Ocean Deoxygenation Conference | Kiel 2018

Stable isotopes (14,15N, 16,18O) of dissolved inorganic nitrogen (N) were measured in sediment porewaters and benthic flux chambers across the Peruvian oxygen minimum zone (OMZ) from 74 to 1000 m water depth. Sediments at all locations were net consumers of bottom water NO3-. In waters shallower than 400 m, this sink was largely attributed to dissimilatory nitrate reduction to ammonium (DNRA) by communities of filamentous nitrate-storing bacteria (Marithioploca and Beggiatoa) and to denitrifying foraminifera. The δ15N of their collective intracellular NO3- pool was >30 ‰. The apparent N isotope effect of benthic NO3- loss was 7.4 ± 0.7 ‰ at microbial mat sites and 2.5 ± 0.9 ‰ at the lower fringe of the OMZ (400 m) where foraminifera were abundant. Model simulations of the data generally support a previous hypothesis (Prokopenko et al., 2013) attributing the 15NH4+ enrichment to a close coupling of DNRA and anammox (DAX) using NO2- supplied by Marithioploca and NH4+ form the porewater. The model predicts that 40 % of NO3- actively transported into the sediment by Marithioploca is lost as N2 by DAX. This enhances N2 fluxes by a factor of 2 – 3 and accounts for 70 % of fixed N loss to N2. By limiting the flux of 15NH4+ back to the ocean, DAX tends to decrease overall benthic N fractionation. Knowledge of the sink of NH4+ once it leaves the sediment is critical for understanding how the benthos contributes to the N isotope effect in the water column.