Earth and Planetary Science Letters 300 (2010) 287–298
Contents lists available at ScienceDirect
Earth and Planetary Science Letters
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l
Limited hydrologic response to Pleistocene climate change in deep vadose
zones — Yucca Mountain, Nevada
James B. Paces a,⁎, Leonid A. Neymark a, Joseph F. Whelan a, Joseph L. Wooden b,
Steven P. Lund c, Brian D. Marshall a
a
b
c
U.S. Geological Survey, Box 25046, MS963, Denver Federal Center, Denver, CO 80225, USA
U.S. Geological Survey, Stanford University, Stanford, CA 94305, USA
Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089, USA
a r t i c l e
i n f o
Article history:
Received 6 June 2010
Received in revised form 1 October 2010
Accepted 5 October 2010
Available online 3 November 2010
Editor: T.M. Harrison
Keywords:
vadose zone
paleohydrogeology
paleoclimate
U-series dating
secondary ion mass spectrometry
Yucca Mountain
a b s t r a c t
Understanding the movement of water through thick vadose zones, especially on time scales encompassing
long-term climate change, is increasingly important as societies utilize semi-arid environments for both water
resources and sites viewed as favorable for long-term disposal or storage of hazardous waste. Hydrologic
responses to Pleistocene climate change within a deep vadose zone in the eastern Mojave Desert at Yucca
Mountain, Nevada, were evaluated by uranium-series dating of finely layered hyalitic opal using secondary
ion mass spectrometry. Opal is present within cm-thick secondary hydrogenic mineral crusts coating floors of
lithophysal cavities in fractured volcanic rocks at depths of 200 to 300 m below land surface. Uranium
concentrations in opal fluctuate systematically between 5 and 550 μg/g. Age-calibrated profiles of uranium
concentration correlate with regional climate records over the last 300,000 years and produce time-series
spectral peaks that have distinct periodicities of 100- and 41-ka, consistent with planetary orbital parameters.
These results indicate that the chemical compositions of percolating solutions varied in response to nearsurface, climate-driven processes. However, slow (micrometers per thousand years), relatively uniform
growth rates of secondary opal and calcite deposition spanning several glacial–interglacial climate cycles
imply that water fluxes in the deep vadose zone remained low and generally buffered from the large
fluctuations in available surface moisture during different climates.
Published by Elsevier B.V.
1. Introduction
Hydrologically unsaturated zones (vadose zones) more than 100 m
thick are common in arid and semi-arid regions and preserve regional
climate variations as changes in rates and compositions of infiltrating pore
water (Edmunds and Tyler, 2002; Glassley et al., 2002; Walvoord et al.,
2004). An understanding of the long-term movement of water through
thick vadose zones is increasingly important as limited water resources in
these environments are evaluated in terms of both quantity and quality,
and as these environments are being considered as sites for long-term
isolation of toxic waste. However, the flow of small amounts of water
through thick vadose zones, usually as thin films, is difficult to measure,
especially at time scales required for waste isolation (thousands to
hundreds of thousands of years). In particular, chronologies of pore water
records are poorly constrained and may not extend beyond the last fullglacial episode (10 to 20 ka). In contrast, long and detailed histories
spanning hundreds of thousands of years are common in speleothems
and travertine veins formed in karst environments (Fairchild et al., 2006;
⁎ Corresponding author. Tel.: + 1 303 236 0533; fax; + 1 303 236 4930.
E-mail address: jbpaces@usgs.gov (J.B. Paces).
0012-821X/$ – see front matter. Published by Elsevier B.V.
doi:10.1016/j.epsl.2010.10.006
Winograd et al., 1992). These types of mineral records that allow
evaluation of paleohydrologic responses to climate have not been
reported from non-karst settings due to the apparent absence of materials
that clearly preserve the history of unsaturated flow in thick vadose zones.
Secondary fracture coatings exposed in underground workings at
depths of 200–300 m within Yucca Mountain, Nevada (Fig. 1), offer a
unique opportunity to investigate long-term hydrologic responses of a
thick vadose zone to Pleistocene climate change. Although the
morphologies and growth histories are distinct from speleothems
deposited in karst settings, they formed from similar depositional
processes involving downward-percolating vadose water (Paces et al.,
2001; Whelan et al., 2002). An understanding of the hydrologic
response to climate-induced changes in moisture availability
throughout the Pleistocene is critical to evaluations of sites such as
Yucca Mountain, which was studied as a geologic repository for highlevel radioactive waste (Department of Energy, 2008; Levich and
Stuckless, 2007; Sharpe, 2007; Stuckless and Dudley, 2002, p. 2-6).
Deep percolation represents the primary mechanism for release and
transport of long-lived radionuclides. The Yucca Mountain site is no
longer being considered as a viable option for long-term storage of
spent nuclear fuel, in part, because of the perception that the fractured
rock may become saturated with water during wetter climates (e.g.,
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Fig. 1. A) Shaded-relief map showing the locations of underground tunnels at Yucca Mountain, Devils Hole, and pluvial lakes in Death Valley and adjacent basins (Lowenstein et al.,
1999; Smith and Bischoff, 1997). B) Shaded-relief map of central Yucca Mountain with traces of the Exploratory Studies Facility (black) and East–West Cross Drift (red) tunnels.
Locations of secondary mineral coatings used in this paper are shown as blue-filled circles. Locations of coatings sampled for other studies are shown as white-filled circles (Whelan
et al., 2008). Block-bounding normal faults are shown as solid gray lines with symbols on downthrown side (Day et al., 1998). C) East–west section across Yucca Mountain showing
major hydrogeologic units, high-angle normal faults, proposed repository horizon, and static water table (Day et al., 1998).
NEI Nuclear Notes at http://neinuclearnotes.blogspot.com/2010/03/whynot-yucca-mountain.html; accessed on October 1, 2010).
