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The proposal has funded PI, one postdoctoral scholar from France Dr. Erwan Martin, a professor from an undergraduate institution (Cal-State Northridge, Dr. Jorge Vazquez) as a summer visitor (supplemental funding), and a number of undergraduate students (Dan Sakrisson, Luke Sitts, Nicholas Weldon) who worked in the lab in the summers on 2008 and 2009.

The main results of our investigation that stems from an earlier study by the PI Bindeman, (GCA, 2007, 71, 2326), demonstrated that volcanic ash layers, no matter how altered they are, contain measurable quantities of gypsum resulting from atmospheric photochemical reaction upon volcanic ash transport. In wet climate, gypsum is lost to the environment relatively quickly, but in arid and dry climate it is preserved for millions of years. An unusual isotopic signature characterizes the SO2 gas, upon its release during the course of volcanic eruptions. As SO2 gets oxidized in the upper atmosphere by so called mass-independent compounds, among which ozone plays the major role, the resultant sulfate records this mass-independent isotopic signature that is preserved in the geologic record in the form of leachable sulfate. In particular, volcanic sulfate possess excess of 17-O, inherited from SO2 oxidation by ozone and ozone-derived compounds. The main motivation behind this PRF proposal was to use this mass-independent isotopic signature of the volcanic sulfate as a robust stratigraphic correlation tool. This hypothesis result from the fact that any secondary processes such as alteration, loss due to leaching, microbial sulfate reduction, and others are mass-dependent processes, unable to modify initial and unique mass-independent signature of the volcanic sulfate, absorbed on volcanic ash particles in ash layers.

For that, we completed our investigation of three thick ash layers that belong to three largest eruptions in N America for the past 2 million years: Lava Creek Tuff (0.6 Ma, Yellowstone), Huckleberry Ridge tuff (2.1 Ma, Yellowstone), Bishop Tuff (0.75 Ma, Long Valley Caldera, California). Ash from these eruptions covered most of the territory of the western US, and occurs as thick layers in many outcrops from Kansas to California. These three ash layers are wonderfully preserved in the dry Lake Tecopa, California.

We found that each layer is indeed possess rather specific values of 17-O excess, as well as rare sulfur isotope anomalies (33-S and 36-S) that result from photochemical reaction in the upper atmosphere. However, due to heavy dilution by sedimentary sulfate which is derived from the lake Tecopa watershed area, the signature can get rather irregular, and thus multi isotope investigation of leached sulfates, including sulfur and conventional oxygen isotope analyses, is required.

Besides stratigraphic applications, the results of this work have important implication for the effects of volcanic eruptions on climate: we estimate that up to 50% of the global ozone layer is getting consumed in the course of volcanic SO2 oxidation in the upper atmosphere. This, however, does not necessarily lead to the increased UV radiation flux to the earth's surface as SO2 molecule has very similar absopbtion bands to ozone. If, however, SO2 removal is accelerated, there could be temporal conditions of increased UV radiation months to years following these supereruptions.

Research by the summer visitor Dr Jorge Vazquez included investigation of the zircon crystal size distributions in the the same ash layers in order to supplement our isotopic methods with novel textural characterization. We found again that each ash layer has a rather characteristic distribution of this tiny, alteration resistant mineral, that was transported over hundreds of miles from the respective eruption centers. Only diminished concentrations of the largest (>250 microns) zircons is observed. Notice that zircon is the only mineral that is not lost due to Aeolian differentiation in the volcanic plume. Thus, zircon CSD, can be potentially used for lateral cross correlation of ambiguous or heavily altered ash layers, and U-Pb ages in this zircon can be used to constrain the ages of these ash layers.

The following presentations were made:

Geological Society of America Annual meeting, Portland, October 2009
Martin E., Bindeman I.
IAVCEI conference Reykiavik, Iceland (August 2008)
Rare Isotope Insights into Supereruption: Sulfur and Triple Oxygen Isotope Geochemistry of Sulfate Aerosols Absorbed on Volcanic Ash Particles by Martin E, and Bindeman I,
International Goldschmidt conferene on Geochemistry
Martin, E; Bindeman, I Detailed "supervolcanic" ash record in dry Lake Tecopa, California: delta S-34, delta O-18 and Delta O-17 of soluble volcanic sulfate

Additionally there is one paper published in Earth and Planetary Science Letters this year.