SO2 Oxidation Kinetics Leave a Consistent Isotopic Imprint on Volcanic Ice Core Sulfate

Abstract

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 SO2 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 Δ33S versus Δ36S and δ34S versus Δ33S slopes to be constrained at −1.56 (1σ = 0.25) and 0.09 (1σ = 0.02), respectively. The Δ33S versus Δ36S slope refines a prior determinations of Δ36S/Δ33S = −4 and overlaps the range observed for sulfur seen in early Earth samples (Archean). In recent volcanogenic sulfate, the Δ33S versus δ34S differs, however, from the Archean record. The similitude for Δ36S/Δ33S and the difference for Δ33S/δ34S 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.