| Summary: | Arctic Ocean contributes to the global oceanic uptake of CO2 by about 5% to 14% in taking up from 66 to 199 TgC yr-1. However, the role of the marine cryosphere was ignored because it is considered as an impermeable barrier, impeding the gas exchanges between the ocean and the atmosphere [Bates and Mathis, 2009]. However, a growing body of evidence suggests that gases exchange could occur between sea ice and the atmosphere. In this context, two Arctic surveys were carried out in the framework of the International Polar Year (IPY). From there, we present a snapshot of the partial pressure of CO2 (pCO2) dynamics firstly during the initial sea ice growth and secondly from early spring to the beginning of the summer. We confirmed previous laboratory measurement findings that growing young sea ice acts as a source of CO2 to the atmosphere by measuring CO2 efflux from the ice (4 to 10 mmol m-2 d-1). We also confirmed the precipitation of calcium carbonate as ikaite in the frost flowers and throughout the ice and its negligible role on the effluxes of CO2. In early spring, supersaturations in CO2 (up to 1834 µatm) were observed in sea ice as consequence of concentration of solutes in brines, CaCO3 precipitation and microbial respiration. As the summer draw near, brine shifts to a marked undersaturation (down to almost 0 µatm) because of the brine dilution by ice meltwater, dissolution of CaCO3 and photosynthesis during the sympagic algal bloom. Out of the winter, soon as the ice becomes permeable, CO2 fluxes were observed: (i) from the ice to the atmosphere, as the brine were supersaturated, (ii) from the atmosphere to the ice, as brine shift to an undersaturation. Temperature appears to be the main driver of the pCO2 dynamics within sea ice. It mainly controls the saturation state of the brine (where others processes may be added, e.g., CaCO3 precipitation, primary production) and thus, the concentration gradient of CO2 between sea ice and the atmosphere. It also controls the brine volume and so the brine connectivity, allowing the gas exchanges between sea ice and the atmosphere. We also present a new analytical method to measure the pCO2 of the bulk sea ice. This method, based on equilibration between an ice sample and a standard gas, was successfully applied on both artificial and natural sea ice. However, this method is only applicable for permeable sea ice (i.e., brine volume > 5% [Golden et al., 1998; 2007]) to allow the equilibration between the ice and the standard gas.
|
|---|