The start of the Cambrian period is marked by "fluctuations" in a number of geochemical records, including Strontium, Sulfur and Carbon isotopic excursions.
The chemical signature may be related to continental break-up, the end of a "global glaciation", or a catastrophic drop in productivity caused by a mass extinction just before the beginning of the Cambrian.
The weight difference means that some isotopes are discriminated against in chemical processes – for example, plants find it easier to incorporate the lighter 12C than heavy 13C.
The ratios of three major isotopes, 87Sr / 86Sr, 34S / 32S and 13C / 12C, undergo dramatic fluctuations around the beginning of the Cambrian.
Some carbon reservoirs are very isotopically light: for instance, biogenic methane, produced by bacterial decomposition, has a δ13C of −60‰ – vast, when 1‰ is a large fluctuation!