Middle to Late Pleistocene Paleoenvironmental Reconstructions from Lake El\u27gygytgyn, Arctic Russia

Climate change is a major issue challenging the world today. Our global society faces rising temperatures, variable weather patterns, and rising sea level among other associated issues. Our action (or inaction) to address current changes will have serious ramifications for life on our planet in the coming centuries and millennia. In order to provide context for these present and future changes, we can utilize the paleo record to understand the natural variability of Earth’s climate system. One region of the world is changing more rapidly than the global average. Over recent decades, the Arctic has experienced warmer temperatures, reduced sea ice, melting permafrost, and shifts in the amount and seasonality of precipitation. Unfortunately, paleoclimate and environmental records from the terrestrial Arctic, particularly beyond the last 120 ka, are few. This is due to the repeated extensive glaciation of the northern hemisphere high latitudes during the ice ages of the Quaternary. One area of the Arctic, in the Anadyr mountains of Chukotka, has remained unglaciated through the Pleistocene. Lake El’gygytgyn, a meteorite impact crater lake formed 3.6 million years ago lies in this area and so provides a continuous sedimentary sequence from the Mid-Pliocene to present. This dissertation includes four studies of Lake El’gygytgyn sediments over the last 800 thousand years. A variety of biogeochemical and stable isotope proxies are used to reconstruct climate and environmental variability throughout the study interval. These studies provide novel information about the natural variations of terrestrial Arctic climate on glacial-interglacial timescales. Chapter 2 of this dissertation involves the analysis of bacterial membrane lipids called branched glycerol dialkyl glycerol tetraethers (brGDGTs) and plant leaf wax n-alkanes to provide records of relative temperature change and terrestrial vegetation turnover in response to aridity during the glacial-intergacial cycles of the Mid-Pleistocene. Our data suggests that regional temperature is strongly influenced by local summer insolation while aridity changes derive from sea level driven changes in continentality. Comparison of our data with previously published paleoclimate records from Lake El’gygytgyn highlights the difference in proxy response to climatic variables and the utility of a multi-proxy approach. Additionally, we use our extensive records to identify the presence of a global climatic transition, the Mid-Brunhes Event (MBE), for the first time in the terrestrial Arctic. In Chapter 3, we analyze algal lipid biomarkers from the same samples used in Chapter 2. We also incorporate stable carbon and nitrogen isotopes to determine changes in primary production and organic matter preservation in Lake El’gygytgyn across multiple glacial-interglacial cycles. Chapter 4 spans the Holocene and Late Pleistocene (280 ka to present). We use the hydrogen isotopic composition of long chain plant leaf wax n-alkanes to reconstruct temperature and hydroclimate changes. We find that MIS 7 was a stronger interglacial period than MIS 5e in the terrestrial Arctic and attribute the variability in hydrogen isotopes predominantly to temperature and moisture source changes. In Chapter 5, we compare the hydrogen isotope composition of n-alkanes measured in Chapter 4 to the isotopic composition of n-alkanoic acids in the same samples previously analyzed by Wilkie (2012). This is a novel approach for paleoclimate records. Our results indicate that the type of compound selected for analysis can have a significant impact on the paleoclimatic interpretations of a study. Finally in Chapter 6, a summary and avenues for future research are provided. Overall, this dissertation is a testament to the utility of biomarkers and stable isotopes in Arctic lake sediments. Each study provides unique information about the terrestrial Arctic climate during the Quaternary and contributes to our understanding of climatic variability and the dynamics of this sensitive region