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
