Numerical modeling of the impact of sediment driven processes on bottom water chemistry over the Louisiana continental shelf

In the northern Gulf of Mexico’s hypoxic zone, previous results have demonstrated that in the summer, acidified bottom waters are confined to a thin bottom boundary layer where the production of CO2 is dominated by benthic metabolic processes. Recently developed numerical models for this region have estimated benthic fluxes of dissolved inorganic carbon as a function of sediment O2 consumption, which in turn has been parameterized as a function of temperature. However, these models often overestimated seabed oxygen consumption. Accordingly, coupled studies based on models that include a more process-based representation of the seabed biogeochemistry are essential. Here, a one-dimensional, fully coupled hydrodynamic, sediment transport, and biogeochemical model was developed to represent the summer hypoxic zone on the Louisiana continental shelf to evaluate the degree to which sediment and resuspension events supply benthic flux of dissolved inorganic carbon to the overlying water column. The model, developed within the Regional Ocean Modeling System in the Coupled Ocean-Atmosphere-Wave-and-Sediment Transport modeling framework, was run to represent summer conditions when field observations were available. The model estimates bed roughness, total bed shear stress, and skin friction shear stress, which are important to the estimates of near-bed turbulence and suspended sediment concentrations. These values are compared with observations from four locations on the shelf, varying from about 20 m to 50 m depth. Results show that this model provided reasonable estimations. Then the model was run with inclusion of biogeochemical reactions within the seabed to account for flux of dissolved inorganic carbon and alkalinity. Preliminary results revealed that sediment-associated processes played an important role in contributing DIC to the bottom waters over the shelf, which decreased pH and carbonate saturation states.