Ikaite Nucleation At 35 Degrees C Challenges The Use Of Glendonite As A Paleotemperature Indicator

Glendonites have been found worldwide in marine sediments from the Neoproterozoic Era to the Quaternary Period. The precursor of glendonite, ikaite (CaCO3 . 6H(2)O), is metastable and has only been observed in nature at temperatures <7 degrees C. Therefore, glendonites in the sedimentary record are commonly used as paleotemperature indicators. However, several laboratory experiments have shown that the mineral can nucleate at temperatures>7 degrees C. Here we investigate the nucleation range for ikaite as a function of temperature and pH. We found that ikaite precipitated at temperatures of at least 35 degrees C at pH 9.3 -10.3 from a mixture of natural seawater and sodium carbonate rich solution. At pH 9.3, we observed pseudomorphic replacement of ikaite by porous calcite during the duration of the experiment (c. 5hours). These results imply that ikaite can form at relatively high temperatures but will then be rapidly replaced by a calcite pseudomorph. This finding challenges the use of glendonites as paleotemperature indicators.