Abstract: Internal Waves and Nepheloid Layer on Continental Shelf, Gulf of Mexico

David W. McGrail, Richard Rezak

The nepheloid layer on the South Texas continental shelf exhibits a distinct relation to a mixed (isohaline-isothermal) layer at the base of the water column. This mixed layer is a boundary phenomenon produced by turbulence generated during the adjustment of the various modes of motion to the no-slip condition at the shelf and sea interface. The boundary layer is capped by markedly stratified water which inhibits further upward mixing. Sediment, either advected from nearshore regions or resuspended locally, is limited primarily to this mixed layer. There are, however, places where layers containing suspended sediment maxima occur above and isolated from the mixed layer. These usually occur at some step in the temperature and salinity profiles. A simple flow-visualization experiment ca ried out proximal to Southern and Hospital Banks offers both an explanation for the isolated maxima and a mechanism for substantial vertical diffusion of suspended sediment. The experiment consisted of recording the behavior of plumes shed from dye packets deployed within 1 m of the bottom from the R/V Diaphus. Both super 8 mm movie film and videotape were used. Analyses of these recordings indicate that unusually high-frequency internal waves (periods less than 1 minute) occurred at both locations. Similar motions also were picked up as isolated events on the Precision Depth Recorder. At Hospital Bank these motions dominated the flow, whereas at Southern Bank the waves were superimposed on a unidirectional current. Classical theory and laboratory experiments have shown that when these w ves propagate onto a slope they may become unstable, break, and inject mixed fluid from the boundary seaward, which would account for the isolated suspended-sediment maxima and their occurrence in stepped profiles of temperature and salinity. Both the internal waves and intermittent burst of turbulence caused upward diffusion of suspended sediment which would maintain the nepheloid layer.

AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas