TEMPORAL AND SPATIAL VARIATIONS IN SEDIMENT CHARACTERISTICS ON THE MISSISSIPPI-ALABAMA CONTINENTAL-SHELF

Sediment characteristics on the Mississippi-Alabama shelf are dynamic and change in time scales varying from less than 6 months to more than 2 years. Some sediment properties varied by more than an order of magnitude over the 2 years of study. Individual sediment components vary independently and can be described as cyclic, steadily increasing, random, or unchanging. Many of the variations are linked to influxes of terrestrial material associated with river discharge, non-point terrestrial run-off and/or outflow from coastal environments during storm events. Carbonate content and grain size varied from clay-rich fine-grained sediments associated with the Mississippi River delta complex to coarse-grained shell hash on the eastern shelf. Organic carbon content, extractable organic matter and hydrocarbons were elevated in sediments near the Mississippi River delta complex and in a band of sediments between the 100 and 200 m isobaths. TOC values in excess of 2% were observed, however, average TOC concentrations were 0.8%. Hydrocarbons in sediments were present at low concentrations and are a mixture of biological and petroleum hydrocarbons. Terrestrial plant biowaxes were ubiquitous and, when present, petroleum hydrocarbons were associated with elevated barium concentrations. Aromatic hydrocarbon compositions were indicative of unprocessed petroleum and were dissimilar to combustion PAH detected in adjacent bays. PAH concentrations were as much as six times lower than adjacent coastal sediment concentrations. Spatial and temporal heterogeneity in sediments is due to variations in inputs, preservation, diagenetic alteration and oceanographic setting.