Re: [asa] geological dating

From: David Campbell <>
Date: Tue Oct 13 2009 - 13:08:24 EDT

A couple of minor caveats:

In addition to 14C, there are some fossils containing radioisotopes
that can be used for dating. For example, corals often contain enough
thorium to date, and various types of replacement may involve
radioactive elements , e.g., the often uranium-rich dinosaur bones in
parts of the western U.S. or glauconitic molds of marine organisms
(though of course, the date will reflect when the replacement
occurred, not the original organism, and glauconite has a number of

However, in general an igneous rock is the best for radiometric
dating. (A metamorphic high-pressure carbon isomorph might do better
for some other dating). Obtain dates on several different minerals
and isotopes from a single rock, and you've got a very
well-constrained age, with the caveat that a given rock may
crystallize slowly. A volcanic ash layer associated with fossils is
thus about the best-case scenario for dating.

All sorts of long-term trends or variations can provide relative dates
and then be calibrated with radiometric dates. These include, among
others, changes in stable isotope ratios, magnetic reversals,
Milankovitch cycle-related changes, impact layers, and evolution. The
evolution is not integral to the dating; it just is the explanation
for why you see change in organisms over time and can therefore be
confident that, e.g., a layer with Chesapecten jeffersonius is older
than a layer with low rib count Chesapecten madisonius, which is older
than normal Chesapecten madisonius, just as we know that an undated
scrap of paper that identified Jefferson as the current president
would be older than one citing Madison as the current president.

Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"
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Received on Tue Oct 13 13:08:47 2009

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