Isotopic fractionation of dissolved nitrate during denitrification in the eastern tropical North Pacific Ocean

The isotopic composition (delta15N) of dissolved nitrate was measured at five stations within the oxygen-deficient region of the eastern tropical North Pacific Ocean (ETNP) and at one station 900km northeast of Hawaii which was considered to be representative of all major water masses of the Pacific. At this last station, the delta15N composition of dissolved nitrate decreased systematically from about 0.6% at 400m to approximately 0.5% at 5,000m; these results are consistent with other estimates from the western Pacific. In contrast, vertical profiles of delta15N of dissolved nitrate from the ETNP showed a marked departure from the above observed trend and correlated with losses of nitrate arising from denitrification. Instantaneous fractionation factors (α) were estimated using the one dimensional vertical diffusion-advection model. These results suggest that 14NO3- is consumed 3-4% faster than 15NO3-, significantly larger than fractionations (<=2%) observed under laboratory conditions. Maximum rates of denitrification at 100m were also evaluated and ranged from 0.6 to 8 μg-atI-1yr-1 for the stations investigated. The above upper limit is probably excessive, but the average maximum for the four stations analyzed is estimated to be 3.5 μg-at NO3-1-1yr-1. These results compare favorably with suitably corrected oxygen utilization rates derived from electron transport activity measurements