We propose to study the history of late Quaternary environmental change preserved in Arolik Lake, southern Alaska, to obtain a paleoclimate record with centennial-scale resolution. We already have two continuous 6-to-8m-long cores from the lake, including one that extends back at least 22,000 yr (22 14C ka). Now we propose to return to the lake armed with geophysical equipment for obtaining more detailed bathymetric charts and subsurface stratigraphic profiles to place these and new cores into a basin-wide stratigraphic context, to identify features indicative of lake-level fluctuations, and to locate sites within the basin for additional coring where a longer record (>60 ka) might be obtainable. We will also measure the sub-bottom stratigraphy at a second lake (Nimgun Lake), 26 km northwest of Arolik Lake, to locate targets for cores extending into the last full-glacial period. All cores will be processed for basic physical parameters and the most promising core will be selected for concentrated study of complementary physical (grain size, bulk density, magnetic susceptibility), biochemical (organic carbon, biogenic silica), and ecological (pollen, diatoms) proxy climate indicators, and for geochronology (AMS 14C and tephrochronology). The goal is to obtain centennial-scale resolution for the proxy-climate indicators. The analyses will be a collaborative effort by workers with experience in obtaining sediment cores and acoustic-stratigraphic data from lakes (Kaufman, Werner), and by experts in the analysis of pollen (Hu), diatoms (Smol), and tephra (Riehle) from southwestern Alaska. The results will contribute to previous and on-going work by PALE researchers in Beringia whose goal is to provide highly resolved records of paleoclimate to compare with climate model simulations and with records from glacier ice and marine sediment.
Arolik Lake is in a good position to record paleoenvironmental changes. Among other factors, the sedimentology of the lake should reflect changes in the regional glacial activity. It is situated at the optimal distance from a source of eolian sediment derived from a late Wisconsin outwash plain: close enough to deliver a signal, but not so close as to overwhelm the lake deposits. Nimgun Lake was fed distally by a small cirque glacier during the late Wisconsin. Both lakes are within the area where we have conducted extensive research into the extent and timing of late Pleistocene glacier fluctuations, where previous studies in the adjacent lowlands provide an independent framework of Quaternary paleoenvironmental change, and where we are developing a tephrostratigraphy for precise intra-and inter-basinal stratigraphic correlation's. The lakes are dammed by moraines of early Wisconsin age (~110 -60 ka).
Because lake sediments accumulate continuously over tens of thousands of years and are often dateable, they afford one of the best natural archives of past climate variability on the continents. A continuous record of climate change extending back through the full-glacial period has not yet been obtained from southwestern Alaska, but is important for assessing whether the rapid climate oscillations known in marine and ice cores from North Atlantic region also affected continental regions of northwestern North America. Assessing whether rapid climate changes occurred synchronously between these regions, or whether the spatial and temporal patterns are random, or even anti-phase, is fundamental to understanding the mechanisms that control climate oscillations. Developing this understanding for the full-glacial period is important in southwestern Alaska because deglacial records (15-10 ka) are potentially influenced by regional-climatic effects resulting from the submergence of extensive continental shelf.
The record of full-glacial climate will also be used in a data-model comparison of a fundamental feature of atmospheric circulation simulated by global circulation models: The glacial anticyclone. At 18 ka, the simulated anticyclone generates strong southerly flow that warms Alaska, especially during the winter. A candidate for full-glacial warming in eastern Beringia is the so-called "Hanging Lake thermal event" dated in paleovegetation records in northern Alaska and western Canada at ~22-20 ka. This interval is represented in the core from Arolik Lake by increased plant macrofossil abundance. We will evaluate the hypothesis that this warming is attributable to the glacial anticyclone as simulated by global climate models by assessing the fossil (pollen, plant macrofossils, and diatoms) evidence for higher winter temperatures and increased snowfall (and therefore for reduced lake ice), and the physical evidence for glacier fluctuations.
