Triple oxygen isotopes in lake waters, lacustrine carbonates and soil carbonates: An indicator for evaporation

Oxygen isotopic compositions of minerals and natural waters have been widely used in paleoclimatology and paleohydrology to infer climate parameters. However, 18O/16O ratios alone are often very complicated to interpret because 18O/16O ratios can be affected by temperature and topography as well as hydrological processes such as evaporation and precipitation. Deviations in 17O/16O relative to 18O/16O [often denoted as Δ17O = ln(δ17O +1) - λln(δ18O +1)] have been shown to be sensitive to evaporation due to the difference in isotopic fractionation exponent [θ = ln(δ17O +1) / ln(δ18O +1)] between water-vapor equilibrium and vapor diffusion in the air. Thus, the triple oxygen isotope systems of minerals and natural waters may provide additional information on hydrologic processes that are not discernable from analysis of 18O/16O ratios alone. Chapter 2 of the thesis explores the possibility of using Δ17O in lake waters and lacustrine carbonates as an indicator for local evaporation. Samples were collected from salt lakes, freshwater lakes as well as their associated rivers in western USA. Triple oxygen isotope data of all samples were presented to show the variation of Δ17O under different hydrological and climatic scenarios. They are shown to be generally consistent with theoretical models adapted from the Craig-Gordon model. An open pan evaporation experiment was also conducted to study the effect of dissolved salt on triple oxygen isotopes in natural waters. Contrary to salt effect on δ18O, we did not observe significant salt effect on Δ17O. Chapter 3 examines how triple oxygen isotopes in soil carbonates can be used to distinguish soil carbonate δ18O which has been modified by significant evaporation. Soil carbonate samples from East Africa, USA and China were measured. Results show that most deep soil carbonates samples (>50cm) do not show significant deviation of Δ17O from primary precipitation. Meanwhile, some shallow soil carbonates have different Δ17O compared with primary precipitation due to evaporative modification. This is basically consistent with a theoretical model developed in this chapter which simulates triple oxygen isotopes profiles in drying soil columns. The overall object of this thesis is to investigate the usage of triple oxygen isotopes in terrestrial environments, especially in soil carbonates and lacustrine carbonates, as an indicator for evaporation. This will be helpful to decouple the δ18O variation caused by evaporative modification and other climatic factors and therefore to elucidate δ18O recorded in carbonate minerals. In addition, in an Appendix of this thesis, results from a series of heating experiments on dolomite samples were conduct to study 13C-18O bond reordering process in dolomite. The clumped isotope measurements show that the clumped isotope reordering progress within dolomite is much slower compared to previously reported results for calcite