Isotopic composition of nitrate in the central Arabian Sea and eastern tropical North Pacific: A tracer for mixing and nitrogen cycles

The delta(15)N composition of nitrate and N-2 gas was measured in the easte rn tropical North Pacific (ETNP) and central Arabian Sea (AS) suboxic regio ns. The delta(15)N of nitrate increased from 6 parts per thousand at 2,500 m to 15 parts per thousand at 250-350 m in both regions, while the delta(15 )N of N-2 concurrently decreased from 0.6 parts per thousand to 0.25 parts per thousand. The denitrification isotopic fractionation factor (epsilon(de nit)) for each region was estimated using both advection-reaction and diffu sion-reaction models. Values for epsilon(denit) in the ETNP ranged from 25 +/- 2 (advection-reaction) to 30 +/- 3 (diffusion-reaction). Values for eps ilon(denit) in the central AS varied from 22 +/- 3 (advection-diffusion) to 25 +/- 4 (reaction-diffusion) using a starting nitrate isotopic compositio n of 6 parts per thousand but were indistinguishable from calculated values from the ETNP when an initial value of 5 parts per thousand was employed. Based upon the model results, an average global epsilon(denit) Of 27 +/- 3 is proposed for marine suboxic water columns. Isotopic enrichment of nitrat e in oxic waters beneath the active denitrification regions was observed an d indicates the presence of significant cross-isopycnal ventilation at dept h. The isotopic composition of nitrate decreased above 250 m to similar to 80 m, and this pattern is hypothesized to be caused by the input of isotopi cally light nitrogen from nitrogen fixation in the euphotic zone. A simple isotopic mass balance indicates that a significant percentage of primary pr oductivity in the central AS may be fueled by nitrogen fixation.