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Fighting fire with water: microstructural and geochemical imprint of explosive volcanism in coral skeletons

James Vincent , Sebastian Flöter, Alexandra Tsay, Tom Sheldrake

Abstract

Characterising the eruptive history of a volcano is essential for hazard assessment and mitigation. On tropical volcanic islands however, terrestrial deposits are often limited as large quantities of tephra are directly deposited into the surrounding seawater or subsequently remobilised during weathering processes. Volcanic tephra leaches multiple chemical elements on contact with water, which significantly impacts local and regional environments. Coral reefs growing around volcanic islands in the tropics thus experience periodic disruptions from explosive volcanism. These disruptions influence the seawater geochemistry and ecophysiology of the coral organism, resulting in microstructural and geochemical imprints within their aragonitic skeletal archives. We sampled coral cores along the north-west coast of St. Vincent in July 2024, three years after the April 2021 eruption of La Soufrière. Element/Ca ratios were measured using LA-ICP-MS analyses and microstructural porosity investigated by micro-CT scanning. Our results show elevated Mn/Ca concentrations at the time of the eruption coinciding with elevated skeletal porosity. Mn is leached rapidly from tephra when deposited in seawater and is an important micronutrient used in photosynthesis and subsequently skeletal growth. The skeletal concentration of REE's elements such as Y, La and Nd are also elevated after the eruption and exhibit clearer seasonality decreasing in amplitude with each year after the eruption. We interpret this prolonged signal as an indication of continued tephra leaching from lahars and weathering during the rainy season. These results highlight the utility of massive corals in reconstructing past explosive volcanism, but challenges distinguishing primary tephra deposition from remobilisation due to lahars.