The influence of cold seepage on the grain size and geochemistry of sediments from the Laptev Sea shelf

Abstract

The East Siberian Arctic shelf is renowned for its active methane-rich fluid seeps, confirmed by numerous gas flares in the water column and direct observations of methane release from the seafloor. These cold-seep sites serve as natural laboratories for studying the impact of methane seepage on early diagenesis, authigenic mineral formation, and the geochemical cycles of carbon, sulfur, and other elements. This study examines the grain size and chemical composition of two sediment types from the Laptev Sea shelf: cold-seep sediments influenced by sulfate-driven anaerobic methane oxidation, and unaffected “seep-free” sediments (i.e., control site). High methane concentrations (up to 649 μM/L) in the surface layer of sediments indicate that the sulfate–methane transition zone is found close to the sediment–water interface approximately 5–6 cm below the seabed. At the cold-seep site, thin layers containing up to 40% sand fraction were discovered among the fine-grained sediments. We infer that upward-migrating fluids may contribute to the remobilization of the fine fraction, resulting in concentration of coarse particles. Cold-seep sediments are characterized by molybdenum (Mo) enrichment compared to the lithological background, although one “seep-free” core exhibits evidence of authigenic Mo deposition. The Mo concentration (up to 16.8 ppm) indicates that enrichment occurs under sulfidic conditions restricted to pore water, which are caused by anaerobic methane oxidation. The high Mo concentration in one “seep-free” core likely implies past cold-seep activity. A general Mo–U covariation indicates that the molybdenum enrichment in cold-seep sediments from the Laptev Sea outer shelf is controlled by a weak particulate shuttle process.