RARE ISOTOPE INSIGHTS INTO SUPERERUPTION: SULFUR AND TRIPLE OXYGEN ISOTOPE GEOCHEMISTRY OF SULFATE AEROSOLS ABSORBED ON VOLCANIC ASH PARTICLES

We discuss O and S isotopes in sulfate leached from volcanic ash of a series of large-volume eruptions: Lava Creek and Huckleberry Ridge tuffs of Yellowstone, Bishop Tuff, and a set of smaller volume eruptions. This sulfate dataset spans significant range in δ³⁴S, δ¹⁸O, and Δ¹⁷O (29, 30‰ and 3.3‰ respectively) with O and S isotopes recording mass-independent fractionation. Detailed study of 45 volcanic samples and 23 enclosing sediments that host meter-thick ash layers was performed in dry Lake Tecopa in California. This lakeexisted as dry intermountain basin for >2Ma during accumulation of these ashes. Sulfate in sediments does not possess significant isotopic anomalies and the existence of Δ¹⁷O up to 0.5 ‰ can be interpreted as coming from the ash. We present sulfate mixing model in the sedimentary basin of lake Tecopa and calculate that up to 78% of sulfate in ash is derived from sediments. Mass-independent sulfur isotopic compositions was recalculated based on measured values and estimated proportions of mixing in the lake and is –0.35‰ for Δ³³S and +1.1 ‰ for Δ³⁶S. The slope of Δ³³S vs Δ³⁶S suggests that sulfate-forming reactions involved some photolysis of SO₂ in the stratosphere within high-altitude plinian plumes. Additionally, kinetic fractionation of sulfur isotopes by reaction of volcanic SO₂ with OH, and stratospheric oxidation of SO₂ has lead to significant ranges in isotope ratios. The largest caldera-forming eruptions have the highest Δ¹⁷O values, which may be due to stratospheric reaction with ozone-derived OH following exhaustion of tropospheric OH radicals. We estimate that the initial volcanic sulfate possessed the maximum undiluted +8‰ Δ¹⁷O value, signifying ozone as ¹⁷O source. We consider possibilities if these results imply that massive eruptions are capable of drying out the stratosphere from water (and OH) or watering it up as a result of warming up the tropopause and introducing the volcanic plume water, and finally the temporal depletion of the global ozone layer. If the ozone depletion is scaled with the amount SO₂ released during supereruption and with the Δ¹⁷O measured in the corresponding volcanic sulfate such depletion may be many times that of the measured eight percent depletion following 1991 Pinatubo eruption.