Ikaite nucleation at 35 °C challenges the use of glendonite as a paleotemperature indicator

Abstract

Glendonites have been found worldwide in marine sediments from the neoproterozoic era to the Quaternary period. the precursor of glendonite, ikaite (caco 3 • 6H 2 o), is metastable and has only been observed in nature at temperatures <7 °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 °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 °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. 5 hours). 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. Calcium carbonate, one of the most common naturally occurring minerals, has an important role in the global carbon cycle. Calcite is the stable form of calcium carbonate at the Earth's surface. However, growth of calcite is often inhibited by a range of external factors 1-3. This explains why metastable hydrous and anhydrous carbonate minerals tend to form instead of calcite. A common example is the precipitation of aragonite instead of calcite in the oceans. The Mg/Ca ratio of the seawater controls this precipitation: when this ratio exceeds 2, aragonite is favoured because Mg acts as an inhibitor of calcite growth 1,4. Two hydrous forms of calcium carbonate occur instead of calcite under certain conditions: monohydrocalcite (CaCO 3 •H 2 O) and ikaite (CaCO 3 •6H 2 O). Monohydrocalcite (MHC) has been found as calcareous incrustations and the main form of calcium carbonate in Lake Issyk-Kul, Republic of Kyrgyzstan 5 and as beach rocks around two salt lakes 6. Experimental results and investigation of natural samples indicate that the formation of MHC requires high Mg/Ca ratios and a pH >8 in the solution from which it forms 6-8. The second hydrous form, ikaite, is more common than MHC despite the narrow temperature range of its stability. The mineral was first discovered as tufa columns in Ikka Fjord, SW Greenland 9,10. Ikaite has only been observed in nature at temperatures between −2 and 7 °C 11. At temperatures >7 °C, ikaite transforms to calcite and water pseudomorphically or by decomposition. In spite of the narrow temperature range of its stability, ikaite has been widely reported: in organic rich marine sediments 12-14 , in sea ice 15,16 , in speleothems 17 , as seasonal tufa columns in alkaline lakes 18 , as precipitates in sediments or on the shores of alkaline springs or lakes 19,20 , and as precipitates in riverbeds caused by anthropogenic pollution 21. In 1982, Suess et al. discovered authigenic ikaite crystals in marine sediments, and the resemblance of these crystals to the pseudomorph glend...

Cold spells in the Nordic Seas during the early Eocene Greenhouse

“…The flora and fauna of the Fur Formation suggest local tropical to sub-tropical climates 5 . Thus, the presence of numerous, unusually large glendonites at several levels in the succession has long been a conundrum 16 , and appears to support the findings that ikaite may grow at much higher temperatures than near-freezing 9 . Quantitative temperature reconstructions for the glendonites are needed to test if the glendonites of early Eocene Denmark are indeed indicative of very low temperatures (≤4°C 8 ).…”

“…The glendonites contain higher P and Mg concentrations than the concretions in which they are found, suggesting that these elements were inhibiting calcite and allowing ikaite nucleation in sediment 8,9 (Fig. 3).…”

“…"Glendonite" is the name given to the calcite pseudomorphs after the mineral ikaite (CaCO 3 •6H 2 O), a hydrated form of calcium carbonate that in general is unstable at Earth surface temperature and pressure conditions 7 . A combination of low temperatures (≤4°C in the natural marine sedimentary realm 8 ), and chemical inhibitors of the more stable polymorphs, are needed to promote the precipitation and stabilisation of ikaite over calcite and aragonite 9 . In the marine sedimentary realm, ikaite growth is thought to be facilitated by either the anaerobic oxidation of methane (AOM) or bacterial sulphate reduction, both of which generate high alkalinity and high phosphate and/or sulphate concentrations which may act as chemical inhibitors of the anhydrous CaCO 3 polymorphs 8,10,11 .…”

“…The glendonites and potentially ice-rafted erratics found in the Paleocene-Eocene strata of Svalbard have given rise to speculation of multiple transitions between cold and warm phases during this time 4 . However, since the successful laboratory growth and stabilisation of synthetic ikaite at 35°C 9 , the significance of glendonites in the geological record has been questioned. No truly quantitative temperature reconstructions were possible for the Svalbard glendonites due to the poor preservation of the entire succession on Svalbard 4 .…”

Database of global glendonite and ikaite records throughout the Phanerozoic

“…However, experiments carried out during the last few years have revealed at least short-term ikaite stability at much higher temperatures, up to 30-35 o C (Clarkson et al, 1992;Rodríguez-Ruiz et al, 2014;Purgstaller et al, 2017;Tollefsen et al, 2020). Ikaite stability at high temperatures had only previously been demonstrated under high pressures (van Valkenberg et al, 1971).…”