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Special Event

Sea ice, sink or source for atmospheric CO2?

Monday, 22 June, 10:30 am
Dr. Nicolas-Xavier Geilfus, Postdoctoral Fellow
Fairbanks—201 O'Neill • Seward— Rae Building • Juneau—212 Lena Point bldg.

The Arctic Ocean represents an important CO2 sink, with current estimates of net air to sea CO2 fluxes from -66 to -199 Tg C yr-1. However, the magnitude and role of sea ice in the air-sea CO2 exchange remain uncertain. Studies from both polar regions show that CO2-carbonate chemistry in sea ice and brine is heterogeneous and variable, leading to complex CO2 dynamics. However, these studies suggest that sea ice may act as an important control on the CO2 partial pressure in the ocean surface layer.
Sea ice formation and resulting brine rejection has been postulated to drive CO2 fluxes to deep seawater. However, during the early stage of sea ice growth, we reported a significant release of CO2 from sea ice to the atmosphere due to CO2-supersaturated brine expelled upward through the sea ice matrix. Studies of the spring-summer transition investigated the importance of temperature and salinity changes, the precipitation/dissolution of ikaite (CaCO3.6H2O), and biological activity on the inorganic carbon dynamics in sea ice. Temperature appears to be the main driver of the sea ice pCO2 dynamics. It mainly controls the saturation state of the brine and the sea ice-atmosphere CO2 concentration gradient. As temperature decreases, salts concentration, including inorganic carbon, increases within the brine and may promote a precipitation of ikaite. Both processes lead to an increase of the brine pCO2. As temperature increases, brines are diluted by fresh meltwater within the ice matrix and ikaite crystals are dissolved. The combination of these processes, associated with a biological production, lead to a strong decrease of the brine pCO2.
Sea ice melt, melt pond formation and their resultant dilution of the surface seawater lead to a reduction of the surface seawater pCO2 and promote an additional uptake of atmospheric CO2. Melting Arctic seasonal sea ice represents an additional uptake of atmospheric CO2 of -10 Tg of C yr-1. However, additional data are needed to flesh out these initial results.
While it is clear that sea ice can represent a strong source or sink of atmospheric CO2, depending on the season, the contribution of seasonal processes to the net sequestration of carbon over the yearly cycle remains elusive.