ISOFERIX-EUCFe
Scientific Rationale
The Eastern Equatorial Pacific is a High Nutrient Low Chlorophyll (HNLC) area. Primary production there is thought to be controlled, notably, by iron supply upwelled from the Equatorial Undercurrent flowing at around 200m depth all the way across the Pacific from the Papua New Guinea area (see the iron section below). Understanding the processes associated to these sources and transports was therefore essential to understand what drives primary production in the Eastern Equatorial Pacific.
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Organization
In order to do so, LEGOS participated to the EUCFe cruise, (PI J. Murray, Univ. Washington, R/V Kilo Moana, 45 days at sea in August-Sept. 2006), which cruise track is shown on the left. There were a total of 20 scientists on board, biologists, geochemists and physicists. Atmospheric deposition was also studied. The Toulouse Marine Isotopy group collected samples for the analyses of 1) neodymium (Nd) isotopes and Rare Earth Element (REE) concentrations and 2) iron (Fe) isotopes, in the framework of the ISOFERIX project (LEFE-CYBER).
In addition, samples have been taken by our physicist colleagues from the Flusec cruise in the Coral Sea (sea crosses on the map) for Nd isotopes.
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Map of the EUCFe cruise. Locations of Fe and Nd isotope sampling are indicated by
black circles. The 2 black crosses indicate samples collected during the FLUSEC cruise
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Fe (unfiltered) concentration along the equator from this cruise.
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3 LEGOS PhD students worked on theses samples. A. Radic and M. Labatut for Fe isotopes, and M. Grenier for Nd isotopes and Lagrangian trajectory modelling. This project is tightly linked to the SOLWARA project built between the LEGOS physic and geochemistry groups, focusing on the Salomon Sea, located just upstream of the areas studied in the ISOFERIX-EUCFe project.
Results and schedule
Iron isotopes have been measured in both the dissolved fraction and the suspended particles. The dissolved isotopic composition in the EUC all along the Equatorial section (not shown) displays the exact same value confirming the fact that the iron is transported all along that way without major alteration and that its source in the Eastern Equatorial Pacific is indeed the western Equatorial Pacific.
As shown in the figure on the right, in the source area (around Papua New Guinea) dissolved iron is systematically heavier (larger values of d56Fe) than particulate Fe. Because the dissolved Fe is though here to be derived from particulate iron (because of huge particulate iron inputs and very large particulate Fe concentrations), these isotopic data suggest that the release of dissolved iron from these particles occurs without any reduction process (otherwise the dissolved Fe would be lighter), contrary to what was suggested in all studies published recently on this topic. This work lead us to introduce the new concept of "non reductive release of dissolved Fe" (Radic et al 2011, Labatut at al. Sub).
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Neodymium isotopic compositions and Rare Earth Element (REE) concentrations measured in the dissolved phase allowed to study the fluxes of terrigeneous matter entering the ocean in the Papua New Guinea area, and thereby fertilizing the water masses with micronutrient (notably iron). They suggest that they result from the dissolution of erosion products from the surrounding basaltic formations, deposited on the continental shelves and slopes (Grenier et al. 2013).
Nd isotopes also contributed to reveal and/or confirmed water mass trajectories.
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This project is now over. Its scientific thematic are pursued in the framework of the Solwara project.
Participants: F. Lacan, M. Labatut, C. Jeandel, M. Grenier, A. Radic, C. Venchiarrutti, C. Pradoux, J. Murray
Partners: LEGOS : University of Washington (Seattle, USA)
Fundings: NSF, INSU