Mass-independent isotopic signatures of volcanic sulfate from three supereruption ash deposits in Lake Tecopa, California

Hundreds to thousands of megatons of sulfur dioxide released by supereruptions can change chemical and physical properties of the atmosphere and thus induce climate perturbations. We present oxygen and sulfur isotope analyses of sulfate in 48 volcanic ash samples, and 26 sediment samples from dry lake beds in the Tecopa basin, California, USA. These ash layers represent three supereruptions, including the 0.64 Ma Lava Creek Tuff, 2.04 Ma Huckleberry Ridge Tuff and 0.76 Ma Bishop Tuff. Mass-independent oxygen signatures (Delta O-17 Up to 2.26 parts per thousand) that are present in these ash units, and not in associated sediments, indicate oxidation of volcanic SO2 by mass-independent ozone and its products. In this study, we consider the formation, deposition, preservation and dilution of mass-independent volcanic sulfate (MIVS). Using the isotopic compositions of the sulfates, we construct a mixing model that demonstrates that the main source of sulfate in Lake Tecopa is mass-dependent sediment-derived sulfate (MDSDS, >77%). However, ash beds still preserve up to 23% of MIVS that initially had undiluted Delta O-17 value around 8 parts per thousand, and Delta S-33 as low as -0.35%., and Delta S-36 Up to 1.08 parts per thousand. Therefore, despite potential dilution by MDSDS, the MIVS signatures can be preserved in the geologic record for few million years, if deposited as gypsum in and environments, alkaline or saline lake. The oxygen and sulfur mass-independent signatures of the volcanic sulfates indicate that photolysis and oxidation of volcanic SO2 has been achieved in the upper atmosphere. Since only supervolcanic eruptions were shown to generate massive amount of mass-independent sulfate, it requires that up to 20-60% of the global ozone layer is consumed as a result of supervolcanic SO2 released. This may occur as a result of a strong physical and chemical degradation of the tropopause; we speculate that the distinction between the high-troposphere and the low-stratosphere, at least locally, could be erased by supereruptions, and recorded by MIVS. (C) 2009 Elsevier B.V. All rights reserved.