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Scenarios of Deoxygenation of the Eastern Tropical North Pacific During the Past Millennium as a Window Into the Future of Oxygen Minimum Zones

Abstract.

"Diverse studies predict global expansion of Oxygen Minimum Zones (OMZs) as a consequence of anthropogenic global warming. While the observed dissolved oxygen concentrations in many coastal regions are slowly decreasing, sediment core paleorecords often show contradictory trends. This is the case for numerous high-resolution reconstructions of oxygenation in the Eastern Tropical North Pacific (ETNP). [...]"

Source: Frontiers in Marine Science
Authors: Konstantin Choumiline
DOI: 10.3389/feart.2019.00237

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Dinitrogen fixation across physico‐chemical gradients of the Eastern Tropical North Pacific oxygen deficient zone

Abstract.

"The Eastern Tropical North Pacific (ETNP) Ocean hosts one of the world's largest oceanic oxygen deficient zones (ODZs). Hotspots for reactive nitrogen (Nr) removal processes, ODZs generate conditions proposed to promote Nr inputs via dinitrogen (N2) fixation. In this study, we quantified N2 fixation rates by 15N‐tracer bioassay across oxygen, nutrient and light gradients within and adjacent to the ODZ. [...]"

Source: Global Biogeochemical Cycles
Authors: C.R. Selden et al.
DOI: 10.1029/2019GB006242

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Glacial expansion of oxygen-depleted seawater in the eastern tropical Pacific

Abstract.

"Increased storage of carbon in the oceans has been proposed as a mechanism to explain lower concentrations of atmospheric carbon dioxide during ice ages; however, unequivocal signatures of this storage have not been found. In seawater, the dissolved gases oxygen and carbon dioxide are linked via the production and decay of organic material, with reconstructions of low oxygen concentrations in the past indicating an increase in biologically mediated carbon storage. [...]"

Source: Nature
Authors: Babette A. A. Hoogakker et al.
DOI: 10.1038/s41586-018-0589-x

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Rapid nitrous oxide cycling in the suboxic ocean

Abstract.

"Nitrous oxide (N2O) is a powerful greenhouse gas and a major cause of stratospheric ozone depletion, yet its sources and sinks remain poorly quantified in the oceans. We used isotope tracers to directly measure N2O reduction rates in the eastern tropical North Pacific. Because of incomplete denitrification, N2O cycling rates are an order of magnitude higher than predicted by current models in suboxic regions, and the spatial distribution suggests strong dependence on both organic carbon and dissolved oxygen concentrations. Furthermore, N2O turnover is 20 times higher than the net atmospheric efflux. The rapid rate of this cycling coupled to an expected expansion of suboxic ocean waters implies future increases in N2O emissions. [...]"

Source: Science (2015)
Authors: Andrew R. Babbin, Daniele Bianchi, Amal Jayakumar, Bess B. Ward
DOI:10.1126/science.aaa8380

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Dependence of nitrite oxidation on nitrite and oxygen in low-oxygen seawater

Abstract.

"Nitrite oxidation is an essential step in transformations of fixed nitrogen. The physiology of nitrite oxidizing bacteria (NOB) implies that the rates of nitrite oxidation should be controlled by concentration of their substrate, nitrite, and the terminal electron acceptor, oxygen. The sensitivities of nitrite oxidation to oxygen and nitrite concentrations were investigated using 15N tracer incubations in the Eastern Tropical North Pacific. Nitrite stimulated nitrite oxidation under low in situ nitrite conditions, following Michaelis-Menten kinetics, indicating that nitrite was the limiting substrate. [...]

Source: Geophysical Reasearch Letters
Authors: Xin Sun, Qixing Ji, Amal Jayakumar, Bess B. Ward
DOI: 10.1002/2017GL074355

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Biological nitrogen fixation in the oxygen-minimum region of the eastern tropical North Pacific ocean

Abstract.

"Biological nitrogen fixation (BNF) was investigated above and within the oxygen-depleted waters of the oxygen-minimum zone of the Eastern Tropical North Pacific Ocean. BNF rates were estimated using an isotope tracer method that overcame the uncertainty of the conventional bubble method by directly measuring the tracer enrichment during the incubations. Highest rates of BNF (~4 nM day−1) occurred in coastal surface waters and lowest detectable rates (~0.2 nM day−1) were found in the anoxic region of offshore stations. [...]"

Source: The ISME Journal
Authors: Amal Jayakuma et al.
DOI: 10.1038/ismej.2017.97

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