Giant Pit Craters in Scotia Sea, off Antarctica: A Modern Analog for Seafloor Gas Ejection from Intrusive Magma Hydrothermal Vent Complexes

Abstract Massive injection of 13C depleted carbon to the ocean and atmosphere coincided with major environmental upheaval multiple times in the geological record. For several events, the source of carbon has been attributed to explosive venting of thermogenic gas produced when volcanic sills intrude organic-rich sediment. The concept mostly derives from studies of sedimentary basins with ancient hydrothermal vent complexes (HTVCs), because good examples younger than the Early Eocene were not known. We present geophysical data documenting at least 150 large (km-scale) craters on the modern seafloor across ~148,000 km2 of Scan Basin in the southern Scotia Sea, offshore Antarctica. Seismic and bathymetric information further reveals the craters relate to saucer-shaped igneous sills and vertical fluid pipes. Magmatic intrusions produce thermogenic gas, where buoyant hydrothermal fluids migrate upwards from sill flanks through V-shaped gas chimneys. Dome-shaped gas reservoirs develop, deforming overlying sediment and forming forced-folds. Fluid expulsions, driven by excess pore pressure, generate vertical conduits that cause blow-out collapses on the seafloor. Age estimates for crater formation come from correlations of seismic reflectors with bore hole data from recently drilled IODP Expedition 382. Based on this, we suggest a minimum age of 900 ka (Early Pleistocene) for the onset of magma intrusion and first formation of V-shaped craters on the flanks of sills. Subsequent crater reactivations have occurred since, perhaps episodically. Importantly, numerous craters related to sills and gas chimneys populate the seafloor above a sedimentary basin but this does not necessitate rapid and massive release of gas. A crucially important result is that this modern oceanic HTVCs system showing giant craters as present-day seafloor expression needs consideration when interpreting ancient HTVCs and gas expulsion from the seafloor.