Oxygen Isotope Fractionation Between Diatomaceous Silica and Water

Abstract

The temperature dependence of the oxygen isotope fractionation between diatomaceous silica and water was determined by analyzing frustules of freshwater diatoms cultured in the laboratory at temperatures ranging from 3.6 to 20.0°C. Within the limits of experimental reproducibility, measured oxygen isotope fractionations were independent of species and of the isotopic composition of the water. The fractionation varied regularly with temperature according to the equation 1000lnα (silica-water) = 15.56 (10 3 T −1) − 20.92. This relation corresponds to a temperature coefficient of roughly 0.2‰ per °C, significantly lower than published coefficients estimated from analyses of fossil diatoms from sediments and from extrapolation of experimentally determined quartz-water fractionations to low temperatures. The magnitude of the fractionation at a given temperature was 3–8‰ lower than previously published fractionations that were determined from analyses of fossil diatoms and from experimental data for quartz. The discrepancies between the new results and those of previous studies are attributed mainly to intrinsic differences in the oxygen isotope characteristics of fresh and fossil diatoms. Fresh diatomaceous silica appears to have an isotopically anomalous surficial layer containing large amounts of readily exchangeable, relatively low- 18O oxygen, including abundant oxygen in hydroxyl groups, with the result that partial dissolution or diagenesis may systematically shift the δ 18O values of fossil diatom frustules to higher values by removing this relatively unstable surficial material. If the effects of partial dissolution and diagenesis are regular or predictable, then the temperature information recorded during diatom growth may prove useful for paleoclimate studies.