Major-element composition of the particulate matter in the near-bottom nepheloid layer of the gulf of Mexico

Abstract

During cruises 71-A-12 and 73-A-3 of the R/V “Alaminos” eighty-six samples of suspended matter at eleven near-bottom stations in the Gulf of Mexico and northwestern Caribbean Sea were collected, and simultaneously, values for light scattering were measured. Selected samples of the suspended matter were analyzed for particulate aluminum, silicon, iron, calcium, magnesium, organic carbon and inorganic carbon. The results indicate that a permanent but highly variable near-bottom nepheloid layer exists in the Gulf of Mexico but not in the northwestern Caribbean Sea. Average total-suspended loads in the Gulf of Mexico nepheloid layer are two times higher than in the clear water above the nepheloid layer. Since there is a significant increase in the alumino-silicate fraction and a corresponding decrease in the organic fraction of the suspended matter in the nepheloid layer relative to the overlying water, it appears that sediments are the most probable source of the increased concentrations of suspended matter in the nepheloid layer. This hypothesis is supported by X-ray diffraction analyses on the nepheloid material collected at one station which show the same mineral assemblages as the underlying sediments. Time studies over periods of one week and one and one-half years showed large total-suspended-matter variations which indicate that non steady-state processes, primarily vertical eddy diffusion and possibly advection, are controlling the distribution of suspended matter in the nepheloid layer.