Nutrient regime and upwelling in the northern Benguela since the middle Holocene in a global context - a multi-proxy approach

Abstract

The last 5500 years of climate change and environmental response in the northern Benguela Coastal Upwelling are reconstructed by means of three sediment cores from the inner shelf off central Namibia. The study is based on nutrient (delta(15)N, delta(13)C) and productivity proxies (accumulation rates of total organic carbon; AR(TOC)). Reconstructed sea surface temperatures (alkenone-derived SST) and temperatures at subsurface depths (T(delta 18O); based on tests of planktonic foraminifers) reflect the physical boundary conditions. The selection of proxy indicators proved a valuable basis for robust palaeo-climatic reconstructions, with the resolution ranging from multi-decadal (NAM1) over centennial (core 178) to millennial scale (core 226620). The northern Benguela experienced pronounced and rapid perturbation during the middle and late Holocene, and apparently, not all are purely local in character. In fact, numerous correlations with records from the adjacent South African subcontinent and the northern hemisphere testify to global climatic teleconnections. The Holocene Hypsithermal, for instance, is just as evident as the Little Ice Age (LIA) and the Roman Warm Period. The marked SST-rise associated with the latter is substantiated by other marine and terrestrial data from the South African realm. The LIA (at least its early stages) manifests itself in intensified winds and upwelling, which accords with increased rainfall receipts above the continental interior. It appears that climate signals are transferred both via the atmosphere and ocean. The combined analysis of SST and T(delta 18O) proved a useful tool in order to differentiate between both pathways. SSTs are primarily controlled by the intensity of atmospheric circulation features, reflecting changes of upwelling-favourable winds. T(delta 18O) records the temperature of the source water and often correlates with global ocean conveyor speed due to varying inputs of warm Agulhas Water. It seems as though conveyor slowdown or acceleration not only affected the temperature of the source water but also its nutrient content. This relationship between source water quality and conveyor speed is already known from glacial times

Constraining the conditions of phosphogenesis: Stable isotope and trace element systematics of Recent Namibian phosphatic sediments

“…The δ 13 Corg values of Core 25005 (Fig. 5a) show that marine organic matter was the dominant source of organic carbon in the sediment, consistent with the absence of large riverine input, low eolian delivery relative to marine bioproduction and low Corg/N ratios measured from the Namibian Shelf (Meisel et al, 2011b(Meisel et al, , 2011a. The δ 13 Corg values are rather stable at approximately -19.5 to -20‰ in the lower non-phosphatic zone but decrease to -21 ‰ in the phosphatic zone, with TOC abundances and δ 13 Corg values showing a clear positive correlation (r = 0.96,n = 18;Fig.…”

System controls of coastal and open ocean oxygen depletion

“…However, sedimentary core analyses have provided insight into past surface water temperatures, upwelling intensity and productivity over multi-decadal to glacial-interglacial time periods (e.g. Berger and Wefer, 2002;West et al, 2004;Meisel et al, 2011) from which inferences about the bottom-water oxygen condition can be made.…”

Spatial dynamics of the bearded goby and its key fish predators off Namibia vary with climate and oxygen availability

“…Advection across the ABF is highly variable both on bi‐annual (Rouault, ) and geological (Meisel et al ., ) time scales. Paleo‐ecological studies of fish scales and proxies for environmental factors in sediment cores suggest that the presence of advected waters has been associated with large fluctuations in fish abundance and changes in community structure over the last 3200 yr (Struck et al ., ), which probably reflects differences in species‐specific responses to hypoxia.…”