RT journal article
T1 Autonomous observations of biogenic N2 in the Eastern Tropical North Pacific using profiling floats equipped with gas tension devices
A1 McNeil, C.L.
A1 D’Asaro, Eric A.
A1 Altabet, Mark A.
A1 Hamme, Roberta C.
A1 García Robledo, Emilio Guillermo
A2 Biología
K1 denitrification
K1 biogenic nitrogen
K1 oceanographic floats
K1 gas tension device
K1 argon
AB Oxygen Deficient Zones (ODZs) of the world’s oceans represent a relatively smallfraction of the ocean by volume (<0.05% for suboxic and<5% for hypoxic) yet arereceiving increased attention by experimentalists and modelers due to theirimportance in ocean nutrient cycling and predicted susceptibility to expansionand/or contraction forced by global warming. Conventional methods to studythese biogeochemically important regions of the ocean have relied on welldeveloped but still relatively high cost and labor-intensive shipboard methodsthat include mass-spectrometric analysis of nitrogen-to-argon ratios (N2/Ar) andnutrient stoichiometry (relative abundance of nitrate, nitrite, and phosphate).Experimental studies of denitrification rates and processes typically involve eitherin-situ or in-vitro incubations using isotopically labeled nutrients. Over the lastseveral years we have been developing a Gas Tension Device (GTD) to study ODZdenitrification including deployment in the largest ODZ, the Eastern TropicalNorth Pacific (ETNP). The GTD measures total dissolved gas pressure from whichdissolved N2 concentration is calculated. Data from two cruises passing throughthe core of the ETNP near 17 °N in late 2020 and 2021 are presented, withadditional comparisons at 12 °N for GTDs mounted on a rosette/CTD as well asmodified profiling Argo-style floats. Gas tension was measured on the float withan accuracy of< 0.1% and relatively low precision (< 0.12%) when shallow (P< 200dbar) and high precision (< 0.03%) when deep (P > 300 dbar). We discriminatebiologically produced N2 (ie., denitrification) from N2 in excess of saturation dueto physical processes (e.g., mixing) using a new tracer – ‘preformed excess-N2’.We used inert dissolved argon (Ar) to help test the assumption that preformedexcess-N2 is indeed conservative. We used the shipboard measurements toquantify preformed excess-N2 by cross-calibrating the gas tension method tothe nutrient-deficit method. At 17 °N preformed excess-N2 decreased fromapproximately 28 to 12 μmol/kg over s0 = 24–27 kg/m3 with a resulting precisionof ±1 μmol N2/kg; at 12 °N values were similar except in the potential densityrange of 25.7< s0< 26.3 where they were lower by 1 μmol N2/kg due likely tobeing composed of different source waters. We then applied these results to gastension and O2 (< 3 μmol O2/kg) profiles measured by the nearby float to obtain the first autonomous biogenic N2 profile in the open ocean with an RMSE of ±0.78 μM N2, or ± 19%. We also assessed the potential of the method to measuredenitrification rates directly from the accumulation of biogenic N2 during thefloat drifts between profiling. The results suggest biogenic N2 rates of ±20 nMN2/day could be detected over >16 days (positive rates would indicatedenitrification processes whereas negative rates would indicate predominantlydilution by mixing). These new observations demonstrate the potential of the gastension method to determine biogenic N2 accurately and precisely in futurestudies of ODZs.
PB Frontiers Media S.A.
SN 2296-7745
YR 2023
FD 2023
LK http://hdl.handle.net/10498/31545
UL http://hdl.handle.net/10498/31545
LA eng
DS Repositorio Institucional de la Universidad de Cádiz
RD 13-nov-2026