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|Title:||Oxygen and Boron Isotope Effects in Synthetic Calcite|
|Abstract:||Natural calcium carbonates, such as foraminifera and speleothems, are ubiquitous on the Earth’s surface and are essential subjects of study to the field of paleoclimatology. In particular, climate proxies using stable isotope and/or trace element concentrations of marine or continental carbonates largely rely on laboratory calibration studies in which carbonates are synthesized and their geochemical properties are carefully quantified. The boron isotope compositions of marine carbonates are a potentially powerful tool for the reconstruction of ancient seawater pH. If proven reliable, this tracer will allow systematic recording of paleooceanographic conditions, greatly assisting climatologists identifying historical fluctuations in atmospheric carbon dioxide concentrations. The boron isotope-pH proxy relies on the hypothesis that only the charged borate ion, possessing a distinct boron isotope composition as a function of pH, is incorporated into the carbonate crystal lattice. In this study, abiotic calcite was synthesized in high ionic strength solutions (0.7 mol/kg) across a range of controlled pH conditions (low pH ~ 7.15, mid pH ~ 8.35 and high pH ~ 9.15). The magnitude of boron isotope fractionation (δ11B) was determined to quantify the pH-dependence of δ11Bcalcite between the carbonate and the precipitating solution. The observed increase in boron isotope composition of calcite with pH is consistent with preferential incorporation of borate ion into the crystal lattice, however the sensitivity of the acid dissociation and isotope equilibrium constants render it difficult to ascertain its exclusive contribution to the boron isotope composition of calcite. Observed non-equilibrium effects further mask the interpretation of the underlying mechanisms, which must be understood precisely to validate the proxy. Ultimately, the relationship between the boron isotope composition of marine carbonates and ocean pH may provide objective evidence for ocean-CO2 system alterations possibly stemming from human-induced climate change. This research also evaluated the temperature- and pH-dependence of oxygen isotope fractionation (between calcite and water) in high ionic strength systems at 10, 25 and 40 °C over a pH range of 7.46-9.43. Our study is the first to assess oxygen isotope effects and fractionation behaviour under these varied conditions while employing the constant addition method. Preliminary results support several working hypotheses in the field and have proven consistent with previously untested theoretical predictions.|
|Appears in Collections:||Open Access Dissertations and Theses|
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