Quaternary Science Reviews 29 (2010) 1445e1452
Contents lists available at ScienceDirect
Quaternary Science Reviews
journal homepage: www.elsevier.com/locate/quascirev
Evidence for a variable and wet Younger Dryas in southern Alaska
Darrell S. Kaufman a, *, R. Scott Anderson a, Feng Sheng Hu b, c, Edward Berg d, Al Werner e
a
School of Earth Sciences and Environmental Sustainability, Northern Arizona University, Flagstaff, AZ 86011, USA
Department of Plant Biology, University of Illinois, Urbana, IL 61801, USA
c
Department of Geology, University of Illinois, Urbana, IL 61801, USA
d
US Fish and Wildlife Service, Kenai National Wildlife Refuge, Kenai, AK 99669, USA
e
Department of Earth and Environment, Mount Holyoke College, South Hadley, MA 01075, USA
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 September 2009
Received in revised form
20 February 2010
Accepted 23 February 2010
Pollen, macro- and micro-fossils, and sedimentologic indicators in sediment cores from Discovery Pond
(DP) in south-central Alaska indicate that the coldest interval of the last deglacation was coincident with
the onset of the Younger Dryas (YD), around 12.8 cal ka. The multi-proxy record from DP together with
a compilation of recently published YD records from southern Alaska and the adjacent northern Pacific
Ocean shows that, during the course of the YD, temperatures increased, then reached a maximum
sometime around 11 cal ka. At DP, a pronounced increase in the abundance of Isoëtes and Pediastrum,
including species associated with oligotrophic lakes and known to respond to increased precipitation,
combined with a reduction in wetland aquatics and an increase in the minerogenic component of the
sediment, all indicate a shift from wetland to open-water conditions at around 12.2 cal ka. Similar to
other evidence from southern Alaska, our proxy record from DP indicates an increase in temperature and
effective moisture during the second half of the YD. An increase in winter precipitation might be
associated with a deepening of the Aleutian low-pressure system and a northward shift in winter storm
tracks, consistent with recent simulations by climate system models.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
The Younger Dryas (YD) cold reversal has received extensive
research attention because the spatial-temporal pattern of climate
changes can reveal the interactions among components of the
climate system. The YD climate perturbation is most strongly
expressed around the North Atlantic, where it has been ascribed to
an influx of meltwater leading to a reduction in deepwater
formation, and thereby a decline in ocean thermohaline circulation
(THC) (e.g., Alley, 2000). The impact of a diminished THC on North
Pacific climate has been studied in recent simulations by climate
system models (Hu et al., 2008; Okumura et al., 2009). These
experiments show that reduced THC leads to cooling in the North
Pacific through both oceanic and atmospheric connections.
A pronounced climate fluctuation coincident with the YD has
previously been documented in multiple proxy climate records
from southern Alaska. In their recent summary of the YD across
Beringia, Kokorowski et al. (2008) reported that 10 of 15 proxy sites
in southern Alaska registered a climate reversal during the YD, and
that those that lack evidence of a YD oscillation have coarse
* Corresponding author. Tel.: þ1 928 523 7192; fax: þ1 928 523 9220.
E-mail address: [email protected] (D.S. Kaufman).
0277-3791/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.quascirev.2010.02.025
sampling or poor chronologies. The compilation was based
primarily on evidence from pollen, most with assemblages that lack
modern analogs, which hampers the ability to derive secure
paleoclimatic inferences. Nonetheless, a moraine record of glacial
advance (Briner et al., 2002) and multiproxy lake-sediment records
from southwestern Alaska (Hu et al., 2002, 2006; Hu and Shemesh,
2003), as well as lake-sediment records from other areas of
southern Alaska (Engstrom et al., 1990; Peteet and Mann, 1994;
Brubaker et al., 2001; Yu et al., 2008), provide strong evidence for
a pronounced climate perturbation sometime during the YD. Our
new proxy record from south-central Alaska adds to the emerging
evidence of the complexity of climate change during the YD from
both the north Atlantic (e.g., Bakke et al., 2009), and the north
Pacific (e.g., MacDonald et al., 2008) regions. It shows that
temperature and moisture in south-central Alaska increased
through the YD.
2. Study site and setting
Discovery Pond (DP; informal name; 60 47.30 N, 150 50.20 W) is
located near the Discovery Oil Well in the lowlands of the Kenai
Peninsula, 11 km from the southeast shore of upper Cook Inlet
(Fig. 1). DP is a small lake (0.04 km2) at 97 m asl, with a simple
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D.S. Kaufman et al. / Quaternary Science Reviews 29 (2010) 1445e1452
Fig. 1. (A) North Pacific region with the location of two marine cores with recently
published evidence for Younger Dryas conditions; grey areas are Northern Hemisphere
ice sheets; dashed line is 100 m isobath. (B) Alaska with locations of other Younger
Dryas records shown in Fig. 5. AL ¼ Arolik Lake; DP ¼ Discovery Pond; HML ¼ Hundred
Mile Lake; GL ¼ Greyling Lake; dashed line is 50 m isobath. (C) Aerial photograph of
Discovery Pond and Swanson Fen with the locations of core sites.
bathymetry (maximum depth ¼ 4 m). The depression originated as
a kettle in late-Wisconsin drift deposited during the Moosehorn
stade of the Naptowne glaciation sometime prior to about 19 cal ka
(Reger et al., 2008). The lake is topographically closed with a relatively small ratio of drainage-basin to lake-surface area (20:1), and
probably receives most of its water through underwater seepage. It
is situated on a local topographic high that coincides with a mound
in the unconfined water table, as suggested by the surface elevations of surrounding lakes. This configuration makes the lake
sensitive to fluctuations in effective moisture that influence the
elevation of the groundwater table.
DP is within the boreal maritime climate of the Kenai Peninsula.
The mean annual temperature at the Homer climate station, the
longest record in the Kenai lowlands, averages 3.1 C (1932e2006;
http://www.wrcc.dri.edu). An air temperature logger installed near
the shore of DP recorded a mean daily temperature of 3.7 C for the
period of July 1, 2004 to June 30, 2005 (Fig. 2; hourly temperature
data are available at: http://jan.ucc.nau.edu/wdsk5/S_AK/),
compared with a mean monthly temperature of 5.5 C reported for
the same interval at Homer (http://climate.gi.alaska.edu/Climate/).
