Physical and chemical controls on ikaite (calcium carbonate hexahydrate) precipitation in seawater

Ikaite, CaCO3•6H2O, is a metastable polymorph of calcium carbonate which is the parent mineral to glendonites (stellate calcite pseudomorphs found throughout the geological record). In the modern ocean, ikaite may be found below the sediment-water interface, or in tufa columns, at temperatures of<7 °C (Buchardt et al., 2001; Zhou et al., 2015; Vickers et al., 2022), yet the presence of glendonites in marine sediments believed to have been deposited under much warmer conditions (i.e. > 10 °C) suggests that this ikaite could have formed under warmer conditions than it does today, or that short-duration cooling interrupted hyperthermal episodes of the early Cenozoic (Vickers et al., 2024). As much uncertainty surrounds the timing, and physical and chemical conditions of the formation, and transformation, of the precursor ikaite, the usefulness of glendonite for (semi-)quantitatively reconstructing temperature and environmental conditions is uncertain, and the explanation for the early Cenozoic glendonites remains obscure. This study examines the physical and chemical controls on ikaite nucleation, growth and transformation in seawater, with a view to understanding trace element and isotope partitioning into ikaite and ikaite transformation products, and if and how certain elemental and isotopic environmental proxies may be applied to glendonite calcite phases. References Buchardt, B., Israelson, C., Seaman, P. and Stockmann, G., 2001. Ikaite tufa towers in Ikka Fjord, southwest Greenland: their formation by mixing of seawater and alkaline spring water. Journal of Sedimentary Research, 71(1), 176-189. Zhou, X., Lu, Z., Rickaby, R.E., Domack, E.W., Wellner, J.S. and Kennedy, H.A., 2015. Ikaite abundance controlled by porewater phosphorus level: Potential links to dust and productivity. The Journal of Geology, 123(3), 269-281. Vickers, M.L., Vickers, M., Rickaby, R.E., Wu, H., Bernasconi, S.M., Ullmann, C.V., Bohrmann, G., Spielhagen, R.F., Kassens, H., Schultz, B.P. and Alwmark, C., 2022. The ikaite to calcite transformation: Implications for palaeoclimate studies. Geochimica et Cosmochimica Acta, 334, 201-216. Vickers, M.L., Jones, M.T., Longman, J., Evans, D., Ullmann, C.V., Wulfsberg Stokke, E., Vickers, M., Frieling, J., Harper, D.T., Clementi, V.J. and Expedition, I.O.D.P., 2024. Paleocene–Eocene age glendonites from the Mid-Norwegian Margin–indicators of cold snaps in the hothouse?. Climate of the Past, 20(1), 1-23.