This paper focuses on determining the last 300,000 years (300 ka) of
opal growth in samples of secondary mineral coatings from fracture and
lithophysal cavities present in the deep vadose zone. Because of the very
slow growth rates of these coatings (micrometers per thousand years, or
μm/ka; Neymark et al., 2002; Paces et al., 2004), U-series disequilibrium
ages were determined at spatial resolutions of ~20 μm using secondaryion mass spectrometry (SIMS). If variations in Pleistocene climate
produced large differences in infiltration and deep percolation, vadose
zone minerals are likely to preserve systematic variations in growth rate
caused by fluctuations in solute supply or degree of mineral saturation in
percolating water. Importantly, variations in growth rates at depth should
be correlated with fluctuations in surface moisture known to have
affected the region (Sharpe, 2007, and references therein). Such
correlations have been observed in speleothems along with chemical
and isotopic data that reflect climate-induced variations in vadose
hydrochemistry (Fairchild et al., 2006).
2. Hydrogeologic setting
Yucca Mountain, located on the eastern side of the Mojave Desert
approximately 145 km northwest of Las Vegas, Nevada, consists of a 1–
3-km-thick sequence of gently dipping, ~12.8-million-year (Ma) old
tuffs with a 400–700-m-thick vadose zone (Keefer et al., 2007; Stuckless
and Dudley, 2002). The repository horizon was proposed to be located in
the lower of two densely welded and fractured ash-flow tuffs separated
by 25 to N100 m of nonwelded ash-flow tuffs (Fig. 1C). The tuffs
constituting the vadose zone have been grouped into informal
hydrogeologic units based on their physical properties (Montazer and
J.B. Paces et al. / Earth and Planetary Science Letters 300 (2010) 287–298
Wilson, 1984). The moderately to densely welded Tiva Canyon and
Topopah Spring welded units (Fig. 1C) have relatively high fracture
permeability and low matrix permeability. The intervening, largely
nonwelded to partially welded Yucca Mountain and Pah Canyon Tuffs,
along with basal Tiva Canyon Tuff and uppermost Topopah Spring Tuff,
have been grouped together as the Paintbrush Tuff nonwelded (PTn)
hydrogeologic unit (Moyer et al., 1996) which has relatively high matrix
permeability and low fracture permeability.
Widely accepted conceptual models of infiltration in arid settings
(Flint et al., 2001a,b) posit that infiltration is directly related to available
surface moisture, that infiltration passing beneath the root zone (zone of
evapotranspiration) recharges to the water table, and that infiltration
fluxes were substantially larger than present-day values during
Pleistocene pluvial climate conditions when temperatures were as
much as 10–15 °C cooler and precipitation was 1.4–3 times wetter than
present conditions (Forester et al., 1999; Quade et al., 2003; Sharpe,
2007; Spaulding, 1985; Thompson et al., 1999). Increased surface
moisture during the last several glacial episodes supported large
freshwater lakes in Death Valley and other basins in the region
(Fig. 1A; Forester et al., 2005; Lowenstein et al., 1999; Smith and
Bischoff, 1997) and may have produced substantially higher amounts of
infiltration compared to modern estimates (N100 mm/a versus 0.6–
10 mm/a; Faybishenko, 2007). Despite the clear evidence for increased
surface moisture in the past, the response of deep vadose percolation to
Pleistocene climate fluctuations has remained difficult to quantify.
3. Samples and analytical methods
3.1. Sample description
Secondary mineral coatings consist of 1- to 30-mm-thick irregular
crusts of coarse, sparry calcite often interlayered with lesser clear hyalitic
opal (Paces et al., 2001; Whelan et al., 2002). Coatings are present on a
small percentage of open fracture footwalls and cavity floors exposed
throughout the Exploratory Studies Facility (ESF) and East–West Cross
Drift (EWCD) tunnels (Fig. 1B). Samples used here are from the upper
lithophysal and middle nonlithophysal zones of the Topopah Spring Tuff
in the northern part of the underground workings. This area contains
some of the thicker, more delicate coatings with relatively abundant opal;
however, coatings in this area are fundamentally similar to others
exposed elsewhere in the ~10 km of tunnels excavated.
The sporadic distribution of mineral coatings, their restriction to
sloping floors rather than ceilings or hanging walls of mineralized cavities,
and their highly irregular and convoluted textures are interpreted as
evidence that water never ponded within lithophysae. Instead, physical
characteristics of the deposits are consistent with precipitation from films
of water that descended through fracture pathways and seeped into airfilled cavities (Paces et al., 2001; Whelan et al., 2002). Precipitation of
calcite can be caused by changes in temperature as water flows
downward, by changes in the partial pressure of CO2 within a gas-filled
cavity, by increases in Ca concentrations in solution, and by evaporation
(Dreybrodt, 1980; Marshall et al., 2003). In contrast, warming of
descending fluids causes opal dissolution rather than precipitation
whereas loss of water vapor by evaporation results in opal precipitation
regardless of dissolved silica species:
2−
þ
SiO2 ðOHÞ2 þ 2H ↔SiO2 þ 2H2 O↑as vapor
3−
SiOðOHÞ
þ
þ H ↔SiO2 þ 2H2 O↑as vapor
SiðOHÞ4 ↔SiO2 þ 2H2 O↑as vapor
The absence of other silicate phases within secondary mineral
coatings is considered to be evidence that evaporation, rather than
silicate weathering, is the primary cause of opal precipitation. Therefore,
289
evaporative loss of water from thin films drawn up along the outer
surfaces of minerals by capillary forces (Whelan et al., 2005) is capable
of driving simultaneous precipitation of both calcite and opal. Thin
(b3 mm) coatings of calcite on steep-dipping fractures invariably lack
opal and likely formed by geothermal warming of descending solutions.
However, thicker coatings (5–30 mm) on shallow-dipping cavity floors
commonly contain calcite and opal, often with textures indicating
coeval growth of both phases (Paces et al., 2001; Whelan et al., 2002).
The thickest coatings record the most complete history of seepage and
growth with basal layers that are 10 Ma or older (Neymark et al., 2002;
Whelan et al., 2008; Wilson et al., 2003).
3.2. Analytical methods
Isotopes of Th and U were analyzed by secondary ion mass
spectrometry (SIMS) using the Stanford-U.S. Geological Survey Sensitive High-Resolution Ion Microprobe-Reverse Geometry (SHRIMP-RG)
following methods described elsewhere (Paces et al., 2004). An 18–
40 nA primary beam of 16O− ions was used to sputter and ionize U and
Th or their oxides from spots on polished cross sections of hyalitic opal.