A Remote Automated Weather Station (RAWS) at Swanson River,
located 6.3 km southwest of DP, recorded an average daily
temperature of 4.8 C for the same period (Fig. 2), with daily
temperatures that are strongly correlated (r ¼ 0.992) with those
from our on-site logger. Water temperature in DP tracks air
temperature at the Swanson River RAWS site during the ice-free
season, and shows that the water is well mixed through the lake
depth (Fig. 2).
Mean annual precipitation at Homer is 64 cm, and decreases to
49 cm at Kenai, which is situated in a stronger rain shadow. Winter
(DJF) precipitation at these stations contributes about a quarter of
the annual amount, but appears to disproportionately influence the
hydrologic budget, as suggested by the correlation (r ¼ 0.70)
between DJF precipitation at Homer and annual stream discharge for
Kenai River at Cooper Landing (http://waterdata.usgs.gov). In turn,
winter precipitation along the coast of south Alaska, is strongly
influenced by the Aleutian low-pressure system. As the low
strengthens, southwesterly flow over south-central Alaska is
enhanced, with an attendant increase in precipitation and advection
of warm moist air into the south coastal regions, especially during
winter. At Homer, DJF precipitation is correlated (r ¼ 0.62) with the
North Pacific Index, the area-weighted sea-level pressure over the
North Pacific (between 30e65 N and 160 Ee140 W), a measure of
the Aleutian low strength (Trenberth and Hurrell, 1994).
3. Methods
Multiple cores up to 6 m long were recovered from three sites
using Livingstone and percussion corers. Cores DP-1 and DP-4 were
taken from approximately the same site and were spliced to
generate a composite sequence from the depocenter of DP (hereafter, DP-1/4). The cores were split, photographed, described, and
analyzed for multiple parameters. (1) Magnetic susceptibility (MS)
was measured on all cores at 0.5-cm resolution using a Bartington
Fig. 2. Daily temperature of lake water in Discovery Pond and the air at Swanson River RAWS station located 6.3 km southwest of DP (http://www.wrcc.dri.edu/cgi-bin/rawMAIN.
pl?akASWA). Hourly data from DP posted at: http://jan.ucc.nau.edu/wdsk5/S_AK/.
D.S. Kaufman et al. / Quaternary Science Reviews 29 (2010) 1445e1452
MS meter with a MS2E sensor. (2) Organic-matter (OM) content
was measured at 1-cm intervals by weight loss on ignition by
combusting 1 cm3 of sediment at 550 C for 2 h. (3) Biogenic silica
(BSi) from 1 cm intervals was extracted with 10% Na2CO3 and the
concentration of SiO2 determined with a spectrophotometer
following the procedure of Mortlock and Froelich (1989). (4) Pollen,
spores, and Pediastrum cell nets (coenobia) were analyzed from
2 ml sediment samples spaced an average of 8 cm using a procedure modified from Fægri et al. (1989). (5) Macrofossils were
extracted by sieving 1-cm-thick samples spaced 5 cm apart or less
between 430 and 330 cm. (6) Eleven radiocarbon (14C) ages were
obtained from the last glacialeinterglacial transitional interval of
the cores. The samples comprised macrofossils of detrital wood,
seeds, insects, and mosses (Table 1). All ages were calibrated to
calendar years using CALIB 5.0 (http://calib.qub.ac.uk/calib) based
on the INTCAL04 calibration dataset (Reimer et al., 2004), and are
reported in years before 1950 (cal BP).
The identification of certain Pediastrum (Chlorophyceae)
species is uncertain, and their ecology is not well known.
However, a new study by Weckström et al. (2010) characterized
environmental variables for 14 species of Pediastrum found in
subarctic Finnish lakes, documenting the importance of pH,
dissolved organic carbon (DOC), conductivity, color, and precipitation as the most significant factors. In addition, several other
published sources were used to identify and provide meaningful
ecological reconstructions. Identifications were based on the size
of the Pediastrum coenobia, and the characteristics of individual
cells within the coenobia, including ornamentation and shape and
size of the peripheral horns. Transects across the pollen slides
were scanned, and all identifiable coenobia were counted and
assigned to one of six groups (based on Hielsen and Sørensen,
1992 and other references; Appendix A).
4. Results
4.1. Chronology
The age model for DP sediments was constructed by transferring
all 14C ages (Table 1) onto the single depth scale of core DP-4 on the
basis of correlating tephra layers, lithologic boundaries, and MS
variations among the cores (Fig. 3). We used core DP-4 as the
master core because it contains the longest record and was the
focus of the laboratory analyses. The correlations based on welldefined lithologic boundaries assume that the transitions occurred
simultaneously at the core sites.
1447
We analyzed the 14C age of paired samples of different materials
from two levels (Table 1). For one pair, the median age of a Nuphar
seed is 180 years older than bark fragments from the same depth;
for the other pair, the median age of a Potamogeton seed is 140 years
younger than mosses. The ages for both pairs overlap within the
2-sigma age ranges. All of the ages were included in the age model,
except one (CAMS-113541), which was rejected because it was
much younger than the trend defined by the others.
The age model for 14 to 8 ka is based on a second-order polynomial fit to ten 14C ages (r2 ¼ 0.996) (Fig. 3, inset). The unit
deposited during the YD (unit 2b, see below) did not yield plant
macrofossils for 14C analysis, but its age is bracketed above and
below. The age model is well constrained down to the oldest age at
13.3 ka (411 cm). We extrapolate ages to 14 ka (30 cm below the
oldest 14C age), below which further extrapolation becomes
increasingly tenuous.
