Isotopic fractionation of dissolved nitrate during denitrification in the eastern tropical north pacific ocean

The isotopic composition (δ¹⁵N) 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 900 km northeast of Hawaii, which was considered to be representative of all major water masses of the Pacific. At this last station, the δ¹⁵N composition of dissolved nitrate decreased systematically from about +6‰ at 400 m to approximately +5‰ at 5,000 m; these results are consistent with other estimates from the western Pacific.In contrast, vertical profiles of δ¹⁵N of dissolved nitrate from the ETNP showed 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 100 m were also evaluated and ranged from 0.6 to 8 μg-at 1⁻¹ yr⁻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⁻ 1⁻¹ yr⁻¹. These results compare favorably with suitably corrected oxygen utilization rates derived from electron transport activity measurements.