pH control on oxygen isotopic composition of symbiotic corals

Boron, carbon and oxygen isotopic compositions were determined at the micrometre scale by high-resolution ion microprobe in a sample of modern coral (massive hermatypic coral, Porites lutea). The ion probe data show for B and O much larger isotopic variations at the micrometre scale than those measured at the millimetre scale by conventional techniques: delta(18)O(PDB) values range from -10.6 +/- 0.9parts per thousand to -0.2 +/- 0.5parts per thousand and delta(11)B values range from +18.6 +/- 1.5parts per thousand to +30.6 +/- 1.6parts per thousand. By contrast, delta(13)C values show the same range of variations, from -4.6 +/- 0.65parts per thousand to -2.2 +/- 0.67parts per thousand at the micrometre and millimetre scales. The range of delta(11)B values indicates that significant pH variations, from approximate to7.1 to approximate to9.0, are present at the sites of calcification. The largest delta(18)O variations correspond to the highest delta(11)B values, i.e. to the highest pHs. This measurement of pH allows modelling the oxygen isotopic fractionation occurring during aragonite precipitation. Taking into account the rate of O isotopic equilibrium between dissolved carbonate species (H2CO3, HCO3- and CO32-) and water via the two reactions of hydration and hydroxylation, the full range of delta(18)O values measured at the micrometre scale can be modelled for residence times of dissolved carbonates in the calcifying fluid ranging between approximate to1 h and at maximum approximate to12 h. The pH controls the delta(18)O of the growing carbonate through the relative fractions of dissolved carbonate species and through the kinetics of their isotopic equilibration with water via hydration and hydroxylation. The so-called 'vital effect' systematically observed for delta(18)O in corals can thus be understood as representing an average of rapid pH variations due to coral biology during coral growth. Selectively measuring delta(18)O values in the zones of coral skeletons that have low delta(11)B values (i.e. formed at low pH) should significantly improve the quality of palaeoclimatic reconstructions based on delta(18)O values. (C) 2003 Elsevier B.V. All rights reserved.