Investigating changes in redox conditions off the coast of Namibia by using trace metal proxies

Carbon burial in marine sediments is an effective process for removing carbon dioxide from the atmosphere. Upwelling brings nutrient-rich deep water to the surface, stimulating primary productivity which in turn contributes to burial of organic carbon in the sediments. One of the most productive upwelling systems in the world is the Benguela Upwelling System (BUS), located in the South Atlantic off the coast of SW Africa which lies within Cape Basin. This system is characterized by warm nutrient-depleted North Atlantic Deep Water that overlies cold nutrient-rich, oxygen-depleted Antarctic waters. In the geologic past, these currents have changed in strength and depth, with changes of the oxygen content (current ventilation) of the deep waters impacting the burial and preservation of organic carbon. In this study, trace metal proxies have been applied to reconstruct redox conditions and examine bottom water oxygen changes in the Cape Basin during the late Quaternary. In addition, to identify trace metals associated with specific mineral phases, scanning electron microscopy was carried out to obtain detailed, high-resolution images of the minerals and organic components within the sediment. The results of this study show strong trace metal enrichments throughout the sediment column with slight variations, except for the Holocene deposits, in the northern basin (Site 1082) and the central basin (Site 1084). The trace metal enrichments at the southern Site 1085 suggest a broad, seasonal oxygen minimum zone that extended south between about 500 and 1000 kyr. Strong changes in the intensity of oxygenation existed within the water column and the bottom waters in BUS during full glacial-interglacial cycles, likely related to a combination of changes in overlying productivity and current ventilation. This study enhances our understanding of how the carbon sink in the South Atlantic Ocean has evolved over time, while also providing valuable insights into its potential impact on a changing climate.