Individual analyses required approximately 25 min to count ions at
234
masses 28Si16
U, 238U, 230Th16O, 232Th16O, 235U16O, 238U16O, and
4 O7,
background masses. Analyses resulted in steep-walled, flat-bottomed
pits approximately 25 μm in diameter and 10 μm deep (see Fig. S1 in
online supplementary material). Measured 234U/238U and 230Th16O/
238 16
U O (equivalent to 230Th/238U) atomic ratios for opal in radioactive
secular equilibrium typically are within analytical uncertainty of values
calculated using accepted decay constants (see Fig. S2 in online
supplementary material). Nevertheless, ratios measured in Yucca
Mountain opal were normalized to the weighted mean values for the
same ratios measured in a 1.9-Ma-old opal standard (M21277; Amelin
and Back, 2006) to correct for systematic instrument bias and to obtain
activity ratios (AR). Measured 232Th abundances in Yucca Mountain
hyalitic opal are extremely small (232Th16O/230Th16O atomic ratios
typically much less than 1.0) and essentially all 230Th measured in these
opals is derived from in situ decay of 238U. Two-sigma uncertainties for
isotope ratios are derived from counting statistics (within-run error)
plus external error derived from the reproducibility of frequent analysis
of the M21277 opal standard.
Ages based on 230Th/U disequilibrium were calculated using conventional iterative 230Th–234U–238U dating equations (Broecker, 1963;
Ivanovich and Harmon, 1992; Ludwig, 2003). However, 230Th/U ages
older than ~200–250 ka have large uncertainties due to large analytical
errors associated with 234U and (especially) 230Th. To take advantage of
the longer half-life of 234U (~245 ka versus ~76 ka for 230Th), model
234
U/238U ages (Ludwig and Paces, 2002) were calculated for spots older
than ~200 ka assuming that 234U/238U AR in solutions at a given
site remained constant through time and equal to values obtained from
230
Th/U-dating results for younger layers from the same grain. Model-age
uncertainties were propagated using within-run error for the measured
234
U/238U AR combined with the 2σ range in initial 234U/238U AR
calculated for younger layers.
Concentrations of U were estimated by normalizing 238U16O beam
intensities in analyses of unknown opals to values measured for
M21277 spots determined in the same analytical session and
multiplying by the median value of 981.5 μg/g for M21277 U
concentrations reported in Amelin and Back (2006). Errors reported
for opal U concentrations are large (±30% 2σ) because of the
heterogeneous U distributions in the standard. Quantification of U
concentrations is not necessary for U-series dating.
Cathodoluminescence (CL) images of calcite and opal reveal
alternating bright and dark bands that reflect growth (Fig. 2). This
layering is important for tracing internal microstratigraphy and for
recording compositional variations that reflect changes in percolation
chemistry or vadose-flow conditions. Profiles of relative pixel
brightness on CL images were made using an image-processing add-
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in to Adobe Photoshop™ (The Image Processing Tool Kit™, v4.0,
Reindeer Graphics, Inc.). To compensate for differences in brightness
levels between images, pixel values were normalized to the maximum
value present in each profile (“relative brightness”). Profile distances
were converted to depths in micrometers using scale bars included on
CL images and confirmed by petrographic measurements. The centers
of SIMS ablation pits were projected along CL-defined growth layers
until they intersected profile lines overlain on the CL image. Distances
from spot centerlines to the outer surface of the mineral grain,
measured in micrometers, were used as apparent microstratigraphic
depths. To evaluate relations between CL brightness and U concentration, a mean value of relative brightness was calculated for pixels
within a 10-μm interval on either side of each SIMS spot centerline
projected to the CL profile line.
Grain mounts were oriented to minimize angular corrections for
dipping layers. Verification that layering intersects the polished surface
at high angles was made by petrographic examination. However,
because polished surfaces are not necessarily perpendicular to growth
layering, measured microstratigraphic depths represent apparent
thickness that may overestimate true microstratigraphic thickness. As
such, calculated growth rates represent maximum values. Corrections
for layer dip angle would decrease calculated growth rates by less than
30% for layers dipping up to 45 degrees from vertical.
4. Results
4.1. Cathodoluminescence intensity and uranium concentration
Yucca Mountain hyalitic opal fluoresces bright yellow–green under
short-wavelength ultraviolet light. This characteristic coloration has been
attributed to activation caused by trace amounts of U that maintains a
near-linear O-U-O molecular bond after inclusion in the solid phase
(Hanchar, 1999). To test if UO2+
also causes CL response in Yucca
2
Mountain hyalitic opal, a traverse of 44 SIMS spots across growth layering
in opal sample HD2055 (red circles in Fig. 2) yielded measured 238U16O
beam intensities of 136–13,938 counts per second and relative mean spot
brightness values of 34.2–99.0% (Fig. 3; data in Table S1 of online
supplementary material). CL brightness and 238U16O beam intensity are
positively correlated (R2 value of 0.955). U concentrations determined for
U-series age analyses also are positively correlated with CL brightness.
Spot analyses of other trace elements (Mg, K, Na, Ca, Al, Mn, Fe, Ba) were
determined to evaluate potential correlations with U concentrations or CL
brightness; however, no consistent trends were observed (see section on
SHRIMP-RG Elemental Abundances in online supplementary material).
Therefore, CL intensity in the hyalitic opal is attributed to the amount of U
present. This allowed conversion of measured CL intensities to U
concentrations using linear relations determined for each opal grain.
Resulting U concentrations ranged from 5.4 to 550 μg/g (data in Tables S1
and S2 of online supplementary material).
4.2. Age—depth relations
Fig. 2. Cathodoluminescence images of growth-zoned opal in samples HD2055,
HD2059, and HD2074 from three different lithophysal cavities showing locations of
spots used to determine 238U16O beam intensities (red circles) and U-series ages
(yellow circles). Orange lines trace growth layering from spots to measured opal
profiles.
