The study of ikaite (CaCO3·6H2O) formation and its impact on biogeochemical processes in artificial sea ice brine

Abstract

Calcium carbonate precipitation in polar sea ice has been proposed as one of the driving forces for the carbon pump in sea ice covered regions. After decades of controversial discussion on whether calcium carbonate can be precipitated in sea ice, the mineral ikaite (CaCO3·6H2O) was for the first time discovered in Antarctic sea ice (Dieckmann et al., 2008) and later also found in Arctic sea ice (Dieckmann et al., 2010). However, the mechanism of ikaite precipitation in sea ice is not well known, as is the effect of ikaite precipitation on biogeochemical processes in sea ice. The aim of this thesis was to study, under simulated sea ice brine conditions, whether ikaite is the only phase of calcium carbonate formed in sea ice and to determine the effect of pH, salinity, temperature and phosphate concentrations on the precipitation of ikaite, as well as the effect of ikaite precipitation on biogeochemical processes in sea ice. In the first part of this thesis, I investigate the pathway of ikaite formation in solution and the effect of pH as well as phosphate (PO4) on polymorphism of calcium carbonate in general. It can be shown that the formation of ikaite does not necessarily follow a precursor amorphous calcium carbonate (ACC) pathway and ikaite can be precipitated directly from solution. pH and PO4 can act as a switch for different calcium carbonate polymorphs. At near freezing temperatures, high pH as well as the presence of PO4 favours ikaite formation in fresh water, while low pH and the absence of PO4 are in favor of vaterite formation. In the second part of this thesis, I study the effect of different parameters in sea ice brine (pH, salinity, temperature and phosphate concentrations) on ikaite formation as well as the effect of ikaite precipitation on PO4 removal in artificial sea ice brine. The results show that ikaite is very likely the only polymorph precipitated in natural sea ice. Phosphate is not crucial for ikaite formation in sea ice. The change in pH and salinity has a large impact on ikaite precipitation in sea ice, while the change in temperature and phosphate concentrations has little effect. PO4 can be coprecipitated with ikaite. pH as well as the initial PO4 concentrations greatly affects the PO4 removal by ikaite precipitation, while the change in salinity (S > 0) and temperature shows no effect on the coprecipitation of PO4 with ikaite. These findings may shed some light on the observed variability of PO4 concentrations in natural sea ice.