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Volume 13, issue 19 | Copyright

Special issue: Hydrography, biogeochemistry, and biology of "dead-zone eddies"...

Biogeosciences, 13, 5633-5647, 2016
https://doi.org/10.5194/bg-13-5633-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 10 Oct 2016

Research article | 10 Oct 2016

Oxygen utilization and downward carbon flux in an oxygen-depleted eddy in the eastern tropical North Atlantic

Björn Fiedler1, Damian S. Grundle1, Florian Schütte1, Johannes Karstensen1, Carolin R. Löscher1,a, Helena Hauss1, Hannes Wagner1, Alexandra Loginova1, Rainer Kiko1, Péricles Silva2, Toste Tanhua1, and Arne Körtzinger1,3 Björn Fiedler et al.
  • 1GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
  • 2Instituto National de Desenvolvimento das Pescas (INDP), Mindelo, Cabo Verde
  • 3Christian Albrecht University Kiel, Kiel, Germany
  • anow at: University of Southern Denmark, Nordic Center for Earth Evolution, Odense, Denmark

Abstract. The occurrence of mesoscale eddies that develop suboxic environments at shallow depth (about 40–100m) has recently been reported for the eastern tropical North Atlantic (ETNA). Their hydrographic structure suggests that the water mass inside the eddy is well isolated from ambient waters supporting the development of severe near-surface oxygen deficits. So far, hydrographic and biogeochemical characterization of these eddies was limited to a few autonomous surveys, with the use of moorings, underwater gliders and profiling floats. In this study we present results from the first dedicated biogeochemical survey of one of these eddies conducted in March 2014 near the Cape Verde Ocean Observatory (CVOO). During the survey the eddy core showed oxygen concentrations as low as 5µmolkg−1 with a pH of around 7.6 at approximately 100m depth. Correspondingly, the aragonite saturation level dropped to 1 at the same depth, thereby creating unfavorable conditions for calcifying organisms. To our knowledge, such enhanced acidity within near-surface waters has never been reported before for the open Atlantic Ocean. Vertical distributions of particulate organic matter and dissolved organic matter (POM and DOM), generally showed elevated concentrations in the surface mixed layer (0–70m), with DOM also accumulating beneath the oxygen minimum. With the use of reference data from the upwelling region where these eddies are formed, the oxygen utilization rate was calculated by determining oxygen consumption through the remineralization of organic matter. Inside the core, we found these rates were almost 1 order of magnitude higher (apparent oxygen utilization rate (aOUR); 0.26µmolkg−1day−1) than typical values for the open North Atlantic. Computed downward fluxes for particulate organic carbon (POC), were around 0.19 to 0.23g Cm−2day−1 at 100m depth, clearly exceeding fluxes typical for an oligotrophic open-ocean setting. The observations support the view that the oxygen-depleted eddies can be viewed as isolated, westwards propagating upwelling systems of their own, thereby represent re-occurring alien biogeochemical environments in the ETNA.

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Oxygen-depleted mesoscale features in the open eastern tropical North Atlantic, which are formed in the Mauritanian upwelling region, were discovered recently. This study examines biogeochemical structure and magnitudes of related processes within these isolated water masses. We found very low oxygen concentrations and strongly enhanced acidity at near-surface depth. Oxygen utilization and downward carbon export were found to exceed known values for this ocean region.
Oxygen-depleted mesoscale features in the open eastern tropical North Atlantic, which are formed...
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