TONGA - Ocean productivity and carbon export


Shallow hydroThermal sOurces of trace elemeNts: potential impacts on biological productivity and the bioloGicAl carbon pump


TONGA is a multidisciplinary project dedicated to the study of the control of ocean productivity and carbon export driven by micronutrients from hydrothermal origin. It is based both on a 37-day oceanographic cruise in the Western Tropical South Pacific (R/V L’Atalante) and modelling work and involves hydrothermal geochemists, physical oceanographers, trace element chemists (ocean and atmosphere), biogeochemists, biologists and modellers.


During the cruise (Nov.-Dec. 2019), we will deploy both drifting (5 days) and fixed (one year) mooring lines equipped with sediment traps in the vicinity of active volcanoes releasing hydrothermal Fe-rich fluids.

  1. The drifting mooring line will allow to study, on a high temporal resolution (daily), CNPSi export, mesopelagic remineralization rates, and associated dissolved trace metal release rates, elemental budgets, and diversity of planktonic organisms exported (sampling for stocks/fluxes of metals and CNPSI will be performed daily in the same water mass during the deployment as we performed during former cruises).
    In particular, processes regulating the exchanges between the dissolved and the particulate pools of trace elements (remineralization vs. scavenging) will be investigated using the trace metal clean RESPIRE traps.
  2. The fixed mooring line deployed for one year will study the seasonal variability of export and the contribution of N2 fixation to such fluxes. Using this δ15N budget, we will determine what fraction of export production was supported by N2 fixation during the deployment.
    By multiplying this fraction of export production supported by N2 fixation by the PN sinking flux, we can calculate an absolute N2 fixation rate for the duration of each the sediment trap deployment (along the annual cycle for the fixed line) and correlate this with surface chlorophyll data, POC fluxes and ARGO data.




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