Trace metals distributions in the Southern Ocean : Kerguelen Plateau process study

The atmospheric concentration of carbon dioxide (CO2) is of a great interest for scientists since the largest contribution to Earth total radiative forcing is caused by the increase in the atmospheric concentration of CO2 since 1750 (IPCC 2013). Indeed, from 1750 to 2011, anthropogenic CO2 emissions have released 555 gigatons of carbon (GtC) that accumulated within different ecosystems and mainly in the atmosphere (240 GtC), 160 GtC have accumulated in natural terrestrial ecosystems and 155 GtC have been taken up by the ocean (IPCC 2013). Two processes lead to the transfer of CO2 from the atmosphere to the ocean: the physical and the biological carbon pumps (Fig. I.1). First, the physical carbon pump is initialized by the surface ocean, which constantly exchanges gases and heat with the low atmosphere. The solubility of CO2 in seawater increases when temperatures decrease. Thus, in the high latitudes, low temperatures, strong winds, and waves favour the transfer of CO2 into the ocean. Then, CO2 is exported beneath the ocean surface via the sinking of the cold CO2-rich waters. This physical removal of CO2 is also called the solubility pump‚ÄövÑvp. When these cold waters return to the surface ocean and warm up again, they release CO2 to the atmosphere. Secondly, the organic biological carbon pump is initialized in the surface ocean by phytoplankton, which assimilates carbon (C) via photosynthesis, converting CO2 and water in organic matter and oxygen (Fig. I.1). This process removes aqueous CO2 from the euphotic layer, where light penetrates sea surface, and leads to the storage of around 20% of the surface C inventory in the ocean interior (Laws et al., 2000; Falkowski, 2002). In addition, C can also be fixed by corals, foraminifera, coccolithophore or mollusk and crustacean as calcium carbonate to form protective coating or shells, however, this process is counter balanced by the production of carbonic acid, which increase the concentration of CO2 in seawater initiating a diffusive flux of CO2 from the ocean to the atmosphere, the so called carbonate counter pump. These are the reasons why the biological pump of C is a key regulator of the global C cycle. In its absence, atmospheric CO2 concentration would increase by approximately 50 % (200 ppmv), a considerable fraction compared to the present ~ 400 ppmv (Falkowski, 2002).