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Acceleration of oxygen decline in the tropical Pacific over the past decades by aerosol pollutants

Abstract

Dissolved oxygen in the mid-depth tropical Pacific Ocean has declined in the past several decades1. The resulting expansion of the oxygen minimum zone has consequences for the region’s ecosystem2 and biogeochemical cycles3, but the causes of the oxygen decline are not yet fully understood. Here we combine models of atmospheric chemistry, ocean circulation and biogeochemical cycling to test the hypothesis that atmospheric pollution over the Pacific Ocean contributed to the redistribution of oxygen in deeper waters. We simulate the pollution-induced enhancement of atmospheric soluble iron and fixed nitrogen deposition, as well as its impacts on ocean productivity and biogeochemical cycling for the late twentieth century. The model reproduces the magnitude and large-scale pattern of the observed oxygen changes from the 1970s to the 1990s, and the sensitivity experiments reveal the reinforcing effects of pollution-enhanced iron deposition and natural climate variability. Despite the aerosol deposition being the largest in mid-latitudes, its effect on oceanic oxygen is most pronounced in the tropics, where ocean circulation transports added iron to the tropics, leading to an increased regional productivity, respiration and subsurface oxygen depletion. These results suggest that anthropogenic pollution can interact and amplify climate-driven impacts on ocean biogeochemistry, even in remote ocean biomes.

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Figure 1: Dissolved oxygen in the thermocline waters of the North Pacific.
Figure 2: Pollution-induced deposition of soluble iron and fixed nitrogen based on GEOS-Chem.
Figure 3: Simulated changes in the oxygen and sinking organic flux from the 1970s to the 1990s.
Figure 4: Breakdown of the simulated changes in the oxygen and sinking organic flux.

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Acknowledgements

T.I. is grateful for support from US National Science Foundation, grant number OCE-1242313. A.N. acknowledges support from the Cullen-Peck Faculty Fellowship and the Georgia Power Scholar Chair. J. Valett provided Supplementary Fig. 1. The authors would like to thank D. Jacob and the Harvard University Atmospheric Chemistry Modeling Group for providing the base GEOS-Chem model used during our research. Resources supporting this work were provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at NASA Ames Research Center.

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T.I. and A.N. initiated the research. T.I. was responsible for conducting ocean biogeochemistry simulations, analysis of the results and overall manuscript development. M.S.J. conducted atmospheric chemistry simulations. All authors contributed to the project planning, experimental design, the discussion of the results and their implications, as well as commenting on the manuscript.

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Correspondence to T. Ito.

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The authors declare no competing financial interests.

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Ito, T., Nenes, A., Johnson, M. et al. Acceleration of oxygen decline in the tropical Pacific over the past decades by aerosol pollutants. Nature Geosci 9, 443–447 (2016). https://doi.org/10.1038/ngeo2717

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