An accurate age framework is essential for providing a paleohydrologic
context for these samples. Both 230Th/U ages and initial 234U/238U AR were
calculated for most analyses, as well as model 234U/238U ages for spots
older than ~200 ka (Table S2 in online supplementary material). Resulting
ages range from 41 to 1290 ka and pass three important geochronological
tests: (i) ages increase with increasing microstratigraphic depth,
(ii) coeval layers have similar ages, and (iii) secular equilibrium 234U/
238
U and 230Th/238U values are obtained for ancient opal (e.g., 1.9-Ma-old
opal standard 21277 and deepest layers of Yucca Mountain opal).
Relations between age and microstratigraphic depth are remarkably linear over the last 300 ka and indicate that average opal growth
rates are extremely slow, ranging between 0.42 and 1.54 μm/ka
(Fig. 4). These rates are consistent with those noted previously for
Pleistocene opal dated by U-series (Paces et al., 2004) and U–Pb
J.B. Paces et al. / Earth and Planetary Science Letters 300 (2010) 287–298
291
Fig. 3. Relation between relative mean spot brightness and 238U16O beam intensity for
SIMS spots analyzed across an opal profile from sample HD2055 (red circles in top
image of Fig. 2; data from Table S1). Error bars represent 5% of the given value for spot
brightness and 2σ counting errors for beam intensities (if greater than symbol size).
(Nemchin et al., 2006) methods. Given these slow rates of growth, a
~ 20-μm-diameter spot incorporates opal deposited over a 13–48 ka
time period. Furthermore, spots that include the outermost layers of
opal have 230Th/U ages ranging between 31 and 41 ka (Table S2 in
online supplementary material). Regressions of age versus microstratigraphic depth yield high linear correlation coefficients
(R2 = 0.972 to 0.995) and low MSWD (mean square of weighted
deviates, a measure of the ratio of observed scatter of points from the
best-fit line to the scatter expected from assigned errors; Ludwig,
2003) values (0.46 to 0.71), both indicating that long-term average
opal growth rates in individual grains remained essentially constant
over the last 300 ka. The relatively large age uncertainties for spots
that include layers formed over substantial time spans allow for the
possibility of shorter term (less than ~ 10 ka) variations in growth
rates. Nevertheless, deviations from linear growth are not apparent
over the last 300 ka in these profiles (Fig. 4) even though layers span
several climate cycles thought, on the basis of hydrologic modeling
studies, to have resulted in 10- to 100-fold variations in infiltration
(Faybishenko, 2007).
Unlike opal, U concentrations in Yucca Mountain calcite are too
low for SIMS dating. However, opal interlayered within calcite (Fig. 5)
can be dated. Results from the tips of several calcite blades yield
average calcite growth rates of 0.84 and 1.21 μm/ka (Fig. 6). These
rates are the same as those obtained for opal-only profiles, and with
Miocene growth rates determined for cm-thick calcite-dominated
coatings from the same vadose horizon (Neymark et al., 2002). Even
though opal is typically present as discrete patches within calcite
blades, textural relations along lateral opal/calcite contacts as well as
compositional banding at the same spatial scales in both phases are
interpreted as evidence of coeval growth (Paces et al., 2001; Whelan
et al., 2002; Wilson et al., 2003). Therefore, growth rates determined
from opal profiles are applicable to calcite deposited over the same
time periods.
5. Hydrologic responses to climate change
5.1. Continuity of depositional records
The ability of the dating method to capture and resolve detailed
growth histories is important for using these records to evaluate
hydrologic responses to Quaternary climate. In particular, the
presence of non-zero age intercepts for regressions shown in Figure 4
implies that opal was not deposited on these grains during the last 24
to 33 ka. A depositional hiatus, which includes both the wettest and
Fig. 4. Relations between microstratigraphic depth and uranium-series age (± 2σ) for
opal profiles shown in Figure. 2. Numbered points correspond to spots in Figure 2 and
analyses in Table S2. Regression slopes represent growth rates in μm/ka.
driest conditions during the last 30 ka, may be an indication that
water did not seep into the cavities over this time period. Therefore, it
is important to understand how faithfully the secondary minerals
record a history of percolation and seepage in the vadose zone.
Although the calcite-opal coatings form by general outward growth
from basal layers into open cavities, detailed growth histories can be
complex and difficult to deconvolute. Microstratigraphic discontinuities within opal have been observed from textural relations where
opal layers separated by substantial thicknesses of calcite can be traced
on CL images back to areas lacking the intervening calcite. Measured
age-depth relations in these cases yield very different growth rates
depending on whether the intervening calcite is included or excluded
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Fig. 5. Cathodoluminescence images of the tips of secondary calcite blades from samples HD2310 (A) and HD2313 (B) containing inclusions of opal formed on growth surfaces.
Orange lines trace growth layering from spots dated by SIMS (yellow circles) to measured calcite + opal profiles used to calculate mineral growth rates shown in Figure 6 (data in
Table S2).
(i.e., where the profiles are measured). In the several cases where
these types of profiles were dated, resulting ages indicate depositional
hiatuses on the order of several hundred thousand years. Shorter
hiatuses might not be readily recognized.
Non-zero age intercepts are also dependent on where the origin of
a profile is located. In all three cases shown in Figure 2, outermost
surfaces of opal hemispheres where chosen for zero depths. However,
the opal grain in sample HD2059 (middle image) has approximately
50 μm of calcite lying on top of the outermost layer of dated opal (light
gray material above spots 2 and 6). Given the 1.54 μm/ka growth rate
for this opal, the youngest period of calcite deposition matches the
33 ka hiatus missing from the HD2059 depth profile shown in
Figure 4. Nevertheless, opal deposition appears to have ceased on
the HD2059 grain after about 33 ka for crystallographic or chemical
reasons that are not fully appreciated, although calcite deposition
appears to have continued until recently.
