Fate of vent-derived methane in seawater above the Haakon Mosby mud volcano (Norwegian Sea)

The Ha kon Mosby mud volcano (HMMV) is a cold methane-venting seep situated at the Norwegian-Barents-Spitsbergen continental margin. Methane discharged by the vent creates a plume in the ambient seawater at 1200 m water depth. Here, we study the hydrographic regime to evaluate its influence on the distribution pattern of methane as it is shown by the concentration gradients. The stable carbon isotopic signature of methane is used to trace the vent methane in the lateral and vertical direction and to trace its fate in the hydrosphere. Direct methane release into the bottom water occurs in the central zone of the HMMV. Methane included in the subseafloor reservoir and in the plume above the vent has the same carbon isotopic ratios, which means that the released methane is not oxidized. The shape of the methane plume is determined by spreading predominantly along its original isopycnal. However, vent methane is traceable in seawater up to 800 m above the HMMV by methane values, which exceed the background and high carbon isotopic signature heterogeneity. The fate of vent methane in the hydrosphere is dominated by dilution and mixing with background methane within the bottom water rather than by oxidation as it is shown by the carbon isotopic ratios. Methane released by the HMMV moves northward with deep intermediate waters, which are detached from the ocean surface and may enter the polar Arctic Ocean. Consequently, methane discharged at the HMMV results in a direct input of fossil methane into the Recent methane reservoir of the deeper ocean and reduces the capacity of the deep ocean as a sink for atmospheric methane.