Nitrogen cycling in sediments on the NW African margin inferred from N and O isotopes in benthic chambers

Benthic nitrogen cycling in the Mauritanian upwelling region (NW Africa) was studied in June 2014 from the shelf to the upper slope where minimum bottom water O-2 concentrations of 25 mu M were recorded. Benthic incubation chambers were deployed at 9 stations to measure fluxes of O-2, dissolved inorganic carbon (DIC) and nutrients (NO3-, NO2-, NH4+, PO43-, H4SiO4) along with the N and O isotopic composition of nitrate (delta N-15-NO3- and delta O-18-NO3-) and ammonium (delta N-15-NH4+). O-2 and DIC fluxes were similar to those measured during a previous campaign in 2011 whereas NH4+ and PO43- fluxes on the shelf were 2 - 3 times higher and possibly linked to a long-term decline in bottom water O-2 concentrations. The mean isotopic fractionation of NO3- uptake on the margin, inferred from the loss of NO3- inside the chambers, was 1.5 +/- 0.4 parts per thousand for N-15/14 ((15)is an element of(app)) and 2.0 +/- 0.5 parts per thousand for O-18/16 ((18)is an element of(app)). The mean (18)is an element of(app):(15)is an element of(app) ratio on the shelf (< 100 m) was 2.1 +/- 0.3, and higher than the value of 1 expected for microbial NO3- reduction. The (15)is an element of(app) are similar to previously reported isotope effects for NO3- respiration in marine sediments but lower than determined in 2011 at a same site on the shelf. The sediments were also a source of N-15-enriched NH4+ (9.0 +/- 0.7 parts per thousand). A numerical model tuned to the benthic flux data and that specifically accounts for the efflux of N-15-enriched NH4+ from the seafloor, predicted a net benthic isotope effect of N loss ((15)is an element of(sed)) of 3.6 parts per thousand; far above the more widely considered value of similar to 0 parts per thousand. This result is further evidence that the assumption of a universally low or negligible benthic N isotope effect is not applicable to oxygen-deficient settings. The model further suggests that (18)is an element of(app):(15)is an element of(app) trajectories > 1 in the benthic chambers are most likely due to aerobic ammonium oxidation and nitrite oxidation in surface sediments rather than anammox, in agreement with published observations in the water column of oxygen deficient regions.

Automated summary

Benthic nitrogen cycling in the Mauritanian upwelling region was studied in 2014, revealing higher NH4+ and PO43- fluxes on the shelf possibly linked to declining bottom water O2 concentrations. The isotopic fractionation of nitrate uptake and elevated (18):(15) ratios on the shelf suggest aerobic ammonium oxidation and nitrite oxidation activities due to oxygen deficiency.