Recontructing ocean pH using Coral

δ¹¹B levels preserved in coral skeletons can be used to look at recent changes in ocean pH.  Boron isotopes occur as either Borate ion or Boric acid, the levels of which are affected by ocean pH (Wei et al. 2009).  Borate ions are exclusively incorporated into marine carbonate, whilst Boric acid is not involved.  This means that the relative abundance of Borate ion is controlled by the pH of the seawater at the time of calcification (Vengosh et al. 1991).  Wei et al. (2009) looked at 200 years worth of δ¹¹B data (with a precision of +- 0.02 pH) and deduced that between 1940 and 1998 the Great Barrier Reef saw multiple fluctuations in δ¹¹B which correspond to changes of 0.5pH.

References

Vangosh, A. Starinsky, A. Chivas, A. McCulloch, M. (1991). ‘coprecipitation and isotopic fractionation of boron in modern biogenic carbonates’, Geochim. Cosmochim. Acta, 55, 2901-2910.

Wei, G. McCulloch, M. Mortimer, G. Deng, W. Xie, L. (2009).  ‘Evidence for ocean acidification in the Great Barrier Reef of Australia’, Geochim. Chosmochim. Acta. 73, 2332-2346.