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Evidence for a variable and wet Younger Dryas in southern Alaska
Quaternary Science Reviews 29 (2010) 1445e1452
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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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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/.
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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.
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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.
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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.
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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
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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).
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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
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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
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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. 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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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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/.
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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.
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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.
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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.
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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
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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
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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
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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.
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