Quantification of calcium carbonate (ikaite) in first– and multi–year sea ice

Abstract. Ikaite (CaCO₃•6H₂O) is a metastable calcium carbonate mineral that forms at low temperature and/or high pressure. Ikaite precipitates in sea ice and may play a significant role in air–sea CO₂ exchange in ice covered seas and oceans. However, the spatial and temporal dynamics of ikaite in sea ice are poorly understood due to few available measurements and time consuming analytical techniques. Here, we present a new method for quantifying ikaite in sea ice and compare it with a more time-consuming imaging technique currently in use. In short, sea ice cores were melted at low temperatures (< 4 °C), filtered for ikaite crystals that subsequently were dissolved and analyzed as dissolved inorganic carbon (DIC). The new method was applied on cores from experimental sea ice in Winnipeg (49° N), Canada, first–year sea ice near Cambridge Bay (69° N), Nunavut, Canada, and first– and multi–year sea ice near Station Nord (81° N), Greenland. Ikaite crystals were found in all sea ice types. The new ikaite quantification method is a straightforward technique that generally agrees with the image analysis technique and is both more accurate and precise. The DIC method may give lower concentrations in first–year ice and higher concentrations in multi–year ice than image analysis, likely due to the large spatial variability of ikaite crystals in first–year sea ice and the small crystal size in multi–year ice, both of which make quantification by image analysis more difficult. The new method showed high concentrations of ikaite in 20 cm thick young sea ice (335 µmol kg⁻¹), lower concentrations in 1.5 m thick first–year sea ice (45 µmol kg⁻¹) and low concentrations in 3.3 m thick multi–year sea ice (3 µmol kg⁻¹). Highest concentrations were observed in the upper ice layers at all stations and layers where sea ice algae were present. The higher abundance of ikaite in young first–year sea ice indicates that its concentrations will likely increase in the Arctic as a result of the recent rapid decline of the multi–year ice cover and increasing presence of seasonal sea ice. As a result, it is likely that ikaite will play a more significant role in air–sea CO₂ exchange in ice–covered seas in the future.