledgements
We thank Jerry Trout and Dennis Hoburg for assistance in obtaining samples
from Cave of the Bells. We also thank Austin Long, Heidi Barnett, J. Warren Beck,
David Dettman, Chris Eastoe, Jay Quade, and Trey Wagner all of the University of
Arizona, and Rick Toomey of Kartchner Caverns. NSF Earth System History 03-18480,
UA small Faculty Grant, GSA student grant, and University of Arizona Department of
Geosciences student grants provided funding for this research.
115
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oxygen isotope study of the active water-carbonate system in a karstic
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Sheppard, P. R., Comrie, A. C., Packin, G. D., Angersbach, K., and Hughes, M. K.
(2002). The climate of the US Southwest. Climate Research 21, 219-238.
Wahi, A. K. (2005). "Quantifying mountain system recharge in the Upper San Pedro
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Wright, W. E., Long, A., Comrie, A. C., Leavitt, S. W., Cavazos, T., and Eastoe, C.
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117
Figure captions
Figure 1. Location map
Figure 2. Monthly weighted averages of Tucson precipitation 18O values (red circles,
error bars are one standard deviation) from 1981 to 2005, Tucson average temperature
(green squares), and average amount of precipitation (blue triangles).
Figure 3. Cave dripwater and precipitation 18O values from November 2002 to May
2006. Red squares are dripwaters collected in the Popcorn Room, green triangles from
D’s Climb, and blue diamonds from the Soda Straw Room. Black x’s are precipitation
18
O values from above the cave. Arrows denote negative rain months that led to a slight
decrease in Soda Straw dripwater 18O values after a lag of 0-1 months.
Figure 4. Correlation of COB precipitation 18O values and Tucson precipitation 18O
values over the period of overlap, November 2002 to October 2005. Blue squares
represent winter (Oct-Mar) rain (Jan-2004, open square, is omitted from the correlations,
see text for details), red circles summer monsoon (Jul-Sept) rain, and green triangles rain
in the spring to foresummer (Apr-Jun). The heavy black line shows the relationship for
all months and the thin blue line just for Oct-Mar. Most months plot below the black
dashed line, which denotes the expected relationship between COB and Tuscon
precipitation 18O due to the elevation effect (-0.125‰/100m, estimated from Wahi
(2005)). This could be due to evaporation from the COB rain gauge, but some of the
offset is also likely due to regional variations in precipitation isotopes.
Figure 5. 18O versus D values of COB precipitation (solid dark blue circles for winter
(Oct-Mar) months, open dark blue circles the rest of the year), COB dripwaters (light
blue small circles), and long-term monthly weighted averages of Tucson precipitation
(solid red squares for winter (Oct-Mar) months, open red squares the rest of the year).
Also shown are the meteoric water lines (entire year- thin lines, winter- heavy lines)
defined by these relationships compared to the global meteoric water line (black dashed).
Figure 6. Monthly and seasonal amount weighted averages of temperature versus 18O
values of Tucson precipitation (1984-2004/2005). Solid blue large squares are winter
(Oct-Mar) seasonal averages (one omitted point is shown as an open square) and open
light blue small squares are the monthly data. The heavy dark blue line is the trend
through the winter seasonal values minus the flier and the dashed line includes the flier.
The thin light blue line is the trend in the monthly data. Solid red large circles are
summer (Jul-Sept) seasonal averages and open orange small circles are summer monthly
data. The heavy red line is the trend through the summer seasonal data and the thin
orange line that of the summer monthly data.
118
Figure 1.
119
Figure 2.
120
Figure 3.
121
Figure 4.
122
Figure 5.
123
5.0
y = 0.405x - 16.262
R2 = 0.152
y = 0.396x - 15.831
R2 = 0.138
-5.0
-10.0
y = -0.013x - 8.725
R2 = 0.000
18
O (VSMOW)
0.0
y = -0.175x - 6.810
R2 = 0.057
-15.0
y = 0.144x - 10.201
R2 = 0.033
-20.0
0
5
10
15 20 25 30
Temperature (°C)
35
40
Figure 6.
5.0
y = -0.015x - 2.656
R2 = 0.537
y = -0.020x - 3.719
R2 = 0.102
-5.0
-10.0
18
O (VSMOW)
0.0
y = -0.008x - 7.568
R2 = 0.271
y = -0.006x - 7.995
R2 = 0.154
y = -0.035x - 7.255
R2 = 0.105
-15.0
-20.0
0
100
200
Amount (mm)
Figure 7.
