46262-G2
Rare Oxygen and Sulfur Isotopes of Volcanogenic Sulfate Aerosols in Sedimentary Rocks as a Robust Correlation Tool
We conducted fieldwork and collected > 60 samples, then obtained a detailed record of rare isotopes in nicely preserved ash deposits and enclosing sediments in dry lake Tecopa near Death Valley in California. We studied three large volume eruptions those ash is preserved in the lake: Huckleberry Ridge tuff (Yellowstone caldera, 2 Ma), Bishop tuff (Long Valley caldera, 0.76 Ma) and Lava Creek tuff (Yellowstone caldera, 0.64 Ma). Postdoctoral fellow Erwan Martin directed the research in both the field and the lab in this project with the help of undergraduate students. Rare isotope preservation issues were addressed: sulfate is a soluble compound and is preserved only in dry and arid climates (Bao et al. JGR, 2000). We conducted analyses of 56 samples for 34-S, 18-O, 16-O and O-17 and measured sulfate concentrations along and across ash layers with particularly large coveregae of the youngest deposit – the Lava Creek tuff. We are addressing the following questions: What are the isotopic values of 32,33,36S and 17,18O in sulfate components in volcanic ash? What are the sources of sulfur and condition of preservation and post-depositional modification of conventional isotopic values that should not affect mass dependent rare isotope signature in volcanic ash? How extensive and consistent is the Δ17O signature along a single continuous ash layer, and across it near Death Valley? Is this a tropospheric “dry fog” (Bao et al. JRL, 2003) or a stratospheric (e.g. Bindeman et al. GCA, 2007) phenomenon? Can we onfirm existing observations that small eruptions possess no or minimal Δ17O, and exponents of mass dependent sulfur isotope fractionations close to the equilibrium values of 0.515 (Δ33S) and 1.90 (Δ36S), while large eruptions have significant Δ17O >0, but mass dependent S isotope fractionations that deviate beyond the above equilibrium S exponents? What is the regional potential for correlation of ash deposits using rare isotopes? Finally, given large Δ17O signature of ash from large-volume eruptions ultimately derived from ozone, what is the role of this eruption on atmospheric ozone? We have made one presentation on this topic at an international geochemical Goldschmidt conference in Vancouver, Canada in July 2008.
