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

The precipitation of ikaite (CaCO3 center dot 6H(2)O) 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 degrees N, 20 degrees W) of NE Greenland. Ikaite crystals, ranging in size from a few mu m to 700 mu m, 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 surface-ice values of 700-900 mu mol kg(-1) ice (similar to 25x10(6) crystals kg(-1)) to values of 100-200 mu mol kg(-1) ice (1-7x10(6) crystals kg(-1)) near the sea ice-water interface, 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 the lower half of the sea ice were twice as high. This depth-related discrepancy suggests interior ice processes where ikaite crystals form in surface sea ice layers and partly dissolve in layers below. Melting of sea ice and dissolution of observed concentrations of ikaite would result in meltwater with a pCO(2) of < 15 mu atm. This value is far below atmospheric values of 390 mu atm and surface water concentrations of 315 mu atm. Hence, the meltwater increases the potential for seawater uptake of CO2.