ABSTRACT

As part of the Offshore Ecology Investigation (OEI) of the Gulf Universities Research Consortium (GURC), water clarity and particulate matter in the South Timbalier OCS (Outer Continental Shelf) area offshore of central Louisiana were studied from 1972 to 1974. The principal purpose was to determine whether or not long-term oil production and associated platforms have altered the natural turbidity of continental shelf waters.

Turbidity of surface waters varied seasonally from a mean or 0.8 mg/1 in autumn 1972 to 5.3 mg/1 following the Mississippi River flood of 1973. Mid-depth waters were the clearest, typically containing 0.15 mg/1. A turbid bottom water layer persisted throughout the whole period; turbidity in this layer varied from 1.5 to >10 mg/1. No differences in turbidity existed between the area of numerous production platforms and an undeveloped control area.

Clay minerals filtered from surface, mid-depth, and bottom-water layers are quantitatively similar to Mississippi River suspended sediment, and dissimilar to the cohesive surface of the shelf sediments. Therefore, the bottom turbid layer is not generated by erosion of the cohesive bottom muds, but by the settled residuum of Mississippi River suspended sediment.

The turbid layer of bottom-water is actively by-passing the shelf at present. A process model is outlined, indicating that the suspended river sediment settles from the turbid surface plume through the mid-depth water, into the turbid bottom layer. There it migrates laterally toward shore, in a series of resuspensive pulses, merges with a surf-generated littoral turbid layer, and ultimately is trapped by vegetation in the prograding chenier plain of southwestern Louisiana. Offshore production platforms have no detectable effect on this process or on the turbidity of the central Louisiana shelf waters.