4.2. Multi-proxy record of paleoenvironmental change
The lithostratigraphy of DP sediments is subdivided into three
units, each with distinctive texture, organic content, and color (Figs. 3
and 4; Table 2). Unit 1 (>13.4 ka) is inorganic mud with sand beds in
the lower 1e2 m of all three cores. Unit 2 (13.4e9.0 ka) is 1e2 m of
organic-rich sediment that is further subdivided into three subunits:
(a) Unit 2a (13.4e12.8 ka) is dense peaty mud with abundant plant
remains (including Nuphar seeds) that coarsens at the shallowerwater sites; (b) Unit 2b (12.8e11.1 ka) is a 30-cm-thick interval of silty
gyttja; (c) Unit 2c (11.1e9.0) is peaty mud with abundant Nuphar
seeds and other plant remains that is coarser and more abundant at
the shallow-water sites. The peaty mud grades upward into unit 3
(<9.0 ka), which is fine gyttja with several tephra layers.
MS values are highest with numerous spikes in unit 1 (Fig. 4).
MS generally decreases upward, along with increasing OM, and the
two are inversely correlated (r ¼ 0.81; n ¼ 160; excludes MS peaks
>15 106 SI). Prominent peaks in MS generally coincide with
unidentified tephra layers. Most of the inorganic component of the
sediment above unit 1 is probably wind blown material from glacial
outwash plains, eroding coastal bluffs, and volcanic ash.
OM ranges from about 5e70% in sediment deposited between
14 and 8 ka (Fig. 4). OM rises sharply from background values at
13.8 ka (base of unit 2a), and reverses slightly at 13.6 ka, before
peaking at about 13.2 ka when a tephra was deposited in DP. OM
decreases sharply in the silty gyttja (unit 2b) beginning 12.8 ka. It
registers minimum values between 12.4 and 11.8 ka, and then
increases sharply in the peaty mud (unit 2c) at 11.3 ka. OM remains
Table 1
Radiocarbon data for Discovery Pond.
Sample
DP-1 301.0
DP-1 340.5d
DP-1 346.5
DP-1 360.5
DP-1 410.5
DP-4 358.0
DP-5 80.0
DP-5 80.0
DP-5 150.0
DP-5 150.0
DP-5 186.0
a
b
c
d
Lab ID
(CAMS)a
Materialb
113540
113541
113542
113543
113544
92754
92755
92756
92757
92758
92759
Mixed
Chitin
Mixed
Mixed
Mixed
Mosses
Bark
Nuphar fruits
Mosses
Potamogeton fruits
Mosses
Depth
(cm)
Transferred
to DP-4 (cm)
300e302
340e341
346e347
360e361
410e411
357e359
80
80
150
150
186
306.8
344.2
349.9
363.3
411.2
358.0
327.5
327.5
362.0
362.0
406.3
14
cal agec
C age
(a BP)
(a BP)
7585
7795
9485
9865
11435
9845
8620
8760
9970
9835
11015
45
40
35
40
40
45
35
35
30
35
35
8393
8572
10735
11258
13289
11245
9570
9753
11380
11236
12933
93
93
239
75
85
76
68
150
169
56
83
Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory.
Mixed ¼ fine vegetation and other organic matter of unknown origin.
Calibrated ages are median of the probability distribution based on CALIB 5.0 (calib.qub.ac.uk/calib), with one half of the 2-sigma range.
Rejected because age falls off the trend defined by others.
1448
D.S. Kaufman et al. / Quaternary Science Reviews 29 (2010) 1445e1452
exhibits multi-centennial fluctuations of about 20e30& superposed on an overall increasing trend beginning about 13.5 ka. OM
and BSi co-vary in the older part of the record, but not in the
younger part.
The pollen stratigraphy is similar to that described by previous
studies from elsewhere in south-central Alaska (Ager and Brubaker,
1985), including the Kenai Peninsula (e.g., Anderson et al., 2006).
The pollen assemblages in unit 1 (>13.4 ka) are dominated by
Cyperaceae (Fig. 4) and suggest an herb-dominated tundra (cf Hu
et al., 2002), which was widespread in Alaska at this time.
Around the onset of unit 2a (13.4e12.8 ka), Betula pollen increases
sharply from 1.5 to 55% in <200 years, suggesting the establishment of birch-shrub tundra. Wetland indicators, including Apiaceae
pollen and Equisetum spores, with shallow-water rooted aquatics
(Potamogeton and Nuphar), the alga Botryococcus, and fragments of
bryophytes and bryozoans are also abundant in unit 2a (Table 2). In
unit 2b (12.8e11.1 ka), Betula dominates, while other sub-shrubs
are reduced, and wetland indicators decline, largely replaced by the
clear-water fern ally, Isoëtes, and the colonial alga, Pediastrum, both
of which peak above the middle of unit 2b. The increase in three
species, P. boryanum var. longicorne, P. integrum, and P. angulosum
indicates clear, oligotrophic, open-water conditions (Table 3,
Appendix A). The remains of chironomids, Daphnia, and Chara
oospores also increase in unit 2b. In unit 2c (11.1e9.0 ka), Betula
pollen declines sharply and is ultimately replaced by Populus and
Alnus in unit 3. Also in unit 2c, many wetland and rooted aquatics
become important again, while indicators of oligotrophic, openwater conditions decline.
Fig. 3. Stratigraphy of Discovery Pond sediment cores. Magnetic susceptibility (MS; SI
values 106) profiles shown alongside stratigraphic logs. Ages are in 14C BP. Inset
shows age model for the last glacialeinterglacial segment of Discovery Pond core DP-1/
4. Single rejected age shown as open square. Error bars are 2-sigma ranges; data listed
in Table 1. Age of Younger Dryas based on Greenland ice core (Alley, 2000).
high, comprising >65% of the sediment until about 9.8 ka before
generally decreasing in the fine gyttja (unit 3). BSi content of DP
sediment ranges from about 5 to 90& (SiO2 mg g1; Fig. 4). It
5. Discussion
5.1. Paleoenvironmental interpretation
The peaty mud (unit 2a) that underlies the YD silty gyttja
contains abundant macrophytes, and other plant macrofossils
indicating that, prior to the YD, a fen developed at DP, while the
pollen assemblage indicates that birch-shrub tundra expanded over
Fig. 4. Proxy records of last glacialeinterglacial interval from Discovery Pond. Magnetic susceptibility (MS), organic matter (OM), biogenic silica (BSi), pollen composition, and select
microfossils (see text) from core DP-1/4. Gray lines below 13.8 ka are based on samples from DP-4; black lines are based on DP-1, and the two overlap for MS and OM. Time scale
based on age model shown in Fig. 3, which is secure to about 14 ka.
