ScienceDaily (July 27, 2007) — Hydrothermal circulation in the oceanic basement has widespread effects on seafloor heat flow, ocean chemistry, and geologic carbon cycling.

The occurrence of vigorous water exchange between the ocean and igneous crust depends on whether the high-permeability basaltic basement is hydrologically connected to the seafloor or blocked by low-permeability sediments. Typically, basement edifices that outcrop at the seafloor form the discharge sites for circulating fluids.

The recent observation of curious closed depressions in carbonate sediments overlying basement edifices suggests a process that prolongs seafloor exposure of hydrothermally active basement in thickly sedimented areas.

Formation of these depressions may involve dissolution by discharging basement fluids that have cooled to bottom-water temperatures and have become undersaturated with carbonate. Bekins et al. provide a model of carbonate solubility to estimate the discharging fluid flux required to keep pace with typical equatorial Pacific carbonate mass accumulation rates.

The resulting values are comparable to other published basement fluxes. Recent data from other researchers showing widespread occurrence of these dissolution features in the eastern equatorial Pacific help explain why there are so many indications of vigorous basement ventilation in this region, despite the thick sediment cover that would normally block basement outcrops, including anomalously low regional heat flux, and aerobic and nitrate-reducing microbial activity at the base of the sediments.

Reference: Dissolution of biogenic ooze over basement edifices in the equatorial Pacific with implications for hydrothermal ventilation of the oceanic crust  Barbara A. Bekins et al., U.S. Geological Survey,  Geology, Pages 679-682.

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