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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/bg-2020-82
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-82
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  06 Apr 2020

06 Apr 2020

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A revised version of this preprint was accepted for the journal BG and is expected to appear here in due course.

Present past and future of the OMZ in the northern Indian Ocean

Tim Rixen1, Greg Cowie2, Birgit Gaye3, Joaquim Goes4, Helga do Rosário Gomes4, Raleigh R. Hood5, Zouhair Lachkar6, Henrike Schmidt7, Joachim Segschneider8, and Arvind Singh9 Tim Rixen et al.
  • 1Leibniz Centre for Tropical Marine Research (ZMT), Fahrenheitstr. 6, 28359 Bremen, Germany
  • 2University of Edinburgh, School of Geosciences, James Hutton Road, Edinburgh EH9 3FE, Scotland
  • 3Institute for Geology, Universität Hamburg, Bundesstraße 55, 20146 Hamburg, Germany
  • 4Marine Biology, Department of Marine Biology and Paleoenvironment Lamont Doherty Earth Observatory at Columbia University, 61 Route 9W, Palisades, New York, 10964
  • 5Horn Point Laboratory, University of Maryland Center for Environmental Science, P.O. Box 775, Cambridge, MD 21613
  • 6Center for Prototype Climate Modeling (CPCM), NYU Abu Dhabi
  • 7GEOMAR Helmholtz-Zentrum fuer Ozeanforschung Kiel, Duesternbrooker Weg 20, 24105 Kiel, Germany
  • 8Institute of Geosciences, Christian-Albrechts-Universität Kiel (CAU), Ludewig-Meyn-Straße 10, 24118 Kiel, Germany
  • 9Geosciences Division Physical Research Laboratory (PRL) Navrangpura, Ahmedabad 380 009, India

Abstract. Decreasing concentrations of dissolved oxygen and the resulting expansion of anaerobic ecosystems is a major threat to marine ecosystem services because it favors the formation of greenhouse gases such as methane, endangers the growth of economically important species, and increases the loss nitrate. Nitrate is one of the potential primary nutrients, which availability controls the marine productivity. The Arabian Sea and the Bay of Bengal are home to ~ 59 % of the Earth's marine sediments exposed to severe oxygen depletion and approximately 21 % of the total volume of oxygen-depleted waters (oxygen minimum zones, OMZs). The balance between physical oxygen supply and the biological oxygen consumption controlled the oxygen concentrations. In the Arabian Sea and most likely also in the Bay of Bengal the supply of oxygen sustained by mixing and advection associated with mesoscale eddies compensated the biological oxygen consumption. These steady states maintain low (hypoxic) oxygen concentrations allowing the competition between anaerobic and aerobic processes. However, due to slightly higher oxygen concentrations, the aerobic nitrite oxidization inhibits the anaerobic nitrite reduction and thus denitrification (the reduction of nitrate to N2) to become significant in the Bay of Bengal. A feedback mechanism caused by the negative influence of decreasing oxygen concentrations on the biological oxygen demand helped to maintain these steady states. Furthermore, it might have also counteracted a reduced physical oxygen supply into the Arabian Sea caused by climate-driven changes in the ocean's circulation during the last 6000 years. However, due to human-induced global changes, the OMZs in Arabian Sea and the Bay of Bengal intensified and expanded, which included also the occurrence of anoxic events on the Indian shelf. This affects benthic ecosystems, and in the Arabian Sea it seems to have initiated a regime shift within the pelagic ecosystem structure. Consequences for biogeochemical cycles are unknown, which, in addition to the poor representation of mesoscale features reduces the reliability of predictions of the future OMZ development in the northern Indian Ocean.

Tim Rixen et al.

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Tim Rixen et al.

Tim Rixen et al.

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Short summary
The northern Indian Ocean hosts an extensive oxygen minimum zones (OMZ), which intensified due to human-induced global changes. This includes the occurrence of anoxic events on the Indian shelf, affects benthic ecosystems, and the pelagic ecosystem structure in the Arabian Sea. Consequences for biogeochemical cycles are unknown, which, in addition to the poor representation of mesoscale features reduces the reliability of predictions of the future OMZ development in the northern Indian Ocean.
The northern Indian Ocean hosts an extensive oxygen minimum zones (OMZ), which intensified due...
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