Promotion of ikaite precipitation by bentonite fertilisation in the Eocene Fur formation, northern Denmark

Glendonites are calcite pseudomorphs after the mineral ikaite. Because ikaite typically forms at low temperature, glendonite occurrences in deep time have been interpreted as indication for cold environmental conditions, even when found in sediments associated with greenhouse climates (Vickers et al. 2020). However, the exact conditions that control glendonite formation are not well constrained, and their occurrences often contradict other temperature proxies (Price et al. 2013). Ikaite nucleation experiments have shown that the temperature stability range of ikaite can be extended under the influence of certain chemical compounds (e.g. Mg²⁺, PO₄^3-) and foreign minerals (e.g. Tollefsen et al. 2018; Strohm et al. 2022). Several prominent glendonite intervals worldwide are found in close association with bentonites. Therefore, by using a combination of microscopic and spectroscopic techniques, this study sets out to investigate the ways in which bentonites affect ikaite crystallisation. The investigated bentonite samples were extracted from the early Eocene Fur formation that crops out at the islands Mors and Fur in northern Denmark. This formation is also known to contain some of the largest glendonite crystals recorded to date. 

We followed the method from Tollefsen et al. 2018 to synthesise ikaite and added Fur-formation bentonite and/or solutions that had reacted with this bentonite prior to ikaite synthesis. Laboratory observations combined with ATR-FTIR, Raman spectroscopy and optical microscopy suggest that ikaite precipitation occurred via a dissolution-reprecipitation processes. An ikaite nucleation experiment at high Mg concentrations, reproduced from Tollefsen et al. 2018, yielded 30% ikaite precipitation with 70% co-precipitating nesquehonite. When leachate was used in these experiments, we still observed ikaite precipitation while co-precipitating nesquehonite was almost absent. Crystal agglomeration appeared to occur faster in reactive solution in direct contact with bentonites.

When experimental solutions were reacting with (leaching) bentonite, additional calcium and phosphate release was observed by ICP-OES. Calcium and phosphate contents increased in the solution from about 350 ppm to 700 ppm and about 0.1 ppm to 7.5 ppm, respectively. This fertilisation process is suggested to favour ikaite precipitation over co-precipitating nesquehonite. The effect of element release from the bentonite on the resulting ikaite pseudomorph morphologies appears limited. Natural glendonite morphologies (in the Fur formation) are therefore more likely controlled by the nature of the storage medium and the temperature differential between the crystallisation and transformation process.

 

References

Price, G. D., Twitchett, R. J., Wheeley, J. R., & Buono, G. (2013). Isotopic evidence for long term warmth in the Mesozoic. Scientific reports, 3(1), 1-5.

Strohm, S. B., Inckemann, S. E., Gao, K., Schweikert, M., Lemloh, M. L., Schmahl, W. W., & Jordan, G. (2022). On the nucleation of ikaite (CaCO3x6H2O)–A comparative study in the presence and absence of mineral surfaces. Chemical Geology, 611, 121089.

Tollefsen, E., Stockmann, G., Skelton, A., Mörth, C. M., Dupraz, C., & Sturkell, E. (2018). Chemical controls on ikaite formation. Mineralogical Magazine, 82(5), 1119-1129.

Vickers, M. L., Lengger, S. K., Bernasconi, S. M., Thibault, N., Schultz, B. P., Fernandez, A., ... & Korte, C. (2020). Cold spells in the Nordic Seas during the early Eocene Greenhouse. Nature communications, 11(1), 1-12.