Variability in upwelling intensity and nutrient regime in the coastal upwelling system offshore Namibia: results from sediment archives

Abstract

The coastal upwelling system offshore Namibia is ideally suited to address a focal question of the Integrated Marine Biogeochemistry and Ecosystem Research Programme: what are the mechanisms that drive long-term changes in ecosystems? Considerable interannual variability in climatic forcing is indicated by long time series of meteorological and remote sensing observation; these accompany considerable interannual to interdecadal changes in the upwelling intensity over the last 100 years, as well as a centennial trend. On longer time scales, the only archives available are sediment records spanning the late Holocene. To decipher the sediment record, we mapped surface-sediment patterns of proxies for physical (sea surface temperature/SST from alkenone unsaturation indexes) and nutrient (d 15 N on bulk sedimentary N) variables. Their present-day surface-sediment patterns outline the coastal upwelling cells and filaments and associated high productivity area. Analysed in an array of dated sediment cores, the spatial patterns of SST suggest long-term ([100 years) variability in the location and intensity of individual upwelling cells. The patterns of d 15 N outline an area of intense denitrification near the coast, and advection of water with low-oxygen concentrations in the undercurrent from the North. d 15 N exhibits considerable downcore variability, in particular over the last 50 years. The variability appears to be governed by differences in extent of denitrification and thus of the shelf oxygen balance, which appears to have deteriorated in the last 50 years. Together, the data suggest that SST and denitrification conditions have remained in the narrow bounds outlined by today's patterns in surface sediments, but that spatially small variability in upwelling intensity and make-up of upwelling feed waters induced considerable changes in the lower trophic levels of the coastal upwelling ecosystem over the last 6,000 years. Attempts to correlate proxy records from sediments with observational time series and regional climate reconstructions were not successful, possibly because annual to interannual environmental signals are erased in the process of sediment formation.

Decadal Patterns of Westerly Winds, Temperatures, Ocean Gyre Circulations and Fish Abundance: A Review

“…There has been not-too-convincing evidence of mutidecadal changes in wind intensity in the south subtropical Atlantic. A St Helena Climate index, which apparently reflects the intensity of the southeasterly trades in the South Atlantic, shows strengthening from 1900 to the 1930s, weakening to stable conditions to the 1970s and then a weak strengthening from the 1980s to 2000 [57]. Despite the apparently weak St Helena Climate trends, upwelling intensity followed the expected patterns.…”

Nutrient dynamics and oceanographic features in the central Namibian upwelling region as reflected in δ¹⁵N-signals of suspended matter and surface sediments

“…Second, the central Namibian shelf exhibits pronounced nitrate deficits in coastal proximity (Tyrrell & Lucas 2002;Kuypers et al 2005). The nitrate deficits (i) in combination with the coastal-parallel belt of high d (Figs 4A, 5), and the oxygen depletion in the bottom waters (iv), unanimously support the influence of denitrification in the source waters (see also Pichevin et al 2005;Emeis et al 2009;Lavik et al 2009). Based on a number of nutrient profiles taken at selected stations during Meteor cruise M57-3, Brçchert et al (2004) were able to locate the zones of denitrification along the water column.…”

“…(Villareal et al 1993) and (iii), according to the concept of Rayleigh fractionation kinetics, abundant nitrate supply. To our knowledge, no evidence has yet been found that nitrogen fixation plays an important role in the area investigated (see also Emeis et al 2009). The same applies to vertically migrating diatoms.…”

“…Since then, various authors have put the hypothesis of shelf edge upwelling further forward (e.g. Bang 1971;Barange & Pillar 1992;Brçchert et al 2004;Pichevin et al 2005;Summerhayes et al 2005;Emeis et al 2009). Emeis et al (2009) reported a narrow band of elevated nitrate concentrations situated above the shelf break between latitude 22 and 24 S. Nutrient replenishment via shelf edge upwelling not only accounts for reduced relative nitrate consumption and a corresponding decline in d 15 N in the primary product, but also fits the increase in primary production, as inferred from the minor peak in [TOC] above the outer shelf and upper continental slope (Fig.…”

“…2B) indicates no shelf break cooling is not automatically considered a counter-argument. Upwelled waters do not necessarily reach the uppermost water layer where the UK'37-record is generated (the UK'37-record derives from phytoplankton dwelling between 0 and 2 m depth; see Emeis et al 2009). Besides that, shelf edge upwelling does not bring as cold waters to the surface ocean as the coastal upwelling does (Summerhayes et al 1995 (Fig.…”