Effects of current regimes and oxygenation on particulate matter preservation on the Namibian shelf: Insights from amino acid biogeochemistry


Many reconstructions of past biogeochemical states rely on proxies such as δ15N that in turn are affected by the preservation state of organic matter. N-turnover processes in the upwelling system on the Namibian shelf are difficult to reconstruct because lateral particle advection and differential degradation under oxic to anoxic conditions complicate particle tracing in suspended matter and surface sediments as well as the interpretation of sedimentary δ15N records. We analysed the amino acid composition of suspended matter, sinking particles, phytoplankton biomass and surface sediments in order to investigate the degradation pathway of organic matter from the sea surface to the surface sediment. A principal component analysis carried out with the amino acid data set results in a triangular-shaped model. Two branches evolve from phytoplankton biomass on the apex: The suspended matter branch is characterized by relatively enhanced contents of Glu, Ser and Leu, whereas the non-protein amino acids and Lys mark the sinking particle and sediment branch. The results suggest that both pools have taken different diagenetic pathways and that there is only limited exchange between them. The preservation state of organic matter in surface sediments within the oxygen minimum zone is generally better than on the oxic outer shelf, where organic matter preservation depends on water depth. A vertical increase in δ15N ratios from phytoplankton biomass to the surface sediment by ~ 2.5‰ is attributed to early diagenetic degradation in the water column and at the water/sediment interface. The δ15N ratios of sediments are mainly controlled by the N-isotope signature of nitrate on the outer shelf and upper continental slope. Nutrient-enriched deep water ascends at the shelf breaks due to the influence of internal waves and tides or curl-driven shelf break upwelling. The δ15N of upwelling nitrate at the shelf breaks is ~ 3‰ lower than that of nitrate from the inner shelf. This difference in nitrate sources is expressed in a band of decreasing δ15N ratios of surface sediments at the shelf breaks. The hydrodynamic upwelling regime at the shelf breaks flushes significant amounts of suspended matter from the mesopelagic ocean across the shelf break as indicated by amino acid monomer patterns of suspended matter.
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