Survey of air-ice ocean carbon dioxyde exchange over arctic sea-ice

Sea ice covers about 7% of Earth’s surface at its m axi um seasonal extent, representing one of the largest biomes on the planet. For decades, sea ice has been considered by the scientific community and biogeochemical modelers involved in assessing ocean i CO2 uptake as an inert and impermeable barrier to air-sea exchange of gases. However, this assumption is not supported by studies of the permeability of ice to gases and liquids, which sho w that sea ice is permeable at temperatures above 10°C. Recently, uptakes of atmospheric CO 2 over sea-ice cover have been reported (Delille et al., 2007; Semiletov et al., 2004; Zemmelink et al., 200 6) supporting the need to further investigate pCO 2 dynamics in the sea-ice realm and related CO 2 fluxes. In January 2009, we started a study that aims to ro bustly track CO2 exchange between land-fast sea-ice and the atmosphere during the winter and spring sea son. Towards this aim, a meteorological mast equipped for eddy-covariance measurements was insta lled on land-fast sea-ice near Barrow (Alaska), 1 km off the coast, from the end of January 2009 to the beginning of June 2009, before ice break-up. Due to concerns about using open-path analyzer in c old environment (Burba et al., 2008), the mast was equipped with a CO 2 closed-path analyzer together with a C-SAT 3D soni c a emometer. These data were supported by continuous measurements of s olar radiation, snow depth, ice thickness and temperature profile in the ice. Biogeochemical data necessary for the understanding of the CO 2 dynamics in sea-ice were obtained through regular i ce coring. After data screening, the final dataset consisted 2178 half-hours segments of reliable CO 2 flux data. Two regimes were detected for the CO 2 exchanges linked with the status of the sea-ice: a winter-regime and a spring-summer regime. During the winter period i.e. from start of the mea surements to 27 of April (day 117), air temperature was always below 10°C. The temperature profile in the ice was mostly linear, ranging from 2°C at the sea ice-water interface to below −10°C at the sea-ice surface. Snow depth was almost con tant and around 32 cm. pCO 2 of brines were oversaturated with regard to the at mosphere up to 2200 ppmv. The brine volume at the interface ranged from 3.5 to 6. 8 %. From 27 of March onwards brine volume at the sea ice-snow interface was above the threshold of permeability for liquid according to Golden et a l (1998). During this period, we observed some conspi cuous CO2 fluxes events tightly linked to windspeed. The flux was directed from the sea-ice t o the atmosphere and reached up to 0.6 μmol m s (51.8 mmol m d). This flux to the atmosphere is expected as sea-ice t the air interface is permeable during a large pa rt of the period and brines are oversaturated compared to the atmosphere. CO 2 may accumulate in the snow layer which thus acts as a buffer that is flus hed under occurrence of high wind speeds and associated pressure pumping. During the spring-summer period i.e. from 27 of Apr il (day 117) onwards, we observed a marked increase in sea ice temperature. Temperature profil es suggest that convective events occurred within the ice cover between April 27 and May 05 (day 117 to 125). Within these convective events, two regimes were observed. First, for a period of 5 day s (day 117 to 122), pCO 2 was still above the threshold of saturation and CO 2 fluxes were still mainly positive but lower than i n the winter period, ranging from 0.1 to 0.2 μmol m s. This flux was only moderately controlled by winds peed perhaps due to the reduced snow cover. Further temperature inc ase led to a second flux regime where pCO 2 of the brines were undersatured and sea ice shifted from a source to a sink of CO 2 for the atmosphere, ranging from 0 to 0.1 μmol m s (day 122 to 128). These latter fluxes showed a diu rnal pattern with no exchange during the night and downward fluxes du ring the day. The physical and bio-chemical processes occurring w ithin sea-ice that control these fluxes will be discussed in more depth in the presentation. Fig. 1: Time evolution (day number) of (a) CO 2 flux, (b) wind speed, (c) ice temperatures, (d) ai r temperature and (e) snow cover depth over sea-ice i n Barrow (Alaska).