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Published December 2022 | Published + Supplemental Material
Journal Article Open

Coincident Biogenic Nitrite and pH Maxima Arise in the Upper Anoxic Layer in the Eastern Tropical North Pacific

Abstract

The Eastern Tropical North Pacific (ETNP), like the other marine oxygen deficient zones (ODZs), is characterized by an anoxic water column, nitrite accumulation at the anoxic core, and fixed nitrogen loss via nitrite reduction to N₂O and N₂ gases. Here, we constrain the relative contribution of biogeochemical processes to observable features such as the secondary nitrite maximum (SNM) and local pH maximum by simultaneous measurement of inorganic nitrogen and carbon species. High-resolution sampling within the top 1 km of the water column reveals consistent chemical features previously unobserved in the region, including a tertiary nitrite maximum. Dissolved inorganic carbon measurements show that pH increases with depth at the top of the ODZ, peaking at the potential density of the SNM at σ_θ = 26.15 ± 0.06 (1 s.d.). We developed a novel method to determine the relative contributions of anaerobic ammonium oxidation (anammox), denitrification, nitrite oxidation, dissimilatory nitrate reduction to nitrite, and calcium carbonate dissolution to the nitrite cycling in the anoxic ODZ core. The calculated relative contributions of each reaction are slightly sensitive to the assumed C:N:P ratio and the carbon oxidation state of the organic matter sinking through the ODZ. Furthermore, we identify the source of the pH increase at the top of ODZ as the net consumption of protons via nitrite reduction to N2 by the denitrification process. The increase in pH due to denitrification impacts the buffering effect of calcite and aragonite dissolving in the ETNP.

Additional Information

© 2022. The Authors. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. We are indebted to the captain and crew of the R/V Falkor and R/V Atlantis for their assistance at sea. We also thank the chief scientist Stefan Sievert on the AT37-12 cruise for enabling CTD sampling. We acknowledge the decades of scientists, marine technicians, ships' crew, and engineers who contributed to CLIVAR and GO-SHIP P18 cruises, and NOAA CoastWatch and OceanWatch for providing satellite altimetry data presented in Supporting Information S1. The R/V Falkor ship time was provided by a Schmidt Ocean Institute Grant to KLC and ARB. The AT37-12 research cruise was funded by the NSF OCE-1559042 and OCE-1559198 Grants. This work was funded by Simons Foundation Grant 622065, NSF Grants OCE-2138890 and OCE-2142998 to ARB, NSF Grants OCE-1923312 and OCE-2148468 to RJW, the MIT Ally of Nature Award, and the generous support of Dr. Bruce L. Heflinger. Additionally, DD was partially funded by an NSF Graduate Research Fellowship. Finally, we thank Babbin lab members and Adam Subhas for their support and comments on the manuscript and Rémi Buisson for his assistance in creating Figure 1. Data Availability Statement. Bottle data from the FK180624 cruise are available through BCO-DMO (https://www.bco-dmo.org/dataset/832389), and CTD (Sea-bird SBE-911+) data are available through Rolling Deck to Repository (https://doi.org/10.7284/129668). CTD data from AT37-12 are available through BCO-DMO (https://www.bco-dmo.org/dataset/739098), and bottle data from AT37-12 are included as a supplement (Data Set S1). GO-SHIP and CLIVAR P18 data are available from the GLODAP v2.2021 (Lauvset et al., 2021; Olsen et al., 2019, 2020), cruise numbers 33RO20161119 and 33RO20071215, respectively. The scripts used in analyzing FK180624 data, developing models in the study, and plotting important figures are available at https://github.com/tcinay/bablab_FK180624.git.

Attached Files

Published - Global_Biogeochemical_Cycles_-_2022_-_Cinay_-_Coincident_Biogenic_Nitrite_and_pH_Maxima_Arise_in_the_Upper_Anoxic_Layer_in.pdf

Supplemental Material - 2022gb007470-sup-0001-supporting_information_si-s01.pdf

Supplemental Material - 2022gb007470-sup-0002-data_set_si-s01.csv

Files

Global_Biogeochemical_Cycles_-_2022_-_Cinay_-_Coincident_Biogenic_Nitrite_and_pH_Maxima_Arise_in_the_Upper_Anoxic_Layer_in.pdf

Additional details

Created:
August 22, 2023
Modified:
October 25, 2023