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Results

by TIM last modified Apr 03, 2015 07:24 PM
Results of the LEGOS marine geochemistry group

MAIN RESULTS FOR THE YEARS 2005-2009

The main results achieved by our team over the last four years are grouped together under 4 different chapters, corresponding to our main research focuses: i) analytical developments, ii) margin input studies, including modelling iii) process studies, mostly dissolved/particle exchange and iiii) examples illustrating the interest of pluridisciplinary and multi-tracer studies with the examples of the KEOPS and AMANDES projects, which clearly open the

ANALYTICAL DEVELOPMENTS (GEOMAR RESULT SHEETS # 1 TO 6)

An important part of the work proposed for 2005-2009 was the development of analytical platforms and protocols. With the installation of the LAFARA underground laboratory (GEOMAR SHEET#1), the first 226Ra data measured in seawater and particles by MC-ICP/MS (GEOMAR SHEET#2), the development of the isotopic iron analysis providing the first Fe isotope data ever published in seawater (GEOMAR SHEET#3), the development of the Pa isotope measurement, together with the improvement of the procedure of extraction of the tracers from seawater sample (GEOMAR SHEET#4), these objectives have been successfully achieved. In addition, GEOMAR SHEET#5 shows how we internally assess our analytical procedures by comparing the data obtained with different methods present at Observatoire Midi-Pyrénées (e.g. TIMS vs MC/ICPMS or MC/ICPMS vs gamma countingÄ). Finally, we also validated our analytical procedures by participating in the international intercalibration exercise organized in the framework of GEOTRACES. First results are summarised in the GEOMAR SHEET#6.

MARGIN INPUTS: AN ESSENTIAL COMPONENT OF THE OCEANIC CYCLES (GEOMAR RESULT SHEETS # 7 TO 10)

During the past 4 years, a large part of our research activity was focused on a better understanding and quantification of the role of the margin inputs on the oceanic cycle of the elements. Our approach was firstly to acquire new data. This was conducted on Submarine Groundwater Discharge (SGD, likely underestimated so far) that are easily traced using Ra isotopes and provided original results (GEOMAR SHEET#7). Off Kerguelen Island, we observed again seawater Nd isotope changes close to a margin, confirming that Boundary Exchange (BE) was likely acting in this oceanic area (GEOMAR SHEET#8). Simultaneously to the data collection, we developed coupled geochemical-dynamical models, in order to test the importance of BE processes on the Nd oceanic cycle: the thesis results of T. Arsouze allowed us to quantify the rate of exchange and to compare this input term to the dissolved rivers and aeolian dusts on a global scale, a regional scale and during the Last Glacial Maximum (GEOMAR SHEETS #9 and 10).

DISSOLVED/PARTICLE EXCHANGE: A KEY PROCESSUS GOVERNING THE ELEMENT BEHAVIOUR (GEOMAR RESULT SHEETS # 11 AND 12)

Model and data results reported in GEOMAR SHEET#11 and 12 underlined the role of the particles and the imperative requirement of correct quantification of the dissolved/particulate exchange within the water column.

AN EXAMPLE OF MULTITRACER AND INTEGRATED APPROACHES - KEOPS RESULTS (GEOMAR RESULT SHEETS #8, 13, 14) AND AMANDES PROJECT

One of the goals of the KEOPS project was to identify the sources, pathways and fate of the natural fertilizing input that could explain the recurrent bloom occurring in the wake of the Kerguelen archipelago, where the islands are located in the middle of a High Nutrient Low Chlorophyll area (HNLC). To explain these blooms requires the linking of information on the origin of the fertilizing factor (source tracers) to information on the water trajectory (water mass tracers and dynamic modelling) and finally to information on the fate of the matter in the water column (dissolve/particle tracers and particle dynamic). The KEOPS experiment (PI S. Blain, Banyuls) associated the measurements of oceanographic parameters, micronutrient and geochemical tracers in dissolved and solid phases, and biological requirements and activity on the Kerguelen plateau and in the nearby ocean. In this project, our team was in charge of geochemical tracers, by measuring REE and Nd isotopes (source tracers, GEOMAR SHEET #8) with Ra isotopes (water tracers, GEOMAR SHEET#13) and Th isotopes and Pa/Th ratios (particle fate tracers, GEOMAR SHEET #14). The common conclusion deduced from these tracers is that (a) the main source of micronutrient enrichment to the surface waters is likely the dissolution of lithogenic material (basaltic here) deposited on the shelf, (b) this shelf enrichment is rapidly advected over the plateau and (c) an important fraction is rapidly scavenged from the water column. Finally, because of the importance of the Amazon inputs for the Atlantic Ocean (and therefore the world ocean), we proposed an integrated study of the Amazon River, estuary and mouth, coupling geochemical measurements (tracers) with dynamical data and modelling (current-meter moorings, hydrographic data, satellite data and T-UGO model). This global approach is the aim of the AMANDES project which implied four collaborative cruises in 2007 and 2008 Ädata analysis being underway (GEOMAR SHEETS #15 and 16).

Thanks to our successful participation to the GEOTRACES intercalibration exercise (GEOMAR SHEET#6) we are convinced that multitracer studies, coupled to a thorough dynamic description of the study area, are the key for process and budget quantifications. Therefore, our work in the near future will mainly be based on the development of the international collaborative study GEOTRACES.

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