Boron isotopic compositions of corals: Seawater or diagenesis record?

The first analyses of modern and fossil corals (calcite-free) using a positive ion (Cs2BO2+) TIMS technique and a chemical procedure for B extraction are reported. The degrees(11)B for modem corals fluctuates between 23.3 and 25.5 parts per thousand. The range of variation of fossil corals from the last interglacial period is greater, from 23.5 to 27.3 parts per thousand, and the lowest delta(11)B corresponds to the corals close to Last Glacial Maximum. Boron concentrations show a variation from 49 to 58 ppm in the recent corals, and from 39 to 52 ppm in ancient corals. For the ancient corals, linear correlations are observed in both delta(11)B vs. B concentration and delta(11)B vs. 1/B plots, but these correlations are not observed for modern corals. Two interpretations are presented to account for these observations and to test the potential of delta(11)B for reconstructing paleo-ocean pH. The first interpretation assumes a thermodynamic partitioning of B between seawater and carbonates and shows that both boron isotopic compositions and concentrations can be used to calculate the pH of seawater in which corals grow. For our data, such a model requires that both the pH and the Sigma CO2 of the deglacial ocean was lower than present-day values. The second interpretation is based on the correlation observed for ancient corals between boron isotopic compositions and 1/B, We suggest that The measured boron isotopic compositions of ancient corals are caused by the mixing of a primary aragonite (the coral skeleton) and secondary aragonite precipitated from B-10-enriched reef pore fluids. The characteristics (delta(11)B and B concentrations) of the diagenetic aragonite and the fluids from which it precipitates can be constrained. If this proves correct, only the non-recrystallised parts of ancient corals can be used for paleo-pa reconstructions. These recrystallisation processes did not affect the U-234/U-238 ratios of the corals.