August GEOLOGY and GSA TODAY Media Highlights

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.

Plateau collapse model for the Transantarctic Mountains–West Antarctic Rift System: Insights from numerical experiments
Robert W. Bialas et al., Lamont Doherty Earth Observatory of Columbia University, Earth and Environmental Science, PO Box 1000, 61 Route 9W, Palisades, New York 10964, USA. Pages 687-690.

The Transantarctic Mountains are the highest and longest rift-related mountain belt on Earth and divide the Antarctic continent into its eastern and western halves. Despite their prominence, no model has been able to adequately explain their formation or their juxtaposition with the adjacent West Antarctic Rift System, a broad region of thin, extended continental crust exhibiting wide rift characteristics. Bialas et al. explore the possibility that the Transantarctics represent a remnant edge of a high-elevation plateau that has since rifted and subsided to create the West Antarctic Rift System. This concept revolutionizes the thinking about the Transantarctic Mountains. Previous models discuss ways to make the mountains go up; Bialas et al. propose a scenario where the Transantantarctic Mountains were already high, and the adjacent rift system goes down. They use numerical models and geological and geophysical data to back this new theory of Transantarctic Mountain development.

Late Oligocene initiation of the Antarctic Circumpolar Current: Evidence from the South Pacific
Mitchell Lyle et al., Texas A&M University, Department of Oceanography, 3146 TAMU, College Station, TX 77843-3146, USA. Pages 691-694.