Understanding lower temperature natural systems using stable isotopes

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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, e.g., oxygen-16, oxygen-17 and oxygen-18. Their relative abundances in water, biological tissues and minerals are controlled by environmental conditions. Plant matter and shells, for example, can be proxies of past environments preserved in sediments in chronological order. These proxy isotopic compositions can be translated into records of past temperature, aridity and organic productivity. My research team decodes those isotopic signals and the processes that control or destroy their fidelity. We go "Back to the Future" to reconstruct ancient environments based on robust proxy records and seek patterns that can serve as bellwethers for the future. The program proposed here focuses on fresh water, organic matter and secondary shelly and marl carbonate as proxies for environmental change over the last 10,000 years in the Great Lakes region: (1) We will test a new model that suggests that `in-cloud' processes drive precipitation oxygen- and hydrogen-isotope variations, rather than the classical water-source/distillation model in place for more than 60 years. The Great Lakes region is seasonally affected by air masses of different origins. This complexity provides a needed test of the `in-cloud' hypothesis. If the new model is validated, precipitation isotope compositions could become a bellwether for climate-related hydrological transitions. The resulting paradigm shift would also affect interpretation of proxy isotopic signatures; and (2) We will focus on proxies contained in sediments of the Great Lakes and smaller southern Ontario lakes deposited during the last 3000 years. We seek to detect and distinguish natural isotopic variation from signals of human activities and to evaluate the changing relative contributions of each. First, we must identify those proxies that best preserve original information and those that are not always reliable witnesses to the past (e.g., bulk organics). Then, anchored by robust paleoclimate records obtained from `good' proxies, we can assist policy makers in anticipating shifts in climate, environment and hydrology that this highly populated region may face, and more immediately, in identifying what mitigation of anthropogenic effects will be most effective. Finally: (3) We are using the same tool kit to evaluate the isotopic signature of water trapped in old shales. If those compositions reflect water source, the data can be used to identify the water's origin. If the water has interacted with shale clays after entrapment, original isotopic signatures may have changed. Knowing which scenario is correct is important. Shales are potential repositories for hazardous wastes in Canada; their porewater isotopic signature is commonly assumed to reflect water source and used to determine whether these rocks have been leaky or tight over long periods of time.