Foraminiferal microhabitats and test geochemistry near shallow gas fields in the RÍA of Vigo (NW Spain): Insights from recent sediment CORE records

The Galician rías are coastal embayments characterized by organic-rich sediments, heterogeneous pore-water geochemistry, and in several systems by shallow methane fields. However, the influence of methane-sediment interactions on the geochemical signature of benthic foraminifera in these rías remains unexplored. In this study, we investigate whether methane-influenced pore waters affect the isotopic composition of benthic infaunal foraminifera, and evaluate their relationship with microhabitat distribution. To this end, we analyzed foraminiferal microhabitat structure together with stable carbon (δ 13 C) and oxygen (δ 18 O) isotopes of the benthic infaunal species Bulimina gibba collected from three subtidal sediment cores in the surroundings of shallow gas fields in the Ría of Vigo (NW Spain). Benthic microhabitats are primarily dominated by infaunal species, and measured δ 13 C values range from − 8.05 ± 0.16 to − 0.79 ± 0.09‰, with the most negative values occurring in discrete, centimeter-scale intervals. A two-end-member mixing model provides a first-order estimate suggesting that the dissolved inorganic carbon incorporated into foraminiferal tests may derive from methane-influenced pore waters in specific depth intervals, while most levels reflect background organic-rich conditions. Our results indicate that infaunal foraminifera can record diffuse or intermittent methane seepage in coastal environments, even in very short sedimentary sequences.