[Posted and E-mailed]
markp...@enternet.com.au wrote:In general, isotopic dating methods don't apply well to sedimentary
> I'm doing a study of potassium-argon dating, used for dating fossils.
formations (which is where most fossils are found).
> A lot of evolution is based on K-Ar dates,That is not really true. The succession of fauna, ordering of the
geologic column, and even evolution, were all quite well-established
(mid to late 1800s) long before the technology existed to perform
accurate isotope dating (mid 1900s)... even before scientists were
aware of radioactivity (circa 1890-1900) or the existence of
isotopes (early 1900s).
> so I woould like to hear of are any independent measures to confirm orSee, for example: Dalrymple, G. Brent, 1969. "40Ar/36Ar Analyses
> calibrate or invalidate that method. For example, what dates do we get
> from known eruptions, such as the one that destroyed Pompeii or
> Saint Pierre? Or the eruptions of Krakatoa, Mount St Helens or Ruapehu,
> or any others in recent history?
of Historic Lava Flows" in _Earth and Planetary Science Letters_
vol. 6, pp. 47-55. It lists argon assessments for 26 samples from
several sources of known age. I have a reference to another study
but I have not yet obtained the actual paper, so I can't recommend
> There must be dozens of examples that have precise dates and couldUncertainty in a measured age is a function of uncertainty in the
> be used to evalaute K-Ar as a dating method.
isotope assessments *and* the half-life of the relevant isotope.
Since K-40 has a fairly long half-life (about a billion years),
it's fairly rare to have age uncertainties less than a few tens
of thousands of years.
Measurements on recent samples indicate that the initial conditions
K-Ar dating shows excellent correlations with other long-term
 Dalrymple, G. Brent, 1991. _The Age of the Earth_. California:
 Harland, W. Brian, et al., 1990. _A geologic time scale 1989_.
 Dalrymple, G. Brent, 1986. "Radiometric Dating, Geologic Time,
 Dalrymple, G. Brent, 1986. (Ref. ) See Figs. 2-3 and p. 25
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