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GEOCAL

Par tim — Dernière modification 03/04/2015 19:31

GEOCAL

a French contribution to the GEOTRACES intercalibration effort


Objectives:

 

  • An essential GEOTRACES enabling activity that has been assigned as the highest priority for the early phases of the program, is the intercalibration effort. The objective of GEOCAL was to intercalibrate the measurements conducted at LEGOS of Nd, Ra and Fe isotopes and REE, 227Ac, 230Th and 231Pa concentrations in seawater.


 

Scientific Rationale

Trace elements and isotopes play important roles in the ocean as nutrients, as tracers of processes now and in the past, and as contaminants. Their biogeochemical cycling has direct implications for research in such diverse areas as the carbon cycle, climate change, ocean ecosystems, and environmental contamination.

Despite the recognized importance of trace elements and isotopes, our ability to exploit knowledge of their attributes is limited by uncertainties concerning their sources, sinks, internal cycling and chemical speciation within the ocean.

New analytical methods that allow high-density sampling and new modelling strategies make the 2010-2020 decade the right time to mount a major international research program to study the global marine biogeochemical cycles of trace elements and their isotopes: GEOTRACES.

The first step is to verify that the data that will be produced by the different participating groups will be comparable, so that they we can build a consistent global dataset: this is called an intercalibration.


Organization

Two dedicated cruises have been carried out by the USA GEOTRACES community (PI, Greg Cutter), in the Atlantic (Bermuda to Norfolk; July 2008) and in the Pacific (Honolulu, Hawaii to San Diego, California; May 2009), both funded by the NSF.

Very large volume samples were taken and distributed to the various labs worldwide, so that each lab could perform its own measurement on the exact same seawater sample (as blind tests). More details about the GEOTRACES intercalibration exercise can be found at www.geotraces.org/science/intercalibration

The Toulouse Marine Isotopy Group participated in the intercalibration for radium, neodymium and iron isotopes, REE, 230Th and 231Pa concentrations.


Results and schedule

The results are illustrated in Fig. 1. More precisely we participated to the intercalibrations of:

  • The isotopic composition of Nd both dissolved in seawater and in suspended particles

  • The concentrations of REE elements dissolved in seawater

  • The concentrations of 230Th and 231Pa dissolved in seawater

  • The concentrations of 223Ra, 224Ra, 226Ra and 228Ra dissolved in seawater

  • The isotopic composition of dissolved iron in seawater

geocal_image

In summary, apart from the 230Th measurement (because of the breakdowns of our instruments), the Toulouse Marine Isotopy group participation to the GEOTRACES international intercalibration exercise allowed confirming the expertise in trace element and isotope measurements of the group at the international level. These results are published in [Anderson et al., 2012; Boyle et al., 2012; Charette et al., 2012; van de Flierdt et al., 2012]

Those results allowed our group to confidently step in the next phase of the GEOTRACES program: the GEOTRACES sections and process studies. In this framework we have participated to the BONUS/GoodHope (GIPY4, 2008) Pandora (GP12, 2012), MedBlack (GA04, 2013) and GEOVIDE (GA01, 2014) cruises, and the KEOPS2 (GIpr01, 2011) process study.

Validations of analytical protocols never end. This is perpetual motion! We are still involved in new GEOTRACES intercalibration efforts. A new Fe isotope intercalibration for labs which were not ready for the first round or a new suspended particle trace element concentration intercalibration are currently ongoing.

 

Participants: F. Lacan, C. Jeandel, P. van Beek, C. Pradoux, M. Souhaut, A. Radic

Partners: LEGOS, Old Dominion University (USA), MIT (USA), WHOI (USA), LDEO (USA).

Fundings: INSU, NSF

 

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