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

Abstract

The δ 15 N composition of nitrate and N 2 gas was measured in the eastern tropical North Pacific (ETNP) and central Arabian Sea (AS) suboxic regions. The δ 15 N of nitrate increased from 6‰ at 2,500 m to 15‰ at 250–350 m in both regions, while the δ 15 N of N 2 concurrently decreased from 0.6‰ to 0.25‰. The denitrification isotopic fractionation factor (ε denit ) for each region was estimated using both advection‐reaction and diffusion‐reaction models. Values for ε denit in the ETNP ranged from 25 ± 2 (advection‐reaction) to 30 ± 3 (diffusion‐reaction). Values for ε denit in the central AS varied from 22 ± 3 (advection‐diffusion) to 25 ± 4 (reaction‐diffusion) using a starting nitrate isotopic composition of 6‰ but were indistinguishable from calculated values from the ETNP when an initial value of 5‰ was employed. Based upon the model results, an average global ε denit of 27 ± 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 −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.