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 south-eastern Atlantic, is one of the major EBUs and 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 δ30Si). Burial fluxes and siliceous assemblages are limited to recording the remains reaching the sediment (i.e. export), and δ30Si variations are complicated by species-specific influences and seasonality. Here, we present the first combined δ30Si record of two large centric diatoms from the BUS, encompassing full glacial conditions to the Holocene. In addition to export, our new data allow us to reconstruct the utilization 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 accompanied by strong silicic acid utilization by coastal dwelling diatoms during Marine Isotope Stage 3 (MIS3; 60–40 ka). This pulse of strong silicic acid utilization was followed by a weakening of upwelling and coastal diatom Si utilization into MIS2, before an increase in pelagic diatom Si utilization 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 utilization changes, illustrating the sensitivity of EBUs to climatic change on glacial–interglacial scales.
Highlights
Marine productivity by diatoms represents up to half of the total fixation of organic matter in the oceans and plays a key role in uptake of carbon dioxide (CO2) from the atmosphere (Tréguer et al, 2018)
Valves of the two diatom species were combined to allow for enough material and because the species diatoms are indistinguishable under a low-magnification binocular microscope
These experiments reveal that δ30SiCA was somewhat sensitive to changes in the Southern Ocean (SO) input of [DSi] and the upwelling rate, with reasonable variation within the model of these parameters allowing for a change in δ30SiCA of approximately 0.4 ‰ in each case
Summary
Marine productivity by diatoms represents up to half of the total fixation of organic matter in the oceans and plays a key role in uptake of carbon dioxide (CO2) from the atmosphere (Tréguer et al, 2018). The northward supply of waters with a higher DSi-to-nitrate ratio via AAIW during full glacials would promote low-latitude diatom growth at the expense of other nonsiliceous phytoplankton This assemblage shift would result in a weakening of the carbonate pump and a change in seawater alkalinity that would contribute to atmospheric car-. We present the first late Quaternary δ30Si records from the BUS during the full glacial conditions of Marine Isotope Stage (MIS) 4 to the late Holocene, overcoming these challenges by using only two large centric species from a welldocumented sediment core We will use these records, together with a simple box model, to reconstruct past changes in seawater composition and DSi supply through time
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