SEA-BED BIOGEOCHEMISTRY AND BENTHIC FORAMINIFERAL BATHYMETRIC ZONATION ON THE SLOPE OF THE NORTHWEST GULF-OF-MEXICO

The bathymetric zonation of benthic Foraminiferal taxa in the northwest Gulf of Mexico is summarized and compared to several important environmental parameters measured in boxcores collected along a depth transect. The parameters are bottom water temperature, organic carbon flux, bottom water oxygen content, biogeochemical gradients within the sediments and sedimentation regime. The prominent Foraminiferal boundary between 170 and 200 m is associated with the position of the mud-line in the northwest Gulf. Below this, assemblage changes are more gradational with water depth and, between 200 and 600 m, appear to be related to gradients in temperature, oxygen supply and organic carbon flux. Between 600 and 2000 m bathymetric zonation correlates to the organic carbon flux profile. An analysis of sediment pore water geochemistry and sedimentary features in the box-cores shows that there is a progressive change in the vertical distribution and character of potential microhabitats within the sediments down the slope of the northwest Gulf. From 250 to about 700 m water depth the biogenic structures observed in the sediments are abundant and complex, and the biogeochemical gradients in the sediments are steep. The visible complexity and chemical gradients gradually decrease with increasing water depth so that by 1000 m the anoxic boundary is deeper than 7 cm in our boxcores. At water depths greater than 1200 m the sediments are homogeneous, oxidized, hemipelagites. The published Foraminiferal bathymetric zonation of the N.W. Gulf appears to follow this gradient in sedimentary environments which must influence the generation of benthic Foraminiferal assemblages. The gradient is largely controlled by the organic carbon flux to the sea-bed.