Understanding lower temperature natural systems using stable isotopes

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Earth's air, water, biota, rocks and sediments are dominated by oxygen, hydrogen, carbon and nitrogen. Each element occurs in two or three stable forms called isotopes, for example oxygen-18, oxygen-17 and oxygen-16. Their abundances in low-temperature minerals, and in plant and animal tissues, are determined by environmental conditions. My research team and I will go "Back to the Future" based on the isotopic signatures of climatic and ecological conditions preserved in such proxy records over the last 100,000 years. We will search for patterns in these records that can serve as bellwethers for the future. Two areas in North America are of special interest, the Great Lakes region and the lost Beringian Pleistocene ecosystem in the Yukon.* The modern Great Lakes region is sensitive to shifting air masses on seasonal, decadal and longer time-scales. Freshwater contains an isotopic record of these variations. For ~20 years, we have collected these data for the region. This record's length will now allow testing for climatic trends, and can also serve a baseline for ancient conditions. Modern plants also retain ecosystem isotopic and molecular records, which we will use to interpret data for ancient vegetation. Using oxygen-17, we will also quantify the contribution of atmospheric nitrate to the threat posed by nitrogen deposition in the area. Leakage from bedrock wells from Ontario's early petroleum boom also is a threat to drinking-water aquifers. We will define the isotopic and chemical fingerprints of specific bedrock brines, pinpoint contamination sources, and thus aid cost-effective plugging of the leaks. Further back in time, glacial meltwater outbursts from the retreating Laurentide Ice Sheet may have triggered global cooling by changing oceanic circulation patterns. One pathway to the Atlantic Ocean was through the Great Lakes. We will use shelly fauna and organic biomarkers (n-alkanes) contained in Great Lake sediments to track meltwater movement since ~15,000 years ago. These proxies also record post-glacial ecosystem changes in Great Lakes watersheds. We are especially interested in isotopic signals of climatic factors versus other processes that caused substantial drops in water level - and even hydraulic closure - of some Great Lakes.* Megafauna tissues (bone, tooth) are also superb isotopic proxies for environmental change, recording seasonal variations in climate and forage (vegetation, prey). We will use such data to examine possible causes (climate, over-hunting) of Pleistocene megafauna extinctions in North America. By analyzing mammoth, mastodon and giant beaver, we will learn about the terrestrial ecosystem characteristics of the Great Lakes region at the end of the Pleistocene. Yukon's Beringia also has a fascinating story to tell. Its species diversity (e.g., mammoth, horse, bison, camel, lion, short-faced bear) is much higher than the modern Arctic. Why were its tundra-steppe grasslands so productive, despite the cold and dry climate? Anomalous nitrogen-15 enrichment in mammoths is a special puzzle. This simple observation may signal that the nitrogen cycle in Beringia was quite different from today. To test this idea, we will grow Beringian grasses under Beringian climatic conditions in Beringian loess soils, analyze ancient Beringian vegetation from ~90,000 to 25,000 year-old ground squirrel nests extracted from permafrost, and compare the isotopic results - including individual amino acids - with the region's modern vegetation and soil. In short, we will determine how two different landscapes responded to climate change at the end of the last glacial period. Why some mammals adapted while others became extinct holds important lessons for anticipating modern ecosystem responses to environmental change.