D.S. Kaufman et al. / Quaternary Science Reviews 29 (2010) 1445e1452
1449
Table 2
Macrofossils from Discovery Pond core DP-1/4.
Depth
(cm)
Unit
Age
(cal ka)
Nuphar
seeds
Bryophyte
stems
Bryozoan statoblasts
Potamogeton
fruit
Carex nutlet
(trigonous)
Betula nana
fruit
Chironomids
Daphnia
ephippia
Chara
oospore
Isoetes megaspore
Hydrozetes
330
335
340
345
350
355
360
365
370
375
380
385
390
395
400
404
405
410
415
420
425
429
2c
2c
2c
2c
2c
2c
2b
2b
2b
2b
2b
2b
2b
2b
2a
2a
2a
2a
2a
1
1
1
9.8
10.1
10.3
10.6
10.8
11.0
11.3
11.5
11.7
11.9
12.1
12.3
12.5
12.7
12.9
13.0
13.0
13.2
13.4
13.5
13.6
13.8
0.0
0.0
2.3
4.5
2.3
6.8
4.5
0.0
0.0
0.0
0.0
0.0
0.0
0.0
2.3
6.8
11.3
4.5
11.3
2.3
0.0
0.0
4.5
33.9
24.9
15.8
38.5
210.4
11.3
9.0
9.0
2.3
4.5
4.5
2.3
18.1
4.5
0.0
9.0
633.5
92.8
2.3
4.5
0.0
6.8
13.6
4.5
0.0
2.3
6.8
2.3
2.3
6.8
4.5
2.3
4.5
0.0
2.3
31.7
158.4
43.0
13.6
56.6
31.7
65.6
0.0
0.0
0.0
0.0
0.0
0.0
4.5
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
4.5
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
2.3
0.0
0.0
0.0
0.0
0.0
0.0
2.3
11.3
29.4
18.1
9.0
4.5
9.0
56.6
38.5
13.6
6.8
4.5
79.2
6.8
0.0
2.3
2.3
0.0
0.0
0.0
15.8
4.5
0.0
0.0
0.0
0.0
0.0
11.3
13.6
9.0
4.5
15.8
9.0
6.8
2.3
0.0
0.0
0.0
22.6
11.3
6.8
9.0
9.0
4.5
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
2.3
0.0
0.0
2.3
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
4.5
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
2.3
2.3
2.3
6.8
4.5
0.0
0.0
0.0
0.0
2.3
0.0
13.6
2.3
2.3
0.0
0.0
0.0
0.0
0.0
2.3
0.0
Note: Values are percent relative to terrestrial-pollen total counts.
more well-drained areas. OM and BSi contents exhibit
a pronounced reversal to lower values beginning around 12.8 ka, at
the same time as the onset of the YD cold interval (Fig. 4). We
interpret this decrease, along with the shift in sediment character
from peaty mud (unit 2a) to silty gyttja (unit 2b), as evidence for
reduced lake productivity probably related to lower temperatures
and an extended duration of lake-ice cover, and possibly to
increased input of clastic sediment. A decrease in effective moisture
at the onset of the YD might have reduced the nutrient flux from
the watershed to the lake, contributing to the reduced productivity.
The silty gyttja (unit 2b) that interrupts the peaty mud was
deposited during the YD. An increase in clastic input (including
tephra) during this interval is registered by the decrease in OM,
while sedimentation rates remained relatively constant. Reger et al.
(2008) reported 14C evidence for a glacial re-advance in the Kenai
Mountains during the end of the Naptowne glaciation, possibly
coincident with the YD. We speculate that outwash plains
emanating from these glaciers also expanded, providing a new
source area for eolian sediment.
Among our proxies, the most pronounced change during the YD
is exhibited by the microfossils, Pediastrum and Isoëtes (Fig. 4).
Although no 14C ages are available from within unit 2b, the age
model indicates that this rise occurred about 12.2 ka and peaked
around 11.8 ka, late during the YD. Previous work on lakes in Ontario
(Yu, 2000) also recognized an increase in the abundance of
Pediastrum during the YD, which was interpreted as a decrease in
temperature or an increase in the turbidity. In contrast, the abundance of Pediastrum decreased abruptly near the onset of the YD at
a lake in southwestern Alaska, which coincides with decreased BSi in
the same core and was interpreted as a decrease in aquatic
productivity (Hu et al., 1995). Because no species-level assemblages
were reported, however, we cannot directly compare our interpretation with these previous studies. Much of the older information on
Pediastrum refers to its importance in trophic reconstruction of lakes,
while more recent studies (e.g., Weckström et al., 2010) also recognize the importance of pH and precipitation. Several of the more
important species in the later half of the YD interval (P. boryanum var.
longicorne, P. integrum, P. angulosum) are characteristic of clear, cold,
oligotrophic, open-water conditions. The two dominant Pediastrum
species e P. boryanum var. longicorne and P. boryanum var. boryanum
eare known to respond positively to higher precipitation in subarctic
lakes (Weckström et al., 2010). Associated with the changes in
Pediastrum, shallow-water aquatic plants, including Nuphar and
Potamogeton, were diminished. We interpret these changes in algal,
wetland, and sedimentologic indicators as evidence for elevated
effective moisture, which resulted in an increased lake depth with
decreased nutrients. Thus, an oligotrophic lake apparently replaced
the wetland by about 12.2 ka.
During the 500-year interval following the YD, both OM and BSi
increase, probably in association with increased lake productivity.
Table 3
Discovery Pond core DP-1/4 Pediastrum and Isoëtes.
