Methane-related carbonates formed at submarine hydrothermal springs: a new setting for microbially-derived carbonates?

Numerous small calcite mounds, up to 2.5 m in diameter and 0.75 m in height, accompanied by Ba, Hg and Tl mineralisation, occur in shallow submarine hydrothermal vents on the sea bottom, at 10 m depth, near Punta Mita, on the western coast of Mexico. The hydrothermal activity consists in water and gas (mainly nitrogen and methane) venting at 85°C, through a 100-m-long fissure hosted in basaltic rocks and partially covered by a thin layer of unconsolidated detrital sediments. The mounds consist of travertine-like calcite aggregates that develop around the main submarine hot springs amidst a hydrothermally altered basaltic host rock. Two main calcite generations are texturally recognisable: the first generation shows a radial-fibrous texture; the second is fine-grained calcite, which cements detrital grains and fills the pore spaces. The δ13C analyses of calcite reveal a strong depletion in 13C, with values as low as −39.2‰ (Vienna PeeDee Belemnite), which suggest that microbial communities may have induced calcite precipitation through microbial methane oxidation. Barite, sulphides (mainly pyrite and cinnabar) and phosphates (carbonate–hydroxylapatite) are also present in the mounds in lower concentrations and form by direct precipitation from the hydrothermal fluid. The Punta Mita hydrothermal carbonate mounds represent a potentially novel environment for microbially induced carbonate mineralisation, which is characterised by high temperatures not encountered in areas of cold seep carbonate formation. Stable isotope results suggest that microorganisms responsible for the oxidation of methane may be present and active at temperatures near 85°C at the Punta Mita vents.