Oxygen utilization and downward carbon flux in an oxygen-depleted eddy in the eastern tropical North Atlantic
The occurrence of mesoscale eddies that develop suboxic environments at shallow depth (about 40–100 m) 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...
| Main Authors: | , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-10-01
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| Series: | Biogeosciences |
| Online Access: | http://www.biogeosciences.net/13/5633/2016/bg-13-5633-2016.pdf |
| Summary: | The occurrence of mesoscale eddies that develop suboxic environments at
shallow depth (about 40–100 m) 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 5 µmol kg<sup>−1</sup> with a pH of
around 7.6 at approximately 100 m 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 (0–70 m), 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); 0.26 µmol kg<sup>−1</sup> day<sup>−1</sup>) than typical values for the open North Atlantic. Computed downward fluxes for particulate organic carbon (POC), were around 0.19 to 0.23 g C m<sup>−2</sup> day<sup>−1</sup> at 100 m 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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| ISSN: | 1726-4170 1726-4189 |