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Glendonites are pseudomorphs of the mineral ikaite (CaCO3.6H2O) after loss of hydration water and occur in distinctive euhedral crystalline forms, sometimes clustered as rosettes of up to tens of centimeters in diameter. While it is generally accepted that organic-rich environments, methane seeps, and high phosphate levels are important for ikaite formation, glendonite occurrences in ancient sedimentary sequences are widely considered to reflect nearfreezing temperatures, even at high latitudes during periods of greenhouse climates. To fully understand the paleoenvironmental significance of glendonites, a comprehensive examination of the modern ikaite setting is necessary. Temperature is the most important parameter that has been quantitatively constrained for the presence of ikaite. Low bottom-water temperature, while a required condition for formation of the mineral, is not adequate for its growth; other controls are necessary to explain the absence of ikaite in many cold environments. In this study, we discuss the control of carbonate chemistry on ikaite formation. Our compilation of geochemical data from sediment cores with well-preserved ikaite provide further evidence for the importance of phosphate. A phosphate concentration above ∼400 μM in shallow and cold porewater may be the requisite parameter for extensive ikaite precipitation. Thus, abundant glendonites in ancient successions mark past periods and regions of elevated porewater phosphorus concentrations, which may also be related to high surface productivity and/or iron fertilization. [ABSTRACT FROM AUTHOR]