Submarine volcanic eruptions impact ocean and sediment geochemistry by generating hydrothermal plumes, fluid expulsion, lava flows, and tephra deposition. The amount and impact of such inputs on the sediment composition and biogeochemistry, on short temporal scales, is yet to be quantified. To this end, the multidisciplinary GEOFLAMME cruise was conducted in May 2021 on board the R/V Pourquoi Pas? to study the Fani Maoré edifice, a newly born volcano located on the eastern submarine flank of Mayotte (Comoros archipelago, Indian Ocean). The strategy was to sample sediment cores along two eastward transects from this volcano towards the open ocean. This study aims to allows a comprehensive overview of the diagenetic alterations occurring within sediments in response to the volcanic event. Solid phase characterization using X-ray fluorescence and diffraction, particulate organic carbon (POC) content, carbon isotopic signature (d13C-POC; ∆14C-POC) and pore water analysis (dissolved inorganic carbon-DIC; Alkalinity; NO3–+NO2–; SO42- ; dFe; dMn) reveals low organic matter mineralization and low volcanic material alteration throughout the area. However, close to the volcano and lava flows, DIC, alkalinity and Mg2+ in the sediment show a strong increase at depth while Ca2+ and Sr2+ show a strong depletion. We attribute these concentration gradients to the diffusion of a magmatic CO2-rich fluid with a geochemical signature reflecting strong fluid-rock interactions as evidenced by carbon isotopic signature of DIC (d13C-DIC; ∆14C-DIC) and 87Sr/86Sr ratio followed by carbonate precipitation at depth in the sedimentary pile. This study therefore allows for a better understanding of the sediment biogeochemistry in a volcanic eruptive context, and offers perspective for a better use and development of new volcanic proxies in marine sediments.
