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.
Investigating the patterns of past climate variability over space and time helps to understand and quantify the processes that cause climate to change, which is important as society prepare for the full range of future climate changes due to both anthropogenic and natural factors. Climate varies naturally on long time scales. Centennial and longer time scales are beyond the time frame covered by instrumental measurements. Natural archives offer indirect (proxy) records of past climate variability, which can be tapped to extract information about past climate changes. Future climate projections focus on the influence of human factors, especially the build-up of greenhouse gasses. However, future climate will also be influenced by natural factors, and it will be complicated by internal variability within the climate system. Climate model projections can be improved by accounting for both anthropogenic and natural factors. This project generated highly resolved and well-dated time series of a wide range of physical and biological properties in sediment cores taken from seven lakes across southern Alaska. These have been interpreted as evidence for past environmental and climate change. The results have been reported in three journal articles published so far, and 15 abstracts presented at national and international meetings. They form the basis of two Master’s theses, and a third is nearly complete. Two additional journal articles based on results generated by this project are currently in review, and several are in preparation. Some results from this project have been incorporated into larger community efforts to understand the causes and effects of climate variability on a global scale. This includes a major new synthesis, "Continental-scale temperature variability during the past two millennia," which was published in the May 2013 issue of the journal, Nature Geoscience. The study lakes are situated along an east-west transect at about 60°N latitude, along the center of action for the Aleutian low pressure system. Adak Island is the western study site. Cores were analyzed from a pair of lakes (Heart and Andrew Lakes). These are the first lakes to have been cored on Aleutian Islands. In addition to volcanic eruptions, their sediments record changes in storminess during the last 6000 years. Lone Spruce Pond, Cascade Lake, and Shadow Bay are located in the Bristol Bay area, at the transect midpoint. The cores from Lone Spruce Pond are better dated and contain a more complete tephrostratigraphic sequence than any other previously reported from the area. In addition to a detailed record of paleoenvironmental changes over the entire post-glacial period, the sedimentary sequence provided the framework for the synthesis of the regional tephrochronology, which integrated information from six lakes in the region. Allison and Cabin Lakes are located in the Prince William Sound area, at the east end of the transect. Results shown that the sediment of Allison Lake is marked by annual layers (varves); their thickness correlates with river discharge during fall, which is driven by rainfall events. Cabin Lake contains evidence for changes in the terminal position of nearby Sheridan Glacier during the past 10,000 years, and the sediment contains the first tephras analyzed from the area. This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
