Ikaite is a metastable calcium carbonate hexahydrate (CaCO3·6H2O), which naturally occurs in environments characterized by temperatures of −2 to +7 °C. It is therefore considered as an indicator of cold water conditions, although its suitability as a paleotemperature indicator is still questioned, especially as under certain laboratory conditions, ikaite was formed at temperatures of up to 35 °C. At warmer temperatures, ikaite becomes unstable and quickly decomposes to anhydrous calcium carbonate(s) and water. However, the primary ikaite crystal morphology can occasionally be preserved as a pseudomorph, generally known as ‘glendonite’. Even in early studies, glendonites were considered as indicators of cold climates due to their association with cold water fauna, dropstones and other cold climate indictors.. We are using a global glendonite database with more than 900 occurrences to reconstruct their distribution through Phanerozoic time. Based on these reconstructions we observe that the Early Paleozoic glendonites cannot be directly correlated with cooler global climate episodes, but are restricted to the upwelling of cold waters on the wide shelf of the Baltic paleobasin. Carboniferous–Permian occurrences correlate with the Late Paleozoic Ice Age, one of the longest and most prominent glacial events in Earth's history. Our plate reconstructions suggest that most Late Paleozoic glendonite occurrences are restricted to the high latitudes. Glendonites are absent from Triassic and Upper Cretaceous sediments around the world, which were deposited during periods generally considered as the hottest of the Phanerozoic. However, glendonites are numerous within certain Jurassic and Cretaceous strata deposited during well-known cooling intervals. Our reconstructions suggest that the majority of Mesozoic glendonite occurrences are confined to the polar or near-polar regions. Paleogene–Neogene occurrences are restricted to the middle and high latitudes of the northern hemisphere. In addition to glendonites from marine deposits, a few findings are known from continental environments. Quaternary ikaites and glendonites have a widespread distribution in marine settings close to polar seas (mainly Arctic shelves) or on deep-water continental margins (e.g., Zaire deep fan, Nankai Trough, Sakhalin Island slope), with rare findings in terrestrial environments such as lakes, ice sheets and caves. In contrast to Quaternary occurrences, the older Mesozoic and Paleozoic glendonites are confined to shallow marine shelf environments. This discrepancy is likely to be related to the lower preservation potential of deep-water and terrestrial sediments in deep time, as opposed to sediments deposited at high rates along the margins of epicontinental or marginal seas.
