Kambrium: Difference between revisions

 

=== Dätam Kambriumik ===

 

Dät stralimafik se New Brunswick pladon fini dila balid Kambriuma bü yels za 511-balions. Retons yels 21-balion pro dils votik tel Kambriuma.

 

for the extinction event at the beginning of the Cambrian has recently been submitted. The rationale for this precise dating is interesting in itself as an example of [[Paleontology|paleological]] [[deductive reasoning]]. Exactly at the Cambrian boundary there is a marked fall in the abundance of [[carbon-13]], a "reverse spike" that [[paleontologist]]s call an ''excursion''. It is so widespread that it is the best indicator of the position of the Precambrian-Cambrian boundary in [[stratigraphic]] sequences of roughly this age. One of the places that this well-established carbon-13 excursion occurs is in [[Oman]]. Amthor (2003) describes evidence from Oman that indicates the [[carbon]]-[[isotope]] excursion relates to a mass extinction: the disappearance of distinctive fossils from the pre-Cambrian coincides exactly with the carbon-13 anomaly. Fortunately, in the Oman sequence, so too does a [[volcanic ash]] horizon from which [[zircon]]s provide a very precise age of 542 ± 0.3 Ma (calculated on the decay rate of [[uranium]] to [[lead]]). This new and precise date tallies with the less precise dates for the carbon-13 anomaly, derived from sequences in [[Siberia]] and [[Namibia]]. It is presented here as likely to become accepted as the definitive age for the start of the Phanerozoic eon, and thus the start of the Palaeozoic era and the Cambrian period.