Nitrite Production Mechanisms in the Primary Nitrite Maximum of the Tropical South Pacific Inferred from Nitrite Isotopes Collected during the Geotraces 2013 Cruise

Tuesday, 16 December 2014
Brian D Peters, Stanford Earth Sciences, Stanford, CA, United States and Karen L Casciotti, Stanford University, Los Altos Hills, CA, United States
Nitrite has been observed to accumulate near the bottom of the euphotic zone in many regions of the world ocean. This recurring feature is referred to as the primary nitrite maximum (PNM). The production of nitrite in the PNM has been attributed to both nitrate reduction and ammonium oxidation, depending on the location. However, the factors driving nitrite accumulation are not well understood. We use nitrite isotope data collected during the GEOTRACES 2013 Eastern Pacific Zonal Transect (EPZT) to examine differences in nitrite production mechanisms in the PNM of a gradient from coastal to oligotrophic offshore waters in the tropical south Pacific. Large differences in the nitrogen and oxygen isotopic composition of nitrite (reported as δ15N vs air N2 and δ18O vs VSMOW in units of ‰, respectively) were found between eastern and western parts of the transect (eastern and central south Pacific, respectively). δ15N of nitrite ranged between −5 and −54‰ in the in the eastern PNM, while ranging between 1 and 7‰ in the central PNM. δ18O of nitrite was between 10 and 45‰ in the eastern PNM, and 10 to 14‰ the central PNM. These differences in δ15N and δ18O of nitrite in the eastern versus central PNM may be related to differences in nitrite production mechanisms, namely nitrate reduction and ammonium oxidation. We hypothesize that nitrate reduction may be the more important nitrite-producing process in the eastern PNM, while ammonium oxidation is likely to be more important in the central PNM. Furthermore, since nitrite oxidation has been observed to exhibit an inverse kinetic isotope effect, the strongly negative δ15N values of nitrite in the eastern PNM suggest that nitrite oxidation may be an important nitrite sink there. Analysis of nitrate isotopes will allow us to further constrain the contributions of these processes, and this is currently underway.