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Geotraces GP15

Nitrate, nitrite and nitrous oxide isotopes

[following text copied from ] This 2018 expedition, code named “GP15,” was an important U.S. GEOTRACES mission. The Research Vessel Roger Revelle carried 59 scientists and crew through the Pacific Ocean along 152° W between Alaska and Tahiti. This path allowed us to examine the influence of strong margin fluxes, atmospheric dust deposition, and the distal ends of hydrothermal plumes from the Juan de Fuca Ridge and East Pacific Rise as well as oxygen minimum zones, equatorial upwelling, and some of the lowest-nutrient waters in the world’s oceans in the South Pacific gyre at 20°S. It was the first meridional section of the U.S. GEOTRACES program, and indeed, this transect allowed us to explore virtually all of the processes and fluxes known to introduce trace elements to the ocean. 

Karen was co-chief scientist on the cruise.  She and Colette collected samples for nitrate, nitrite and nitrous oxide isotopes.  The results will be used to better understand nitrogen cycling within the different biomes and biogeochemical provinces in the Pacific Ocean. This cruise is unique in that it will sample the oldest water in the worlds’ ocean, which possesses the highest nutrient levels, as well as some of the lowest oxygen levels in the global ocean. 


Site-specific isotopes of nitrous oxide in the eastern tropical North Pacific Ocean

Nitrogen cycling in marine oxygen deficient zones

When we talk about greenhouse gases, we usually think of carbon dioxide. Of course, however, there are other greenhouse gases, such as methane and nitrous oxide. In particular, each molecule of nitrous oxide emitted to the atmosphere has the greenhouse gas potential of 265 molecules of carbon dioxide — in other words, if carbon dioxide were the currency of climate change, then nitrous oxide would be the $300 bill. But the mechanisms and rates by which nitrous oxide is produced in the ocean remain poorly constrained, especially in regions of the ocean with little to no oxygen. In our lab, we measure the site-specific isotopes of nitrous oxide (the individual isotopes of the two N atoms in the nitrous oxide molecule) to better understand the rates and mechanisms of nitrous oxide cycling in these oxygen deficient zones. Current work includes using the natural abundance, site-specific nitrous oxide isotopes to quantify the cycling of nitrous oxide in the eastern tropical North Pacific (ETNP) oxygen deficient zone. By measuring the isotopes of nitrate, nitrite, and nitrous oxide at high spatial resolution, we are able to better constrain both the mechanisms and spatial variability of nitrous oxide production in the ETNP.

The Primary Nitrite Maximum

Nitrogen cycling in the upper water column

Nitrogen dynamics in the upper ocean are strongly controlled by the microbial community. We can see the impact of this biology reflected in nitrogen concentrations and isotope values. Nitrite is an especially interesting form of nitrogen in the upper ocean because of the accumulation of measurable amounts in the Primary Nitrite Maximum (PNM). Outside of ODZ regions, nitrite concentration is usually near zero because of its intermediate role in many nitrogen metabolisms, yet nitrite at the PNM is typically 100-200nM. 

In the Casciotti Lab, we use combinations of 15N tracer experiments and natural abundance isotope data to understand how nitrogen is transformed and utilized near the PNM. Tracer experiments can tell us instantaneous rates for processes like ammonia oxidation, nitrite oxidation, nitrate reduction and assimilation of various nitrogen forms. Using isotope systematics to interpret bulk natural abundance isotope values can give us an integrated, spatially and temporally robust, understanding of nitrogen cycling that compliments in situ rate experimentation.  

Rates of hybrid archaeal nitrous oxide production from isotopomer measurements

There are several different mechanisms by which nitrous oxide can be produced in oxygen deficient zones. One of these is a so-called “hybrid” process, in which ammonia-oxidizing archaea produce nitrous oxide, potentially from different substrates. Despite the high abundance of ammonia-oxidizing archaea in the ocean, hybrid nitrous oxide production remains poorly characterized, especially in oxygen deficient zones. In this project, we are using a novel combination of site-specific isotope measurements and tracer experiments to quantify the rates of hybrid nitrous oxide production in the eastern tropical North Pacific. By applying site-specific isotope measurements to rate experiments, we are able to more tightly constrain rates; further, our novel application of site-specific isotopes to traditional rate measurements provides insight into the potential pathways by which archaea produce nitrous oxide from different substrates.

Geotraces GA03

Atlantic Nitrate isotopes

The dominant terms in the oceanic fixed N input/output budget are poorly characterized, and we focus our attention here on N fixation. Developing robust estimates of the global rate and distribution of N fixation from ‘‘direct’’ shipboard measurements of N fixing activity is complicated by the inherent spatial and temporal variability of this biologically mediated flux. Thus, geochemical approaches for estimating N fixation inputs have come to the forefront. Currently, nitrate stable isotope measurements, which could provide an integrative estimate of N fixation on a regional or basin scale, are sparse in the Atlantic, being focused primarily in the Sargasso Sea. The GEOTRACES program provides a platform to put these data into a broader context through the illumination of basin-scale patterns.


Global nitrogen cycle model

We have created a global nitrogen (N) cycle model to study the transformations between different forms of N in marine oxygen deficient zones (ODZs). In these regions, oxygen concentrations are very low and microbial processes remove bioavailable N from the ocean. Understanding N loss is important because low N availability can limit carbon uptake in the surface ocean. Previous research has implied that an oxygen-requiring process, nitrite oxidation, may help prevent N loss in and around ODZs. Using our model, which accurately models the N concentrations and isotopes in the ocean and in ODZs, we have shown that nitrite oxidation is indeed necessary in ODZs in order for the model results to match existing measurements. Future work in this area includes refining upper water column biogeochemistry and tracking nitrous oxide as a state variable in the model.