Nutrient utilization and diatom productivity changes in the low-latitude SE Atlantic over the past 70 kyr: Response to Southern Ocean leakage

Abstract

Abstract. Eastern Boundary Upwellings (EBUs) are some of the key loci of biogenic silica (opal) burial in the modern ocean, representing important productive coastal systems that extraordinarily contribute to marine organic carbon fixation. The Benguela Upwelling System (BUS), in the low-latitude SE Atlantic, is one of the major EBUs, which is under the direct influence of nutrient-rich Southern Ocean waters. Quantification of past changes in diatom productivity through time, in response to Late Quaternary climatic change, feeds into our understanding of the sensitivity of EBUs to future climatic perturbations. Existing sediment archives of silica cycling include: opal burial fluxes, diatom assemblages and opaline silicon isotopic variations (denoted by d30Si). Burial fluxes and siliceous assemblages are limited to recording the remains reaching the sediment (i.e. export), and d30Si variations are complicated by species-specific influences and seasonality. Here, we present the first species-specific d30Si record from the BUS, encompassing full glacial conditions to the Holocene. In addition to export, our new data allows us to reconstruct utilisation of dissolved Si in surface waters in an area with strong input from Southern Ocean waters. Our new archives show that there was enhanced upwelling of Southern Ocean Si-rich water, and accompanied strong silicic acid utilisation by coastal dwelling diatoms, during Marine Isotope Stage 3 (60–40 kyr). This pulse of strong silicic acid utilisation was followed by a weakening of upwelling and coastal diatom Si utilisation into MIS2, before an increase in pelagic diatom Si utilisation across the deglaciation. We combine our findings with mass balance model experiments to show that changes in surface water silica cycling through time are a function of both upwelling intensity and utilisation changes, illustrating the sensitivity of EBUs to climatic change on glacial-interglacial scales.