Understanding the oceanic cycling and transport of the climatically relevant greenhouse gas, nitrous oxide (N2O), is imperative for interpreting how it could change with environmental conditions. We studied N2O distributions under biogeochemically and physically diverse environments along the GEOTRACES GP16 section – from the south Pacific oxygen deficient zone (ODZ) to the western oligotrophic gyre – in concert with isotopic measurements of N2O, nitrate, and nitrite, to investigate the interplay of N2O production, consumption, and water mass mixing. We developed an isotope mixing model to determine the relative contributions and distributions of four N2O endmembers. From the model, we found that partial consumption was an essential determinant of the isotopic composition of N2O within the ODZ, but the consumption signal was rapidly diluted outside the ODZ. Keeling model results also demonstrated how N2O can be traced from the ODZ into the Gyre thermocline in the absence of strong production or consumption terms. Outside of the ODZ thermocline, preformed N2O and N2O derived from ammonia oxidizing archaea were largely responsible for its distribution. Lastly, as shown in other modelling work, a moderate positive site preference (~22‰) for N2O production from incomplete denitrification was necessary to produce realistic endmember distributions. Further, our newly developed tracer, D(SP,18), which removes the isotopic impacts of N2O consumption to highlight the role of production, illustrated a bifurcation of δ15Nβ within ODZ waters, highlighting the potential for nitrate and nitrite to contribute differentially to N2O production in on-shelf and off-shelf ODZ waters.

A document by Colette LaMonica Kelly. Click on the document to view its contents.

The El Niño-Southern Oscillation (ENSO) is a natural climate phenomenon that alters the biogeochemical and physical dynamics of the Eastern Tropical Pacific Ocean. Its two phases, El Niño and La Niña, are characterized by decreased and increased coastal upwelling, respectively, which have cascading effects on primary productivity, organic matter supply, and ocean-atmosphere interactions. The Eastern Tropical South Pacific (ETSP) oxygen minimum zone (OMZ) is a source of nitrous oxide (N2O), a potent greenhouse gas, to the atmosphere. While nitrogen cycling in the ETSP OMZ has been shown to be sensitive to ENSO, we present the first study to directly compare N2O distributions during both ENSO phases using N2O isotopocule analyses. Our data show that during La Niña, N2O accumulation increased six-fold in the upper 100 m of the water column, and N2O fluxes to the atmosphere increased up to 100-fold. N2O isotopocule data demonstrated substantial increases in δ18O up to 60.5‰ and decreases in δ15Nβ down to -10.3‰, signaling a shift in N2O cycling during La Niña in the oxycline compared to El Niño. N2O production via the hybrid pathway and incomplete denitrification with overprinting of N2O consumption are likely co-occurring to maintain the high site preference (SP) values (17‰ – 26.7‰), corroborating previous hypotheses. Ultimately, our results illustrate a strong connection between upwelling intensity, biogeochemistry, and N2O flux to the atmosphere, and highlight the importance of repeat measurements in the same region to constrain N2O interannual variability and cycling dynamics under different climate scenarios.

This document is intended as a technical note for pyompa (https://github.com/nitrogenlab/pyompa), a python package for conducting water mass mixing analysis (also known as “Optimum Multiparameter Analysis”, “OMP analysis” or “OMPA” in the literature). It is being made available in advance of a formal publication as an accompaniment to an upcoming paper by Lawrence et al. on the water mass analysis of the GP15 transect, and also so that it may be used as a reference for individuals who wish to use PYOMPA in their research today. PYOMPA contains several significant improvements over previously-published methods for conducting OMP analysis. These include a hard constraint on mass conservation, flexible definitions for both end-member composition and nutrient exchange ratios, a smooth penalty-based approach to encode prior knowledge of end-member distributions, support for reporting ambiguity when the solution is underdetermined, and a way to leverage knowledge from another model (such as an Ocean Circulation Inverse Model or OCIM) to select the best PYOMPA solution. This document also contains a description of additional analyses, such as archetype analysis for selecting end-member subtypes and an automated way to define the thermocline boundary, as they are often useful in conjunction with water mass mixing analysis. The focus of this document is on the mathematical formulations. Please refer to the github README or contact the authors for more information.

