GEOTHERMAL CONVECTION IN CONTINENTAL SHELVES: A SIGNIFICANT CONTRIBUTOR TO SEAWATER CHEMISTRY?

Geothermal convection in submarine settings has been studied with regard to volcanic and carbonate islands, generalized carbonate platforms, and mid-ocean ridges, but not in continental shelves in general. Coupled fluid flow and heat transport simulations were developed to explore density-driven fluid flow in a cross-section based on the continental shelf off the South Carolina coast. The two-dimensional simulation area extended 1000 km offshore and 10 km below sea level. Hydraulic properties representative of coarse, medium, and fine carbonates were tested, and porosity and permeability decreased with depth to account for sediment compaction.

Results show multiple geothermal convection cells associated with seafloor topography, but the number of flow cells decreases with decreasing sediment permeability. Maximum fluid velocities of 0.1-0.5 m/yr develop near steep areas of the seafloor, particularly at the continental slope. For medium-grained carbonates, the total simulated volume of fluid discharge in a 1 meter-wide slice of the continental shelf is approximately 60 m ³/yr. Based on this result, the total discharge along the 320 km stretch of the coast between Cape Fear and the Savannah River could exceed 10 ⁷ m ³/yr, roughly 3 orders of magnitude smaller than river discharge to the ocean. However, concentrations of rare earth elements and other chemical constituents are much higher in the altered seawater that discharges the continental shelf than in river water. Thus chemical transport through continental shelves could affect nutrient and chemical budgets in the ocean.