Water column denitrification rates in the oxygen minimum layer of the Pacific Ocean along 32°S

Abstract

AbstractTo date, estimates of water column denitrification rates in global‐ and basin‐scale nitrogen budgets do not include any values obtained in the open ocean outside the oxygen minimum zones (OMZs). The high‐quality, 32°S Pacific repeat hydrographic surveys conducted in 1992, 2003, and 2010 offer an opportunity to estimate denitrification rates on basin scales in the open ocean away from the Eastern Tropical Pacific (ETP) oxygen minimum zones. Here, extended optimum multiparameter analysis and N* are used to estimate water column denitrification (ΔNdeni) in the oxygen minimum layer (OML; 1100–2000 dbar). Water column denitrification rates for 2003 and 2010 are estimated from the regression analysis using ΔNdeni and transit time distribution ages based on CFC12. Estimated mean denitrification rates in the OML at 32°S range from 3.1 to 7.5 µmol N m−3 yr−1 (=2.6–6.8 mmol N m−2 yr−1 with a thickness of 900 m). The OML denitrification rates are 2–3 orders of magnitudes lower than those estimated in the OMZs of ETP. However, their significance to Pacific nitrogen budget needs to be investigated in the future. Although uncertainties remain, our results may ultimately contribute to an improved understanding of marine nitrogen cycles.