Organic‐rich mud on the western margin of southern Africa: Nutrient source to the Southern Ocean?

Abstract

The biological pump plays a major role in the transfer of CO2 from the atmosphere to the deep Southern Ocean, a transfer which is largely controlled by the supply of iron and which may partially explain glacial to interglacial variations in pCO2. Analogous to the well‐documented, smaller‐scale “island mass effect,” we propose that the lateral advection of iron by south flowing intermediate waters along the southern African margin may sustain high‐productivity blooms of the Subtropical Convergence Zone (SCZ) between 10 and 70°E. We assess the present‐day interglacial (Holocene) reservoirs and fluxes of organic carbon (OC) and terrigenous mud on the western margin of southern Africa in order to estimate the potential supply of Fe to the Southern Ocean. The highly productive Benguela Upwelling System (BUS) appears to be a relatively inefficient coastal biological pump. Repeated sediment resuspension by wave and tidal energy dissipation limits OC burial to <0.2% of net primary production (NPP) in the southern BUS and to between 0.2 to 2.4% in the northern BUS. Productivity and OC‐rich mud accumulation are focused on the inner portion of the 100–200 km wide shelf which, combined with south flowing bottom currents, limits the export of OC beyond the shelf break to 1.2–8.4% of NPP. However, winnowing of 1 million tons yr−1 of clay particles and the potential early diagenetic benthic (dissolved) Fe flux may supply 10 times more Fe than is transported by dust to the open ocean biological pump of the SCZ. Lowering sea level during glacial periods disperses interglacial mud deposits off the shelf and increases particulate Fe export by as much as a factor of 4. Glacial pulses of margin export may enhance the efficiency of the subantarctic Southern Ocean biological pump and contribute to the initial as well as glacial maximum drawdown in pCO2.