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

Abstract

The delta sup(15)N composition of nitrate and N sub(2) gas was measured in the eastern tropical North Pacific (ETNP) and central Arabian Sea (AS) suboxic regions. The delta sup(15)N of nitrate increased from 6 ppt at 2,500 m to 15 ppt at 250-350 m in both regions, while the delta sup(15)N of N sub(2) concurrently decreased from 0.6 ppt to 0.25 ppt. The denitrification isotopic fractionation factor ( epsilon sub(denit)) for each region was estimated using both advection-reaction and diffusion-reaction models. Values for epsilon sub(denit) in the ETNP ranged from 25 plus or minus 2 (advection-reaction) to 30 plus or minus 3 (diffusion-reaction). Values for epsilon sub(denit) in the central AS varied from 22 plus or minus 3 (advection-diffusion) to 25 plus or minus 4 (reaction-diffusion) using a starting nitrate isotopic composition of 6 ppt but were indistinguishable from calculated values from the ETNP when an initial value of 5 ppt was employed. Based upon the model results, an average global epsilon sub(denit) of 27 plus or minus 3 is proposed for marine suboxic water columns. Isotopic enrichment of nitrate in oxic waters beneath the active denitrification regions was observed and indicates the presence of significant cross-isopycnal ventilation at depth. The isotopic composition of nitrate decreased above 250 m to approx 80 m, and this pattern is hypothesized to be caused by the input of isotopically light nitrogen from nitrogen fixation in the euphotic zone. A simple isotopic mass balance indicates that a significant percentage of primary productivity in the central AS may be fueled by nitrogen fixation