Constraining nonequilibrium isotopic effects in carbonates: through clumped isotopes, trace metal analysis, and numerical modelling
File(s)
Author(s)
Cao, Xuan
Type
Thesis
Abstract
Calcium carbonates are important sedimentary rocks that are not only volumetrically important reservoir rocks but also pivotal paleoenvironment archives. The clumped isotope thermometer (△47) is the most advanced paleo-thermometer for carbonates and a powerful tool in related research. However, the kinetic effects on clumped isotopes are ambiguous and controversial. The pairing of clumped isotope and trace metal concentration found in natural calcite cement was recently suggested as a promising tool to identify kinetic effects in clumped isotopes but needs to be further confirmed via experiments. This PhD work breaks down the knowledge gap into three linked research questions: [1] Key controls on polymorphism, [2] Mathematical description of trace metal incorporation, and [3] Potential causes of kinetic effects on clumped isotopes.
To address the gap, I synthesised CaCO3 under controlled laboratory conditions. The obtained precipitates were analysed for various properties. Based on the results, I concluded: [1] Temperature and the combination of temperature > 25℃ with rapid precipitation rates are primary controls on polymorphism, with ionic impurities and precipitation rate as secondary and tertiary controls. [2] The partitioning of Fe3+ and Mn2+ in CaCO3 is independent in the polymorph and exhibited strong correlations with the precipitation rates and temperatures. [3] The examined parameters didn’t solely cause kinetic effects on the clumped isotope. However, the elevated dehydration of cation impurities can lead to kinetic effects. I interpret this as the shift in the crystal growth mechanism. I extended the △47-T calibration to cryogenic temperatures with a range of -15 to 250℃.
This PhD work provides a laboratory framework for the research of carbonate clumped isotope kinetic effects and suggests that the pairing of clumped isotope and trace metal concentration may not be a widely applicable tool.
To address the gap, I synthesised CaCO3 under controlled laboratory conditions. The obtained precipitates were analysed for various properties. Based on the results, I concluded: [1] Temperature and the combination of temperature > 25℃ with rapid precipitation rates are primary controls on polymorphism, with ionic impurities and precipitation rate as secondary and tertiary controls. [2] The partitioning of Fe3+ and Mn2+ in CaCO3 is independent in the polymorph and exhibited strong correlations with the precipitation rates and temperatures. [3] The examined parameters didn’t solely cause kinetic effects on the clumped isotope. However, the elevated dehydration of cation impurities can lead to kinetic effects. I interpret this as the shift in the crystal growth mechanism. I extended the △47-T calibration to cryogenic temperatures with a range of -15 to 250℃.
This PhD work provides a laboratory framework for the research of carbonate clumped isotope kinetic effects and suggests that the pairing of clumped isotope and trace metal concentration may not be a widely applicable tool.
Version
Open Access
Date Issued
2024-09-30
Date Awarded
01/01/2025
Copyright Statement
Creative Commons Attribution NonCommercial Licence
License URL
Advisor
John, Cedric
Weiss, Dominik
Sponsor
China Scholarship Council
Publisher Department
Earth Science & Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)