A New Multi‐Method Assessment of Stratospheric Sulfur Load From the Okmok II Caldera‐Forming Eruption of 43 BCE

Abstract

The 43 BCE eruption of Okmok Volcano has been proposed to have had a significant climate cooling impact in the Northern Hemisphere. In this study, we quantify the climate cooling potential of the Okmok II eruption by measuring sulfur concentration in melt inclusions (up to 1,606 ppm) and matrix glasses and estimate a total of 62 ± 16 Tg S released. The proportion reaching the stratosphere (2.5%–25%, i.e., 1.5–15.5 Tg S) was constrained by physical modeling of the caldera‐collapse eruption. Using the NASA Goddard Institute for Space Studies E2.2 climate model we found a linear response between cooling and stratospheric sulfur load (0.05–0.08°C/Tg S). Thus, the 1–2°C of cooling derived from proxy records would require 16–32 Tg sulfur injection. This study underscores the importance of combining approaches to estimate stratospheric S load. For Okmok II, we find all methods are consistent with a range of 15–16 Tg S.

On the Budget and Atmospheric Fate of Sulfur Emissions From Large Volcanic Eruptions

“…Even with robust measurements of stratospheric sulfur burdens, disambiguating the climatic signatures of external forcing (e.g., volcanism) from internal variability is fraught with difficulties (e.g., Bengtsson & Hodges, 2019;DallaSanta & Polvani, 2022;Mann et al, 2022). Peccia et al (2023) relied on maximum concentrations of sulfur recorded in MI to constrain their volatile budget. However, a critical challenge for such petrologic approaches is our still limited knowledge of the pre-eruptive behavior of volatiles during magma storage in the lithosphere.…”

“…The extent to which these two approaches can give different results is highlighted in a new study by Peccia et al. (2023), who show how petrologic estimates of eruptive sulfur yield can be a poor guide to the climatically‐effective amount of sulfur that reaches the stratosphere. In particular their analysis shows that a significant component of emitted sulfur does not cross the tropopause and thus experiences a much reduced atmospheric lifetime and transport.…”

“…In their new study, Peccia et al. (2023) employed the petrologic approach to probe the sulfur budget of a less renowned explosive eruption that of Okmok volcano in Alaska, dated to 43 BCE. Of note, they argued that only around a quarter of the estimated 62 Tg of gaseous sulfur released on eruption crossed the tropopause.…”

“…Peccia et al. (2023) relied on maximum concentrations of sulfur recorded in MI to constrain their volatile budget. However, a critical challenge for such petrologic approaches is our still limited knowledge of the pre‐eruptive behavior of volatiles during magma storage in the lithosphere.…”

“…Clues can be found in the pumice deposits found around a volcano and, perhaps more surprisingly, in the ice caps of the polar regions, which receive the eventual fallout from large eruptions. Both approaches have limitations that are addressed in a recent work by Peccia et al (2023,…”