Laboratory study on coprecipitation of phosphate with ikaite in sea ice

Ikaite (CaCO 3 ·6H 2 O) has recently been discovered in sea ice, providing first direct evidence of CaCO 3 precipitation in sea ice. However, the impact of ikaite precipitation on phosphate (PO 4 ) concentration has not been considered so far. Experiments were set up at pH from 8.5 to 10.0, salinities from 0 to 105, temperatures from −4°C to 0°C, and PO 4 concentrations from 5 to 50 µmol kg −1 in artificial sea ice brine so as to understand how ikaite precipitation affects the PO 4 concentration in sea ice under different conditions. Our results show that PO 4 is coprecipitated with ikaite under all experimental conditions. The amount of PO 4 removed by ikaite precipitation increases with increasing pH. Changes in salinity (S ≥ 35) as well as temperature have little impact on PO 4 removal by ikaite precipitation. The initial PO 4 concentration affects the PO 4 coprecipitation. These findings may shed some light on the observed variability of PO 4 concentration in sea ice.