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.
Subscribe to Journal
Get full journal access for 1 year
only $8.25 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Tax calculation will be finalised during checkout.
Rent or Buy article
Get time limited or full article access on ReadCube.
All prices are NET prices.
Stramma, L., Johnson, G. C., Sprintall, J. & Mohrholz, V. Expanding oxygen-minimum zones in the tropical oceans. Science 320, 655–658 (2008).
Seibel, B. A. Critical oxygen levels and metabolic suppression in oceanic oxygen minimum zones. J. Exp. Biol. 214, 326–336 (2010).
Codispoti, L. A. et al. The oceanic fixed nitrogen and nitrous oxide budgets: Moving targets as we enter the Anthropocene? Sci. Mar. 65, 85–105 (2001).
Boyd, P. W. et al. Mesoscale iron enrichment experiments 1993–2005: synthesis and future directions. Science 315, 612–617 (2007).
Garcia, H. E. et al. in Dissolved Oxygen, Apparent Oxygen Utilization, and Oxygen Saturation World Ocean Atlas 2009 Vol. 3 (ed. Levitus, S.) (U.S. Government Printing Office, 2010).
Vaquer-Sunyer, R. & Duarte, C. M. Thresholds of hypoxia for marine biodiversity. Proc. Natl Acad. Sci. USA 105, 15452–15457 (2008).
Deutsch, C., Brix, H., Ito, T., Frenzel, H. & Thompson, L. Climate-forced variability of ocean hypoxia. Science 333, 336–339 (2011).
Czeschel, R., Stramma, L. & Johnson, G. C. Oxygen decreases and variability in the eastern equatorial Pacific. J. Geophys. Res. 117, C11019 (2012).
Stramma, L., Oschlies, A. & Schmidtko, S. Mismatch between observed and modeled trends in dissolved upper-ocean oxygen over the last 50 yr. Biogeosciences 9, 4045–4057 (2012).
Deutsch, C. et al. Centennial changes in North Pacific anoxia linked to tropical trade winds. Science 345, 665–668 (2014).
Tagliabue, A., Aumont, O. & Bopp, L. The impact of different external sources of iron on the global carbon cycle. Geophys. Res. Lett. 41, 920–926 (2014).
Meskhidze, N., Chameides, W. L., Nenes, A. & Chen, G. Iron mobilization in mineral dust: can anthropogenic SO2 emissions affect ocean productivity? Geophys. Res. Lett. 30, 2085 (2003).
Mahowald, N. M. et al. Atmospheric global dust cycle and iron inputs to the ocean. Glob. Biogeochem. Cycles 19, GB4025 (2005).
Meskhidze, N., Chameides, W. L. & Nenes, A. Dust and pollution: a recipe for enhanced ocean fertilization? J. Geophys. Res. 110, D03301 (2005).
Johnson, M. S. & Meskhidze, N. Atmospheric dissolved iron deposition to the global oceans: effects of oxalate-promoted Fe dissolution, photochemical redox cycling, and dust mineralogy. Geosci. Model Dev. 6, 1137–1155 (2013).
Aumont, O., Bopp, L. & Schulz, M. What does temporal variability in aeolian dust deposition contribute to sea-surface iron and chlorophyll distributions? Geophys. Res. Lett. 35, L07607 (2008).
Sholkovitz, E. R., Sedwick, P. N., Church, T. M., Baker, A. R. & Powell, C. F. Fractional solubility of aerosol iron: synthesis of a global-scale data set. Geochim. Cosmochim. Acta 89, 173–189 (2012).
Duce, R. A. et al. Impacts of atmospheric anthropogenic nitrogen on the open ocean. Science 320, 893–897 (2008).
Ito, T. & Deutsch, C. Variability of the oxygen minimum zone in the tropical North Pacific during the late twentieth century. Glob. Biogeochem. Cycles 27, 1119–1128 (2013).
Boden, T. A., Marland, G. & Andres, R. J. Global, Regional, and National Fossil-Fuel CO2 Emissions (Oak Ridge National Laboratory U. S. Department of Energy, Carbon Dioxide Information Analysis Center, 2013).
Krishnamurthy, A. et al. Impacts of increasing anthropogenic soluble iron and nitrogen deposition on ocean biogeochemistry. Glob. Biogeochem. Cycles 23, GB3016 (2009).
Krishnamurthy, A., Moore, J. K., Mahowald, N., Luo, C. & Zender, C. S. Impacts of atmospheric nutrient inputs on marine biogeochemistry. J. Geophys. Res. 115, G01006 (2010).
Emerson, S., Watanabe, Y. W., Ono, T. & Mecking, S. Temporal trends in apparent oxygen utilization in the upper pycnocline of the North Pacific: 1980–2000. J. Oceanogr. 60, 139–147 (2004).
Deutsch, C., Emerson, S. & Thompson, L. Physical-biological interactions in North Pacific oxygen variability. J. Geophys. Res. 111, C09S90 (2006).
Boyd, P. W. & Tagliabue, A. Using the L∗ concept to explore controls on the relationship between paired ligand and dissolved iron concentrations in the ocean. Mar. Chem. 173, 52–66 (2015).
Weber, T. & Deutsch, C. Locan versus basin-scale of marine nitrogen fixation. Proc. Natl Acad. Sci. USA 111, 8741–8746 (2014).
Myriokefalitakis, S. et al. Changes in dissolved Iron deposition to the oceans driven by human activity: a 3-D global modelling study. Biogeosciences 12, 3973–3992 (2015).
Cabré, A., Marinov, I., Bernardello, R. & Bianchi, D. Oxygen minimum zones in the tropical Pacific across CMIP5 models: mean state differences and climate change trends. Biogeosciences 12, 5429–5454 (2015).
Curry, R. & Nobre, C. Hydrobase3 (Woods Hole Oceanographic Institution, 2013).
Krishnamurthy, A., Moore, J. K., Zender, C. S. & Luo, C. Effects of atmospheric inorganic nitrogen deposition on ocean biogeochemistry. J. Geophys. Res. 112, G02019 (2007).
Moore, J. K. & Braucher, O. Sedimentary and mineral dust sources of dissolved iron to the world ocean. Biogeosciences 5, 631–656 (2008).
Conway, T. M. & John, S. G. The cycling of iron, zinc and cadmium in the north east Pacific Ocean – insights from stable isotopes. Geochim. Cosmochim. Acta 164, 262–283 (2015).
Rue, E. L. & Bruland, K. W. Complexation of iron (III) by natural organic-ligands in the central North Pacific as determined by a new competitive ligand equilibration adsorptive cathodic stripping voltammetric method. Mar. Chem. 50, 117–138 (1995).
Zhang, L. et al. Nitrogen deposition to the United States: distribution, sources, and processes. Atmos. Chem. Phys. 12, 4539–4554 (2012).
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.
The authors declare no competing financial interests.
About this article
Cite this article
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
npj Climate and Atmospheric Science (2021)
Characteristics and biogeochemical effects of oxygen minimum zones in typical seamount areas, Tropical Western Pacific
Journal of Oceanology and Limnology (2021)
Scientific Reports (2019)
Nature Geoscience (2018)