The role of the Arctic sea ice in carbon dioxide exchange

Carbon dioxide exchange between atmosphere and oceans is one of the most important natural processes controlling CO2 concentrations in atmosphere. In the Arctic Ocean, mainly covered by sea ice, the study of CO2 release and absorption by ice is of particular importance and complexity. Sea ice is a complex physical-chemical system. During freezing of seawater, solvent substances are trapped in the ice and undergo chemical reactions. One such reaction that occurs at low temperatures is Ca2 + 2HCO3- = ↓CaCO3 + ↑CO2 + H2O. In natural conditions such reactions can lead to release of carbon dioxide into the atmosphere. To study this process, measurements were made at the Russian drifting station “North Pole35” from November 2007 to May 2008 of the temporal variability of total inorganic carbon content in snow and first year sea ice. Estimates of total inorganic carbon in sea ice were made from measurements of total alkalinity and pH in water obtained from sea ice samples that were melted without gas exchange with atmosphere and use of calculations based on theoretical models of the seawater carbonate system. It was found that total inorganic carbon content in the main part of sea ice core was in proportion to salinity similar to seawater covered by ice. However, in the thin slush layer and upper part of ice samples the ratio decreases due to destruction of calcium hydrocarbon and formation of CaCO3 and CO2. The value of CO2 released to atmosphere from beginning of sea ice freeze up in autumn to beginning of melting in summer is estimated as 20±4 mmol/m2. This is about 10% of maximum amount of CO2 which could be released to atmosphere if total destruction of hydrocarbon occurred in 1 m thick sea ice. Conversely, during the summer melt, first year sea ice extremely under saturated by carbon dioxide which results in uptake of CO2 from atmosphere. Our experimental data show that during the study period, the formation and consequent melting of sea ice at NorthPole35 led to a net uptake of carbon dioxide from atmosphere. The value of net uptake over the annual cycle is on the order of 30 mmol/m2. Comparison is made between the temporal variability of CO2 concentration in atmospheric surface layer measured on the drifting stations “North Pole 35, 36” and at the NOAA observatory in Barrow. In Barrow, increased annual amplitudes of CO2 concentrations have been recorded over recent years. These results support the conclusion that accounting for arctic sea ice cover is important for understanding the CO2 balance in the polar atmosphere.