HEAT AS A TRACER FOR SUBMARINE GROUNDWATER DISCHARGE (Invited Presentation)

Temperature has been used for decades to detect sub-seafloor groundwater flow systems ranging from large-scale geothermal convection to local submarine groundwater discharge (SGD). As in hyporheic systems on land, there is intense interest in developing thermal methods that can assess transport in zones affected by more than one flow process. MATTSI is a thermal inverse modeling code designed to detect hydrodynamic exchange (shallow benthic flushing) superimposed on larger groundwater flow systems. MATTSI calculates the depth and magnitude of dispersion required to match observed temperatures, creating a record that shows the timing and depth of thermal signals that do not conform to the standard governing equation for heat transport by conduction and advection. Field observations are recommended to identify the causes of these signals, potentially including hydrodynamic exchange, sediment transport, and burrow irrigation. MATTSI was first used to identify periodic transport reaching depths of ~8 cm from shallow thermal records collected 30 km offshore of the Georgia coastline. More recently, and in conjunction with chemical tracer studies, work in a wellfield installed offshore of Charleston, South Carolina, supports pervasive exchange of saline groundwater across the seafloor of the broad continental shelf and reveals summer pulses of groundwater discharge that may explain seasonal variations in Ra activities in the region. Current work on the seafloor of the continental shelf focuses on mapping flow through the zone of hydrodynamic exchange, as part of work to determine the fate of nutrients delivered to the ocean by SGD. We hypothesize that nutrients from saline SGD enter the food chain through benthic microalgae, which serve as key primary producers in continental shelves.