If non-zero age intercepts from dated opal profiles truly reflected an
absence of seepage into these cavities, all opal growth should have
ceased. Therefore, no opal younger than about 24 ka (X-intercept in
bottom plot of Fig. 2) should be present on outer surfaces of opal
hemispheres from sample HD2074. However, the outermost several μm
of opal from 6 other hemispheres from this same cavity have ages
determined by in situ microdigestions that range from 4 ± 1.9 to 11.6 ±
0.8 ka (Table 5 of Paces et al., 2004). Furthermore, previous efforts to
determine age-depth relations for two opal hemispheres from HD2074
resulted in age intercepts of 8 ± 23 ka and 8.9± 8.3 (determined by
SIMS dating; Fig. 9 of Paces et al., 2004), and 7.7 ± 6.3 ka (determined by
in situ microdigestion dating; Fig. 13 of Paces et al., 2004). Cumulatively,
these results indicate that seepage into the HD2074 cavity did not stop
during the current interglacial climate; however, they also indicate that
not all opal grains represent a simple and continuous record of seepage
history.
5.2. Numerical simulations of opal growth rates
A series of simple numerical simulations of opal growth and spot
analysis were constructed to evaluate the sensitivity of the SIMS ageprofiling methods used in this study (see section on Simulations of
Opal Growth Rates in online supplemental material). In these
simulations, opal layers were deposited in 1-μm layers at constant
growth rates from seepage water with constant 234U/238U AR. U-series
isotopic compositions were calculated for each layer assuming closedsystem evolution since the time that each layer was formed. Ages for a
hypothetical SIMS analyses were then simulated by averaging the
J.B. Paces et al. / Earth and Planetary Science Letters 300 (2010) 287–298
293
rates between 1 and 2 μm/ka are more likely to discriminate between
these two scenarios and yield accurate growth histories, especially if
spots are closely spaced.
5.3. Age-calibrated uranium profiles
Fig. 6. Age-depth relations for profiles of the outermost layers of two secondary calcite
blades containing interlayered opal shown in Figure 5. Numbered points refer to spots
shown in Figure 5 and data in Table S2. Errors represent 2σ uncertainties.
isotopic compositions of layers included within a 25-μm-wide “spot”.
Growth rates, hiatus durations, data density, and U concentrations
were varied to evaluate the effects on simulated profiles.
Simulations of a continuously growing layered opal indicate that the
230
Th/238U and 234U/238U values obtained using a 20-μm-diameter spot
yield a close approximation of the true average age for the layers
analyzed. 230Th/U ages calculated using weighted average isotopic
compositions (layers constituting the central part of the spot having the
greatest influence) have a small, but systematic bias toward younger
values due to the non-linear effects of radioactive decay (Neymark and
Paces, 2000; Neymark et al., 2000). However, these effects are typically
negligible relative to the 10% or larger analytical uncertainties obtained
from ion microprobe counting statistics (Fig. S9 of online supplemental
material). These effects are greatest for young samples deposited at slow
rates (b0.5 μm/ka).
Results of simulations that included systematic depositional
hiatuses (i.e., 10 to 50 ka periods of non-deposition out of every
100 ka) indicate that detailed growth histories are likely to be
obscured at slow rates of growth (b1 μm/ka). Increased data density
(i.e., overlapping spots) improves the chances of identifying growth
hiatuses. However, for simulations using growth rates less than 1 μm/
ka, it is not possible to discriminate between (1) episodic deposition
at higher growth rates interspersed with hiatus of 20 ka or less, versus
(2) continuous growth at slower time-averaged growth rates (Fig. S12
of online supplemental material). Deposits formed at higher growth
Regardless of the geochronological limitations and possible gaps in
the depositional record, the high spatial resolution (micrometerscale) of the distinct CL banding present in vadose zone opals captures
detailed compositional information at millennial or shorter time
scales. In order to evaluate whether the variations in U concentration
causing this banding are dependent on cavity-specific conditions or
more site-wide causes, profiles need to be calibrated using the
available age information. To do this, ages were assigned to each pixel
of a CL profile using the slope and intercept values derived by linear
regression of age versus depth data (Fig. 4). Resulting age-calibrated
profiles were checked against age-depth-intensity relations for
individually dated horizons to ensure that ages for dated SIMS spots
were honored. Overall uncertainties for ages estimated for CL profiles
are based on uncertainties calculated for regression slopes.
Resulting age-calibrated U profiles have several distinct Uconcentration peaks present between 40 and 50 ka, 120–140 ka,
230–280 ka, and 300–350 ka (Fig. 7). The timing of peaks is consistent
among profiles from different cavities and generally correlate with the
precisely dated δ18O record of regional paleotemperature preserved in
Devils Hole calcite (Winograd et al., 1992, 2006), located approximately 45 km south–southeast of Yucca Mountain (Fig. 1A). For each
opal profile, four primary peaks (labeled 1–4) and four secondary
peaks (labeled a–d) were assigned ages at either their maximum
values or midpoints of peak width (Fig. 7). The calcite δ18O profile
from Devils Hole was treated similarly using peaks with higher δ18O
(warmer climate). Mean ages for high-U opal layers closely correlate
with the record of warm interglacial climates, matching ages of highδ18O peaks from the Devils Hole profile within b10 ka (Table 1). The
close correspondence between individual opal profiles and the
continuous record of calcite deposition at Devils Hole is viewed as
evidence that these opal grains did not suffer depositional discontinuities long enough to compromise the climate-proxy record.
Discrepancies in timing between peaks in the three opal profiles are
most noticeable for the oldest, least reliably dated layers, particularly
for profile HD2055, which has the poorest geochronological constraints of the three.
5.4. Time-series analyses
Astronomical forcing has been identified as a likely cause for
oscillatory cycles of climate change throughout the Pleistocene
(Milankovitch cycles). Paleoclimate records commonly have periodicities of 100, 41, and 24 to 19 ka corresponding to the Earth's orbital
parameters of eccentricity (departure from a circular orbit), obliquity
(angle of tilt of the pole), and precession (rotation of the pole about its
central axis) (Muller and MacDonald, 1997). Time-series analysis
(spectral analysis) of paleoclimate signals incorporating these types of
periodic features will yield spectrums with a small number of
frequencies that can be related to orbital periods (Muller and
MacDonald, 2000).
