Department of Earth and Atmospheric Sciences

1. What chemical fractions of sediments can be used as a proxy for lakewater isotopic composition? In the past few years, new technology (developed in large part by former IU Geology Professor John Hayes and his students) has enabled us to make unprecedented measurements of hydrogen isotope ratios (D/H) on individual organic compounds. This approach offers several advantages over commonly used methods. Paleolimnologists interested in reconstructing lakewater isotopic composition have generally been restricted to ¹⁸O/¹⁶O ratios of calcium carbonate of aquatic fossils and sediment cellulose, both of which can be subject to severe limitations. The oxygen isotopic composition of carbonate is affected by temperature-dependent fractionation effects, and sediment cellulose can contain significant amounts derived from terrestrial sources, which affects its isotopic composition. For the past few years, I have been pursuing the possibility of using D/H ratios of certain biomarker lipids as a means of overcoming both of these problems.

2. How well can stable isotope ratios in freshwater systems be used as a proxy for climate conditions? Whereas stable isotope ratios of oxygen (¹⁸O/¹⁶O) and hydrogen (D/H) in precipitation are well correlated with mean annual temperature in most cold parts of the world, this signal is not always directly mapped into the isotopic composition of lakewater. For example, evaporation, both in the lake and in the watershed, can be important because it preferentially removes light isotopes, leaving the remaining water enriched in the heavy isotope. In general, these factors can be evaluated only through a rigorous program of hydrologic monitoring. I headed an isotopic monitoring program of lakewater near Iqaluit (Nunavut, Arctic Canada) to examine these effects.

3. How do lake-sediment based records of climate contribute to regional paleoclimate histories, and what does that tell us about climate sensitivity? Many environmental variables must be considered together to define climate. Because different climate proxies are sensitive to different variables, a wide range of paleoclimate proxies must be studied in order to generate the most reliable climate reconstructions. Depending on the hydrologic condition of the lakes, isotopes may reflect mean annual temperatures or the may also contain a significant evaporation signal. In order to make sense of lake-sediment based records, one must interpret them in light of other evidence from other proxies that are sensitive to temperature and water balance. Regional and global paleoclimate syntheses nearly always conclude that climate change is spatially heterogeneous. By comparing my paleoclimate records from one part of the world with records from other parts of globe, it is possible to determine the timing and geographical extent of anomalies, which can then help lead to an understanding of the forcing that caused the anomaly. The ultimate goal of this research is to relate the forcing to the size of the anomaly, thereby gaining a better understanding of how the climate system works.

4. Are hydrogen isotope ratios in terrestrial leaf-wax components a viable proxy for humidity? Humidity is a climate variable that is difficult to reconstruct. However, it may be that D/H ratios of leaf-wax components (n-alkanes, long-chain fatty acids, and long-chain n-alkanols) can be used as a proxy for evaporation. Plant physiologists have determined that water within the leaves of vascular plants becomes isotopically enriched due to evaporation. The degree of isotopic enrichment increases as humidity decreases. If leafwater isotopic enrichment is recorded in the leaf wax components as preliminary data suggest, then it may be possible to use the D/H of leaf wax components in sedimentary systems as an integrated, basin-wide paleo-humidity proxy. If successful, this means of obtaining paleo-humidity information would have important ramifications for paleobotanical and paleoclimate studies. These projects illustrate the kind of study which can be undertaken now using new geochemical approaches. The overarching theme I wish to address in my future research is, What is the climate history of the earth? What caused these changes, and what can we predict about future environmental change based on the paleoenvironmental record?