Nitrogen isotope patterns in the oxygen-deficient waters of the Eastern Tropical North Pacific Ocean

We analyzed the stable nitrogen isotope composition of an extensive set of samples of particulate matter (PM) and seawater nitrate collected during October/November 1997 along the Mexican coastline from 24 degrees to 11.5 degreesN. At the northern and southern end of our study area, the delta N-15 Of PM ranged between 5 and 7 parts per thousand, in the upper 200 m of the water column with higher values at intermediate depths. These data are very similar to those reported from other parts of the open ocean. In the oxygen minimum zone (OMZ), we found significantly higher delta N-15 values for suspended particles. Furthermore, the delta N-15 of nitrate (NO3) was elevated within the OMZ and we found a strong relationship between the oxygen concentration, nitrate deficit and the N-15 content of the nitrate. The core of the OMZ between 22 degreesN, 105 degreesW and 15 degreesN, 110 degreesW coincided with higher nitrate deficits and delta N-15 values relative to the stations near the boundaries. The delta N-15 of nitrate was highest, with values up to 18.7 parts per thousand, where oxygen concentrations were below 1-2 mu mol/l. This pattern is consistent: with an overall nitrogen isotopic enrichment factor of 22.5 parts per thousand for denitrification in the core of the OMZ using the Rayleigh equation (closed-system approach). Results from a diffusion model (open-system approach), however, gave a fractionation factor of 30 +/- 7.5 parts per thousand, implying that the Rayleigh formula only gives a lower estimate of the fractionation factor epsilon. The vertical flux of particles collected in short-term deployments (ca. 35 h) of a drifting sediment trap was not significantly correlated with the water column nitrate deficit. The isotopic signature of the nitrate within the gradient is very similar to the delta N-15 value of sedimenting particles, suggesting that there might be a strong link between the production and sedimentation of particles. Upward flux of nitrate across the thermocline can account for less than half of the particle Aux leaving the mixed layer. Mixing and transport of nitrate across the lower boundary of the OMZ can lead to significant enrichment in the N-15 content of deep waters, and our isotopic data imply that at least 14% of the nitrate in the waters below the OMZ originates from this source. (C) 2001 Elsevier Science Ltd. All rights reserved.