Time-series analyses were made of age-calibrated opal U profiles
using both the multi-taper method (MTM) and maximum entropy
method (MEM). Both methods evaluate total time series, but estimate
spectral content in different ways. In addition, both methods assume
that the SIMS-derived ages used to calibrate CL brightness profiles are
true ages — no “tuning” of the age profiles was performed prior to
analysis. Therefore, only the best-dated profiles were tested by
spectral analysis.
Results of time-series analyses show spectra with strong internal
consistency and frequencies indicating periodicities greater than 10 ka
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J.B. Paces et al. / Earth and Planetary Science Letters 300 (2010) 287–298
evaporative concentration of solutes in soil and fracture water during
warmer, drier climates. Profiles of calcite abundance and δ18O in the
Yucca Mountain subsurface indicate that the effects of evaporation
extend into the fracture network to depths up to 100 m (Whelan et al.,
2002). Studies of other thick vadose zones have recognized the
importance of subsurface evaporation and upward vapor fluxes, as
well as the long periods (10–100 ka) required for equilibration to
near-surface conditions (Glassley et al., 2002; Walvoord et al., 2002,
2004).
Because of the strong climate signal observed in vadose zone opal
formed over the last 300 ka, mineral growth rates might be expected
to fluctuate along with climate-induced changes in surface moisture.
Studies of speleothems from North America, Europe, and Australia
have found that faster growth rates are correlated with Pleistocene
periods of increased surface moisture (Ayliffe et al., 1998; Baker et al.,
1993; Musgrove et al., 2001). Growth rates in modern speleothems
also were shown to correlate well with rainfall and excess water (i.e.;
drip rate; Banner et al., 2007; Genty and Quinif, 1996). Although
growth rates of Yucca Mountain minerals are ~100,000 times slower
than speleothems in karst environments (Fairchild et al., 2006),
calcite precipitation is likely to be controlled by similar processes in
both settings. Consequently, estimates of water-to-calcite volume
ratios for the deep Yucca Mountain vadose zone are similar to values
reported for speleothems and laboratory experiments where both
water and calcite volumes are known (Marshall et al., 2003).
Calcite and amorphous silica have remained abundant in soils at
the surface of Yucca Mountain throughout at least the middle
Pleistocene (Section 10.4 of Ludwig and Paces, 2002). Therefore, it is
likely that water infiltrating into the vadose zone has been chemically
saturated with respect to both phases over the last 300 ka. This
likelihood is supported by data from modern water seeping from
several fractures in shallow parts of the ESF tunnel during the wet
winter of 2005 as well as water perched in the deep vadose zone with
δ13C-corrected radiocarbon ages of 2 to 7 ka (p. 37 of Yang et al.,
1996), all of which have compositions that were saturated with
respect to calcite and silica gel (Oliver and Whelan, 2006; Fig. 36 of
Paces et al., 2001). Furthermore, effects of substantial variations in
surface temperature during the Pleistocene are likely to be negligible
within individual cavities at depths of 200 to 300 m.
Because both saturation indices and temperature likely remained
constant in the deep vadose zone at Yucca Mountain, percolation flux
and associated seepage into cavities is interpreted to be the dominant
factor controlling rates of mineral growth. Thicker coatings have
higher growth rates and are a likely consequence of greater and more
continuous average seepage fluxes. Thinner coatings have lower
growth rates and are interpreted to have formed in cavities that
received smaller or less frequent seepage fluxes. Water-film thickness
and flow velocity are likely critical factors that explain differences
between thinner, opal-poor coatings lining steeply dipping fractures
compared to thicker, opal-rich coatings flooring shallow dipping
lithophysae. Within individual lithophysal cavities, water films
supplying solutes to outer mineral surfaces may only represent a
Fig. 7. Age-calibrated uranium concentration profiles for opals shown in Figure 2 along
with calcite δ18O profile for the Devils Hole climate record (Winograd et al., 1992,
2006). Estimates of 2σ uncertainties are based on errors calculated for slopes in Figure 4
and are shown as error bars for peaks with ages at approximately 50 and 350 ka. Peaks
for uranium concentration and calcite δ18O are sequentially labeled from 1 to 4 for
primary peaks, and a–d for secondary peaks and their ages are given in Table 1.
Interglacial periods based on marine oxygen isotope stages are shown as gray bands
labeled 3, 5e, 7e, 9c, and 11c in italics (Winograd et al., 1992).
(Fig. 8). The most prominent spectral peak in all profiles has a frequency
of approximately 1 × 10− 5 cycles a− 1 corresponding to a periodicity of
100 ka (Fig. 8). This is true for both MTM and MEM methods, but
spectral peaks are better defined and more stable using MEM results. In
addition, higher frequency peaks are present indicating periodicities at
shorter time scales. The most prominent higher frequency spectral
peaks fall between 2.19 × 10− 5 and 2.55× 10− 5 a− 1 corresponding to
periodicities of 46–38 ka. These peaks bracket the 41,000-year
periodicity attributed to orbital obliquity. Additional high-frequency
spectral peaks at 4.2 × 10− 5 to 4.9 × 10− 5 a− 1 353 (24–20 ka) are close
to periodicities expected from orbital precession.
The relatively large uncertainties in the non-tuned ages used for
spectral analyses limit the abilities of these profiles to define accurate
orbital periodicities. However, the two dominant peaks in the spectra
from the best-dated opal profiles are similar to spectral patterns
observed in the Devils Hole calcite δ18O record (Winograd et al.,
2006). Although the spectral analysis of Yucca Mountain opal does not
conclusively demonstrate astronomical forcing as the cause of
variations in U concentration, observation of the 100- and 41-ka
periods in the best-dated profiles is consistent with the presence of a
climate signal deep within the Yucca Mountain UZ.
5.5. Deep vadose zone responses to climate change
The correlation between increased U concentrations in opal and
interglacial stages (along with cyclic variations of Mg concentration in
similar-aged calcite; Wilson et al., 2003) are consistent with greater
Table 1
Ages of peaks in uranium concentrations labeled on the opal profiles shown in Figure 7.
