Nitrogen isotopic studies in the suboxic Arabian Sea

Measurements of N-15/N-14 in dissolved molecular nitrogen (Nz), nitrate (NO 3-) and nitrous oxide (N2O) and O-18/O-16 in N2O [expressed as delta(15)N a nd delta(18)O, relative to atmospheric N-2 and oxygen (O-2), respectively] have been made in water column at several locations in the Arabian Sea, a r egion with one of the thickest and most intense O-2 minima observed in the open ocean. Microbially-mediated reduction of NO3- to N-2 (denitrification) in the oxygen minimum zone (OMZ) appears to greatly affect the natural iso topic abundances. The delta(15)N Of NO3- increases from 6 parts per thousan d in deep waters (2500 m) to 15 parts per thousand within the core of the d enitrifying layer (250-350 m); the delta(15)N Of N-2 concurrently decreases from 0.6 parts per thousand to 0.20 parts per thousand. Values of the isot opic fractionation factor (epsilon) during denitrification estimated using simple advection-reaction and diffusion-reaction models are 22 parts per th ousand and 25 parts per thousand, respectively. A strong decrease in delta( 15)N of NO3- is observed from similar to 200 m (> 11 parts per thousand) to 80 m (similar to 6 parts per thousand); this is attributed to the input of isotopically light nitrogen through nitrogen fixation. Isotopic analysis o f N2O reveals extremely large enrichments of both N-15 and O-18 within the OMZ, presumably due to the preferential reduction of lighter N2O to N-2. Ho wever, isotopically Light N2O is observed to accumulate in high concentrati ons above the OMZ indicating that the N2O emitted to the atmosphere from th is region cannot be very heavy. The isotope data from the intense upwelling zone off the southwest coast of India, where some of the highest concentra tions of N2O ever found at the sea surface are observed, show moderate depl etion of N-15, but slight enrichment of O-18 relative to air. These results suggest that the ocean-atmosphere exchange cannot counter inputs of heavie r isotopes (particularly O-18) associated with the stratospheric back flux, as proposed by previous workers. This calls for additional sources and/or sinks of N2O in the atmosphere. Also, the N2O isotope data cannot be explai ned by production through either nitrification or denitrification, suggesti ng a possible coupling between the two processes as an important mechanism of N2O production.