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Oxygen Deficient Zones (ODZs) of the world’s oceans represent a relatively small fraction of the ocean by volume (<0.05% for suboxic and<5% for hypoxic) yet are receiving increased attention by experimentalists and modelers due to their importance in ocean nutrient cycling and predicted susceptibility to expansion and/or contraction forced by global warming. Conventional methods to study these biogeochemically important regions of the ocean have relied on well-developed but still relatively high cost and labor-intensive shipboard methods that include mass-spectrometric analysis of nitrogen-to-argon ratios (N2/Ar) and nutrient stoichiometry (relative abundance of nitrate, nitrite, and phosphate). Experimental studies of denitrification rates and processes typically involve eitherin-situorin-vitroincubations using isotopically labeled nutrients. Over the last several years we have been developing a Gas Tension Device (GTD) to study ODZ denitrification including deployment in the largest ODZ, the Eastern Tropical North Pacific (ETNP). The GTD measures total dissolved gas pressure from which dissolved N2concentration is calculated. Data from two cruises passing through the core of the ETNP near 17 °N in late 2020 and 2021 are presented, with additional comparisons at 12 °N for GTDs mounted on a rosette/CTD as well as modified profiling Argo-style floats. Gas tension was measured on the float with an accuracy of< 0.1% and relatively low precision (< 0.12%) when shallow (P< 200 dbar) and high precision (< 0.03%) when deep (P > 300 dbar). We discriminate biologically produced N2(ie., denitrification) from N2in excess of saturation due to physical processes (e.g., mixing) using a new tracer – ‘preformed excess-N2’. We used inert dissolved argon (Ar) to help test the assumption that preformed excess-N2is indeed conservative. We used the shipboard measurements to quantify preformed excess-N2by cross-calibrating the gas tension method to the nutrient-deficit method. At 17 °N preformed excess-N2decreased from approximately 28 to 12 µmol/kg over σ0 =24–27 kg/m3with a resulting precision of ±1 µmol N2/kg; at 12 °N values were similar except in the potential density range of 25.7< σ0< 26.3 where they were lower by 1 µmol N2/kg due likely to being composed of different source waters. We then applied these results to gas tension and O2(< 3 µmol O2/kg) profiles measured by the nearby float to obtain the first autonomous biogenic N2profile in the open ocean with an RMSE of ± 0.78 µM N2, or ± 19%. We also assessed the potential of the method to measure denitrification rates directly from the accumulation of biogenic N2during the float drifts between profiling. The results suggest biogenic N2rates of ±20 nM N2/day could be detected over >16 days (positive rates would indicate denitrification processes whereas negative rates would indicate predominantly dilution by mixing). These new observations demonstrate the potential of the gas tension method to determine biogenic N2accurately and precisely in future studies of ODZs.
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Decadal oscillations in the ocean’s largest oxygen-deficient zone
The impact of global warming on the ocean’s oxygen-deficient zones (ODZs) is uncertain, partly because of a lack of data on past changes. We report monthly resolved records of coral skeleton–bound nitrogen isotopes (CS-δ15N) to reconstruct denitrification in the Eastern Tropical North Pacific (ETNP) ODZ over the last 80 years. The data indicate strong decadal variation in ETNP denitrification, with maxima during the cool North Pacific phase of Pacific Decadal Variability. The maxima in denitrification (and thus oxygen deficiency) were likely due to stronger upwelling that enhanced productivity leading to greater oxygen demand in the thermocline. Prior findings of multidecadal-to-centennial ODZ trends were likely biased by this variability. ODZ evolution over the next century will depend on how global warming interacts with the decadal oscillations.
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- PAR ID:
- 10658856
- Publisher / Repository:
- Science
- Date Published:
- Journal Name:
- Science
- Volume:
- 386
- Issue:
- 6725
- ISSN:
- 0036-8075
- Page Range / eLocation ID:
- 1019 to 1024
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1126/science.adk4965
