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 mag nitude over the 2 years of study. Individual sediment components vary independently and can be described as cyclic, steadily increasing, ran dom, or unchanging. Many of the variations are linked to influxes of t errestrial material associated with river discharge, non-point terrest rial run-off and/or outflow from coastal environments during storm eve nts. Carbonate content and grain size varied from clay-rich fine-grain ed sediments associated with the Mississippi River delta complex to co arse-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 b etween the 100 and 200 m isobaths. TOC values in excess of 2% were obs erved, however, average TOC concentrations were 0.8%. Hydrocarbons in sediments were present at low concentrations and are a mixture of biol ogical and petroleum hydrocarbons. Terrestrial plant biowaxes were ubi quitous and, when present, petroleum hydrocarbons were associated with elevated barium concentrations. Aromatic hydrocarbon compositions wer e indicative of unprocessed petroleum and were dissimilar to combustio n PAH detected in adjacent bays. PAH concentrations were as much as si x times lower than adjacent coastal sediment concentrations. Spatial a nd temporal heterogeneity in sediments is due to variations in inputs, preservation, diagenetic alteration and oceanographic setting.