Depth (cm) P. boryanum boryanum P. angulosum P. boryanum brev. brev. P. boryanum var. longicornea P. integrum Pedistrum unidentified Pedistrum total Isoëtes
340
355
363
370
375
380
384
403
410
1
0
19
21
38
3
12
2
0
1
3
0
0
2
0
0
0
2
0
0
4
7
7
0
0
0
0
Note: Values are percent relative to terrestrial-pollen total counts.
a
a.k.a. P. boryanum brevicorne var. granulatum.
0
0
13
16
35
4
7
0
0
0
0
2
0
4
0
2
0
0
0
1
9
28
24
0
4
2
0
2
4
47
72
110
7
25
4
2
0
0
111
945
590
424
328
1
2
1450
D.S. Kaufman et al. / Quaternary Science Reviews 29 (2010) 1445e1452
The accumulation of peaty mud resumed (unit 2c) as effective
moisture decreased after 11.0 ka. Full recovery to pre-YD values was
delayed until about 11 ka, about 500 years following the termination of the YD. We interpret unit 2c, the dense Nuphar-rich peaty
mud with OM approaching 70% that formed between 10.8 and
9.8 ka, as the driest interval in the core. This interval coincides with
the peak in summer insolation, and with the Holocene thermal
maximum in Alaska (Kaufman et al., 2004).
5.2. Comparison with other records from the North Pacific
Evidence for increased moisture during the YD is rare in Alaska.
Had the cooling at the onset of the YD been associated with an
increase in winter precipitation, then we would expect widespread
evidence for mountain glacier advances at that time. Instead, only
minor glacier advances have been documented and in only a few
places in Alaska (Briner and Kaufman, 2008), indicating that cooling was associated with a decrease in winter accumulation, or that
summers remained relatively warm, as has been proposed to
explain the lack of a YD moraine at Matanuska Glacier in southcentral Alaska (Evenson et al., 2005). Pollen records from across
southern Alaska generally indicate colder conditions during the YD
(Kokorowski et al., 2008). On Kodiak Island (Peteet and Mann,
1994), pollen assemblages imply that the YD was cold and dry,
although enhanced southerly atmospheric flow may have increased
winter snowfall in south-central Alaska (Peteet et al., 1997). In the
Bristol Bay area, pollen profiles from Nimgun Lake (Hu et al., 2002)
show an abrupt reversal from Betula- to herb-dominated tundra
coincident with the onset of the YD, although Betula pollen
increases sharply during the second half of the YD. Similarly, at
Grandfather Lake, also in southwestern Alaska, oxygen isotope
values indicate that climatic recovery began during the middle of
the YD (Hu and Shemesh, 2003).
A peat core from Swanson Fen, located just 0.3 km northeast of
DP, was recently studied for pollen and macrofossils (Jones et al.,
2009). Like DP, the proportion of Betula pollen in Swanson Fen
remains high during the YD (zone SF-1b), interpreted as a signal
of increased snowfall (Jones et al., 2009). Zone SF-2a, which
overlies SF-1b in Swanson Fen, exhibits a pronounced increase in
Polypodiaceae (fern) spores and other wet-meadow species,
indicating an increase in moisture. The base of Zone SF-2a was
dated at 11.5 ka (Jones et al., 2009); we suggest that the base of
the zone instead dates to about 12.2 ka. This age revision is based
on a linear interpolation (r2 ¼ 0.998) using four out of five
reported 14C ages from the relevant section of the Swanson Fen
core (Table 2 in Jones et al., 2009). A transition to wetter conditions (Zone SF-2a) at 12.2 ka rather than 11.5 ka at Swanson Fen
would correlate with the pronounced increase in Pediastrum and
other taxa at DP, which is constrained by more 14C ages at DP and
which we interpret as evidence for an increase in lake level,
leading to an open-water, oligotrophic conditions. The fact
that Polypodiaceae spores dominate the percentages through
zone SF-2 at the Swanson Fen, but are relatively unimportant at
DP probably results from the fact that ferns grew on the fen
surface, but were less important immediately surrounding the
lake itself. Thus, comparing pollen/spore assemblages between
Swanson Fen and DP is not straightforward.
Within the last several years, six relatively well-dated lacustrine- and marine-based proxy records that extend through the YD
have been published from southern Alaska, some with decadalscale resolution (Fig. 5). From west to east (Fig. 1), these six records
include: (1) In the far northwestern Pacific Ocean, d18O values
in planktonic foraminifera from core MD01-2416 indicate that seasurface temperatures reached their minimum deglacial value early
during the YD, then increased during the YD (Sarnthein et al.,
2006). (2) In southwestern Alaska, an abrupt decline in BSi abundance, an increase in C:N ratio, and a shift in diatom assemblages at
about 13 ka at Arolik Lake suggest that lake productivity decreased
at the onset of the YD, then increased as temperatures warmed
within the YD (Hu et al., 2006). Isotopic evidence from Arolik Lake
indicates that effective moisture increased markedly around
12.3 ka, consistent with a pollen-based moisture reconstruction
from nearby Nimgun Lake (Hu et al., 2002). (3) On Kenai Peninsula,
BSi abundance from Discovery Pond indicates an increase in
productivity, and macrofossil assemblages suggest an increase in
effective moisture during the second half of the YD (this study). (4)
In south-central Alaska, at Hundred Mile Lake, OM increases from
the base of the sediment core around 13.4 ka, and continues to rise
Fig. 5. Recently published proxy records of climate change during the Younger Dryas in southern Alaska and the North Pacific Ocean. From west to east, records include: core MD012416 (Northwest Pacific Ocean; Sarnthein et al., 2006); Arolik Lake (Hu et al., 2006); Discovery Pond (this study); Hundred Mile Lake (Yu et al., 2008); Greyling Lake (McKay and
Kaufman, 2009), which was analyzed at higher resolution for this study; and core EW0408-85JC (Barron et al., 2009). Bold dashed lines highlight warming trend during and
following the YD. Site locations show in Fig. 1. Data are shown in comparison with the oxygen-isotope record of Greenland ice (units are normalized d18O values; GISP data from
www.ncdc.noaa.gov/paleo/data), and June insolation at 60 N (Berger and Loutre, 1991).
