Dissolved iron distribution in the tropical and sub tropical South Eastern Pacific

Dissolved iron (DFe) distributions (< 0.2 mu m) 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-a evidenced by satellite images. Using the new tracer Fe*= DFe- r(Fe:P) (PO43-) we show that DFe was in deficit compared to PO43- 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.