Abstract
• CaSO4 and halogens in the Hunga volcanic cloud were mostly derived from seawater.
• Sea salts had a strong influence on the stratospheric volcanic cloud at Hunga.
• Small- and large-scale submarine eruptions frequently impact the stratosphere.
• Non-submarine eruptions can also inject seawater and sea salts in the stratosphere.
Submarine volcanic eruptions can form subaerial plumes that reach the stratosphere. Despite this, the impact of submarine eruptions on climate remains unclear due to a lack of clear geological record, with the recent large-scale Hunga eruption on 15 January 2022 being considered as an isolated case. Here, we review the impact of submarine and subaerial volcanoes in island or coastal settings (i.e., near seawater) on volatile/aerosol loading in the stratosphere. Isotopic δ34S signatures of the Hunga ash suggest that CaSO4 salts on the ash surface are dominantly formed by the evaporation of seawater during the eruption. We infer that SO2 scavenging on volcanic ash did not play a major role in the lower-than-expected SO2 detected in the volcanic cloud. Chlorine isotopic compositions (δ37Cl) also argue in favor of a seawater-derived origin of the chlorides from ash leachates. Combining petrological, leachate, isotopic and thermal analysis data, we demonstrate a near-absence of halogen degassing from the Hunga magma prior to and during the eruption, and conclude that the chlorine and bromine contents of tropospheric and stratospheric volatile species very dominantly derive from seawater/sea salts. We generalize our findings to all submarine eruptions, and large-scale, non-submarine eruptions in island or coastal settings, and propose that these may commonly form volcanic clouds that incorporate seawater thereby leading to the injection of related components (water vapor, sea salts, halogens) into the stratosphere. This makes seawater inputs a serious consideration when evaluating the long-term impact of volcanoes on climate.