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

Eva L. Scheller*, Miquela Ingalls, John M. Eiler, John P. Grotzinger, Uri Ryb

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

5 Scopus citations

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 δ18OCARB and Δ47 decreased, while δ13CCARB 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 δ18OCARB 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 δ18OCARB 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, δ18OCARB, δ18Ofluid, and δ13CCARB when considering dehydration diagenetic effects.

Original languageAmerican English
Pages (from-to)146-164
Number of pages19
JournalGeochimica et Cosmochimica Acta
Volume354
DOIs
StatePublished - 1 Aug 2023

Bibliographical note

Publisher Copyright:
© 2023 The Author(s)

Keywords

  • Carbonate
  • Carbonate diagenesis
  • Clumped isotopes
  • Ikaite
  • Monohydrocalcite
  • Paleoclimate
  • Paleotemperature
  • Stable isotopes

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