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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 13, issue 1
Biogeosciences, 13, 211–221, 2016
https://doi.org/10.5194/bg-13-211-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue: Low oxygen environments in marine, fresh and estuarine...

Biogeosciences, 13, 211–221, 2016
https://doi.org/10.5194/bg-13-211-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 15 Jan 2016

Research article | 15 Jan 2016

Millennial changes in North Atlantic oxygen concentrations

B. A. A. Hoogakker1, D. J. R. Thornalley2, and S. Barker3 B. A. A. Hoogakker et al.
  • 1Department of Earth Sciences, University of Oxford, Oxford, OX1 3AN, UK
  • 2Department of Geography, University College London, London, WC1E 6BT, UK
  • 3School of Earth and Ocean Sciences, Cardiff University, Cardiff, CF10 3AT, UK

Abstract. Glacial–interglacial changes in bottom water oxygen concentrations [O2] in the deep northeast Atlantic have been linked to decreased ventilation relating to changes in ocean circulation and the biological pump (Hoogakker et al., 2015). In this paper we discuss seawater [O2] changes in relation to millennial climate oscillations in the North Atlantic over the last glacial cycle, using bottom water [O2] reconstructions from 2 cores: (1) MD95-2042 from the deep northeast Atlantic (Hoogakker et al., 2015) and (2) ODP (Ocean Drilling Program) Site 1055 from the intermediate northwest Atlantic. The deep northeast Atlantic core MD95-2042 shows decreased bottom water [O2] during millennial-scale cool events, with lowest bottom water [O2] of 170, 144, and 166 ± 17 µmol kg−1 during Heinrich ice rafting events H6, H4, and H1. Importantly, at intermediate depth core ODP Site 1055, bottom water [O2] was lower during parts of Marine Isotope Stage 4 and millennial cool events, with the lowest values of 179 and 194 µmol kg−1 recorded during millennial cool event C21 and a cool event following Dansgaard–Oeschger event 19. Our reconstructions agree with previous model simulations suggesting that glacial cold events may be associated with lower seawater [O2] across the North Atlantic below  ∼ 1 km (Schmittner et al., 2007), although in our reconstructions the changes are less dramatic. The decreases in bottom water [O2] during North Atlantic Heinrich events and earlier cold events at the two sites can be linked to water mass changes in relation to ocean circulation changes and possibly productivity changes. At the intermediate depth site a possible strong North Atlantic Intermediate Water cell would preclude water mass changes as a cause for decreased bottom water [O2]. Instead, we propose that the lower bottom [O2] there can be linked to productivity changes through increased export of organic material from the surface ocean and its subsequent remineralization in the water column and the sediment.

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Models predict a decrease in future ocean O2, driven by surface water warming and freshening in the polar regions, causing a reduction in ocean circulation. Here we assess this effect in the past, focussing on the response of deep and intermediate waters from the North Atlantic during large-scale ice rafting and millennial-scale cooling events of the last glacial. Our assessment agrees with the models but also highlights the importance of biological processes driving ocean O2 change.
Models predict a decrease in future ocean O2, driven by surface water warming and freshening in...
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