Abstract

The Cocos Gap is a deeper portion, or saddle, of the Cocos Ridge and forms part of the western boundary of the Panama Basin. It is probably typical of saddles within most submarine ridges. In order to determine the mechanisms controlling sediment dispersal, the nature and sources of the sediments at 23 core locations were defined by hydrodynamic size separation (> 63, 2-63, <2 microns) and microscopic or X-ray diffraction analysis of the individual fractions. In addition, calcium carbonate, organic carbon, opal and quartz determinations were made for the total sediment. The silt-sized fraction was resolved into eight textural modes. The coarse modes reflect the progressive breakage and winnowing of the coarse fraction (foraminifera) under the influence of bottom currents and gravity. Above 2,000 m mechanical breakdown, winnowing and relocation by bottom currents mask the effects of depth related dissolution of the carbonate fraction. Intermediate modes in general represent a transitional facies with both biogenic and terrigenous influences, while the finest modes characterize a distal regime of clay deposition. The clay fraction is amorphous material with very low percentages of well-crystallized clays. Three main sources and transport paths were recognized, including one associated with the circulation of the Panama Basin bottom water. Sedimentation within the Gap is controlled by local processes, predominantly the interaction between tidally induced intensification of bottom water flow and directional (thermohaline) flow. The steepness of the seafloor slope is a major factor controlling the efficiency of winnowing of the sediment away from certain higher elevations (biogenic source areas) to the sheltered parts and flanks of the ridge. Superimposed upon this sediment dispersal is the influx of terrigenous material carried by directional bottom currents that operate as postulated upper and lower contour currents along the flanks of the ridge. The crest of the Cocos Gap acts as a catchment area for the biogenic components, while the adjacent more sloping region, the sub-plateau, acts as a source area. The extreme breakage of the foraminifera is most likely a function of the tidally induced intensification of the bottom water flow, characteristic of many shallow ridges, and is probably most significant in the sub-plateau. Hydrographic data indicate that there is no significant transport of bottom water across the Cocos Gap into the Panama Basin, but downslope transport of carbonate and siliceous fragments and minerals from the Gap into the basin is associated with cyclical tidal bottom water flow.

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