CO2 transfer in landfast sea ice: impact of processes at the interfaces

Sea ice is a biome actively participating in the regional cycling of CO2 both as a source and a sink at different times of the year depending on ice physics, ice chemistry and ice trophic status (autotrophic vs heterotrophic). The porous sea ice provides a dynamic habitat hosting diverse communities of microorganisms (algae, bacteria, heterotrophic protists, fungi and viruses), particularly concentrated at the bottom of the ice at McMurdo Sound (Antarctica). Bacterial and algal productions affect the CO2 dynamics by releasing or consuming CO2, which in turn impacts concentrations of dissolved inorganic carbon (DIC) and total alkalinity (TA) - key parameters to describe the ocean-sea ice-atmosphere CO2 fluxes. The balance between photosynthesis and respiration of both algae and bacteria, expressed as the net community production (NCP), determines the net trophic status of the ice. NCP relates directly to the biogenic contribution of sea ice to CO2 uptake or release.
During the YROSIAE project, which took place at Cape Evans in McMurdo Sound from Nov. 2011 to Dec. 2012, we carried out the first long-term monitoring of pCO2 and CO2 fluxes at sea ice interfaces. The seasonal pattern of air-ice CO2 fluxes was consistent with pCO2 changes, i.e. brine pCO2 over-saturation during late winter (brine concentration of DIC and upward brine expulsion) leading to CO2 degassing, and under-saturation during spring (brine dilution and DIC depletion) leading to atmospheric CO2 uptake. However, diurnal cycles of air-snow-ice CO2 fluxes were superimposed on seasonal changes and appeared to be controlled by the diurnal cycle of basal snow and ice skin temperatures.
Though the ice trophic status is likely to affect CO2 fluxes, it appeared that seasonal and diurnal changes at the sea ice surface were decoupled from the succession of autotrophic and heterotrophic phases observed in the ice interior.
At the bottom of the ice, a large biomass build-up was associated with high remineralisation and heterotrophic activity. Such condition is likely due to the presence of a biofilm (microbial assemblages embedded in extracellular polymeric substances). The biofilm may further promote CaCO3 precipitation in parallel with an increase of salinity-normalized TA. Such sea ice system, where significant heterotrophic activity is maintained independently of the biomass build-up and which supports CaCO3 precipitation jointly with increasing alkalinity, challenges previous insights.