Peak label
HD2055
HD2059
HD2074
Weighted mean
Devils hole δ18O
Age of profile feature (ka)
1
a
2
b
c
3
d
4
53 ± 13
41 ± 3
51 ± 5
44 ± 14
47 ± 7
88 ± 21
77 ± 5
83 ± 8
79 ± 4
81
127 ± 30
131 ± 9
122 ± 12
128 ± 7
129 ± 11
155 ± 37
156 ± 10
167 ± 16
159 ± 8
163
189 ± 45
192 ± 12
189 ± 18
191 ± 10
193
269 ± 64
232 ± 15
230 ± 22
233 ± 12
238 ± 13
298 ± 71
269 ± 17
294 ± 29
277 ± 36
287
350 ± 84
307 ± 20
349 ± 34
319 ± 58
328 ± 10
Uncertainties for individual peaks are calculated from 2σ slope uncertainties reported for regressions in Figure 4. Uncertainties for weighted means are given at the 95% confidence
level and include an additional weighting factor (Student's t times the square root of the mean square of the weighted deviates; Ludwig, 2003) if a set of peaks exhibit scatter beyond
individual errors. Ages for Devils Hole are derived from the data given in Winograd et al. (1992, 2006) with uncertainties for major interglacial periods representing approximate
duration of warm periods based on the widths of the dark bands shown in Figure 7.
J.B. Paces et al. / Earth and Planetary Science Letters 300 (2010) 287–298
295
Fig. 8. Results of time-series spectral analyses for four well-dated CL profiles of opal from Yucca Mountain. Age-calibrated CL profiles were analyzed using the maximum entropy
method with several different filters (section 3.8 of Muller and MacDonald, 2000). Dashed vertical lines shown at 100-, 41-, and 23 ka represent cycles attributed to major
astronomical forcing factors (eccentricity, obliquity, and precession, respectively). CL images for opal samples are shown in Figure 2, except for HD2074, IP1, grain #1 (not shown).
fraction of total water flux flowing through that cavity. Nevertheless,
the presence or absence of water films as well as the thicknesses of
water films will remain proportional to total seepage flux. Therefore, if
climate change produced fluctuations in percolation and seepage
fluxes in the deep vadose zone, mineral growth rates also should have
fluctuated.
Alternatively, mineral growth rates may be limited by the rates of
CO2 and water vapor removal. Although gas loss is an important
mechanism for deposition, a water-limiting mechanism for mineral
growth is considered more likely for several reasons. Air permeability
in the deep vadose zone would tend to decrease during pluvial
climates if increased percolation filled fractures and reduced the
overall air circulation within the deep vadose zone. As a consequence,
mineral growth rates would likely be lower during wetter periods if
gas flow was the driving factor for mineral precipitation. In addition,
age-calibrated profiles from different cavities are strongly correlated
(Fig. 7) indicating that water films were closely connected to gravity
driven percolation regardless of the different geometries involved. If
gas-flow was the dominant factor limiting mineral precipitation,
coherency between profiles from different cavities would be less
likely given the variety of fracture geometries affecting gas permeability. Finally, the dependency of mineral precipitation on water
rather than gas fluxes is supported by geochemical and isotopic data
from the host rocks themselves. Whole-rock samples subjacent to
lithophysal cavities that host 10–30-mm-thick mineral coatings show
greater U losses and 234U/238U disequilibrium than rock samples
subjacent to cavities hosting little or no secondary minerals (Paces et
al., 2006), reflecting greater degrees of water-rock interaction
associated with more-mineralized cavities.
Despite the strong evidence for a meteoric source of vadose zone
water, the effects of large variations in effective surface moisture
known to have occurred throughout the Pleistocene are not apparent
in these deep vadose-mineral records. Although the available spatial
and geochronological resolutions do not allow detailed growth
histories to be defined at century or even millennial time scales,
these data are consistent with more-or-less uniform percolation and
seepage fluxes at depth despite the large differences in surfacemoisture availability between glacial and interglacial time periods.
Variations in surface moisture over the time period recorded by opal
profiles is manifested by the presence or absence of pluvial lakes
throughout the region, including a very wet penultimate glacial cycle
(marine oxygen isotope stage 6) that resulted in pluvial Lake Manly
reaching depths up to 180 m at between ~ 180 and 140 ka followed by
a long, warm interglacial period (stage 5e, Fig. 7) from about 130 to
110 ka that allowed Lake Manly to evaporate (Forester et al., 2005;
Lowenstein et al., 1999). These records of climate change form the
basis for large fluctuations in model infiltration simulated between
glacial and interglacial climates (Faybishenko, 2007).
The uniform rates of mineral growth within the deep vadose zone
at Yucca Mountain are interpreted as strong evidence that the amount
of water flowing through the deep vadose zone was buffered from
variations in surface moisture and shallow infiltration. Hydrogeologic
processes contributing to this buffering may include: down-slope
diversion of water at the contacts between colluvium and welded Tiva
Canyon Tuff and between the Paintbrush nonwelded tuff and the
underlying welded Topopah Spring Formation (Fig. 1C) as first
proposed by Montazer and Wilson (1984); increased evapotranspiration by plant communities that adapt to cooler, wetter climates;
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J.B. Paces et al. / Earth and Planetary Science Letters 300 (2010) 287–298
reduction of water fluxes in shallow fractured tuffs by evaporation
and upward water flux; uniform release of percolation stored in
nonwelded tuffs overlying fractured repository units; and reduction
of deep fracture flux by evaporation and/or imbibition into the welded
tuff matrix of the deep vadose zone. These processes, most likely
working in combination, may be effective at limiting the amount of
water that infiltrates into the subsurface, reducing the percolation flux
as it migrates downward, and buffering the percolation flux that
enters into the deep vadose zone.
5.6. Meteoric versus upwelling water sources
Age-calibrated profiles of U concentration in opal that correlate
with Pleistocene regional climate variations demonstrate that water
flowing through the deep vadose zone was derived from a meteoric
water source. This conclusion strongly contrasts with recently
reported interpretations of measured hydrogen and calculated
oxygen isotope data for fluid inclusions in calcite coatings from
similar depths within Yucca Mountain (Dublyansky and Spötl, 2010).
