# How was the onset of the Cambrian period dated?

What dating method was used to determine the beginning of the Cambrian, and what was its margin of error? Was only one fossil dated, or were a number of similar fossils used?

• It doesn't answer my question. I already knew the dating was radiometric. Mar 13 '20 at 13:35
• @MichaelWalsby other people don't know that until you say so. Can you add some more detail to your question? I think I can see what you are asking. While radiometric dating can generate the age of a given object, to determine the beginning or end of a period in geological history requires much more than that.
– uhoh
Mar 15 '20 at 14:39
• @uhoh i removed my comment,i still think the question needs some work. Mar 15 '20 at 17:02

## 2 Answers

Fossils are only in very rare cases dated directly, because they do not contain sufficient radioactive isotopes for dating. Most geological periods are first and foremost defined by biostratigraphy- fossil assemblages, typically. The Cambrian period starts with the Fortunian stage - this stage is defined by the appearance of a certain trace fossil in Fortuna peninsula, eastern Canada, but is correlable with similar formations world wide. The whole discussion on how this stage is defined is available here. As outlined in this paper, in addition to fossils, the Fortunian stage hosts various volcanic rocks which contain zircons, which can be dated by U-Pb. The age of the actual boundary is then interpolated from these data.

Just to give some complementary informations from the other answer: the base of the Fortunian is indeed defined in eastern Canada on the basis of a trace fossil (which obviously cannot be itself dated since it is just an imprint), Trichophycus pedum. This trace fossil however being spread worldwide help correlating various sections throughout the world. One of such section is the Birba-5 well in Oman where the radiometric age was actually measured (at least the one currently considered as the proper age of the base of the Cambrian): volcanic ash bed BB5 could be dated using $$\ce{^{206}Pb/^{238}U}$$ radiometric dating at 541 +/- 0.63Ma, based on the analysis of 8 different zircon grains.

It also corresponds to a negative carbon isotope excursion.

See the Cambrian chapter and the Appendix 2 on radiometric ages in Gradstein et al. 2012. Or Bowring et al. 2007 for the original data.

• Note that the margin of error given here is the one from Gradstein et al. 2012 where they recalculated it apparently (though given i am not a specialist I don't think i can explain correctly that computation). Jun 17 '20 at 13:20