SOUTH ATLANTIC DEEP-WATER OXYGENATION CHANGES IN THE LATE QUATERNARY

Through the capacity of the ocean to sequester carbon dioxide, ocean deep water currents are strongly intertwined with changes in climatic conditions. Any changes in deep-water oxygenation, and thus, circulation patterns, are recorded in the geochemical signature of elements in the sediment. Here we present redox-sensitive trace metal data from sediments collected along the continental margin slope off the coast of Namibia in the South Atlantic – an upwelling system with very high primary productivity. Our data indicate increased organic carbon accumulation rates at the lower slope due to shelf erosion during sea-level lowstand. However, trace metal signatures observed in sedimentary surface layers indicate that down-slope transport of sediments resets the primary depositional signal of trace metal concentrations and record environmental conditions prevailing on-site. Additionally, our data show that a decrease in surface water productivity occurred during the Holocene at the upper slope, compared to the glacial interval, while the lower slope sediments recorded high concentrations of redox-sensitive metals. The observed increase of organic carbon burial and trace metal enrichments recorded in the lower slope glacial sediments suggests seaward-shifted primary productivity in the surface waters. However, our results indicate that the observed anoxic deep water conditions in the Eastern Cape Basin cannot alone be explained by increased organic matter deposition. Instead, a change in ocean circulation to more sluggish (less ventilated) conditions caused by a slow-down of the Atlantic meridional overturning circulation during the Last Glacial Period is the most likely scenario.