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 ¹²C than heavy ¹³C. Other isotopes are only produced as a result of the radioactive decay of other elements, such as ⁸⁷Sr, the daughter isotope of ⁸⁷Rb. Rb, and therefore ⁸⁷Sr, is common in the crust, so abundance of ⁸⁷Sr in a sample of sediment (relative to ⁸⁶Sr) is related to the amount of sediment which originated in the crust, as opposed to from the oceans.

The ratios of three major isotopes, ⁸⁷Sr / ⁸⁶Sr, ³⁴S / ³²S and ¹³C / ¹²C, undergo dramatic fluctuations around the beginning of the Cambrian.

Carbon isotopes

Further information: δ13c

Carbon has 2 stable isotopes, carbon-12 (¹²C) and carbon-13 (¹³C). The ratio between the two is denoted δ¹³C, and represents a number of factors.

Because organic matter preferentially takes up the lighter ¹²C, an increase in productivity increases the δ¹³C 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 δ¹³C 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 δ¹³C.

Causes often suggested for changes in the ratio of ¹³C to ¹²C found in rocks include:

• A mass extinction. Chemistry is largely driven by electro-magnetic forces, and lighter isotopes such as ¹²C respond to these more quickly than heavier ones such as ¹³C. So living organisms generally contain a disproportionate amount of ¹²C. A mass extinction would increase the amount of ¹²C available to be included in rocks and therefore reduce the ratio of ¹³C to ¹²C.
• 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 ¹²C 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 ¹²C-rich methane and thus reduce the ratio of ¹³C to ¹²C 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).