The study deals with the modern situation of the northern Benguela Upwelling, directing particular attention to the shelf region off central Namibia (21 to 24° S). At the centre of the investigation is the comparison of δ15N‐records in surface sediments (δ15Nsediment) with suspended particulate matter (δ15NSPM) from the surface ocean. In addition to that, water column profiles (including hydrographic data) provide an insight into changes of δ15NSPM with depth and elucidate potential offsets between δ15NSPM and δ15Nsediment. The parallel spatial trend of δ15Nsediment and surface ocean δ15NSPM shows that secondary processes are not so pronounced as to obliterate the signal generated in the surface waters. Highest δ15N‐signatures are found right off the coast where water temperatures are lowest. Concomitantly high productivity rates and low bottom oxygen suggest the upwelling of denitrified source waters. With increasing distance offshore, δ15N declines unexpectedly, reaching a minimum above the shelf break. Beyond that, the trend reverses to “normal” with δ15N‐signals continuously increasing towards the mesopelagic ocean. The decrease in δ15Nsediment and surface ocean δ15NSPM with increasing distance to the coast disagrees with the concept of Rayleigh fractionation kinetics, viz. the progressive 15N‐enrichment of the nitrate pool as it is gradually used up by phytoplankton growth. On the basis of the available evidence, the downward trend of δ15N results from decreased relative nitrate consumption, resting on a combination of reduced primary production and the existence of an ulterior source of nutrients. Nutrient replenishment seems to occur via an additional upwelling front at the edge of the shelf as well as tapping of subsurface nitrate through sufficiently deep penetration of wind‐ and wave‐induced mixing over large areas of the shelf. Both mechanisms are considered capable of working against the expected nutrient drawdown (i.e. 15N‐enrichment) as surface waters travel offshore. It is important to keep these caveats in mind when interpreting δ15N‐variations in sediment cores retrieved from this area. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

No 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

The study deals with the modern situation of the northern Benguela Upwelling, directing particular attention to the shelf region off central Namibia (21 to 24° S). At the centre of the investigation is the comparison of &delta;¹⁵N-records in surface sediments (&delta;¹⁵N_sediment) with suspended particulate matter (&delta;¹⁵N_SPM) from the surface ocean. In addition to that, water column profiles (including hydrographic data) provide an insight into changes of &delta;¹⁵N_SPM with depth and elucidate potential offsets between &delta;¹⁵N_SPM and &delta;¹⁵N_sediment. The parallel spatial trend of &delta;¹⁵N_sediment and surface ocean &delta;¹⁵N_SPM shows that secondary processes are not so pronounced as to obliterate the signal generated in the surface waters. Highest &delta;¹⁵N-signatures are found right off the coast where water temperatures are lowest. Concomitantly high productivity rates and low bottom oxygen suggest the upwelling of denitrified source waters. With increasing distance offshore, &delta;¹⁵N declines unexpectedly, reaching a minimum above the shelf break. Beyond that, the trend reverses to "normal" with &delta;¹⁵N-signals continuously increasing towards the mesopelagic ocean. The decrease in &delta;¹⁵N_sediment and surface ocean &delta;¹⁵N_SPM with increasing distance to the coast disagrees with the concept of Rayleigh fractionation kinetics, viz. the progressive ¹⁵N-enrichment of the nitrate pool as it is gradually used up by phytoplankton growth. On the basis of the available evidence, the downward trend of &delta;¹⁵N results from decreased relative nitrate consumption, resting on a combination of reduced primary production and the existence of an ulterior source of nutrients. Nutrient replenishment seems to occur via an additional upwelling front at the edge of the shelf as well as tapping of subsurface nitrate through sufficiently deep penetration of wind- and wave-induced mixing over large areas of the shelf. Both mechanisms are considered capable of working against the expected nutrient drawdown (i.e. ¹⁵N-enrichment) as surface waters travel offshore. It is important to keep these caveats in mind when interpreting &delta;¹⁵N-variations in sediment cores retrieved from this area. <br><br> doi:<a href="http://dx.doi.org/10.1002/mmng.201100005" target="_blank">10.1002/mmng.201100005</a>

We examined the susceptibility of &delta;¹⁵N-signals and C_org/N ratios in organic-rich sediments to pre-analysis sample treatment. Each sample was subjected to three different kinds of processing. For comparative purposes, the first measurement series (MS-1) was carried out on untreated sediment. In MS-2, the sediment was rinsed with distilled water. In MS-3, analyses were carried out on decalcified and rinsed material, in MS-4 the samples were decalcified without being subsequently washed. The sediment yielded conspicuously different results depending on the type of processing it was subjected to. Rinsing, irrespective of whether acidification was included or not, induced substantial modifications in &delta;¹⁵N accompanied by a pronounced loss of NH₄⁺ (up to 14 wt% of the initial N-content). Molar C_org/N ratios, on the other hand, were only affected by a combination of acidification and rinsing. The discrepancies are ascribed to the influence of decomposition-derived ammonium (NH₄⁺). In untreated sediment (MS-1), NH₄⁺ seems to produce misleading shifts in both &delta;¹⁵N-signals and C_org/N ratios. Presumed mechanisms involved are as follows: Firstly, nitrogen isotopes fractionate during NH₄⁺-volatilisation in the heating oven, where the sediment is put to desiccate. Secondly, NH₄⁺-ions are able to escape that fractionation when adsorbed to negatively-charged SiO₂-surfaces. The adsorption capacity of SiO₂ increases with increasing pH of the pore water and hence with increasing carbonate content. Our findings raise serious doubts about whether untreated sediment (MS-1) can provide reliable C_org/N and &delta;¹⁵N-records. Pre-analysis acidification plus rinsing (MS-3) seems to eliminate the deceptive influence of NH₄⁺-adsorption and -outgassing. <br><br> doi:<a href="http://dx.doi.org/10.1002/mmng.201100004" target="_blank">10.1002/mmng.201100004</a>