Abstract
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 (N 2 /Ar) and nutrient stoichiometry (relative abundance of nitrate, nitrite, and phosphate). Experimental studies of denitrification rates and processes typically involve either in-situ or in-vitro incubations 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 N 2 concentration 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 N 2 (ie., denitrification) from N 2 in excess of saturation due to physical processes (e.g., mixing) using a new tracer – ‘preformed excess-N 2 ’. We used inert dissolved argon (Ar) to help test the assumption that preformed excess-N 2 is indeed conservative. We used the shipboard measurements to quantify preformed excess-N 2 by cross-calibrating the gas tension method to the nutrient-deficit method. At 17 °N preformed excess-N 2 decreased from approximately 28 to 12 µmol/kg over σ 0 = 24–27 kg/m 3 with a resulting precision of ±1 µmol N 2 /kg; at 12 °N values were similar except in the potential density range of 25.7< σ 0 < 26.3 where they were lower by 1 µmol N 2 /kg due likely to being composed of different source waters. We then applied these results to gas tension and O 2 (< 3 µmol O 2 /kg) profiles measured by the nearby float to obtain the first autonomous biogenic N 2 profile in the open ocean with an RMSE of ± 0.78 µM N 2 , or ± 19%. We also assessed the potential of the method to measure denitrification rates directly from the accumulation of biogenic N 2 during the float drifts between profiling. The results suggest biogenic N 2 rates of ±20 nM N 2 /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 N 2 accurately and precisely in future studies of ODZs.