Post-mortem oxygen isotope exchange within cultured diatom silica

Rationale Potential post-mortem alteration to the oxygen isotope composition of biogenic silica is critical to the validity of palaeoclimate reconstructions based on oxygen isotope ratios (δ18O values) from sedimentary silica. We calculate the degree of oxygen isotope alteration within freshly cultured diatom biogenic silica in response to heating and storing in the laboratory. Methods The experiments used freshly cultured diatom silica. Silica samples were either stored in water or dried at temperatures between 20 °C and 80 °C. The mass of affected oxygen and the associated silica-water isotope fractionation during alteration were calculated by conducting parallel experiments using endmember waters with δ18O values of –6.3 to –5.9 ‰ and –36.3 to –35.0 ‰. Dehydroxylation and subsequent oxygen liberation was achieved by stepwise fluorination with BrF5. The 18O/16O ratios were measured using a ThermoFinnigan MAT 253 isotope ratio mass spectrometer. Results Significant alterations in silica δ18O values were observed, most notably an increase in the δ18O values ollowing drying at 40-80 °C. Storage in water for seven days between 20-80 °C also led to significant alteration in δ18O values. Mass balance calculations suggest that the amount of affected oxygen is positively correlated with temperature. The estimated oxygen isotope fractionation during alteration is an inverse function of temperature, consistent with the extrapolation of models for high temperature silica-water oxygen isotope fractionation. Conclusions Routinely used preparatory methods may impart significant alterations to the δ18O values of biogenic silica, particularly when dealing with modern cultured or field collected material. The significance of such processes within natural aquatic environments is uncertain; however, there is potential that similar processes also affect sedimentary diatoms, with implications for the interpretation of biogenic silica-hosted δ18O palaeoclimate records