Those authors concluded that an apparent enrichment of δ18O in
water from calcite-hosted fluid inclusions (calculated using measured
δ2H values, fluid inclusion homogenization temperatures, and the
assumption of O isotopic equilibrium between calcite and water)
required water/rock exchange and a deep-seated hydrothermal water
source. However, elevated temperatures are only applicable to the
oldest parts of the mineral coatings (Whelan et al., 2008; Wilson et al.,
2003): secondary minerals formed in the last 2 to 4 million years were
deposited under modern thermal conditions. If the isotope measurements reported by Dublyansky and Spötl were from inclusions formed
during this younger time frame, then temperature corrections are
inappropriate. Without temperature corrections, measured δ2H and
δ18O values (Table 1 of Dublyansky and Spötl, 2010) scatter on either
side of the global meteoric water line.
Other than the well-documented hydrothermal alteration of tuffs
well below the water table at Yucca Mountain that are associated with
Timber Mountain caldera eruptions at ~10.5 Ma (e.g., Bish and Aronson,
1993; Whelan et al, 2008), there is no credible evidence requiring
hydrothermal activity. Preservation of fragile fumarolic δ18O signatures
in vadose zone nonwelded tuffs precludes post-depositional hydrothermal modification (Holt, 2002). Furthermore, seismic pumping, the
mechanism that Dublyansky and Spötl (2010) invoke to inject calciteand opal-depositing hydrothermal fluids hundreds of meters up into the
vadose zone is not widely accepted (Bredehoeft, 1992; Carrigan et al.,
1991; Rojstaczer, 1999). In contrast, the opal climate-proxy records
presented here are consistent with a simple mechanism of gravitydriven, vadose zone flow, and a large body of other evidence from both
surface and subsurface hydrogenic deposits (Marshall et al., 2005;
National Research Council, 1992; Neymark et al., 2005; Paces et al.,
2001; Sonnenthal et al., 2005; Stuckless et al., 1991, 1998; Vaniman and
Whelan, 1994; Vaniman et al., 1994, 2001; Whelan and Stuckless, 1992;
Whelan et al., 1994, 2002, 2005, 2008; Wilson and Cline, 2005; Wilson
et al., 2003) that overwhelmingly supports a meteoric source.
6. Conclusions
The hydrologic response of deep vadose zones to changes in
climate-induced surface moisture is an important aspect of understanding travel times, recharge rates, and potential water fluxes in
arid environments, especially at sites chosen for isolating toxic wastes
over long periods of time. The accepted paradigm of a direct
correlation between effective surface moisture and deep fracture
flow is shown to be questionable at Yucca Mountain within the arid
Mojave Desert of southern Nevada. Instead, data from secondary
hydrogenic minerals indicate that water fluxes through the deep
vadose zone are at least partly decoupled from surface moisture and
shallow infiltration, both of which have experienced large fluctuations
over the past million years.
Data supporting this conclusion were obtained from detailed
geochronological studies of U-rich opal formed in natural cavities at
depths of 200 to 300 m below land surface. Profiles across individual
hemispherical grains were dated by U-series methods using secondary
ion mass spectrometry with spatial resolutions of approximately 20 μm.
Ages obtained for 1-mm-thick grains range from 41 to 1290 ka. Opal
grains also exhibit intricate growth banding that is manifested most
notably in cathodoluminescence images. Oscillatory CL intensity is
shown to be a function of U concentration which varies from 5.4 to
550 μg/g. Opal growth rates calculated over the past 300 ka are
extremely slow, ranging from 0.42 to 1.54 μm/ka for different grains.
Growth rates of associated calcite are shown to be similar. In spite of the
variations in growth rates between different opal grains, rates remained
more-or-less constant within individual profiles over time given the
current degree of spatial and geochronological resolution. Ages are used
to calibrate the more detailed U concentration profiles which show a
remarkable correspondence with climate-proxy records and with
frequencies indicative of planetary orbital forcing factors. Major peaks
in U concentration correspond closely with interglacial episodes and are
interpreted to be the result of increased amounts of evaporative
concentration in soil water and shallow infiltration.
The uniform, linear rates of opal growth indicate that hydrologic
conditions in the deep vadose zone remained stable throughout the
Pleistocene despite multiple climate cycles, including the particularly
cold and wet penultimate glacial episode (Sharpe, 2007). Long-term
hydrologic stability also is implied by similar growth rates for older parts
of the same coatings (Neymark et al., 2002; Whelan et al., 2008) despite
Miocene and Pliocene climates that were warmer and wetter relative to
Pleistocene conditions (Thompson, 1991; Winograd et al., 1985). These
results imply that seepage fluxes in the deep vadose zone are decoupled
from effective surface moisture and buffered from large variations in
shallow infiltration. A number of hydrogeologic processes likely
contribute to this long-term hydrologic stability including runoff and
down-slope diversion at contacts between soil and bedrock and
between nonwelded and welded tuffs, changes in evapotranspiration
rates, evaporation and upward flux of shallow fracture water, storage
and uniform releases of percolation from nonwelded tuffs, and
reduction of deep fracture flux by evaporation and imbibition. Other
deep vadose settings likely contain similar mineral records that could be
used to evaluate hydrologic response to climate change.
Acknowledgements
We thank L. Benson, K. Maher, R. Roback, I. Winograd, M. Cosca, S.
Ewing and three anonymous journal reviewers for technical reviews of
this manuscript and earlier drafts. Funding and field support for this work
was provided by the U.S. Department of Energy (DOE) under Interagency
Agreement DE-AI28-07RW12405; however, DOE had no involvement in
the study design, analyses, or interpretation of results. Author contributions are as follows: Paces had primary responsibility for the study design,
data collection, interpretation, and report writing. Neymark and Wooden
were instrumental in helping collect U-series data. Lund contributed
insights into the comparison of opal and Devils Hole profiles and provided
time-series analyses and interpretations. Neymark, Whelan, and Marshall
contributed to interpretations of vadose zone mineral genesis and
hydrologic processes, and helped refine the final presentation. Any use
of trade, firm, or product names is for descriptive purposes only and does
not imply endorsement by the U.S. Government.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
doi:10.1016/j.epsl.2010.10.006.
J.B. Paces et al. / Earth and Planetary Science Letters 300 (2010) 287–298
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