Ikaite crystal distribution in Arctic winter sea ice and implications for CO2 system dynamics

• Main Authors: Rysgaard, Søren, Søgaard, D. H., Cooper, M., Pućko, M., Lennert, Kunuk, Papakyriakou, T. N., Wang, F., Geilfus, Nicolas-Xavier, Glud, Ronnie Nøhr, Ehn, J., McGinnnis, D. F., Attard, Karl, Sievers, Jakob, Deming, J. W., Barber, D.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2012
• Subjects: Arctic; Greenland; Sea ice; The Cryosphere; The Cryosphere Discussions
• Online Access: https://pure.au.dk/portal/en/publications/617eace0-3737-4581-ba0d-3d7021263552; https://doi.org/10.5194/tcd-6-5037-2012; https://pure.au.dk/ws/files/82099753/Rysgaard_et_al._2013_Ikaite_crystal_distribution_in_winter_sea_ice_and_implications_for_CO2_system_dynamics.pdf; http://adsabs.harvard.edu/abs/2012TCD.6.5037R

The precipitation of ikaite (CaCO3·6H2O) in polar sea ice is critical to the efficiency of the sea ice-driven carbon pump and potentially important to the global carbon cycle, yet the spatial and temporal occurrence of ikaite within the ice is poorly known. We report unique observations of ikaite in unmelted ice and vertical profiles of ikaite abundance and concentration in sea ice for the crucial season of winter. Ice was examined from two locations: a 1 m thick land-fast ice site and a 0.3 m thick polynya site, both in the Young Sound area (74° N, 20° W) of NE Greenland. Ikaite crystals, ranging in size from a few µm to 700 µm were observed to concentrate in the interstices between the ice platelets in both granular and columnar sea ice. In vertical sea-ice profiles from both locations, ikaite concentration determined from image analysis, decreased with depth from surfaceice values of 700-900 µmol kg-1 ice (~ 25 × 106 crystals kg-1) to bottom-layer values of 100-200 µmol kg-1 ice (1-7 × 106 kg-1), all of which are much higher (4-10 times) than those reported in the few previous studies. Direct measurements of total alkalinity (TA) in surface layers fell within the same range as ikaite concentration whereas TA concentrations in bottom layers were twice as high. This depth-related discrepancy suggests interior ice processes where ikaite crystals form in surface sea ice layers and partly dissolved in bottom layers. From these findings and model calculations we relate sea ice formation and melt to observed pCO2 conditions in polar surface waters, and hence, the air-sea CO2 flux.