The mechanisms and stable isotope effects of transforming hydrated carbonate into calcite pseudomorphs

Creators: Scheller, Eva L.^{1, 2}; Ingalls, Miquela³; Eiler, John M.²; Grotzinger, John P.²; Ryb, Uri⁴

1. Massachusetts Institute of Technology
2. California Institute of Technology
3. Pennsylvania State University
4. Hebrew University of Jerusalem

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Abstract

Ikaite (CaCO₃·6H₂O) and monohydrocalcite (CaCO₃·H₂O; MHC) are hydrated carbonates that form at frigid (<9 °C) temperatures. During gradual heating and dehydration, the more thermodynamically stable anhydrous calcite replaces and pseudomorphs ikaite and MHC. Previously, ikaite pseudomorphs have been identified in the sedimentary record by characteristic replacive macro- and microtextures and interpreted as evidence for near-freezing marine paleotemperatures. Prior to this study, we lacked an understanding of isotopic exchange during mineral dehydration necessary to interpret isotopic compositions of such fabrics. Specifically, do the stable isotopic compositions of ikaite pseudomorphs preserve the primary environmental signal, or are they altered during mineral transformation? Through heating experiments of MHC from Ikka Fjord, we find that δ¹⁸O_CARB and Δ₄₇ decreased, while δ¹³C_CARB remained nearly unchanged during progressive dehydration. An oxygen isotopic exchange model fitted to experimental data suggests that the isotopic changes reflected partial re-equilibration of δ¹⁸O_CARB and Δ₄₇ towards the new diagenetic conditions due to oxygen equilibrium exchange between CO₃²⁻ and H₂O within the MHC lattice. However, this process never reaches full equilibrium, an effect we argue reflects the fact that structural H₂O escapes the solid carbonate structure faster than isotopic exchange can reach equilibrium. In addition, labelled water experiments demonstrated that oxygen isotopic exchange also occurs with secondary external waters during dehydration.

We apply this new framework to interpret the isotopic compositions of Pleistocene ikaite pseudomorphs from Mono Lake (CA, USA) high-stand deposits, which have undergone subaerial dehydration. Specifically, dehydration diagenetic overprinting of Δ₄₇ similar to the results of our controlled heating experiments can explain the warm temperatures, 6–26 °C, recorded by ikaite pseudomorphs and the higher δ¹⁸O_CARB recorded by ikaite pseudomorph compared to other Pleistocene Mono Lake calcite tufas. We show that Mono Lake ikaite pseudomorph tufa isotopic data can be explained in a model scenario where approximately coeval precursor ikaite and other calcite tufas in Pleistocene Mono Lake formed from similar fluid compositions but at different water temperatures, likely related to changing seasons. In summary, we show that the isotopic composition of ikaite and MHC pseudomorphs can be used for paleoclimate reconstruction of water temperature, δ¹⁸O_CARB, δ¹⁸O_fluid, and δ¹³C_CARB when considering dehydration diagenetic effects.

Copyright and License

This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Acknowledgement

This work was initiated under the Simons Collaboration for the Origin of Life with support to J.P.G. E.L.S. was supported by NESSF #80NSSC18K1255. We would like to thank Dr. Hao Xie and Dr. Nami Kitchen for their assistance to the project. We thank Dr. Nathan Stein for contributions in the field. We would also like to thank Prof. Peter Stougaard for supplying the sample from Ikka Fjord. In addition, we thank Prof. Gabrielle Stockmann and Dr. Christine Chen for their support and discussion. We thank Prof. Gregory A. Henkes and two anonymous reviewers for their edits that have improved the manuscript. We humbly acknowledge the Kutzadika’a, the communities of Mono Basin, and other indigenous peoples whose identities may have been lost to colonialism, on whose land this work relies. The Mono Lake tufas were sampled in BLM and Forest Service lands under study permissions associated with the International Geobiology Course (Caltech/USGS) in 2017 and 2018. We acknowledge that the studied MHC sample is the property of the Greenland, and that it was used with permissions in agreement with the Greenlandic Ministry of Domestic Affairs, Nature and Environment for the sole purposes of basic research.

Data Availability

Table 1 and 2 datasets, raw clumped isotope data, and measured XRD patterns are available on the following repository: https://doi.org/10.5281/zenodo.7526537.

Supplemental Material

The supplementary materials include two figures. Fig. S1 shows the XRD calibration dataset and curve fit used for converting XRD peak height ratios to wt% carbonate and to demonstrate uncertainties to the fitting algorithm parameters. Fig. S2 shows an example of a more detailed depiction of the simulations summarized in Fig. 6 to understand the individual modelled behavior of MHC, calcite, and ultimately how that is reflected in the MHC-calcite mixture. Supplementary material to this article can be found online at https://doi.org/10.1016/j.gca.2023.04.025.

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