MULTI-SCALE FLUID FLOW IN THE NEAR-SHORE SEAFLOOR

Interactions between shallow and deep fluid flow systems extend beyond continental settings to the coastal seafloor, where seafloor fluid flow can be an important control on porewater biogeochemistry and chemical budgets in the coastal ocean. Shallow flow in this setting is dominated by tides, waves and ocean-current interactions with seafloor topography (ripples). At a larger scale, the coastline also represents a fundamental regional discharge zone for large-scale fresh water and buoyancy-driven saline flow systems. Thermal and geochemical data from offshore piezometers suggest that seafloor fluid flow may be controlled alternately by shallow ocean interactions and long-term upward flow during different periods, but controls on long-term flow from depth have not been widely explored. Numerical flow and transport models were used to compare large-scale hydrogeologic driving forces that exist in coastal settings and to determine the possible depth and extent of these regional flow systems. In the simulations, the large-scale flow systems contributing to SGD spanned 10-20 km surrounding the coast and continental slope and reached depths exceeding 1 km. Land surface and seafloor topography for the models were based on cross-sections of North Carolina, but sediments were assumed to be a single rock type to aid the comparison of driving forces. Simulations were also based on the assumption that sea level and the simulation domain remain static. In reality, regional flow is affected by sediment heterogeneity and may vary over time scales of months (seasonal variations) to thousands of years (sea level change). In contrast, shallow flow systems operate on time scales of hours to days. Mixing between old, reducing, chemically evolved waters and young, oxidizing incoming seawater suggests an important biogeochemical interface.