OPTICAL CHARACTERISTICS OF THE SUSPENDED SEDIMENT IN THE HIGH ENERGY BENTHIC BOUNDARY LAYER EXPERIMENT

Profiles of light transmission versus depth have been studied in the region of the High Energy Benthic Boundary Layer Experiment (HEBBLE) at bottom depth betwen 4900 m and 5000 m. A component model has been developed and consists of five components of transmission which can be combined to accurately duplicate any given transmission profile. The components are shown to be representative of three basic phenomena: the particle concentration within an even flow; the separation of a benthic nepheloid layer; and the trailing edge of a benthic cloud. In the case of even flow it is the relative magnitudes of settling and eddy diffusion which determine the shape of the transmission profile. Separation of the benthic nepheloid layer was inferred to be caused by an occasional cross-slope velocity component. The component model yields diffusion coefficients comparable to those estimated by a simple diffusion/settling model.

Using the components, various features of the HEBBLE area have been studied. Integration of the transmission profiles together with reasonable estimates of the size of the benthic turbidity cloud indicate mass fluxes on the order of a metric ton per second over a period of four or five days past a cross-sectional area of 10 km('2). In addition, Eulerian and Lagrangian transformations have been performed on the data obtained over several weeks and covering distances of hundreds of kilometers. The Eulerian transformation shows that benthic storms that were detected at one location and time appear nearly identical at a later time downstream. Similarly, Lagrangian transformation shows that separate benthic storms can be detected over a large distance. Distance and time scales obtained from these transformations show the HEBBLE area to be one characterized by bottom storms which keep their general form over periods of at least two weeks and for distances travelled of at least 400 kilometers.

An excellent correlation exists between transmission and particle volume concentration. A nearly identical correlation was obtained in the same area at a time eighteen months after the initial correlation was determined. Index of refraction determination yield indices ranging from 1.55 to 1.60, characteristic of typical marine clays.