Pacific Decadal Oscillation
0.6
0m
Freeze
Core
University of Nebraska-Lincoln, 214 Bessey Hall Department of Geosciences, Lincoln, NE 68588
-0.6
Negative Phase
Positive Phase
(Modified from NASA Jet Propulsion Lab)
Drought Frequency
Varve Ages
0-2773 ybp
+PDO/-AMO
+PDO/+AMO
Multidecadal changes in sea surface
temperatures in the Pacific Ocean and Atlantic
Ocean basins have been shown to have a
profound influence on long-term drought
frequencies in North America. Together, the
Atlantic Multi-decadal Oscillation (AMO) and
Pacific Decadal Oscillation explain more than
half of the temporal and spatial variance in
pervasive drought in North America (McCabe et
al. 2004). Arrows on the diagram to the left
indicate the position of Foy Lake, Montana.
1m
Lake-Climate Response
Foy Lake, MT 1954
Foy Lake, MT 1937
Sediment Chronology
Foy Lake is a deep lake located
in the northern part of the Flathead
River Basin, inside the Rocky
Mountains. The lake is roughly a
mile across and the basin is isolated
in the hills to the west of Kalispell,
Montana.
Foy Lake Age Model
The age model for Foy Lake is
based upon varved sediments,
radiocarbon dates, and ash from two
volcanic events (Mazama and Glacier
Peak). An annually-resolved varve
chronology was compared with Pb210 dates for the last century to verify
that varves represented annual
laminations. Radiocarbon dates in the
late Holocene were used to provide
chronological controls and verify age
estimations.
Increasing
aridity
As lake levels fluctuate, diatom
habitat areas change. Decreasing
lake levels always reduce planktic
habitat area, but benthic habitat
area (black margin) may increase
or decrease because of basin slope.
Full Stage
Complete Isolation
-2000
-4000
-6000
-8000
-10000
-12000
-14000
Dated Material
Varved Samples
100
Age Model
2m
25% = Normal Drought Frequency
Needle (3989 -40/+40)
200
(Modified from McCabe et al. 2004)
Late Holocene (0-1350 ybp)
500
700
Mazama Ash (7550)
-0.1
-2
-0.3
-4
-0.5
-0.7
Dry
More Benthic
300
1100
2000
1980
In northwestern Montana, regional drought
(represented by the Palmer Hydrologic Drought
Index for Climate Division 1) is strongest during the
positive phase of the PDO and long-term moisture
increases are associated with negative phases of the
PDO. Drought variations remain in-phase with
low-frequency changes in the PDO throughout the
past century (data from the NOAA website).
Early Holocene & Late Glacial
(9701-13187ybp)
3700
500
1200
11700
40
25
29
6700
22
25
20
1100
12200
33
6200
29
1000
50
5700
33
11200
67
5200
50
40
900
10700
100
4700
67
700
10200
200
4200
200
100
600
200
12700
10200
300
3200
Benthic
400
5m
lla
600
ith
Su
rr
ire
em
io
id
br
eb
iss
on
ii
500
Ep
Ps
eu
do
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St
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St
(s
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)
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ia
C
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id
cf.
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da
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va
ep
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no
m
an
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us
m
Fr
in
ag
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ca
pu
cin
Fr
a
ag
cf.
i la
va
ria
uc
te
he
ne
ria
ra
e
1960
3200
400
4m
Glacier Peak Ash (13100)
Tychoplanktic
m
ich
ig
a
C
yc
lo
te
l
1940
Needle (11200 -310/+350)
0
700
500
1000
800
900
1500
2000
6m
Wood
7210 -205/+95
1000
3700
4200
4700
5200
5700
6200
6700
10700
11200
11700
12200
12700
1100
200
67
100
50
40
200 100
2500
67
50
40
33
29
25
22
Frequency (years)
Frequency (years)
20
200
100
67
50
40
33
29
25
Frequency (years)
3000
Evolution of Diatom-inferred Drought Frequencies in Northwestern Montana Through Time
-6
3500
1917 1922 1927 1932 1937 1942 1947 1952 1957 1962 1967 1972 1977 1982 1987 1992 1997
Years
Evolutive spectral analyses show that drought has not consistently reoccurred at the same frequencies throughout the past 13,000 years.
Prior to the early Holocene (far right panel) diatom-inferred drought frequencies are poorly developed and are generally strongest at lowfrequencies. As the transition into the early Holocene occurs, weak trends become significant frequencies (black contour) developing strong
spectral ridges at frequencies around 36, 45, 60-80, and some low-frequency ridges around 285. In contrast, the mid-Holocene shows a
series of well-developed ridges that continue throughout nearly the entire period from 7000-3000 ybp. Between 4,500-4,000 ypb, most of
the significant high-frequencies break down, leaving only a few scattered low-frequency trends, which is a pattern that appears to continue
into the late Holocene. The apparent ‘plateau’ in the late Holocene occurs at a transition point in the diatom record when the isolated lake
basins join and diatom-response is amplified as a result of a dramatic increase in benthic habitat area (Stone & Fritz, 2004).
Periods shown are divided by diatom dissolution zones with a 3-dimensional view of the spectra and a plan view of the same time
period with 90% confidence intervals contoured. Early, Middle, and Late Holocene periods are all high-resolution, but have slightly
different sample spacing (7.5, 10, and 5 year intervals, respectively). Evolutive spectral analyses were run with a 100-point moving window
with a 3-data-point offset (Mann & Park 1999; Mann & Lees 1995).
4500
7m
Middle Holocene
5000
5500
Mazama Ash
7550 ybp
6000
HOLOCENE
Years Before Present (1950)
4000
Fossil diatom assemblages from annually-resolved
sediments from Foy Lake mimic both low-frequency trends and
often high-frequency drought variability (represented by the
Palmer Hydrologic Drought Index for Climate Division 1).
Diatom assemblages recovered from Foy Lake show a clear
sensitivity to high-frequency drought occurences; decreases in
the diatom index shown here usually represent very minor
changes (3-7% shifts in relative dominance of planktic or
benthic diatoms), while persistent drought (1930s-1940s) can
produce a threshold response because of the unique lake-basin
morphometry (Stone & Fritz 2004).
1920
1000
Dissolution
Zone
Planktic Diatom Index
0
2
1900
Age (years before present)
0.1
PDO
Charcoal (9275 -185/+190)
900
Late Holocene
2
1
Age (years before present)
0.3
0
800
Age (years before present)
4
Palmer Hydrologic Drought Index
0.5
-6
Bulk Sed.
4835 -90/+30
Bulk Sed. (8440 -50/+150)
Diatom Stratigraphy of Foy Lake, MT
6
-1
3m
40
Wet
-4
600
50
More Planktic
-2
-2
Wood (7210 -205/+95)
67
0.7
0
Middle Holocene (2202-7550 ybp)
800
Planktic
2
400
100
Diatom Response to Drought
4
Needle
3989-40/+40
Bulk Sed. (4835 -90/+30)
300
The varves are carbonate/organic
in nature and occur sporadically
through the Late Glacial and early
Holocene and continuously through
the late Holocene. The persistence of
varved sediment in the late Holocene
supports the diatom inference that
modern lake levels are probably
significantly deeper than any time
since the early Holocene.
Increasing
aridity
Partial Isolation
0
-PDO/+AMO
0
Depth (cm)
During periods of pervasive
drought, common in the late 30s
and early 40s, the lake level
lowered dramatically, exposing a
broad shallow platform. At less
than a meter deep, it effectively
isolated the two sub-basins. With
continued lake-level decline, these
two basins become completely
isolated.
-PDO/-AMO
Years (before 1950)
6
Div. 1 MT PHDI
Dry
Jeffery Robert Stone & Sherilyn C. Fritz
0
The Pacific Decadal Oscillation (PDO) is an
index of long-term trends in sea-surface
temperatures (SST) in the northern Pacific basin.
Positive and negative phases of the PDO have
two frequencies, a shorter frequency of about
20-25 years and a longer multi-decadal
frequency ranging from 50-80 years.
Last-Century Comparison: PDO & PHDI
Wet
A 13,000-YR HIGH-RESOLUTION RECORD OF ROCKY MOUNTAIN
CLIMATE CHANGE
Sedimentology of the
Foy Lake Core
Teleconnections & Drought in
Northwestern Montana
6500
7000
7500
8000
8m
Bulk Sed.
8440 -50/+150
8500
Dissolution
Zone
9000
9500
Early Holocene
10000
Increasing benthic area Increasing planktic area
4
6
8
10
12
14
1005
Lake Elevation (m)
9m
11000
Charcoal
9275 -185/+190
100000
LATE
GLACIAL
Diatom habitat areas can be
estimated in lake basins by
1000
calculating the surface area of the
lake (planktic) and surface area of
995
the lake basin and the depth of
light penetration (benthic). The
990
availability of these two habitat
areas can be modeled under
different lake-level elevations to
Photic
985
Penetration
produce an estimate of the
Depth
proportional habitat area. The
3m
980
4.5 m
result is a model of the relative
7.5 m
9m
proportion of benthic and planktic
diatoms that a lake is likely to
0
2
4
6
8
10 12 14 16 18
Mean depth (m)
support.
By comparing changes in planktic:benthic ratios in lake
sediment records to the model, it is possible to estimate past lake
depths. At Foy Lake, the unique shape of the basin results in a
threshold response with declining lake levels. As lake levels decline,
relative benthic habitat area increases dramatically and then
declines dramatically as the two lake basins begin to isolate. This
threshold response is evident in the curve representing mean lake
depth (Stone & Fritz 2004).
