Near-surface hydrocarbon anomalies in shelf sediments off Spitsbergen: Evidences for past seepages

As global warming occurs, the dissociation of bound methane on Arctic shelves due to ocean current temperature changes may become a major contributor to the global methane budget, and thus contribute to strong positive climate feedback mechanisms. However, little is known about the magnitude and fate of methane emissions from shallow submarine sediments to the atmosphere in the peculiar area. In this paper, we present one of the first direct evidences for seepage on the northwestern Barents Sea shelf. By studying the molecular and isotopic signatures of low-molecular-weight hydrocarbons in seawater, near-surface sediment pore space and the sediment matrix at 26 locations, we provide a detailed view on the partitioning of gaseous hydrocarbons in the sediment-water interface off Spitsbergen. In the free gas phase, low concentration of methane (similar to28 ng/g wet Sediment) paired with constantly high isotopic values (similar to-65%) is consistent with high impact of methane oxidation on the isotopic composition. In contrast, high concentrations of adsorbed CH4 (up to 5292 ng/g wet <63 μm) and C2+ (ethane through pentane) ( up to 1724 ng/g wet <63 mum) in the sediment matrix suggest that the adsorbed gas is reasonably well protected against microbial degradation. Moreover, the isotopic and molecular signatures of adsorbed CH4 (-38-60%; similar to100-similar to5300 ng/g wet <63 μm) and C2H6 (-20--36%; ∼30-∼1700 ng/g wet <63 mum) indicate an integrated, but strongly varying signal of historic thermally derived hydrocarbon plumes plus in situ adsorbed gas of biological origin. This suggests a minimal exchange and unlike histories of free and adsorbed gas. The majority of gaseous hydrocarbons in the adsorbed pool may be the result of past seepages. Past upward flow of thermogenic gas and impregnation with mature hydrocarbons was particularly strong along tectonic lineaments. Active petroleum source rocks along the continental margin and migration of gaseous hydrocarbons at re-activated fault systems might explain the anomalies along these conduits.