The study of ikaite formation in sea ice

• Main Authors: Hu, Y.-B., Nehrke, Gernot, Dieckmann, Gerhard, Völker, Christoph, Wolf-Gladrow, Dieter
• Format: Conference Object
• Language: unknown
• Published: 2012
• Subjects: Sea ice
• Online Access: https://epic.awi.de/id/eprint/30423/; https://epic.awi.de/id/eprint/30423/1/The%20study%20of%20ikaite%20formation%20in%20sea%20ice.pdf; https://hdl.handle.net/10013/epic.39530; https://hdl.handle.net/10013/epic.39530.d001

Ikaite (CaCO3.6H2O) is a metastable mineral of calcium carbonate, which is usually found in environments characterized by low temperature (below 5°C), high pH, high alkalinity, high concentration of phosphate and organic matter. Although synthetic CaCO3.6H2O was already known from laboratory studies in 1865, ikaite was first observed in nature in 1963. Recently, Dieckmann et al. (2008, 2010) discovered this mineral in sea ice, which at the same time, was the first direct proof of CaCO3 precipitation in sea ice. However, little is known about the mechanism of ikaite formation in sea ice. Our study focuses on how physicochemical processes in sea ice affect the formation of ikaite. Experiments were set up at pH ranging from 8.5 to 9.0, and salinity ranging from 0 to 105 at 0 °C, in order to examine the effect of pH, salinity and also phosphate on the formation of ikaite. Preliminary results read: (1) Experiments show that ikaite can form at different pH levels (8.5, 9.0). At high pH, the induction time (the time when the crystals start to precipitate) is shorter which means high pH favours the formation of ikaite. This might be expected given higher CO32- concentrations and thus higher saturation levels for ikaite with increasing pH. (2) The results of experiments with different salinities show that ikaite can form over wide range of salinities from 0 to 105 both in Artificial SeaWater (ASW) and NaCl solution in the presence of phosphate. In ASW, the induction time increases with salinity from S = 0 to S =105; while in NaCl solution, the induction time first increases with salinity and then decreases with the further increase of salinity. Salinity plays both positive and negative roles in the formation of ikaite. On the one hand, the increase in salinity will increase the fraction of CO32- in DIC. On the other hand, the increase in salinity means more ions are involved in the solution, which will reduce the activities of Ca2+ and CO32- by forming ion pairs with them. This effect is more obvious in ASW, as there are ...