D.S. Kaufman et al. / Quaternary Science Reviews 29 (2010) 1445e1452
through the YD chron (Yu et al., 2008). A decrease in both carbonate
abundance and d18O values, and an increase in OM, indicate
a climatic shift around 12.3 ka at Hundred Mile Lake. The extent to
which these changes represent changes in temperature versus
effective moisture is unclear, however, because carbonate
abundance increases while d18O values decrease during the early
Holocene, suggesting that a combination of factors influence these
proxies at this site. (5) In the northeastern Chugach Mountains, OM
content in sediment of proglacial Greyling Lake is low prior to the
YD, then increases through the YD, indicating that lake productivity
increased as mountain glaciers retracted in the headwaters of the
drainage during the YD (McKay and Kaufman, 2009). (6) In the
northeastern Gulf of Alaska, sediment in a new core from nearby
the southern coast of Alaska (EW0408-85JC) contains microfossils
and geochemical indicators of decreased productivity coincident
with the onset of the YD (Barron et al., 2009). The indicators then
rise during the course of the YD. In particular, the proportion of
diatom taxa common in subtropical to temperate ocean water
increases from 12.9 ka to a maximum at 11.0 ka, concurrent with
a decrease in sea-ice related taxa (Barron et al., 2009).
This compilation includes the most-recent and highest-resolution YD records available from across southern Alaska. Although
sample spacing and geochronological control varies among the
sediment cores, the records share similar trends. All of the proxies
reach their lowest values of the last 13 ka at around the start of the
YD. These proxies indicate that sea-surface temperature and lake
productivity reached minimum deglacial values around the onset
of the YD. The extent to which the onset of the YD is marked by
a reversal in these proxy values differs among these sites, however.
Nonetheless, all records show increasing values, as sea-surface
temperatures and lake productivity increased through the YD
chron, reaching peak values around 11 ka (bold dashed lines in
Fig. 5). Furthermore, our new multi-proxy record from the Kenai
lowland indicates that effective moisture increased as an oligotrophic lake developed at DP at around 12.2 ka, during the second half
of the YD.
The evolution of temperature and moisture during the YD has
recently been reported from other high-resolution marine and
terrestrial records from the northeastern Pacific region south of
Alaska. Proxy records from both marine (e.g., Barron et al., 2003)
and lacustrine (e.g., MacDonald et al., 2008) sediment indicate that
temperatures decreased with the onset of the YD, then increased
through the YD, consistent with the evidence from Alaska
summarized here. In contrast to the temperature evolution during
the YD, changes in the effective moisture appear to have been
opposite in Alaska compared with the western conterminous
United States. MacDonald et al. (2008) recently summarized the
terrestrial evidence from southwest North America for the evolution of climate during the YD. Although not all of the records show
the same trend, most indicate wet conditions early during the YD,
followed by a shift to drier conditions during the second half of the
YD. The opposite sense of moisture change through the YD points to
a progressive northward shift in storm tracks concurrent with
overall warming. A northward shift of winter storm tracks in the
North Pacific has also been simulated by climate models under
projected global warming (e.g., Salathé, 2006).
6. Conclusion
Our compilation of recently published YD records from southern
Alaska and the adjacent North Pacific Ocean agrees with previous
work that suggests that the coldest interval of the last deglaciation
was coincident with the onset of the YD. The most recently
published records, including our new reconstruction from
Discovery Pond, illustrate with greater clarity that temperatures
1451
increased during the course of the YD, reaching a maximum
sometime around 11 ka (Fig. 5). In contrast to proxy records from
the Greenland ice sheet, most records from southern Alaska do not
show an abrupt termination to the YD. Instead, the YD warming
trend continued into the early Holocene, and peak warmth may
have coincided with the maximum summer insolation at 65 N
latitude.
Our new evidence for the rise of water level in a groundwaterfed lake in subarctic Alaska late during the YD suggests that the first
half of the YD was drier than the second half. An increase in
effective moisture might be explained by a strengthening of the
Aleutian low-pressure system, which steers winter storms toward
southern Alaska. A deepening of the Aleutian low was recently
simulated by a coupled ocean-atmosphere model used to study the
teleconnections associated with a freshwater pulse to the North
Atlantic (Okumura et al., 2009). In addition to a 2e4 C cooling of
the North Pacific region, a principal feature of the model output is
an increase in cyclonic activity in the eastern North Pacific, which
can be ascribed to a strengthened, eastward-shifted low-pressure
system. An intensification of the Aleutian low, with a concomitant
northward shift in the position of winter storm stacks is also
a prominent feature of climate simulations for future global
warming (e.g., Salathé, 2006). Evidence for decreased moisture
during the late YD in southwest North America (MacDonald et al.,
2008) contrasts with evidence for increased moisture in Alaska
presented here. Together the opposite trends are consistent with
the climate-model output, suggesting that winter storms shift
northward during overall warming.
Acknowledgements
C. de Fontaine, K. Kathan, E. Kingsbury, and staff of Kenai
National Wildlife Refuge helped core Discovery Pond; J. Bright and
C. Schiff analyzed the OM and BSi; C. McCracken and A. Bair assisted
with paleobotanical analyses; and T. Brown analyzed the 14C ages.
NSF awards ATM-0318341, EAR-0823522, Kenai National Wildlife
Refuge, and the Alaska Volcano Observatory (K. Wallace) supported
this research. We thank M. Jones and D. Peteet for valuable
discussions of the YD on Kenai Peninsula, and T. Ager, J. Barron, T.
Lowell, G. MacDonald, V. Markgraf, Y. Okumura, Z. Yu, and two
anonymous reviewers for their input on an earlier version of the
manuscript. Laboratory of Paleoecology Contribution 97.
Appendix A. Groups of Pediastrum coenobia distinguished
in this study
(1) Pediastrum boryanum var. boryanum. This species is the most
widely distributed taxa today, occurring in waters of various
trophic conditions. Weckström et al. (2010) found highest
correlation with precipitation, pH, and conductivity.
(2) Pediastrum angulosum. This species is a good indicator of
oligotrophic conditions. Crisman (1978) found that it dominated lakes surrounded by conifer forest in Minnesota, correlating with low pH, low alkalinity, and with low productivity.
