The suboxic intermediate waters of the world's oceans, though constituting only a small fraction of total oceanic volume (0.1%), play a vital role in the ocean's nitrogen cycle and its biogeochemical and isotopic signatures. Suboxic conditions are required to activate the microbial processes that convert nitrogen to N2 gas. Because of this, suboxic waters act as a predominant sink, removing nitrogen from the biosphere. An ocean chemist from the University of Massachusetts at Dartmouth is working in collaboration with scientists from GEOMAR/Kiel University to complete an extensive field study of the Eastern Tropical South Pacific Ocean off the coast of Peru, one of the three major open ocean suboxic zones. Water samples, collected from a series of transects off the Peruvian margin, would undergo isotopic and gas ratio analyses to assess the following: (1) insights on the net impact of the Peru suboxic zone on oceanic nitrogen and nitrogen isotope budgets and (2) the relationship between the intensity of the nitrogen sink and the nitrogen isotope signal preserved in margin sediments. Results from this research would provide the science community with a greater understanding of the relationship between the oceanic nitrogen cycle and suboxic waters.
As regards broader impacts, this research would refine the interpretation of paleo-records with respect to past interactions between the oceanic nitrogen cycle and climate variability and contribute to numerical modeling studies of the global oceanic cycle. This study represents a collaborative effort between U.S., German, and Peruvian scientists. One graduate student would be supported and trained as part of this project.
Intellectual Merit Oxygen minimum zones (OMZ’s), though constituting a small fraction of total oceanic volume (0.1%), host globally important biogeochemical processes and are central to understanding the ocean’s N cycle and its isotopic distribution. In particular, very low O2 is required to ‘turn-on’ the microbial processes that convert chemically combined N to N2 gas (‘N-loss’), a form of nitrogen that cannot be biologically assimilated except by specialized diazotrophs. As such, biogenic production of N2 is the predominant global sink for combined N and OMZ’s host a large portion of total marine N-loss and dominate the ocean N isotope budget through co-generation of 15N and 18O enriched NO3- We highlight here our discovery of an OMZ N-loss hotspot in apparent association with a mesoscale eddy. For context, regional biogeochemical maps were created of the upper Peru OMZ (Fig. 1). Where O2 became sufficiently low to enable N-loss processes (<5 µmol/kg) there was a corresponding appearance of the intermediate NO2- and an N deficit (N’ – a measure of the net removal of nitrogen). The biogeochemical maps also reveal a N-loss hotspot with extreme values for these indicators. Station 7 is characterized by strongly elevated NO2- and extreme N’ as compared to surrounding stations (Fig. 2). Although the maxima are relatively shallow, these N-loss indicators extend broadly over the water column from 80 to 400 m depth. The extreme NO3- drawdown resulted in extreme stable isotope (15N and 18O) enrichment in residual NO3- (Fig. 2). These are the highest NO3- d15N and d18O values reported for OMZ’s. NO3- d15N and d18O co-varied with a slope near 1 indicating the dominance of NO3- reduction as the source of these isotope enrichments. A Rayleigh plot (Fig. 2d) to assess to strength of microbial isotope alteration suggests a value lower than typically reported for OMZ’s with implications for the global ocean N isotope budget The N-loss hotspot was associated with an anticyclonic coastal eddy detected in satellite maps of sea level anomaly (Fig. 3). These eddies also have low surface chlorophyll concentrations (SSC) at their centers but high values around most of their peripheries. We hypothesize that these coastal eddies increase OMZ N-loss through offshore transport of highly productive coastal waters, visible as high SSC streamers. We have other observations of higher than expected N2 yield in OMZ’s at sites in proximity to the coast that would be consistent with this interpretation. These observations support a highly dynamic OMZ environment impacted by transient forcings that calls into question current experimental approaches that assume OMZ homogeneity as well as underscores a need for new observational tools. Not only could individual microbial processes be patchy in time and space but could successively respond to events in such a way that increases overall biogeochemical output, especially N-loss. Broader Impacts – This work has provided new insights into N-loss processes in the Peru OMZ with implications for global ocean N and N isotope biogeochemistry. It also facilitated international collaborations with German and Peruvian scientists. The work of 5 graduate students have been supported as well as that of a post-doc. Undergraduate summer interns involved in this project were exposed to advanced analytical techniques.
