Spring 2008 Seminar Series

Dr. Alexandra Rao

Abstract

Biogeochemical cycles of carbon, trace metals and nutrient elements in permeable coastal and shelf deposits are uniquely impacted by fluid advection associated with groundwater discharge and pressure gradients generated by waves, tides, and topographically-induced deflection of bottom currents. To investigate elemental cycling in coastal and shelf sands in the presence of pore fluid advection, column reactor experiments have been conducted with homogenized sediments flushed with natural, amended and artificial solutions. Column experiments with surface sands from the South Atlantic Bight continental shelf have shown high rates of organic matter respiration and efficient N removal in these deposits. Laboratory column reactor experiments are also being conducted with aquifer sands from Waquoit Bay, MA to investigate the factors controlling the flux of Ba associated with submarine groundwater discharge, including seasonal fluctuations in salinity, pH, O2, DOC, and metal (Mn, Fe) redox cycling in the subterranean estuary. Submarine groundwater discharge (SGD) also contributes significant fluxes of nutrients to the coastal ocean, which are controlled by inputs to coastal aquifers and geochemical processes that alter aquifer fluid composition in the freshwater-seawater mixing zone. Field sampling in the northwestern Yucatán Peninsula, México revealed the geochemical composition of surface water and porewater in coastal lagoons and aquifer fluids, and groundwater-derived nutrient fluxes to the coastal ocean estimated using radiogenic tracers of SGD. Together, these studies show that the interaction of fluid advection and biogeochemical transformations in permeable coastal sediments may present important challenges to our understanding of biogeochemical cycling in ocean margins.