Parra Barrientos (1979) suggested that it prefers neutral to
slightly acid waters. Weckström et al. (2010) identified it most
often in lakes with high DOC and color. It is rarely dominant.
(3) Pediastrum boryanum var. brevicorne f. brevicorne. The taxonomic status of this form is uncertain. Parra Barrientos (1979)
found that it occurs with P. boryanum var. boryanum, a widespread species.
(4) Pediastrum boryanum var. longicorne (a.k.a. P. boryanum var.
brevicorne f. granulatum; Komárek and Fott, 1983). Jankovská
and Komárek (1982) regarded this taxon as a relict, being
more common during the late glacial than now. Today it is
1452
D.S. Kaufman et al. / Quaternary Science Reviews 29 (2010) 1445e1452
more abundant in temperate and subarctic zones of the
Northern Hemisphere, occurring in oligotrophic to moderately
dystrophic waters. Komárek (1997) considered this species to
indicate clear water, often with the presence of dystrophic
conditions. Weckström et al. (2010) found the best correlation
with precipitation.
(5) Pediastrum integrum. Komárek (1997) and Komárek and
Jankovská (2001) considered this taxon to indicate clearwater lakes, with an oligo- to dystrophic environment. It often
occurs with P. boryanum var. longicorne. Hielsen and Sørensen
(1992), quoting various sources, conclude that it is rare, and
relict, most frequently found in cold, clear waters. Some
authors (i.e., Bigeard, 1933; Whiteside, 1965) considered
P. integrum to be a benthic form of P. boryanum. Crisman (1978)
found it to be dominant in the same conifer lakes as P. angulosum, suggesting similar ecological conditions.
(6) Pediastrum unidentified. This category included those types not
specifically identified.
References
Ager, T.A., Brubaker, L.B., 1985. Quaternary palynology and vegetational history of
Alaska. In: Holloway, V.M., Bryant, R.G. (Eds.), Pollen Records of Late-Quaternary
North American Sediments. AASP Foundation, Dallas, Texas, pp. 353e384.
Alley, R.B., 2000. The Younger Dryas cold interval as viewed from central Greenland.
Quaternary Science Reviews 19, 213e226.
Anderson, R.S., Hallett, D.J., Berg, E., Jass, R.B., Toney, J.L., de Fontaine, C.S.,
DeVolder, A., 2006. Holocene development of boreal forests and fire regimes on
the Kenai Lowlands of Alaska. The Holocene 16, 791e803.
Bakke, J., Lie, Ø, Heegaard, E., Dokken, T., Haug, G.H., Birks, H.H., Dulski, P., Nilsen, T.,
2009. Rapid oceanic and atmospheric changes during the Younger Dryas cold
Period. Nature Geoscience 2, 202e205.
Barron, J.A., Bukry, D., Dean, W.E., Addison, J.A., Finney, B., 2009. Paleoceanography
of the Gulf of Alaska during the past 15,000 years: Results from diatoms,
silicoflagellates, and geochemistry. Marine Micropaleontology 72, 176e195.
Barron, J.A., Heusser, L., Herbert, T., Lyle, M., 2003. High-resolution climatic evolution
of coastal northern California during the past 16,000 years. Paleoceanography
18, 1020.
Berger, A., Loutre, M.F., 1991. Insolation values for the climate of the last 10 million
years. Quaternary Science Reviews 10, 297e317.
Bigeard, E., 1933. Les Pediastrum d'Europe. Etude biologique et systématique. Travaux
du Laboratoire de Botanique de l'Université Catholique d'Angers 5, 1e192.
Briner, J.P., Kaufman, D.S., 2008. Late Pleistocene mountain glaciation in Alaska: key
chronologies. Journal of Quaternary Science 23, 659e670.
Briner, J.P., Kaufman, D.S., Werner, A., Caffee, M., Levy, L., Kaplan, M.R., Finkel, R.C.,
2002. Glacier readvance during the late glacial (Younger Dryas?) in the Ahklun
Mountains, southwestern Alaska. Geology 30, 679e682.
Brubaker, L.B., Anderson, P.M., Hu, F.S., 2001. Vegetation ecotone dynamics in
southwest Alaska during the late Quaternary. Quaternary Science Reviews 20,
175e188.
Crisman, T.L., 1978. Algal remains in Minnesota lake types: a comparison of modern
and late-glacial distributions. Verhandlungen der Internationalen Vereinigung
fur Theoretische und Angewandte Limnologie 20, 445e451.
Engstrom, D.R., Hansen, B.C.S., Wright Jr., H.E., 1990. A possible Younger Dryas
record in southeastern Alaska. Science 250, 1383e1385.
Evenson, E.B., Yu, Z.C., Walker, K.N., Hajdas, I., Alley, R.B., Lawson, D.E., Larson, G.L.,
Lowell, T.V., 2005. Stability(?) of the Matanuska Glacier over the last 14.5 cal ka
and Younger Dryas cooling in south-central Alaska. Geological Society of
America Abstract 37 (7), 542.
Faegri, K., Kaland, P.E., Kryzywinski, K., 1989. Textbook of Pollen Analysis. Wiley,
Chichester.
Hielsen, H., Sørensen, I., 1992. Taxonomy and stratigraphy of late-glacial Pedistrum
taxa from Lysmosen, Denmark e a preliminary study. Reviews of Paleobotany
and Palynology 74, 55e75.
Hu, F.S., Brubaker, L.B., Anderson, P.M., 1995. Postglacial vegetation and climate
change in the northern Bristol Bay region, southwestern Alaska. Quaternary
Research 43, 382e392.
Hu, A., Otto-Bliesner, B.L., Meehl, G.A., Han, W., Morrill, C., Brady, E.A., Briegleb, B.,
2008. Response of thermohaline circulation to freshwater forcing under
present-day and LGM conditions. Journal of Climate 21, 2239e2258.
Hu, F.S., Shemesh, A., 2003. A biogenic-silica d18O record of climatic change during
the last glacialeinterglacial transition in southwestern Alaska. Quaternary
Research 59, 379e385.
