Re: [asa] geological dating

From: Don Winterstein <>
Date: Wed Oct 14 2009 - 10:16:02 EDT

Correction: "at least as old as" should be "no older than."

  ----- Original Message -----
  From: Don Winterstein<>
  To: asa<>
  Sent: Wednesday, October 14, 2009 7:02 AM
  Subject: Re: [asa] geological dating

  "evolution is not integral to the dating...."

  Index fossils are widely used for relative dating of rocks, so in that sense evolution is integral to such dating. That is, if you find fossil x, you know that the formation is at least as old as the time at which fossil x first appeared. Without evolution you wouldn't be able to say this.


    ----- Original Message -----
    From: David Campbell<>
    Sent: Tuesday, October 13, 2009 10:08 AM
    Subject: Re: [asa] geological dating

    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 Wed Oct 14 10:16:22 2009

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