# Early Cambrian geochemical fluctuations

The start of the Cambrian period is marked by "fluctuations" in a number of geochemical records, including Strontium, Sulfur and Carbon isotopic excursions. While these anomalies are difficult to interpret, a number of possibilities have been put forward. They probably represent changes on a global scale, and as such may help to constrain possible causes of the Cambrian explosion.
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.

## Isotopes

Isotopes are different forms of elements; they have a different number of neutrons in the nucleus, meaning they have very similar chemical properties, but different mass. The weight difference means that some isotopes are discriminated against in chemical processes – for example, plants find it easier to incorporate the lighter $^{12}$C than heavy $^{13}$C. Other isotopes are only produced as a result of the radioactive decay of other elements, such as $^{87}$Sr, the daughter isotope of $^{87}$Rb. Rb, and therefore $^{87}$Sr, is common in the crust, so abundance of $^{87}$Sr in a sample of sediment (relative to $^{86}$Sr) is related to the amount of sediment which originated in the crust, as opposed to from the oceans.
The ratios of three major isotopes, $^{87}$Sr / $^{86}$Sr, $^{34}$S / $^{32}$S and $^{13}$C / $^{12}$C, undergo dramatic fluctuations around the beginning of the Cambrian.

### Carbon isotopes

Carbon has 2 stable isotopes, carbon-12 ($^{12}$C) and carbon-13 ($^{13}$C). The ratio between the two is denoted δ$^{13}$C, and represents a number of factors.
Because organic matter preferentially takes up the lighter $^{12}$C, an increase in productivity increases the δ$^{13}$C of the rest of the system, and vice versa. Some carbon reservoirs are very isotopically light: for instance, biogenic methane, produced by bacterial decomposition, has a δ$^{13}$C of −60‰ – vast, when 1‰ is a large fluctuation! An injection of carbon from one of these reservoirs could therefore account for the early Cambrian drop in δ$^{13}$C.
Causes often suggested for changes in the ratio of $^{13}$C to $^{12}$C found in rocks include:
* A mass extinction. Chemistry is largely driven by electro-magnetic forces, and lighter isotopes such as $^{12}$C respond to these more quickly than heavier ones such as $^{13}$C. So living organisms generally contain a disproportionate amount of $^{12}$C. A mass extinction would increase the amount of $^{12}$C available to be included in rocks and therefore reduce the ratio of $^{13}$C to $^{12}$C.
* A methane “burp”. In permafrosts and continental shelves methane produced by bacteria gets trapped in “cages” of water molecules, forming a mixture called a clathrate. This methane is very rich in $^{12}$C because it has been produced by organisms. Clathrates may dissociate (break up) suddenly if the temperature rises or the pressure on them drops. Such dissociations release the $^{12}$C-rich methane and thus reduce the ratio of $^{13}$C to $^{12}$C as this carbon is gradually incorporated into rocks (methane in the atmosphere breaks down into carbon dioxide and water; carbon dioxide reacts with minerals to form carbonate rocks).