SO_2 Oxidation Kinetics Leave a Consistent Isotopic Imprint on Volcanic Ice Core Sulfate

摘要

This work presents measurements of time-resolved mass-independently fractionated sulfate of volcanic origin from Antarctic ice core records that cover the last 2,600 years. These measurements are used to evaluate the time dependence of the deposited isotopic signal and to extract the isotopic characteristics of the reactions yielding sulfate from stratospheric volcanic eruptions in the modern atmosphere. Time evolution of the signal in snow (years) with respect to the fast SO_2 oxidation in the stratosphere suggests that photochemically produced condensed phase is rapidly and continuously separated from the gas phase and preserved during transportation and deposition on the polar ice cap. On some eruptions, a nonzero isotopic mass balance highlights that a part of the signal can be lost during transport and/or deposition. The large number of volcanic events studied allows the Δ~(33)S versus Δ~(36)S and δ~(34)S versus Δ~(33)S slopes to be constrained at -1.56 (1σ = 0.25) and 0.09 (1σ = 0.02), respectively. The Δ~(33)S versus Δ~(36)S slope refines a prior determinations of Δ~(36)S/Δ~(33)S = -4 and overlaps the range observed for sulfur seen in early Earth samples (Archean). In recent volcanogenic sulfate, the Δ~(33)S versus δ~(34)S differs, however, from the Archean record. The similitude for Δ~(36)S/Δ~(33)S and the difference for Δ~(33)S/δ~(34)S suggest similar mass-independently fractionated sulfate processes to the Archean atmosphere. Using a simple model, we highlight that a combination of several mechanisms is needed to reproduce the observed isotopic trends and suggest a greater contribution from mass-dependent oxidation by OH in the modern atmosphere.