OXYGEN-ISOTOPE FRACTIONATION BETWEEN DIATOMACEOUS SILICA AND WATER

The temperature dependence of the oxygen isotope fractionation between diatomaceous silica and water was determined by analyzing frustules o f freshwater diatoms cultured in the laboratory at temperatures rangin g from 3.6 to 20.0 degrees C. Within the limits of experimental reprod ucibility, measured oxygen isotope fractionations were independent of species and of the isotopic composition of the water. The fractionatio n varied regularly with temperature according to the equation 10001n a lpha((silica-water)) = 15.56 (10(3)T(-1)) - 20.92. This relation corre sponds to a temperature coefficient of roughly 0.2 parts per thousand per degrees C, significantly lower than published coefficients estimat ed from analyses of fossil diatoms from sediments and from extrapolati on of experimentally determined quartz-water fractionations to low tem peratures. The magnitude of the fractionation at a given temperature w as 3-8 parts per thousand lower than previously published fractionatio ns that were determined from analyses of fossil diatoms and from exper imental data for quartz. The discrepancies between the new results and those of previous studies are attributed mainly to intrinsic differen ces in the oxygen isotope characteristics of fresh and fossil diatoms. Fresh diatomaceous silica appears to have an isotopically anomalous s urficial layer containing large amounts of readily exchangeable, relat ively low-O-18 oxygen, including abundant oxygen in hydroxyl groups, w ith the result that partial dissolution or diagenesis may systematical ly shift the delta(18)O values of fossil diatom frustules to higher va lues by removing this relatively unstable surficial material. Lf the e ffects of partial dissolution and diagenesis are regular or predictabl e. then the temperature information recorded during diatom growth may prove useful for paleoclimate studies. Copyright (C) 1998 Elsevier Sci ence Ltd.