Along-shelf Direction Research Articles

Low-Frequency Currents and Continental Shelf Waves in the Southern Weddell Sea

Abstract The salient features of low-frequency current fluctuations, obtained from an analysis of eight current meter records from the continental shelf and slope of the southern Weddell Sea, are compared to baroclinic and barotropic theories. A simple baroclinic theory of internal waves is used successfully to predict high-frequency spectral cutoff values from low-frequency velocity ellipse calculations made from the continental slope mooring data. The success of this theory indicates that the higher spectral energy levels observed over the slope compared to the shelf are probably due to baroclinic motions. A barotropic model of free continental shelf waves probably by Saint-Guily (1976) is adapted for the local topography and the predictions of the model compared to observations. Coherences and phases between moorings separated by 10 and 160 km in the along-shelf direction provide substantial evidence of the existence of shelf waves as predicted by the theory for periods of 3–60 days and for the lowest ...

The formation of sedimentary strata in an allochthonous shelf environment: The Washington continental shelf

Models of fine-scale (<10° m, <10 2 yr) strata formation can enhance geological interpretations of the stratigraphic record. The Washington continental shelf represents a good locality to develop such a model for shelf strata, because it is undergoing sedimentation of material supplied by the Columbia River. Modern sand accumulates on the inner (<40–60 m water depth) and mid-shelf regions, and modern mud (mostly silt) accumulates on the mid and outer (>120 m) shelf regions. The inner shelf sand and mid-shelf sandy silt are the predominant accretionary deposits on the shelf. Important sedimentological observations obtained by boxcoring the upper 1 2 m of these deposits are: (a) progressive decrease in grain size with distance from the Columbia, (b) downward coarsening within the seabed, (c) loss of distinct sedimentary structure (homogenization of sediment) with distance from the Columbia. Non-dimensional parameters based on sediment transport and benthic biological studies can be used to relate rates of sediment mixing (reworking) to rates of accumulation. These parameters have the potential for quantitative prediction of strata formation, from measurements of active processes. Physical and biological mixing can be assumed to remain relatively constant in an alongshelf direction on the Washington shelf, and accumulation rate is known (from Pb-210 geochronology) to decrease away from the Columbia. Together these factors tend to increase the ratio of mixing to accumulation with distance from the Columbia, and predict the sedimentological observations listed above.

Development of the benethic nepheloid layer on the south Texas continental shelf, western Gulf of Mexico

A monitoring study of suspended sediment on the South Texas Continental Shelf indicates that a turbid benthic nepheloid layer is regionally persistent. A sequence of quasi-synoptic measurements of the water column obtained during six cruises in an 18-month period indicates substantial spatial and temporal variability in nepheloidlayer characteristics. Regionally, the thickness of the shelf nepheloid layer increases both seaward and in a convergent alongshelf direction. Greatest thicknesses occur over a muddy substrate, indicating a causal relationship; maximum observed local thickness is 35 m which occurs along the southern shelf break. Analyses of suspended particulate matter in shelf bottom waters indicate mean concentrations ranging from 49 · 104 to 111 · 104 particle counts/cc; concentrations persistently increase shoreward throughout the region. Bottom particulate matter is predominantly composed of inorganic detritus. Admixtures of organic skeletal particles, primarily diatoms, are generally present but average less than 10% of the total particulate composition. Texturally, the particulate matter in bottom waters is predominantly poorly sorted sediment composed of very fine silt (3.9–7.8 μm).The variability in nepheloid-layer characteristics indicates a highly dynamic shelf feature. The relationship of nepheloid-layer characteristics to hydrographic and substrate conditions suggests a conceptual model whereby nepheloid-layer development and maintenance are the results of the resuspension of sea-floor sediment. Bottom turbulence is attributed primarily to vertical shear and shoaling progressive internal waves generated by migrating shelf-water masses, especially oceanic frontal systems, and secondarily to shoaling surface gravity waves.