Nitrogen isotopic studies in the suboxic Arabian Sea

Abstract

Measurements of sup(15)N/ sup(14)N in dissolved molecular nitrogen (N sub(2)), nitrate (NO sub(3) sup(-)) and nitrous oxide (N sub(2)O) and sup(18)O/ sup(16)O in N sub(2)O [expressed as delta sup(15)N and delta sup(18)O, relative to atmospheric N sub(2) and oxygen (O sub(2)), respectively] have been made in water column at several locations in the Arabian Sea, a region with one of the thickest and most intense O sub(2) minima observed in the open ocean. Microbially-mediated reduction of NO sub(3) sup(-) to N sub(2) (denitrification) in the oxygen minimum zone (OMZ) appears to greatly affect the natural isotopic abundances. The delta sup(15)N of NO sub(3) sup(-) increase from 6 ppt in deep waters (2500 m) to 15 ppt within the core of the denitrifying layer (250-350 m); the delta sup(15)N of N sub(2) concurrently decreases from 0.6 ppt to 0.20 ppt. Values of the isotopic fractionation factor ( epsilon) during denitrification estimated using simple advection-reaction and diffusion-reaction models are 22 ppt and 25 ppt, respectively. A strong decrease in delta sup(15)N of NO sub(3) sup(-) is observed from approx 200 m (> 11 ppt) to 80 m (approx 6 pp5); this is attributed to the input of isotopically light nitrogen fixation. Isotopic analysis of N sub(2)O reveals extremely large enrichments of both sup(15)N and sup(18)18 within the OMZ, presumably due to the preferential reduction of lighter N sub(2)O to N sub(2). However, isotopically light N sub(2)O is observed to accumulate in high concentrations above the OMZ indicating that the N sub(2)O emitted to the atmosphere from this region cannot be very heavy. The isotope data from the intense upwelling zone off the southwest coast of India, where some of the highest concentrations of N sub(2)O ever found at the sea surface are observed, show moderate depletin of sup(15)N, but slight enrichment of sup(18)O relative to air. These results suggest that the ocean-atmosphere exchange cannot counter inputs isotopes (particularly sup(18)O) associated with the stratospheric back flux, as proposed by previous workers. This calls for additional sources/or sinks of N sub(2)O in the atmosphere. Also, the N sub(2)O isotope data cannot be exaplained by production through either nitrification or denitrification, suggesting a possible coupling between the two processes as an important mechanism of N sub(2)O production.