300
400
124
Table 1
Cave of the Bells precipitation and dripwater data
Date
Location
Volume (ml)
18
O
D
03/14/03Rain
36
-8.8
-56.2
04/10/03Rain
6
-3.8
-26.8
05/14/03Rain
06/13/03Rain
07/11/03Rain
08/11/03Rain
163
09/14/03Rain
10/22/03Rain
11/25/03Rain
29
-7.7
-58.6
12/24/03Rain
45
-7.4
-47.1
01/23/04Rain
320
-15.7
-111.4
02/27/04Rain
-7.6
-51.1
03/24/04Rain
-6.4
-46.0
05/07/04Rain
400
-3.7
-34.9
07/08/04Rain
60
-3.6
-36.6
08/06/04Rain
385
-3.2
-26.9
09/13/04Rain
380
-1.9
-21.0
11/17/04Rain
172
-4.0
-30.9
01/13/05Rain
710
-12.0
-87.4
02/16/05Rain
486
-11.1
-81.8
03/18/05Rain
250
-5.1
-39.7
04/25/05Rain
37
-12.4
-97.0
06/05/05Rain
162
-2.5
-22.0
08/07/05Rain
676
-7.1
-55.3
09/10/05Rain
817
-4.4
-30.4
10/12/05Rain
9
-1.0
-8.2
Winter 2005 was very dry, no rain recorded for several months
03/14/03Soda Straw
04/10/03Soda Straw
05/14/03Soda Straw
06/13/03Soda Straw
07/11/03Soda Straw
08/11/03Soda Straw
09/14/03Soda Straw
10/22/03Soda Straw
11/25/03Soda Straw
12/24/03Soda Straw
01/23/04Soda Straw
75
10
15
30
50
45
55
55
53
45
45
-10.3
-9.9
-9.7
-9.5
-9.3
-9.4
-9.4
-9.3
-9.4
-9.4
-9.6
-69.5
-67.1
-66.7
-66.2
-64.5
-64.9
-65.2
-65.7
-65.6
-64.5
-65.9
d-excess
notes
14.2
3.6outter gauge cracked,
not replaced until Jun
no water in gauge
water not collected
gauge missing
gauge disturbed
2.8
12.1
14.0
9.5
5.3
-5.5
-7.6
-1.3
-5.610/15 gauge disturbed
1.3
8.8
6.6
1.0
2.3
-2.3
1.47/6 gauge disturbed
4.8
-0.1
12.9
12.1
10.9
9.8
9.9
10.4
9.7
9.1
9.4
10.4
10.6
125
Table 1 continued
Date
Location
02/27/04Soda Straw
03/24/04Soda Straw
05/07/04Soda Straw
06/08/04Soda Straw
07/08/04Soda Straw
08/06/04Soda Straw
09/13/04Soda Straw
10/15/04Soda Straw
11/17/04Soda Straw
01/13/05Soda Straw
02/16/05Soda Straw
03/18/05Soda Straw
06/05/05Soda Straw
07/06/05Soda Straw
08/07/05Soda Straw
09/10/05Soda Straw
10/12/05Soda Straw
11/02/05Soda Straw
12/08/05Soda Straw
01/05/06Soda Straw
03/08/06Soda Straw
04/19/06Soda Straw
05/19/06Soda Straw
03/14/03Popcorn Room
04/10/03Popcorn Room
05/14/03Popcorn Room
06/13/03Popcorn Room
08/11/03Popcorn Room
09/14/03Popcorn Room
10/22/03Popcorn Room
11/25/03Popcorn Room
12/24/03Popcorn Room
01/23/04Popcorn Room
02/27/04Popcorn Room
03/24/04Popcorn Room
05/07/04Popcorn Room
06/08/04Popcorn Room
07/08/04Popcorn Room
08/06/04Popcorn Room
09/13/04Popcorn Room
01/13/05Popcorn Room
04/25/05Popcorn Room
Volume (ml)
18
O
D
d-excess
notes
50
43
58
40
40
40
50
44
39
55
35
27
40
28
32
35
20
39
60
29
30
31
-9.7
