Oxygen-17 excesses in products of stratospheric volcanic eruptions and depletion of the ozone layer

Volcanic eruptions inject tens to many hundreds of megatons of SO_2 into the troposphere and stratosphere. The intensity and duration of climate cooling are dependent on the residence time of sulfate aerosols, which in turn are controlled by oxidative pathways of SO_2 by ozone, H_2O_2, or OH radicals. These reactions lead to mass independent fractionations (MIF) of sulfur and oxygen isotopes in sulfate aerosols (Bao et al. 2003 and Savarino et al. 2003a,b), measured in ash leachates and Antarctic ice. Previous studies have not revealed a clear relationship between the sizes of volcanic eruption and magnitude of MIF's in the sulfate deposited by those eruptions. We present results of a survey of stable isotope compositions, including Δ^(17)O, of sulfate associated with 9 explosive eruptions between 2 Ma and the present that cover much of the diversity of sizes and the character of volcanic eruptions. We studied the 0.64Ma, Lava Creek Tuff of Yellowstone in particular detail. We also measured pedogenic sulfate cements in loess, till, soil, and salts that underlie and overly ash layers; we found that they either do not contain sulfate, or contain sulfate with Δ^(17)O <0.2‰. Thus, the observed anomaly in the ash is a property of the volcanogenic sulfate and not of other sulfate.