EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

No ion is an island: Multiple ions involved during boron incorporation into CaCO3

Michael Henehan1,2, Christa Klein-Gebbinck3, Gavin Foster2, Jill Wyman3, Mathis Hain4, and Sang-Tae Kim
Michael Henehan et al.
  • 1GeoForschungsZentrum Potsdam, Sek. 3.3, Earth Surface Geochemistry, Germany (
  • 2School of Ocean and Earth Science, University of Southampton Waterfront Campus, National Oceanography Centre, Southampton, European Way, Southampton SO14 3ZH, Hampshire, UK.
  • 3School of Geography and Earth Sciences, McMaster University, 1280 Main Street West, Hamilton, ON, L8S 4K1, Canada.
  • 4Earth and Planetary Sciences Department, University of California Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA.

Boron isotope ratios, as measured in marine calcium carbonate, are a proven tracer of past seawater and calcifying fluid pH and thus a powerful tool for the reconstruction of past atmospheric CO2 and monitoring of coral physiology. For such applications, understanding the inorganic baseline upon which foraminiferal vital effects or coral pH upregulation are superimposed should be an important prerequisite. Yet, investigations into boron isotope fractionation in synthetic CaCOpolymorphs have often reported variable and even conflicting results, implying that we may not fully understand pathways of boron incorporation into calcium carbonate.  Here we address this topic with experimental data from calcite and aragonite precipitated across a range of pH in the presence of both Mg and Ca. We confirm the results of previous studies that the boron isotope composition of inorganic aragonite precipitates closely reflects that of aqueous borate ion, but that calcites display a higher degree of scatter, and diverge from the boron isotope composition of borate ion at low pH. We discuss these findings with reference to the simultaneous incorporation of other trace and minor elements, and highlight a number of mechanisms by which crystal growth mechanisms may influence the concentration and isotope composition of boron in CaCO3. In particular, we highlight the potential importance of surface electrostatics in driving variability in published synthetic carbonate datasets. Importantly for palaeo-reconstruction, however, these electrostatic effects are likely to play a much more minor role during natural precipitation of biogenic carbonates.

How to cite: Henehan, M., Klein-Gebbinck, C., Foster, G., Wyman, J., Hain, M., and Kim, S.-T.: No ion is an island: Multiple ions involved during boron incorporation into CaCO3, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10652,, 2021.

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