Nitrogen and oxygen isotope measurements of nitrate along the US GEOTRACES Eastern Pacific Zonal Transect (GP16) yield insights into nitrate supply, remineralization, and water mass transport

In this study, stable isotope measurements of nitrate (δ15NNO3 and δ18ONO3) and suspended particulate nitrogen (δ15NSPN, > 51 μm) were made on the 2013 US GEOTRACES Eastern Pacific Zonal Transect (GP16). This cruise extended from the nutrient-rich coastal waters of the Peru upwelling system to the oligotrophic waters of the south Pacific subtropical gyre, where low surface nitrate (NO3−) concentrations (< 4 μM) and high δ15NNO3 and δ18ONO3 values (up to 28‰ and 25‰, respectively) were generated from consumption of NO3− by phytoplankton. Analysis of δ15NNO3 and δ18ONO3 patterns in surface waters relative to δ15NSPN provided insight into supply of nutrients to surface waters along the transect and remineralization at depth. In the subsurface oxygen deficient zone (ODZ), dissimilatory NO3− reduction to nitrite (NO2−) generated distinct elevations in δ15NNO3 and δ18ONO3 (up to ~ 37‰ and 31‰, respectively). Below the oxygen deficient zone, δ15NNO3 values (6 to 10‰) in the intermediate waters (σ0 = 27.0 to 27.5 kg m− 3; 500 m to 1500 m) were elevated relative to deep waters across the entire length of GP16, while intermediate δ18ONO3 values (2 to 3‰) were more similar to deep water values. These observations suggested that both NO3− regeneration and the transport of NO3− from the ODZ influence the intermediate waters of the central and eastern south Pacific. In order to explore this further, results from the GP16 water mass analysis and a NO3− isotope mixing model were employed to quantify the importance of those processes in the thermocline and intermediate waters across GP16. The contribution of NO3− from the ODZ contributed up to 10 μM across the thermocline, but generally < 5 μM in the intermediate waters. In contrast, it was estimated that remineralization contributed 5 to 15 μM of NO3− across the thermocline and intermediate waters. Overall, these results from GP16 confirm that NO3− reduction in the ODZ and organic matter remineralization play important roles in maintaining the widespread elevation in δ15NNO3 observed across the thermocline and intermediate waters of the eastern South Pacific.