Constraints on the dynamics of pockmarks in the SW Barents Sea: Evidence from gravity coring and high-resolution, shallow seismic profiles

Newly acquired sediment cores and high-resolution seismic lines were studied in order to constrain the timing and mechanism of pockmark formation in the Loppa High area in the southwestern Barents Sea. The evolution of the pockmarks after their formation was also investigated, together with the possibility that they are currently active features. The cores retrieved within the pockmarks lack a layer of ice-rafted debris and an overlying sequence of laminated glaciomarine sediments that were deposited on the undisturbed seafloor at the inception of the Bّlling interstadial (~ 15 cal kyr BP). The pockmarks are suggested to have formed in that period, probably due to climate change-induced destabilisation of methane hydrates and the subsequent release of free gas. The depressions deepened as a result of reduced sedimentation above active seeps. The pockmark cores contain a series of gravel-rich layers, which are attributed to the winnowing of sediment during episodic fluid venting activity. The seismic images reveal little disturbance of the most ancient sediments underneath the pockmarks, indicating that the pockmark chimneys are localised features and were not imaged. The Holocene marine sediment cover in the pockmarks is thinner than on the surrounding seafloor, indicating relatively low sedimentation rates inside the depressions. The gas seepage may thus have been active until the recent past, although no indications of present-day methane flux were found.