Hu, F.S., Lee, B.Y., Kaufman, D.S., Yoneji, S., Nelson, D.M., Henne, P.D., 2002. Response
of tundra ecosystem in southwestern Alaska to Younger Dryas climatic
oscillation. Global Change Biology 8, 1156e1163.
Hu, F.S., Nelson, D.M., Clarke, G.H., Rühland, K.M., Huang, Y., Kaufman, D.S., Smol, J.
P., 2006. Abrupt climatic events during the last glacial-interglacial transition in
Alaska. Geophysical Research Letters 33, L18708.
Jankovská, V., Komárek, J., 1982. Das Vorkommen einiger Chlorokokkalalgen in
bohmishen Spatglazial and Postglazial. Folia Geobotanica Phytotaxon
Bohemoslov 17, 165e195.
Jones, M.C., Peteet, D.M., Kurdyla, D., Guilderson, T., 2009. Climate and vegetation
history from a 14,000-year peatland record, Kenai Peninsula, Alaska. Quaternary Research 72, 207e217.
Kaufman, D.S., 29 others, 2004. Holocene thermal maximum in the western Arctic
(0e180 W). Quaternary Science Reviews 23, 529e560.
Kokorowski, H.D., Anderson, P.M., Mock, C.J., Lozhkin, A.V., 2008. A re-evaluation
and spatial analysis of evidence for a Younger Dryas climatic reversal in
Beringia. Quaternary Science Reviews 27, 1710e1722.
Komárek, J., 1997. Palaeoalgological analyses, implication for pollen-analytical
research: Seminarium, Krakow (notes from a seminar).
Komárek, J., Fott, B., 1983. Chlorophyceae (Grunalgen). Ordnung: Chlorococcale. In:
Huber-Pestalozzi, G. (Ed.), Das Phytoplankton des Susswassers Systematik and
Biologie. 7.1. Schweizerbart, Stuttgart, p. 1044.
Komárek, J., Jankovská, V., 2001. Review of the green algal genus Pediastrum;
implications for pollen-analytical research. Bibliography of Phycology 108, 127.
MacDonald, G.M., Moser, K.A., Bloom, A.M., Porinchu, D.F., Potito, A.P., Wolfe, B.B.,
Edwards, T.W.D., Petel, A., Orme, A.R., Orme, A.J., 2008. Evidence of temperature
depressions and hydrological variations in the eastern Sierra Nevada during the
Younger Dryas stade. Quaternary Research 70, 131e140.
McKay, N.P., Kaufman, D.S., 2009. Holocene climate and glacier variability at Hallet
and Greyling lakes, Chugach Range, south-central Alaska. Journal of Paleolimnology 41, 143e159.
Mortlock, R.A., Froelich, P.N., 1989. A simple method for the rapid determination
of biogenic opal in pelagic marine sediments. Deep-Sea Research 36,
1415e1426.
Okumura, Y.M., Deser, C., Hu, A., Timmermann, A., Xie, S.P., 2009. North Pacific
climate response to freshwater forcing in the subarctic North Atlantic: ocean
and atmospheric pathways. Journal of Climate 22, 1424e1445.
Parra Barrientos, O.O., 1979. Revision der Gattung Pediastrum Meyen (Chlorophyta).
Bibliotheca Phycologica 48, 183.
Peteet, D.M., Mann, D.H., 1994. Late-glacial vegetational, tephra, and climatic history
of southwestern Kodiak Island, Alaska. Ecoscience 1, 255e267.
Peteet, D., Del Genio, A., Lo, K.K., 1997. Sensitivity of Northern Hemisphere air
temperatures and snow expansion to North Pacific sea surface temperatures in
the Goddard Institute for Space Studies general circulation model. Journal of
Geophysical Research 102, 781e791.
Reger, R.D., Sturmann, A.G., Berg, E.E., Burns, P.A.C., 2008. A guide to the late
Quaternary history of the northern and western Kenai Peninsula, Alaska. Alaska
Division of Geological and Geophysical Surveys, Guidebook 8.
Reimer, P.J., Baillie, M.G.L., Bard, E., Bayliss, A., Beck, J.W., Bertrand, C.J.H.,
Blackwell, P.G., Buck, C.E., Burr, G.S., Cutler, K.B., Damon, P.E., Edwards, R.L.,
Fairbanks, R.G., Friedrich, M., Guilderson, T.P., Hogg, A.G., Hughen, K.A.,
Kromer, B., McCormac, G., Manning, S., Ramsey, C.B., Reimer, R.W., Remmele, S.,
Southon, J.R., Stuiver, M., Talamo, S., Taylor, F.W., van der Plicht, J.,
Weyhenmeyer, C.E., 2004. INTCAL04 terrestrial radiocarbon age calibration,
0-26 cal kyr BP. Radiocarbon 46, 1029e1058.
Salathé Jr., E.P., 2006. Influences of a shift in North Pacific storm tracks on western
North American precipitation under global warming. Geophysical Research
Letters 33, L19820.
Sarnthein, M., Kiefer, T., Grootes, P.M., Elderfield, H., Erlenkeuser, H., 2006. Warmings in the far northwestern Pacific promoted pre-Clovis immigration to
America during Heinrich event 1. Geology 34, 141e144.
Trenberth, K.E., Hurrell, W., 1994. Decadal atmospheric-ocean variations in the
Pacific. Climate Dynamics 9, 303e319.
Weckström, K., Weckström, J., Yliniemi, L.-M., Korhola, A., 2010. The ecology of
Pediastrum (Chlorophyceae) in subarctc lakes and their potential as paleobioindicators. Journal of Paleolimnology 43, 61e73.
Whiteside, M.C., 1965. On the occurrence of Pediastrum in lake sediments. Journal of
the Arizona Academy of Science 3, 144e146.
Yu, Z., Walker, K.N., Evenson, E.B., Hajdas, I., 2008. Lateglacial and early Holocene
climate oscillations in the Matanuska Valley, south-central Alaska. Quaternary
Science Reviews 27, 148e161.
Yu, Z., 2000. Ecosystem response to Lateglacial and early Holocene climate oscillations in the Great Lake region of North America. Quaternary Science Reviews
19, 1723e1747.