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 "
Isotopes
The ratios of three major isotopes, 87Sr / 86Sr, 34S / 32S and 13C / 12C, undergo dramatic fluctuations around the beginning of the Cambrian.[1]
Carbon isotopes
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:[2]
- 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 carbonate rocks).
References
- doi:10.1038/320258a0.
- Further documentation on these variations is available at the following URLs: [1] Archived 2012-06-23 at the Wayback Machine[2][3][4] Archived 2012-07-28 at the Wayback Machine[5] Archived 2016-03-04 at the Wayback Machine[6] (All listed at this Scholar results page
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