Pockmarks on the South Aquitaine Margin continental slope: the seabed expression of past fluid circulation and bottom currents

Abstract : Inactive and mostly elongated pockmarks of 100-200 m in dimension were recently discovered on the South Aquitaine Margin continental slope. They are distributed at water depths greater than 350 m in both interfluve and sediment wave areas, and are strongly controlled by the sedimentary morphology and architecture. Water column and seafloor backscatter and sub-bottom profiler data do not exhibit present-day or past gas evidence, e. g. massive and continuous gas releases at the seabed and fossil methane-derived authigenic carbonates. It is thus proposed that the pockmarks originated from a shallow source and result from relatively recent and short-duration gas or water expulsion events. Former near-bottom currents may have contributed to the elongation of these WNW-ESE oriented pockmarks whereas present-day weaker near-bottom currents may induce upwelling, contributing to the maintenance of the elongated shapes of the pockmarks.