Collaborative research: Coupled glacial and lacustrine evidence for decadal- to millennial-scale variability in the climatologic Aleutian low, southern Alaska

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The overall goal of this project is to reconstruct the low-frequency behavior of the climatological Aleutian Low on decadal to millennial time scales, and to assess how its variability has related to past shifts in the mean state of climate during the Holocene. To confidently reconstruct past climate change, the inter-related processes that control the proxy climate indicators must be well understood. This project will continue on-going monitoring and lake-core-based investigations at nine lakes in southern Alaska with the dual aims of (1) understanding the primary controls on the sedimentary changes, and (2) applying this understanding to generate the highest quality time series of paleoclimate proxies that relate quantitatively to summer temperature and winter precipitation. The investigators recently discovered two lakes that contain annually laminated (varved) sediment. These are among the few varved lakes currently known in Alaska, and they present an opportunity to develop annually resolved time series of past climate variability. The low-frequency component of lamination thickness and grain-size variability at these glacier-fed lakes probably reflects the extent of ice cover in the catchment, while inter- and intra-annual variability is likely related to melt-season temperature and hydrologic factors, particularly large rainfall events. In addition to the new laminated lakes, this project will generate complementary records from lakes more suitable for ecologically based proxies, including chironomid and pollen/macrofossil assemblages, and lake-productivity indicators that respond to growing-season temperature. In addition, oxygen-isotope ratios in diatoms will be analyzed to reconstruct moisture-source variations, and geomorphic evidence will be used to assess Holocene glacier fluctuations within the lake catchments. The intellectual merit of this study includes its multi-proxy approach designed to probe a key feature of ocean-atmospheric circulation in the North Pacific: the Aleutian Low pressure system. This prominent center of action is linked to indices of climate variability across the Pacific. It is most strongly expressed in winter when it steers southwesterly storms inland, thereby governing the spatial pattern of surface temperature and snowfall across northwestern North America. This project includes glacier and glacial-lacustrine records because glacier size depends on winter precipitation, which varies with the Aleutian Low. Glacier size also depends on summer temperature necessitating complementary time series of summer temperature. The basic study design is to pair a glacial-fed lake (lamination record) with an organic-rich lake (chironomid record) in each of three study areas separated by 2100 km: Adak (middle Aleutian Islands) in the far west, Ahklun Mountains in the southwest, and Chugach Range in the Gulf of Alaska. These three areas straddle the prominent dipole in the influence of the Aleutian Low. When the Aleutian Low strengthens, winter precipitation and temperature increase in the Gulf of Alaska, while it decreases in the west. Moisture sources also shift with the Aleutian Low, and these are recorded in the oxygen-isotope ratios of lake water and the diatoms that grow within it. The multi-proxy time series will extend through the Holocene, with higher-resolution analyses over two periods that encompass past warm intervals, namely the Holocene thermal maximum, which took place early during the current epoch, and the last 2000 years, which includes the so-called medieval warm period, a key benchmark for 20th century warmth.The broader impacts of this study involve its synergistic activities with resource managers at US Fish and Wildlife Service who are developing a premier environmental monitoring program in the Togiak National Wildlife Refuge, geoscientists at the US Geological Survey's Alaska Volcano Observatory who are striving to assess the frequency of eruptions of Aleutian Arc volcanoes and to identify widespread tephra-stratigraphic markers, and the broader community of multi- and inter-disciplinary scientists aiming to understand the causes and effects of climatic change around the North Pacific, the Arctic, and globally. This project contributes to understanding climatic variability, a key challenge facing society. The US Climate Change Science Program identifies "Past Climate Variability and Change in the Arctic and at High Latitudes" as one of its primary research objectives. The project is training three graduate and several undergraduate students in global-change research, and supports a Research Assistant Professor in her early career. The PIs have involved high school science teachers in their field research as part of NSF's PolarTREC program.