Processes on the continental slope off North Carolina with special reference to the Cape Hatteras region

Abstract

Historical data from the slope off Cape Hatteras show this environment to be atypical of the rest of the Atlantic slope in terms of high rates of sedimentation and high benthic/demersal standing stocks. This paper focuses on identifying potential sources for sediments and nutrients on the Hatteras slope and on likely transport mechanisms. Sediments consist mainly of subequal mixtures of sand, silt, and clay and contain an average of 1% carbon. This pool of carbon represents weathered organic matter containing polyunsaturated fatty acids (PUFA) and sterols typical of relatively refractory shelf/estuarine sediments. The labile organic fraction is derived from phytoplankton and zooplankton as reflected in short chain fatty acids (<C22), planktic sterols, and chlorophylla that are found in higher concentrations than observed elsewhere on the eastern U.S. continental slope. The inventory of organic nitrogen is much higher on the mid-slope (800–1 000 m) than shallower or deeper bottom areas as predicted from plots of organic nitrogen versus grain-size for the U.S. Atlantic continental margin. The midslope region is the major focusing area for sedimentation. Bioturbation is an important diagenetic process that has profound influence on sediment profiles of sulphate, methane, fatty acids, sterols, chlorophylla, viable diatoms, and metals. The low inventory of relatively refractory carbon (1%) stands in contrast to high measured rates of organic carbon sedimentation (28–121 g organic C m−2 year−1). This paradox is probably related to high remineralization rates in the water column and on the bottom. The refractory nature of the small residual pool of deposited organic matter may define the trophic niche filled by those benthos found on this slope that are more typically encountered on the shelf (e.g. oligochaetes and opportunistic polychaetes). A likely mechanism for high input rates of both organic and inorganic particulates to the Hatteras slope may be attributed to the position of Cape Hatteras relative to the adjacent narrow shelf. The Cape extends outward almost to the shelf edge topographically diverting sediment and primary production seaward that is moving southward along the outer shelf (the ‘funnel’ hypothesis). Transport of outer shelf sands to the slope may take place by the impingement of Gulf Stream eddies on the outer shelf and upper slope as well as by storm-generated waves. Once shelf sediment is deposited on the upper slope, it is apparently redistributed, as no strong on-shore to off-shore or depth-related gradients are observed in any of the measured sedimentary parameters. The Hatteras slope apparently represents an estuarine type of sedimentary/nutrient regime that is displaced to an otherwise oceanic deep-sea environment by outwelling and funnelling of nutrients and sediments from the shelf to the slope.