The coastal upwelling system offshore Namibia is well-known for its high primary productivity in surface waters and denitrification in OMZ. Both processes gradually enrich the residual nitrate pool in 15N that is assimilated by phytoplankton to synthesize Chl-a and particulate organic nitrogen. To unveil the connection between the upwelled nitrate pool, molecule-specific nitrogen and bulk sediment nitrogen, we analyzed the δ15N of nitrate in seawater, and of Chl-a and bulk nitrogen in 27 surface sediments samples of northern BUS (17°S – 25°S). An offshore increase in both δ15Nsed and δ15NChl-a in the northern (17°-20°S,11°–14°E) sector is consistent with 15N enrichment during progressive nitrate assimilation by phytoplankton. In the southern sector (20°-23°S, 11°–14°E) where the OMZ is prominent, an offshore decrease of δ15Nsed and δ15NChl-a reflect assimilation of nitrate enriched in 15N by water column denitrification. The δ15Nsed and δ15NChl-a values also increase along the inner shelf from 17° to 23°S, interpreted to reflect a time-averaged imprint of denitrification and concomitant increase in δ15N of upwelled nitrate. Despite of the overall similarity in spatial patterns, paired δ15Nsed and δ15NChl-a unexpectedly lack a significant correlation (R2 = 0.22). The deviation between δ15Nsed and δ15NChl-a (△) suggests a varying post-depositional bias of diagenetic processes on the δ15Nsed whereas the original δ15NChl-a is retained. We conclude that intense diagenetic alteration of bulk N in temporally anoxic sediments shifts the δ15Nsed, whereas the sedimentary δ15NChl-a captures the integrated N-loss in the water column, and is an unbiased isotope proxy for denitrification in the OMZ.

Supplement to: Xin, Yu; Dähnke, Kirstin; Emeis, Kay-Christian (submitted): Nitrogen isotope composition of chlorophyll-a and bulk sediment in the northern Benguela Upwelling System and their application in characterizing nitrogen cycling. Organic Geochemistry