Dynamiques du CO2 et du N2O dans le système océan - glace de mer - atmosphère

Greenhouse gases such as carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) are well known to be indirectly responsible for many changes in the sea ice cover in the polar regions, as these regions are sensitive to global warming. The objective of this manuscript is to look at the two climatic gases addressed (CO2 and N2O) and their behaviour in the ocean – sea ice – atmosphere system in the actual warming climate, thus more specifically in the Arctic. On the one hand, the dynamic of CO2 has been studied through artificial sea ice during an experiment performed on two series of mesocosms: one was filled with seawater (SW) and the other one with seawater with added filtered humic-rich river water (SWR). The addition of river water almost doubled the dissolved organic carbon (DOC) concentration in SWR and consequently affected the partial pressure of carbon dioxide (pCO2). This experiment supports previous observations showing that the pCO2 in sea ice brines is generally higher in Arctic sea ice compared to that from the Southern Ocean, especially in winter and early spring. Indeed, DOC is larger in the Arctic seawater: higher concentrations of DOC would be reflected in a greater DOC incorporation in sea ice, enhancing bacterial respiration, which in turn would increase the pCO2 in the ice. Within the same experiment, air–ice CO2 fluxes were measured continuously using automated chambers from the initial freezing of a sea ice cover until its decay. Cooling seawater prior to sea ice formation acted as a sink for atmospheric CO2, but as soon as the first ice crystals started to form, sea ice turned to a source of CO2, which lasted throughout the whole ice growth phase. Once ice decay was initiated, sea ice shifted back again to a sink of CO2. Combining measured air–ice CO2 fluxes with the pCO2 in the air and sea ice, we determined two strongly different gas transfer coefficients of CO2 at the air–ice interface between the growth and the decay phases (2.5 mol m−2 d−1 atm−1 and 0.4 mol m−2 d−1 atm−1 respectively). In the other hand, we present in this work the first winter to spring N2O observations in the sea ice and the upper ocean of the western Nansen Basin (Arctic Ocean). In the seawater, a general N2O undersaturation with respect to the atmosphere was identified at the surface, with a clear enrichment north of 82°N originating from the water masses passing through the Arctic polar circle. We show that the main source of N2O enrichment originates from the East Siberian Arctic Shelf, a key place for benthic denitrification and sea ice formation rejecting brine – including N2O – in the seawater. Sea ice shows N2O oversaturation throughout winter and spring in both first-year (FYI) and second-year ice (SYI). Further, SYI, with expected lower salinity than FYI, is enriched in N2O compared to the dilution curve for salinity. This non-conservative N2O content of SYI with salinity is likely due to (i) SYI formation further east than FYI, (ii) in situ biological activity, (iii) flushing of N2O-rich ice surface meltwater (i.e. brine skim) through decaying FYI and (iv) strongly reduced permeability in SYI. Finally, we suggest that the high N2O concentrations observed in the snow cover results from a combination of brine rejection at the top of the ice (brine skim) and chemical N2O production (under the process of chemodenitrification).CO2 and N2O dynamics in the ocean - sea ice - atmosphere syste