Late Quaternary Carbonate Deposition and Oceanographic Processes in the Southeast Atlantic Ocean
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In the modern ocean, important water masses originating in the North Atlantic Ocean and Southern Ocean intersect the Namibian margin of southwest Africa at various water depths: Antarctic Intermediate Water (AAIW) occurs from 500 to 800 m, Upper Circumpolar Deep Water (UCDW) occurs from 800 to 1400 m, North Atlantic Deep Water (NADW) occurs from 1400 to 3800 m, and southern-sourced bottom water fills the remaining basin up to 3800 m water depth. Changes in the depth distributions and properties of these water masses through time are reflected by variations in the nature of the sediments accumulating along this margin. To investigate these variations, we examined a suite of sediment cores collected from water depths ranging from 550 to 3700 meters. Oxygen isotope and radiocarbon data indicate these cores have records corresponding to the past ~165 kyr, which corresponds to the two most recent glacialinterglacial cycles (MIS 1 through MIS 6). Discrete measurements of CaCO3 content, Xray fluorescence Fe/Ca ratios, and sediment density were made at regular intervals in the cores, and used to determine downcore profiles of CaCO3 content and the mass accumulation rates (MAR) of total sediment, and the CaCO3 and non-CaCO3 components of the sediment. CaCO3 content varies with glacial and interglacial stages such that lowest CaCO3 contents occur during glacial MIS 2, 4 and 6, and the highest CaCO3 contents occur during interglacial MIS 1 and 5. Temporal variations in MARtotal, MARCaCO3 and MARnon-CaCO3 indicate that these differences are primarily due to greater input of non-CaCO3 sediment during glacial periods, rather than dissolution of CaCO3 on the seafloor or decreased primary productivity at the sea surface. Comparison of the depth distributions of CaCO3 content and MARCaCO3 corresponding to the present time and the last glacial maximum (MIS 2) suggest that during MIS 2 the central depths of AAIW/UCDW and NADW were about 750 m and 2000 m, respectively, and southernsourced bottom water occurred at depths below 3100 m. The relative abundances and MAR of quartz (indicated by X-ray diffraction) is greater during glacial stages than interglacial stages, because southwest Africa was more arid and offshore winds were stronger during glacial times.
