Stable Isotope Systematics of Calcite
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Date
2024-01-09
Authors
Olsen, Ellen
Journal Title
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Publisher
University of Oregon
Abstract
The oxygen isotopic composition of calcite is widely used in paleoclimate studies to infer the temperatures of carbonate formation across a wide range of geologic environments including hydrothermal veins, caves, lakes, surface oceans, and in marine sediments. Carbonate-based temperature reconstructions depend on empirical d18O-T relationships that are affected by factors such as carbonate growth rate, solution composition, pH, and source(s) of dissolved inorganic carbon (DIC). We carried out calcite growth experiments over a range of pH (7.5-12.8), temperature (T = 10-25°C), ionic strength (I = 0.1-1.6; [NaCl] = 0-1.4 M) and concentration of the enzyme carbonic anhydrase ([CA] = 0-3 μM). The enzyme CA promotes isotopic equilibration of the DIC pool, which in turn, has a strong influence on the isotopic composition of the mineral. We divide the experimental results into two categories: (1) calcite grown from an equilibrated DIC pool, and (2) calcite grown from a non-equilibrated DIC pool. Results from (1) are used to determine the kinetic isotope effects (KIEs) attending the crystal growth reaction as a function of pH and ionic strength. No evidence of an ionic strength effect on oxygen isotope partitioning between calcite and DIC was found for NaCl concentrations up to 0.35 M, but in higher ionic strength solutions, NaCl was found to inhibit the efficacy of CA and prevent complete isotopic equilibration of the DIC pool, resulting in lower and more variable oxygen isotope fractionations. The oxygen isotope partitioning between calcite and water was found to systematically decrease with increasing pH. Results from (2) are used to determine the KIEs attending the CO2 hydration and hydroxylation reactions as a function of T, pH, ionic strength and [CA]. This study is the first to separately quantify the kinetic fractionation factors (KFFs) for CO2 and OH- separately during CO2 hydroxylation. The experimental results have been used to develop a generalizable model of oxygen isotope effects in the CaCO3-DIC-H2O system. The model can be used to predict the d18O of calcites grown from variably-equilibrated DIC pools and can explain why different experimental setups have yielded different d18O-T relationships for inorganic calcite.This dissertation includes previously published and unpublished co-authored material.
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Keywords
calcite, carbonate, fractionation, geochemistry, isotopes, paleoclimate