-9.7
-9.3
-9.5
-9.3
-9.7
-9.5
-9.5
-9.5
-9.8
-9.5
-9.6
-9.7
-9.9
-9.8
-9.6
-9.6
-9.7
-9.7
-9.8
-9.9
-9.9
-9.9
-66.1
-66.4
-66.2
-66.7
-65.7
-66.1
-66.2
-66.4
-66.1
-66.3
-66.7
-67.4
-67.3
-67.8
-67.1
-67.5
-67.3
-66.3
-68.5
-68.1
-69.1
-68.6
-68.7
11.4
10.9
8.4
9.1
8.9
11.9
9.8
9.9
9.7
12.4
9.7
9.4
10.6
11.1
11.1
9.1
9.8
10.9
9.2
10.5
9.9
10.3
10.6
45
75
40
4
45
155
235
225
140
140
105
80
114
80
70
55
35
-9.7
-9.8
-9.6
-9.6
-9.5
-9.4
-9.4
-9.5
-9.4
-9.4
-9.3
-9.4
-9.5
-9.5
-9.3
-9.3
-9.3
-9.5
-9.5
-66.7
-66.8
-67.2
-66.6
-66.3
-66.4
-66.6
-66.4
-67.0
-65.8
-65.8
-66.1
-66.4
-66.2
-66.1
-66.0
-65.0
-65.6
-67.3
10.9
11.6
9.6
10.2changed formation
9.9
9.0
8.5
9.3
8.4
9.3
8.7
9.4
9.3
10.110/15 tube disconnected
8.111/17 &1/13 dry
8.22/16 trace
9.83/18 tube disconnected
10.4
8.6bottle overflowing
250
126
Table 1 continued
Date
Location
Volume (ml)
18
O
D
d-excess
06/05/05Popcorn Room
07/06/05Popcorn Room
08/07/05Popcorn Room
09/10/05Popcorn Room
10/12/05Popcorn Room
11/02/05Popcorn Room
12/08/05Popcorn Room
01/05/06Popcorn Room
03/08/06Popcorn Room
04/19/06Popcorn Room
05/19/06Popcorn Room
235
175
188
225
250
250
250
140
75
75
-9.3
-9.6
-9.7
-9.4
-9.4
-9.6
-9.7
-9.5
-9.6
-9.7
-9.7
-65.4
-67.0
-66.3
-66.1
-66.5
-65.4
-67.3
-66.7
-66.7
-67.1
-65.2
9.3
9.9
11.4
9.3
9.0
11.0
9.9
9.2
10.4
10.4
12.3
03/14/03D's Climb
04/10/03D's Climb
300
120
-10.1
-9.9
-69.0
-69.0
11.8
10.2
05/14/03D's Climb
150
-10.0
-70.0
10.0
06/13/03D's Climb
175
-10.0
-70.0
10.0
07/11/03D's Climb
125
-9.9
-68.5
10.6
08/11/03D's Climb
135
-9.8
-68.0
10.4
09/14/03D's Climb
185
-9.8
-68.7
10/22/03D's Climb
11/25/03D's Climb
12/24/03D's Climb
01/23/04D's Climb
02/27/04D's Climb
03/24/04D's Climb
05/07/04D's Climb
06/08/04D's Climb
07/08/04D's Climb
08/06/04D's Climb
09/13/04D's Climb
10/15/04D's Climb
11/17/04D's Climb
01/13/05D's Climb
02/16/05D's Climb
03/18/05D's Climb
04/25/05D's Climb
06/05/05D's Climb
07/06/05D's Climb
08/07/05D's Climb
09/10/05D's Climb
10/12/05D's Climb
11/02/05D's Climb
190
185
70
65
85
60
90
62
65
60
90
75
65
-9.8
-9.8
-9.7
-9.7
-9.7
-9.7
-9.7
-9.6
-9.8
-9.7
-9.7
-9.9
-9.8
-9.8
-9.7
-9.5
-9.9
-9.8
-9.6
-9.8
-9.6
-9.7
-9.9
-68.7
-68.7
-68.8
-68.7
-68.8
-68.8
-68.7
-68.1
-68.4
-68.4
-67.8
-67.9
-68.4
-68.1
-67.6
-67.8
-69.4
-68.4
-67.1
-68.3
-67.7
-68.4
-69
10
30
250
250
197
261
250
235
250
notes
9.8
10.0
9.8
8.8
9.0
8.7
8.9
9.1
9.0
9.9
9.4
10.0
11.0
10.3
10.3
10.3
8.5tube disconnected
10.1
10.0
10.1
10.0
9.0
8.9
10.1
