Factors controlling the precipitation of ikaite crystals within sea ice in a laboratory experiment

Abstract

Laboratory experiments were carried out with natural seawater that was frozen in a tank at －25 ℃ in a low-temperature room to understand the factors controlling the precipitation of ikaite (CaCO₃・6H₂O) crystals in sea ice. The seawater remained frozen for 1, 7, or 21 days and was then melted at ＋4 ℃．Measurements of the numbers and sizes of ikaite crystals revealed no ikaite crystals after one day of freezing, but there were 565 and 4998 crystals per liter of seawater after 7 and 21 days of freezing, respectively. The distribution of crystal sizes depended on the duration of freezing. We examined the effect of changing the carbonate chemical components of the seawater on the precipitation of ikaite crystals by adding NaOH to the natural seawater prior to freezing. The number of ikaite crystals increased with increasing amounts of added NaOH because conditions became more favorable for precipitation of ikaite crystals at high pH.