The effect of species on lacustrine [delta]18Odiatom and its implications for palaeoenvironmental reconstructions

The oxygen isotope composition of diatom silica ([delta]18Odiatom) is increasingly being used to reconstruct climate from marine and lacustrine sedimentary archives. Although diatoms are assumed to precipitate their frustule in isotopic equilibrium with their surrounding water, it is unclear whether internal processes of a given species affect the fractionation of oxygen between the water and the diatom. We present [delta]18Odiatom data from two diatom size fractions (3-38 and >38µm) characterized by different species in a sediment core from Heart Lake, Alaska. Differences in [delta]18Odiatom between the two size fractions varies from 0 to 1.2[per thousand], with a mean offset of 0.01[per thousand] (n=20). Fourier transform infrared spectroscopy confirms our samples consist of pure biogenic silica (SiO2) and [delta]18Odiatom trends are not driven by contamination. The maximum offset is outside the range of error, but the mean is within analytical error of the technique (±1.06[per thousand]), demonstrating no discernible species-dependent fractionation in [delta]18Odiatom. We conclude that lacustrine [delta]18Odiatom measurements offer a reliable and valuable method for reconstructing [delta]18Owater. Considering the presence of small offsets in our two records, we advise interpreting shifts in [delta]18Odiatom only where the magnitude of change is greater than the combined analytical error. [PUBLICATION ABSTRACT]