Nitrous oxide (N2O) is a powerful greenhouse gas, and oceanic sources account for up to one third of total flux to the atmosphere. In oxygen-deficient zones (ODZs) like the Eastern Tropical North Pacific (ETNP), N2O can be produced and consumed by several biological processes that are controlled by a variety of oceanographic conditions. In this study, the concentration and isotopocule ratios of N2O from a 2016 cruise to the ETNP were analyzed to examine heterogeneity in N2O cycling across the region. Along the north-south transect, three distinct biogeochemical regimes were identified: background, core-ODZ, and high-N2O stations. Background stations were characterized by less dynamic N2O cycling. Core-ODZ stations were characterized by co-occurring N2O production and consumption at anoxic depths, indicated by high δ18O (> 90‰) and low δ15Nβ (< -10‰) values, and confirmed by a time-dependent model, which indicated that N2O production via denitrification was significant and may occur with a non-zero site preference. High-N2O stations were defined by [N2O] reaching 126.07±12.6 nM, low oxygen concentrations expanding into near-surface isopycnals, and the presence of a mesoscale eddy. At these stations, model results indicated significant N2O production from ammonia-oxidizing archaea and denitrification from nitrate in the near-surface N2O maximum, while bacterial nitrification and denitrification from nitrite were insignificant. This study also represents the first in the ETNP to link N2O isotopocule measurements to a mesoscale eddy, suggesting the importance of eddies to the spatiotemporal variability in N2O cycling in this region.

Marine oxygen deficient zones (ODZs) are dynamic areas of microbial nitrogen cycling. Nitrification, the microbial oxidation of ammonia to nitrate, plays multiple roles in the biogeochemistry of these regions, including production of the greenhouse gas nitrous oxide (N2O). We present here the results of two oceanographic cruises investigating nitrification, nitrifying microorganisms, and N2O production and distribution from the offshore waters of the Eastern Tropical South Pacific (ETSP). On each cruise, high-resolution measurements of ammonium ([NH4+]), nitrite ([NO2-]), and N2O were combined with 15N tracer-based determination of ammonia oxidation, nitrite oxidation, nitrate reduction and N2O production rates. Depth-integrated inventories of NH4+ and NO2- were positively correlated with one another, and with depth-integrated primary production. Depth-integrated ammonia oxidation rates were correlated with sinking particulate organic nitrogen flux but not with primary production; ammonia oxidation rates were undetectable in trap-collected sinking particulate material. Nitrite oxidation rates exceeded ammonia oxidation rates at most mesopelagic depths. We found positive correlations between archaeal genes and ammonia oxidation rates and between -like 16S rRNA genes and nitrite oxidation rates. N2O concentrations in the upper oxycline reached values of greater than 140 nM, even at the western extent of the cruise track, supporting air-sea fluxes of up to 1.71 umol m-2 d-1. Our results suggest that a source of N2O other than ammonia oxidation may fuel high rates of nitrite oxidation in the offshore ETSP and that air-sea fluxes of N2O from this region may be higher than previously estimated.

A water mass analysis is a tool for interpreting the effect of ocean mixing on the distributions of trace elements and isotopes (TEI’s) along an oceanographic transect. The GEOTRACES GP15 transect along 152°W covers a wide range in latitude from Alaska to Tahiti. Our objective is to present the nutrients and hydrography of GP15 and quantify the distributions of water masses to support our understanding of TEI distributions along GP15. We used a modified Optimum Multiparameter (OMP) analysis to determine the distributions of water masses with high importance to nutrient and hydrographic features in the region. In the thermocline, our results indicated the dominance of Pacific Subarctic Upper Water (PSUW) in the subpolar gyre, Eastern North Pacific Central Water (ENPCW) in the northern subpolar gyre, and Equatorial Subsurface Water (ESSW) in the equatorial region. South Pacific Subtropical Water (SPSTW) dominated the top of the thermocline in the southern subtropical gyre, while South Pacific Central Water (SPCW) dominated the lower thermocline. Antarctic Intermediate Water (AAIW), Equatorial Intermediate Water (EqIW), and North Pacific Intermediate Water (NPIW) in the southern hemisphere, equatorial region, and northern hemisphere, respectively, occupied waters just below the thermocline. Dominant water masses in the deep waters of the southern hemisphere include Upper Circumpolar Deep Water (UCDW) and Lower Circumpolar Deep Water (LCDW) with minimal contributions from Antarctic Bottom Water (AABW). Pacific Deep Water (PDW) dominated the deep water in the northern hemisphere. Our results align well with literature descriptions of these water masses and related circulation patterns.