The role of sea ice dynamics in carbonate mineral production and its fate in the Polar Oceans

The interaction between the marine and atmospheric carbon cycle is a critical factor in understanding climate change. The polar oceans play an important role in mediating the Earth's climate, for example, by providing an appreciable part of the global carbon sink in their surface waters and up to 80% of reflection of solar radiation by polar ice. Although the interplay between biology and climate change is a major focus of current studies, less attention has been paid to abiotic drivers that may influence carbon cycling and sequestration. Carbonate minerals and their production in sea ice is an unquantified component of the polar carbon cycle, and key aspects of their dynamics need to be studied before a true appreciation of their role can be assessed. Sea ice is a layer of frozen seawater typically seen floating on the polar oceans. It varies in thickness from a few centimetres to tens of meters and at its maximum extent (in winter) covers up to 13% of the Earth's surface. Seawater begins to freeze at -1.85 oC, leaving its salts in the water (brine) that remains. At -10 oC the brine is four times saltier than seawater. Thermodynamic principles predict that under these conditions minerals, such as calcium carbonate, should precipitate, and there is some indirect evidence to support this from laboratory experiments. The consequences of carbonate formation and its subsequent dissolution are complex and may have a strong bearing on the carbon cycle in polar oceans. For example, it has been estimated that 720,000,000 t of carbon may be removed from surface to deep polar waters as a consequence of carbonate mineral formation in sea ice. Until recently, these estimates have remained subjective and speculative, because neither carbonate minerals nor their mineral form had been described, while the effects of physical-chemical properties on their precipitation and dissolution in sea ice was unknown. The recent discovery (G. S. Dieckmann, pers. comm.) of ikaite, a metastable phase of hydrated CaCO3 (CaCO3.6H2O), has confirmed the presence and form of the carbonate mineral in sea ice but has left us unable to assess the significance of its role in carbon cycling in polar oceans because controls on its production and dissolution in sea ice are unknown. Our aim is to perform laboratory experiments that will determine 1] whether previous thermodynamic modelling of ikaite formation correctly describes the onset and rate of precipitation and dissolution of this mineral from brine solutions typical of sea ice; 2] whether the confined conditions found in the brine channels of sea ice affect the manner and rate at which these processes occur. We will then use our findings to refine our understanding of the importance of ikaite in carbon cycling in the polar oceans. Our second aim is to use the stable isotopic composition of ikaite crystals to reconstruct some of the conditions under which the mineral precipitates in sea ice. This is currently impossible in the natural environment because we cannot probe individual brine pockets to derive the information that we need directly. A combination of oxygen and carbon isotopic measurements of CaCO3.6H2O will allow us to record the temperature and the properties of the brine at the scale of the brine pocket.