Modern and Fossil Pockmarks in the New England Mud Patch: Implications for Submarine Groundwater Discharge on the Middle Shelf

Abstract

AbstractGeophysical evidence is presented for the existence of modern and fossil pockmarks within the New England Mud Patch, an anomalous region of fine‐grained deposition on the middle shelf south of Cape Cod, USA. Modern pockmarks, indicators of focused subsurface fluid flow, are observed as seafloor depressions up to 80‐m wide and 1.2‐m deep, with reflective centers evincing hard material. Probable fossil pockmarks are identified by predominantly u‐shaped bright lenses within the sub‐bottom data and have similar size and distribution as the seafloor pockmarks; they likely indicate long‐term (>10 ka) persistence of episodic‐focused fluid flow. Given the lack of seismic evidence for gas, these features are attributed to submarine groundwater discharge. Based on recent observations and modeling, it is suggested that submarine groundwater discharge is induced by salinity‐driven convection cells associated with overpressured fluids, emplaced either by modern, shore‐connected aquifers, or during Pleistocene glacial advances and retreats.