THE HYDROGEOLOGY OF PASSIVE CONTINENTAL MARGINS: AN EVOLVING CONCEPTUAL MODEL (Invited Presentation)

Groundwater flow through the seafloor affects seawater chemistry, and the influence of this flow varies widely from the shoreline to the mid-ocean ridge. Near the shoreline, tidally-driven groundwater flow occurs over relatively small temporal and spatial scales (hours, meters) and influences nutrient and carbon budgets. At mid-ocean ridges, hydrothermal convection occurs on much larger temporal and spatial scales and influences the major ion chemistry of the ocean. Continental shelves are emerging as a region where groundwater flow has the potential to influence rapid nutrient and carbon cycling as well as the long-term evolution of the major ion chemistry of the ocean. New field evidence shows that variations in sea level drive nutrient- and Ra-enriched submarine groundwater discharge (SGD) tens of km from shore, much farther than commonly depicted in conceptual models. At continental shelf scales, new analysis suggests that flow associated with density gradients and overpressures could influence the major ion chemistry of seawater. The degree to which sea level rise affects these flow processes and chemical fluxes varies, largely related to the migration of the shoreline over a wide range of temporal scales. On human time scales, the most immediate effects are for groundwater flow and transport through intertidal wetlands, but coastal water supply is closely linked to the history of sea level rise over thousands of years.