Oxygen isotopic exchangeability of diatom valve silica; interpretation and consequences for paleoclimatic studies

Problems of analytical reproducibility have impeded the application of ${}^{18}\text{O}{}^{16}\text{O}$ ratio measurements of biogenic opal to paleoclimatology. Lack of reproducibility has been attributed to the labile nature of part of the oxygen of the silica. This has been studied using oxygen isotopic exchange between diatom silica and water vapor. This reaction involves the reversible hydrolysis of weak Si-O bonds. Using controlled exchange with waters of different oxygen isotopic composition, the isotopic ratio of both the exchanged and the unexchanged silica-bound oxygen were estimated. Optimum conditions for reproducible measurement of the oxygen isotopic composition of diatom silica are defined.