Early stages impact of volcanism on sedimentary diagenesis: The case of the newly born volcano, Fani Maoré

Abstract

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 for a comprehensive overview of the diagenetic alterations occurring within sediments in response to this volcanic event. Solid phase characterization using X-ray fluorescence and diffraction, particulate organic carbon (POC) content, carbon isotopic signature (δ¹³C-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 (δ¹³C-DIC; ∆¹⁴C-DIC) and ⁸⁷Sr/⁸⁶Sr ratio followed by carbonate precipitation at depth in the sedimentary pile. This study demonstrated that the interaction between magmatic CO₂ degassing and volcanic rocks leads to natural carbonation at depth, as evidence by vertical profiles that differ from those that can be described in more established volcanic systems. These findings enhance our 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.