Hydrothermal calcite formation in Campi Flegrei caldera, Italy: unraveling carbon sink processes in alkaline volcanic systems.

Understanding carbon dioxide emissions variability in volcanic regions is vital for detecting instabilities in the subvolcanic plumbing system, crucial for managing both volcanic and environmental risks. While changes in magmatic sources drive these variations, non-magmatic processes can complicate signal interpretation, especially in caldera environments. Here, geothermal systems can sequester CO within the bedrock through hydrothermal calcite precipitation, significantly impacting surface-level CO emissions. Unfortunately, few studies have explored this phenomenon, examining hydrothermal calcite origins and their effects on carbon balances and temporal gaseous patterns in active volcanic settings. Our study developed a specialized methodology for quantifying CO sequestered in hydrothermal calcites within alkaline caldera systems. We focused on analyzing hydrothermal calcite in lithics from volcanic deposits of eruptions of varying ages, Volcanic Explosivity Index (VEI), and eruptive vent locations to enhance the representativeness of the entire caldera bedrock. Unlike core samples from geothermal wells, which are infrequent and limited to specific depths, lithics can be easily collected, offering a comprehensive understanding of CO sequestration. Through extensive 3D textural characterization and isotopic investigations on hydrothermal calcite within lithic fragments from selected alkaline volcanic deposits in the Campi Flegrei caldera, our findings emphasized the significant influence of calcite sinks on the overall CO budget released by volcanoes throughout their evolution.