10500
11500
12000
12500
60
122
10000
67
Power
2
42
49
44
53
Late Holocene
0-1350ybp
34
1000
23
21
100
99%
95%
90%
50%
13000
0
40
80
0
40 0
40 0
40
80
0
40
80 0
40
80 0
40
80
0
40
0
40
80 0
0
40
0
0
0
0
0
40
80
0
40
80
0
40
80
0
0
40
0
40
0
0
0
40
80
0
0
40
0
0
0
0
40 0
0
40
0
40
80
10
0
Percent Abundance
The diatom stratigraphy of Foy Lake can be broken into several periods based on assemblage and preservational changes.
During the Late Glacial, the lake was dominated by Navicula diluviana, a common pioneer species in oligotrophic lake systems.
Roughly at the boundary with the Holocene, the diatom assemblage undergoes a major shift towards planktic and tychoplanktic
species, including Stephanodiscus minutulus and numerous colonial Fragilaria species, indicating lake levels increased. This
interpretation is strengthened by the sudden appearance of well-developed varves and dark sediment bands, suggesting
undisturbed sediment, which is common in deep, alkaline lakes with anoxic bottom-waters. Around 9,500 ybp, planktic
dominance begins to give way to tychoplanktic species and eventually benthic species and prolonged periods of diatom
dissolution, suggesting increased alkalinity and lowered lake depth. Sediment laminations disappear and benthic diatoms
dominate most of the mid-Holocene, punctuated by periods of planktic dominance, probably representing general aridity with
numerous millennial-scale shifts in effective moisture. A brief period of increased planktic production is centered at 8.2k ybp,
lasting roughly 300 years. The transition into the late Holocene is also marked by a period of prolonged diatom dissolution, with
an apparent assemblage change resulting from differential preservation. By the middle of the Late Holocene, the assemblage has
shifted to dominance by a planktic species similar to Cyclotella bodanica var. lemanica, suggesting the lake became much deeper
as a result of greatly increased effective moisture. This interpretation is also supported by a the sudden reappearance of welldeveloped varved sediments, which continue throughout the remainder of Holocene.
1
100000
10m
Spacing = 5yrs
1667
Needle
11200 -310/+350
Mid Holocene
2202-7557ybp
125
167
66 53
97 71
60
10000
Power
0
Common Drought Frequencies in Northwestern Montana
Over the Past 13,000 Years
1000
43
46
32
100
10
Spacing = 10yrs
1
100000 588
10000
11m
Early Holocene
& Late Glacial
9701-13187ybp
285
65
111
Power
Modeling Diatom
Habitat Areas
Ratio of Planktic to Benthic Area
1000
42
77
47
32
36
28
24
100
Spacing = 7.5yrs
99%
95%
90%
50%
10
Glacier Peak
Ash 13100 ybp
1
0
0.01
0.02
0.03
0.04
Frequency (1/yr)
0.05
0.06
Multi-tapered spectral analyses of highresolution changes in fossil diatom
assemblages (inferred drought) result in
numerous significant frequencies
throughout the past 13,000 years. Highfrequency periodicities center around 21-24
years and lower frequency occurrences
typically ranged from 40-80 years, similar to
both high-frequency and low-frequency
variability observed in Pacific Decadal
Oscillation (McCabe et al. 2004).
In the past century, pervasive drought
in northwestern Montana appears to
strongly influenced by multi-decadal
changes in Pacific SST, showing similar
frequencies and remaining in-phase with
major shifts. The presence of significant
recurrences of drought at the same
frequencies within all three periods analyzed
suggests that teleconnective processes
probably had a similar influence throughout
the Holocene. (Lees & Park 1995)
Acknowledgments & References
We would like to thank Erik Ekdahl, who assisted with spectral
analyses; Lora Stevens and Mitch Power assisted in the collection
of the core, sampling, and the chronology; Joshua Campbell
analyzed the freeze core and Brandi Bracht assisted with laboratory
preparation of the samples. Lynn Brewster-Wingard of the USGS
provided historical aerial photographs. This research was funded
by a grant from the National Science Foundation.
Lees, J. M. & Park, J., 1995, Multiple-taper spectral analysis: A
stand-alone C-subroutine, Computers & Geosciences: 21, 199236.
McCabe, G. J., Palecki, M. A., & Betancourt, J. L., 1994, Pacific
and Atlantic Ocean influences on multidecadal drought frequency
in the United States, PNAS 101(12):4136-4141.
Mann, M. E., & Lees, J., 1996, Robust Estimation of
Background Noise and Signal Detection in Climatic Time Series,
Climatic Change 33:409-445.
Mann, M. E., & Park, J., 1999, Oscillatory Spatiotemporal Signal
Detection in Climate Studies: A Multiple-Taper Spectral Domain
Approach, Advances in Geophysics 41:1-131.
Stone, J. R., & Fritz, S. C, 2004, Three-dimensional modeling of
lacustrine diatom habitat areas: Improving paleolimnological
interpretation of planktic:benthic ratios, Limnology &
Oceanography 49(5):1540-1548.
NASA Jet Propulsion Lab Website: http://www.jpl.nasa.gov/
NOAA Website: http://www.noaa.gov