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Free keywords:
NITROGEN ISOTOPE BIOGEOCHEMISTRY; BENGUELA CURRENT REGION; INTER-OCEAN
EXCHANGE; SINKING PARTICLES; ATLANTIC-OCEAN; SEASONAL VARIABILITY;
NATURAL ABUNDANCE; NORTHERN BENGUELA; NITRATE UTILIZATION; SPATIAL
VARIATIONSBenguela Current; denitrification; nitrogen isotopes; nutrient
utilisation; two-celled upwelling; SST;
Abstract:
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 degrees S). At the centre of the investigation is the comparison of delta N-15-records in surface sediments (delta N-15(sediment)) with suspended particulate matter (delta N-15(SPM)) from the surface ocean. In addition to that, water column profiles (including hydrographic data) provide an insight into changes of delta N-15(SPM) with depth and elucidate potential offsets between delta N-15(SPM) and delta N-15(sediment). The parallel spatial trend of delta N-15(sediment) and surface ocean delta N-15(SPM) shows that secondary processes are not so pronounced as to obliterate the signal generated in the surface waters. Highest delta N-15-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 N-15 declines unexpectedly, reaching a minimum above the shelf break. Beyond that, the trend reverses to "normal" with delta N-15-signals continuously increasing towards the mesopelagic ocean. The decrease in delta N-15(sediment) and surface ocean delta N-15(SPM) with increasing distance to the coast disagrees with the concept of Rayleigh fractionation kinetics, viz. the progressive N-15-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 N-15 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. N-15-enrichment) as surface waters travel offshore. It is important to keep these caveats in mind when interpreting delta N-15-variations in sediment cores retrieved from this area.
