Abstract: Biota and Biologic Processes of Continental Slope

Gilbert T. Rowe, Richard L. Haedrich

The continental slope is characterized by benthic faunal assemblages distinct from those on the continental shelf and continental rise. Populations are distributed in zones parallel with depth contours, with zonation more marked on the slope than at either lesser or greater depths. The cause of the marked zonation is believed to be intense competition along a resource gradient that itself is correlated negatively with depth. In some regions the zonation is accentuated by a restricting oxygen minimum.

The upper continental slope assemblages constitute a fauna of transition, called the bathyal or archibenthal fauna, living between the seasonally variable continental shelf, where erosion of sediments and reworking mainly are physically controlled, and the deeper abyss, which is not subjected to seasonality. Within this depth range pelagic sound-scattering fishes impinge on the bottom and there is evidence that they are an important part of the diets of the upper-slope benthic fishes, the dominant forms between 1,000 and 1,400 m. At midslope depth (1,500 m in the northwest Atlantic) the fauna becomes dominated by echinoderms, and biologically controlled bedforms become common. In areas that have been intensively sampled we now can map the distribution of the epibenthic fauna and the l rger modifications of the surface sediments that they produce. However, little is known of the rates at which these modifications and reworking in general occur.

On the continental slope the rates of the cycling of organic matter by organisms is intermediate between the very low rates on the static, cold seafloor at greater depths and high rates found in shallow seas. As depth from the shelf to the abyssal plains increases, the bottom biota live farther away from the primary source of organic matter, the phytoplankton. As a result, abundance and biomass tend to decrease exponentially with depth. Exceptions to this general trend of the continental slope are believed to result from down-slope transport by gravity, sediment slumps, turbidity currents or seasonal cascading of cold, dense water, all of which carry organic detritus down to the lower depths of the slope or to the slope-rise junction. The larger epifauna and fishes do not decrease in verall abundance with depth as markedly as the infauna. This suggests that their reliance on the infauna for food is decreasing with depth and that selection has favored species that find and utilize packaged detritus, such as fecal pellets and carcasses, either in the water column or on the bottom.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma