How effective are boron isotopes in reconstructing past ocean pH?

Pagani et al. (2005) deduced that the boron isotope technique needs to be greatly improved in order to accurately reconstruct long term changes in ocean pH.   The boron isotope pH model assumes that the boron isotopic composition of carbonates is the same as that of the boron isotopic concentration borate in solution.   An accurate and precise isotopic fractionation index is required in order to see how pH affects the boron isotopic composition of borate.  There is much uncertainty surrounding the value used for the isotope fractionation index, theoretically the value should be 0.981 but research suggests that a value of 0.974 should be used instead (Kakihana et al. 1977).  Small changes in this isotopic fractionation index cause large changes in the resulting pH and so, future work needs to focus on determining the correct value so that more accurate and realistic ocean pH’s can be reconstructed.  Kasemann et al. (2009) believe that even when a more robust fractionation index is used there are still some reconstructions of cenezoic ocean pH which are too high.  This could be caused by the assumptions of the foraminiferal vital effects in dissolved organic carbon.  It has also been proposed that there is much difficulty in determining the boron isotope compositions of materials and many different values can be obtained from the same sample depending on the analytical techniques that have been used.  Respiration and photosynthesis are thought to alter the pH in the calcifying part of marine organisms which can alter the boron isotopic composition of the carbonate, resulting in a reconstructed pH value which is not a true representation of the ocean pH at the time (Rink et al. 1998).  It is also believed that post depositional dissolution can alter the boron isotope composition, resulting in inaccurate reconstructions (Kasemann et al. 2009).  Despite these problems, Boron isotopes are still a very effective method for reconstructing ocean acidity but improvements are required in order to improve the accuracy of results, especially over long timescales.

References

Kakihana, M. Kotaka, S. Satoh, M. Nomura, M. Okamoto, M. (1977). ‘Fundamental studies on the ion-exchange separation of boron isotopes’, Bull. Chem. Soc. J., 50, 158-163.

Kasemann, S. Schmidt, D. Bijma, J. Foster, G. (2009). ‘In situ boron isotope analysis in marine carbonates and its application for foraminifera and palaeo-pH’, Chemical Geology, 260, 1-2, 138-147.

Pagani, M. Lemarchand, D. Spivack, A. Gaillardet, J. (2005). ‘A critical evaluation of the boron isotope-pH proxy: the accuracy of ancient pH estimates’, Geochimica et Cosmochimica Acta, 69, 4, 953-961.

Rink, S.  Kuhl, M. Bijma, J. Spero, H. (1998). ‘Microsensor studies of photosynthesis and respiration in the symbiotic foraminifer Orbulina universa’, Marine Biology, 131, 4, 583-595.