THE EFFECT OF SEDIMENT DYNAMICS ON THE ECOLOGY OF THE NORTHWESTERN CONTINENTAL SHELF: GULF OF MEXICO

Sediment transport dynamics have a significant effect on physical, chemical, and biological processes on continental shelves and slopes. For the past four decades, short-lived radioisotopes have been used to determine transport and deposition on continental shelves. These data have allowed construction of transport models to predict changes that occur in response to varying environmental conditions.

Short-lived radioisotopes, clay mineralogy, and trace metal concentrations were measured in the sediments on the northwestern Gulf of Mexico shelf. Radioisotope concentration patterns define three centers of sediment accumulation: a large region at the shelf edge off the central Texas coast and two small areas off Matagorda Bay and at the mouth of the Rio Grande River. The clay mineralogy of bottom and suspended sediment assists in determining the origin of the persistent nepheloid layer and bottom sediment, by classifying the sediment into terrestrial and hemipelagic sources. Barium, a major component of drilling mud, tags sediment movement from areas of hydrocarbon exploration. Lead tags sediment originating from major urban areas on the coast. Manganese, an indicator of sediment diagenesis, is concentrated in areas of very slow sedimentation.Biologic data of the shelf show that the region of high sediment accumulation is also a region of high infauna productivity.

Combined data show that sediment transport and deposition on the shelf are the result of dynamic conditions produced by changing weather patterns. Prior to a frontal passage, the wind increases from the southeast, causing turbidity in coastal waters. With passage of the front, winds shift to the north and transport the turbid nutrient-rich coastal water offshore where the sediment load is deposited on the central shelf. It is this part of the shelf that is the most biologically productive.