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 C than heavy C. Other isotopes are only produced as a result of the radioactive decay of other elements, such as Sr, the daughter isotope of Rb. Rb, and therefore Sr, is common in the crust, so abundance of Sr in a sample of sediment (relative to 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, Sr / Sr, S / S and C / C, undergo dramatic fluctuations around the beginning of the Cambrian.
Carbon isotopes
Further information: δ13cCarbon has 2 stable isotopes, carbon-12 (C) and carbon-13 (C). The ratio between the two is denoted δC, and represents a number of factors.
Because organic matter preferentially takes up the lighter C, an increase in productivity increases the δC 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 δC 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 δC.
Causes often suggested for changes in the ratio of C to C found in rocks include:
- A mass extinction. Chemistry is largely driven by electro-magnetic forces, and lighter isotopes such as C respond to these more quickly than heavier ones such as C. So living organisms generally contain a disproportionate amount of C. A mass extinction would increase the amount of C available to be included in rocks and therefore reduce the ratio of C to 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 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 C-rich methane and thus reduce the ratio of C to 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).
References
- Magaritz, M.; Holser, W.T.; Kirschvink, J.L. (1986). "Carbon-isotope events across the Precambrian/Cambrian boundary on the Siberian Platform". Nature. 320 (6059): 258–259. Bibcode:1986Natur.320..258M. doi:10.1038/320258a0.
- Further documentation on these variations is available at the following URLs: Archived 2012-06-23 at the Wayback Machine Archived 2012-07-28 at the Wayback Machine Archived 2016-03-04 at the Wayback Machine (All listed at this Scholar results page
- Marshall, C.R. (2006). "Explaining the Cambrian "Explosion" of Animals". Annual Review of Earth and Planetary Sciences. 34: 355–384. Bibcode:2006AREPS..34..355M. doi:10.1146/annurev.earth.33.031504.103001.