127
Table 1 continued
Date
Location
12/08/05D's Climb
01/05/06D's Climb
03/08/06D's Climb
04/19/06D's Climb
05/19/06D's Climb
Location
Soda Straw
Popcorn Room
D's Climb
Volume (ml)
18
O
250
200
225
220
Average
-9.6
-9.5
-9.8
-10.1
-10.0
-10.1
-10.0
-9.9
18
O
D
d-excess
-69
-69
-69
-69
-68
Stdev
0.22
0.14
0.14
notes
11.8
10.7
11.6
10.6
11.1
Average d-excess
10.3
9.7
10.0
Stdev
1.05
1.03
0.86
18
a
b
a
b
18
-0.08
-0.15
-0.06
O /10 mm
0.41
-0.01
-0.18
O /°C
0.06
2
R
0.27
0.54
0.15
0.15
<0.001
2
R
-9.2
-5.6
154
162
O (VSMOW) Amount (mm)
18
18
p<0.025
p<0.01
p<0.10
Significance
p<0.10
Significance
c
N
N
20
21
21
21
20
21
Jun to Sept
Oct to Mar
Monthly
Jun to Sept
Oct to Mar
Monthly
18
0.40
0.14
O /°C
-0.20
-0.35
O /10 mm
18
c
Significance determined by one-sided Student's t -test
Monthly weighted averages (weighted by amount) for each parameter were further combined into weighted
seasonal averages.
b
minus 1989 which is an outlier
a
Oct to Mar
Jun to Sept
Season
Seasonality
Oct to Mar
Jun to Sept
Oct to Mar
Seasonal
Amount
Jun to Sept
Oct to Mar
Oct to Mar
Seasonal
Temperature
c
O values and
temperature, amount, and seasonality of precipitation.
Relationships between Tucson precipitation
Table 2
2
R
2
R
0.10
0.11
0.14
0.03
c
p<0.01
p<0.01
Significance
p<0.01
p<0.025
c
Significance
61
103
N
61
103
N
128
Table 3
18
Jul-04
Aug-04
Sep-04
Month
Sep-04
Aug-04
Jul-04
Month
Amount
(mm)
5.1
5.1
31.2
4.8
3.3
2.5
17.5
36.6
Temp.
(°C)
32.2
30.6
27.8
29.4
28.3
27.8
26.1
23.9
-15.23
-17.58
-2.26
-3.48
-2.17
-2.91
-0.85
-0.65
8.46
16.15
16.01
12.08
-33
-6
-54
-5.1
-1.5
-6.6
24.6
28.1
28.7
Seasonally weighted
Seasonally weighted total
18
18
Amount
D
O
Temp.
D
O
Temp.
(mm)
(VSMOW) (VSMOW)
(°C)
(VSMOW) (VSMOW)
(°C)
41.4
-22.13
-2.69
11.71
5.8
-0.33
-0.09
1.62
54.1
-17.51
-2.71
13.14
101.3
-40
-5.5
26.5
41.4
3.3
2.5
5.8
17.5
36.6
54.1
Monthly weighted
Monthly weighted total
18
18
Amount
D
O
Temp.
D
O
Temp.
(mm)
(VSMOW) (VSMOW)
(°C)
(VSMOW) (VSMOW)
(°C)
5.1
-2.94
-0.16
3.95
5.1
-1.47
-0.09
3.75
31.2
-49.75
-6.34
20.96
D
O
(VSMOW) (VSMOW)
-1.3
-24
-0.7
-12
-8.4
-66
-32
-1.5
-4
-1.5
-8
-6.7
-47
-4.3
-26
*Precipitation events with missing data were excluded from calculations.
Jul-Sept-04
Season
9/4/04
9/19/04
8/13/04
8/19/04
7/11/04
7/17/04
7/27/04
*7/28/04
Date
7/11/04
7/17/04
7/27/04
*7/28/04
8/13/04
8/19/04
9/4/04
9/19/04
Date
Example of Tucson precipitation data by event and amount weighted monthly and seasonal averages
129