Biogeosciences
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2008
10.5194/bg-5-269-2008
http://www.biogeosciences.net/5/269/2008/
http://www.biogeosciences.net/5/269/2008/bg-5-269-2008.html
http://www.biogeosciences.net/5/269/2008/bg-5-269-2008.pdf
269
280
2008-02-27
Dissolved iron distribution in the tropical and sub tropical South Eastern Pacific
S. Blain
stephane.blain@obs-banyuls.fr
S. Bonnet
C. Guieu
Laboratoire d'Océanographie et de Biogéochimie, Campus de Luminy, case 901, 13288 Marseille cedex, France
Laboratoire d'océanographie de Villefranche sur mer, Quai de la darse, Villefranche sur mer, France
present address: UPMC Univ. Paris 06, UMR 7621, Laboratoire d'Océanographie Biologique, 66650 Banyuls/mer, France
Dissolved iron (DFe) distributions (<0.2 μm) were determined in the upper water column
(0–400 m) of the south eastern tropical and subtropical Pacific, in October–November 2004. Data
were collected along a transect extending from the Marquesas Islands to the Chilean coast
with most of the stations located in the south Pacific gyre. The concentrations of DFe
presented large variability with highest values observed at both extremities of the transect. In
the Chilean upwelling, DFe concentrations ranged between 1.2–3.9 nM. These high values
result from inputs from the continental margin and are likely maintained by anoxic conditions
in the water corresponding to the Oxygen Minimum Zone (OMZ). In subsurface waters near
the Marquesas, that were also associated with the extension of the OMZ, DFe concentrations
varied between 0.15–0.41 nM. Vertical transport of this water by mesoscale activity eastward
of the archipelago may explain the dissymmetric east-west distribution of
chlorophyll-<i>a</i> evidenced by satellite images. Using the new tracer Fe*=DFe−<i>r</i><sub>Fe:P</sub>
(PO<sub>4</sub><sup>3−</sup>) we show that
DFe was in deficit compared to PO<sub>4</sub><sup>3−</sup> resulting from the remineralisation of organic matter.
This suggests that the Marquesas islands and the surrounding plateau are not a significant
source of DFe. In the gyre, DFe concentrations in the upper 350 m water column were around
0.1 nM and the ferricline was located well below the nitracline. These low concentrations
reflect the low input of DFe from the atmosphere, from the ventilation of the upper
thermocline with water containing low DFe, and from the low biological activity within
this ultra oligotrophic gyre.
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