Geochemical evidence for seabed fluid flow linked to the subsea permafrost outer border in the South Kara Sea

Abstract

Driven by rising bottom water temperatures, the thawing of subsea permafrost leads to an increase in fluid flow intensity in shallow marine sediments and results in the emission of methane into the water column. Limiting the release of permafrost-related gas hydrates and permafrost- sequestered methane into the global carbon cycle are of primary importance to the prevention of future Arctic Ocean acidification. Previous studies in the South Kara Sea showed that abundant hydro-acoustic anomalies (gas flares) induced by seafloor gas discharge into the water column occur in water whose depth is ≥20 m. This distribution of gas flares could indicate the outer extent to which continuous permafrost restricts upward fluid flow. This paper reports on a geochemical analysis of a 1.1 m long sediment core located in an area of shallow fluid flow off of the Yamal Peninsula coast (South Kara Sea) using high-resolution seismic data. Our results reveal a thin zone of Anaerobic Oxidation of Methane (AOM), a sharp shallow sulfate-methane transition (SMT) located at a sub-bottom depth of 0.3 m, and significant temporal variation in methane discharge confirmed by the pyrite (FeS2) distribution in the core sample. A concave up pore water chloride profile depicts upward fresh/brakish water advection in subsurface sediments. The terrestrial/fresh water genesis of methane from the sampled core is deduced from the stable isotopic signatures (δ13 C and δD). We propose two mechanisms for the observed fluid flow: i) convection of thaw water from subsea permafrost; and/or ii) lateral sub-permafrost ground water discharge marking the outer extent of continuous permafrost off of the central Yamal Peninsula coast at ˜45 m water depth.