Geochemical characterization of gas emission sites for volcanic surveillance at Deception Island, Antarctica

Abstract

Deception Island is the most active volcano in the Bransfield Strait region (South Shetlands Islands, Antarctica), where the latest eruptions occurred between 1967 and 1970. This horseshoe-shaped island hosts a magmatic-hydrothermal system, developing areas with thermal anomalies and gas emissions primarily along the internal seacoast. The manifestations in Primero de Mayo bay constitute the largest fumarole field on the island showing the highest temperature gas emission (90-100ºC). Previous studies have shown that gas emissions from this area are enriched in acid gases CO2 and H2S, contrasting with other hydrothermal areas on the island, where no species of S were detected. The absence of magmatic gases (SO2-HCl-HF) in fumarolic compositions suggests gas scrubbing by a hydrothermal reservoir.Our study aims to generate a comprehensive gas-water chemistry dataset to characterize the Deception Island volcanic system and establish a baseline for future monitoring efforts. From 2022 to 2024 Antarctic summer (January-March), we have sampled gases by direct sampling alkaline trap (NaOH – ZnAc) and waters from hydrothermal sites. Gas samples collected in 2024 were analyzed for CO2, H2S and water alkalinity at the Deception Island Argentine Station. For Primero de Mayo bay we found vapor/gas ratios averaged on 90%, with CO2 and H2S values reaching up to 13% and 0.7%, respectively, resulting on CO2/H2S ratios between 40 and 110. Additionally, we detected for first time the presence of H2S on Whalers Bay. Water pH and alkalinity on hydrothermal sites ranged between 5.6 to 6.9, and 120 to 470 mg/l CaCO3, respectively. The results obtained in 2024 show gas composition and water alkalinity similar to previous data (2022 and 2023). Developing a robust compositional baseline including isotopic information, as well as simple and rapid resolution sampling and analysis techniques, would greatly contribute to volcanic surveillance efforts and the feeding system undersanting on Deception Island.