Reconstructing and Interpreting the Dust Record and Probing the Plumbing of Mono Lake.

This dissertation is comprised of a study of the fluxes of dissolved species into Mono Lake and three studies involving the reconstruction and interpretation of past changes in dust emissions from the world's dust source areas. In the first study (Chapter 2), chlorine (Cl) and helium (He) isotopes are used to estimate inputs of hydrothermal fluids into Mono Lake, a closed-basin lake in eastern California. We demonstrate that the lake's low 36Cl/Cl ratio is consistent with a long residence time for Cl in the basin and hydrothermal inputs that are at most 50% of total solute inputs, in agreement with previous estimates based on strontium, lithium and helium isotopes and major ion concentrations. Based on these findings, we suggest that a dominant role for hydrothermal inputs in setting the lake's solute budget is not necessary to explain the lake's Cl isotope composition. Our water column He isotope data show evidence of hydrothermal 3He accumulation beneath the pycnocline at rates similar to those measured in a previous study. We combine these 3He accumulation data with published CO2/3He ratios in local hydrothermal fluids to test hypothesized rates of hydrothermal CO2 additions to the lake, finding rates that are inconsistent with several previous estimates. Chapter 3 presents a study of the effects of lateral sediment advection on sedimentary accumulation rates estimated using the constant flux proxies excess thorium-230 (xs230Th) and extraterrestrial helium-3 (3HeET). Two cores at the Blake Ridge, a drift deposit in the western North Atlantic, receive different amounts of laterally advected sediments but should have similar rain rates. We find no evidence for fractionation of xs230Th from 3HeET in sediments containing >90% advected material, and we find good agreement in rain rate estimates based on xs230Th and 3HeET between the cores despite differences in sediment advection. Measurements of grain size fractions in the sediments indicate that both proxies are concentrated in fine grains, but these differences do not appear to be sufficiently large to affect rain rate estimates in settings with laterally advected sediments enriched in fine grains. In Chapter 4, we present a preliminary study of the dust proxies helium-4 (4He) and thorium-232 (232Th) in dust source area samples and ocean sediments. Our results corroborate previous data indicating that 232Th concentrations are roughly constant in dust source areas. 4He concentrations are much more variable, ranging over almost 3 orders of magnitude in source area samples. Preliminary grain size data suggest a marked decrease in 4He concentrations with decreasing grain size. Comparison of 4He/232Th ratios, a potential provenance tracer, shows that