Stable isotopes in diatom silica

Diatom silica is a form of biogenic opal (SiO2.nH2O, Figure 1) containing oxygen, silicon, carbon and nitrogen isotopes that can be used in lacustrine and marine paleoenvironmental studies. Since diatoms bloom following a seasonal pattern defined partly by the variability of climate, nutrient supply, mixing regimes, and in high latitudes the period of ice cover, the isotope signature acquired by diatoms will be skewed toward their major growing season specific to the lake or oceanic region under consideration. The isotope ratio (e.g. 18O/16O, 30Si/28Si, 13C/12C, 15N/14N) of diatom silica are expressed on the deltascale (δ) in terms of per mille (or per mille) (‰): δ = [(Rsample/Rreference) – 1] • 1000 ‰ Where R is the particular isotope ratio (e.g. 18O/16O, 30Si/28Si), and ‘reference’ means the appropriate universally accepted reference material. The ‘δ’ for each element takes its name from the heavy isotope, thus δ18O, δ30Si, δ13C, δ15N. For diatom oxygen the reference is VSMOW (Vienna Standard Mean Ocean Water) calibrated through the quartz NBS28, for silicon it is referenced and measured alongside NBS28, for carbon the reference is VPDB (Vienna PeeDee Belemnite) calibrated against NBS19 and NBS22, and for nitrogen it is atmospheric nitrogen, commonly shortened to AIR. There are no universally accepted standard materials to analyse alongside diatoms although most laboratories use their own standard diatomites as well as NBS quartz and low %C and %N organic materials.