Transport of Fine-Grained Estuarine Sediment by Gravity-Driven Movement of the Nepheloid Layer, Long Island Sound

Interpretation of sidescan-sonar imagery provides evidence that down-slope gravity-driven movement of the nepheloid layer constitutes an important mode of transporting sediment into the basins of north-central Long Island Sound. In the Western Basin, these currents have formed dendritic drainage systems characterized by branching patterns of low-backscatter on the seafloor that exceed 7.4 km in length. The channels progressively combine and widen down-slope, reaching widths of over 0.6 km at their southern distal ends. Although much smaller, similar patterns are also present in the northwestern part of the Central Basin. Bathymetric gradients along the lengths of these systems are low (slightly less than 0.2 degrees) and relatively constant. Sediment samples collected from within the channels and on the surrounding seafloor were mud (primarily clayey silt), but the areas could not be clearly differentiated based on texture.

Earlier work has shown that intense benthic biological reworking, coupled with resuspension due to wave- and current-induced bottom stress, is responsible for the formation of the Sound's sediment-laden near-bottom nepheloid layer. Our work suggests that the density contrast between the heavier nepheloid layer and the lighter ambient seawater subsequently creates an instability that causes the layer to flow down-slope across the seafloor. The extent and widespread occurrence of the dendritic patterns suggest that the density currents are a significant, previously unrecognized, persistent process by which fine-grained sediments are remobilized, transported, and dispersed in the basins of central and western Long Island Sound. Because many contaminants preferentially adsorb onto fine-grained organic-rich sediments and because the Sound is affected by seasonal hypoxia, mechanisms and dispersal pathways by which these fine-grained sediments are transported are important factors determining the eventual fate of contaminants and their potential impact on the environmental health of the estuary. These findings may have applications to other estuarine basins where similar conditions exist.