Seafloor methane seepage under anoxic conditions in the Early Jurassic Laurasian Seaway

The Early Jurassic was punctuated by repeated intervals of enhanced organic matter preservation. Subsequent methane release from organic-rich strata via methanogenesis is locally recorded by seep structures. The geochemical characteristics of these seep structures remain poorly constrained. This study presents a comprehensive inorganic geochemical analysis of carbonate-cemented seep mounds from Sinemurian and Toarcian cold seep systems at Kilve (Somerset, U.K.) and Ravenscar (Yorkshire, U.K.), situated along the Laurasian Seaway. Iron speciation, redox-sensitive trace metals, and carbon and sulfur isotope analyses indicate that methane seepage at these mounds occurred under primarily anoxic ferruginous depositional conditions. Pyrite δ34S values suggest microbial sulfate reduction in active sulfate-methane transition zones (SMTZ), reflecting sulfate-driven anaerobic oxidation of methane (SD-AOM), partly under sulfate-limited conditions. The results suggest that methane flux from underlying organic-rich strata was partially mitigated by microbial processes within the sediment before reaching the water column. Variations in sulfate supply and methane may have influenced the efficiency of this methane filter and led to changes in the methane flux across the sediment-water interface.