Sediment Deposition and Reworking: A Modeling Study using Isotopically Tagged Sediment Classes

A sediment transport model within the Regional Ocean Modeling System (ROMS) was used to examine how repeated cycles of deposition, erosion, and bioturbation influence flood and storm event bed character offshore of a significant fluvial source. Short-lived radioisotopes Beryllium-7 (7Be) and Thorium-234 (234Th) can be used as tracers of deposition and reworking on the continental shelf, and modeled profiles of these radioisotopes, along with simulated profiles of sediment bed grain size distributions, were analyzed for various model runs.The presence of an atmospherically derived radionuclide,7Be, in seafloor sedimentindicates terrestrial (riverine derived) sediment deposition offshore of a fluvial source.In contrast,234Th naturally occurs in seawater through the decay of its generally conservative parent, 238U, and its presence in the seabed indicates the recent suspension of sediment in oceanographic water. Simulated profiles of 7Be and 234Th weredirectly related to the flood and storm sequences used as model input.The model results showedthat the radioisotopic profiles are sensitive to the timing of 7Be input, phasing of wave and current energy, and intensity of bioturbation; complicating the relationship between simulated profiles andmodel input of flood and hydrodynamic forcing. Sediment grain size and geochronological tracers were used as markers of event beds for flood and storm deposition scenarios

Sediment Transport Model Including Short-Lived Radioisotopes: Model Description and Idealized Test Cases

Geochronologies derived from sediment cores in coastal locations are often used to infer event bed characteristics such as deposit thicknesses and accumulation rates. Such studies commonly use naturally occurring, short-lived radioisotopes, such as Beryllium-7 (Be-7) and Thorium-234 (Th-234), to study depositional and post-depositional processes. These radioisotope activities, however, are not generally represented in sediment transport models that characterize coastal flood and storm deposition with grain size patterns and deposit thicknesses. We modified the Community Sediment Transport Modeling System (CSTMS) to account for reactive tracers and used this capability to represent the behavior of these short-lived radioisotopes on the sediment bed. This paper describes the model and presents results from a set of idealized, one-dimensional (vertical) test cases. The model configuration represented fluvial deposition followed by periods of episodic storm resuspension. Sensitivity tests explored the influence on seabed radioisotope profiles by the intensities of bioturbation and wave resuspension and the thickness of fluvial deposits. The intensity of biodiffusion affected the persistence of fluvial event beds as evidenced by Be-7. Both resuspension and biodiffusion increased the modeled seabed inventory of Th-234. A thick fluvial deposit increased the seabed inventory of Be-7 and Th-234 but mixing over time greatly reduced the difference in inventory of Th-234 in fluvial deposits of different thicknesses

A model archive for a numerical model of geochronological tracers for sediment deposition and reworking applied to the Mississippi subaqueous delta

This dataset includes model input, code, and output used in the publication Birchler et al. (2018, Journal of Coastal Research), which used a coupled hydrodynamic-sediment transport-geochemical model to investigate the roles of resuspension, deposition, on biodiffusion on the behavior of short-lived radioisotopes on the Mississippi sub-aqueous delta, USA. Model development for this project focused on incorporating radioisotope tracers into the sediment transport module in the Regional Ocean Modeling System (ROMS). As described in Birchler et al. (2018, Journal of Coastal Research), the model can account for supply and sorption of radioisotope tracers in the water column; biodiffusion of radioisotope tracers within the seabed; storage of radioisotope activity and inventory in the seabed; radioisotope decay in the seabed; and resuspension and removal of radioisotope tracers

A model archive for sediment transport model including short-lived radioisotopes: Model description and idealized test cases

This dataset includes model input, code, and output used in the publication Birchler et al. (2018, Journal of Marine Science and Engineering), which used a coupled hydrodynamic-sediment transport-biogeochemical model to investigate the roles of resuspension, deposition, on biodiffusion on the behavior of short-lived radioisotopes in an idealized one-dimensional model setting. Model development for this project focused on incorporating radioisotope tracers into the sediment transport module in the Regional Ocean Modeling System (ROMS). As described in Birchler et al. (2018, Journal of Marine Science and Engineering), the model can account for supply and sorption of radioisotope tracers in the water column; biodiffusion of radioisotope tracers within the seabed; storage of radioisotope activity and inventory in the seabed; radioisotope decay in the seabed; and resuspension and removal of radioisotope tracers