Here is my promised post on the KBS tuff.
Thu, 06 Nov 1997 05:48:18 -0600 you replied to my questions about it --
you may have wondered why I asked:
>>Do you really believe in the accuracy of the dates claimed for these
>>fossils? Even the KBS tuff -- after all we saw in Nature magazine in the
>>1970's on that (re: dating Skull 1470)?
>
>The vertebrate paleontological data from the strata below the Tuff
>contradicted the original radioactive date. The pigs found in that strata
>were from a younger time than the original radioactive date. It was this
>inconsistency between two different dating methods which forced a re-dating
>by other methods. this then resolved the discrepancy.
Now what actually happened with this? Watch the sequence of events:
Early attempts to date the KBS tuff (1969) gave an age of 212-230 million
years which was immediately rejected as an extraneous argon age
discrepancy, because of the presence of Australopithicine and other
mammalian fossils beneath the tuff (Fitch & Miller 1970, Nature 226:226-8).
The dating was complicated because the tuff is a water-transported mixture.
But they did find an acceptable date, which was then confirmed by several
"independent methods":
In the early 1970's the KBS tuff was "securely dated" at 2.6 million years
based on:
- Vertebrate faunas -- Elephant, Suid (pig), Australopithicus, and
tools
(Maglio, 1972; Nature 239:379-85, Leaky, 1967-69, etc.)
- Potassium-Argon dating -- selected crystals (K-Ar and Ar40-Ar39)
(Fitch & Miller '70, Nature 226:226-8 and see 251:214)
- Paleomagnetism -- polarity data, based on 247 samples below KBS tuff
(Brock & Isaac, 1974, Nature 247:344-48)
- Fission Track Dating -- involving uranium, noting possible reanealing
(Hurford, 1974, Nature 249:236; '76, 263:738)
In the mid 1970's Leaky's team found Skull 1470 below the KBS Tuff
(Leaky,'73, Nature 242:447, National Geographic June, 1973 pp 819-829) and
Leaky said, "either we toss out this skull, or we toss out our theories of
early man". Skull 1470 was "too modern" to be found at that level, if the
Australopithicenes (found both above and below the KBS tuff) were ancestral
to modern man.
Other anthropologists, notably Johanson's team at Berkeley, couldn't accept
any claim of such modern hominids in strata dated almost 3 million years
old. They tried to redate the KBS. It was a complex problem because the
tuff is a slurry of volcanic debris. But they did find and publish a date
that was suitable to them: 1.8 million years, based on:
- Hominid Fossils -- Skull 1470 and other similar skulls below KBS tuff
- Potassium-Argon (K-Ar) -- on pumice from the KBS tuff
and Ar-Ar -- on selected feldspar crystals (Ar40-Ar39)
(Curtis et al,1975, Nature 258:395 (& see 284:229,230 &
294:123)
Then in the late 1970's, a remarkable thing happened. One by one (with
much heated controversy apparent in the papers) the other "independent
methods" re-evaluated their work in light of the new radiometric date, and
confirmed the new age:
- Paleomagnetism -- pinpointing a different polarity reversal,
in light of the change in the K-Ar date
(Hillhouse et al, 1977, Nature 265:411)
- Vertebrate Faunas -- three suid (pig) species (based on teeth)
suggesting possible phylogenetic branching and its timing
in relationship to the new radiometric date
(Cooke, 1978; Science 201:460-63 (&198:13-21)
- Fission Track Dating -- (U-238 in zircon) emphasizing re-anealing,
in light of the change in the accpeted K-Ar date
(Gleadow, 1980, Nature 284:229-230)
By 1980 there was a new "remarkably concordant" well-accepted radiometric date.
Do you see what happened? Many more dates than those mentioned here were
obtained by radiometric methods, but the choice of which one to accept was
made on the basis of the fossils (as you pointed out), because the
acceptable range of dates for each fossil form was known (by evolutionary
theory).
At each change, the authors may well have sincerely felt that they were
separating truth from error. How did they know whether a newly calculated
date was right or wrong? What was the basis by which they could judge the
acceptability of the date, at 1.8 or 2.6 or 200 million? Was it the fossil
record, or accepted ideas about the correct ages of the fossils?
If the deposit had Australopithecines, it could not be 200 million years
old, but 2.6 million was alright. If it had more modern hominid skulls,
and these evolved from the Australopithicenes, it had to be younger than
2.6 million years old. So the it was the fossils that gave the constraints
for the dates -- or, rather, the accepted evolutionary age of the fossils
was the criterion that gave the constraints for the acceptable geologic age
of the strata. When the known fossil data changed (with their attendant
acceptable ages), the chosen date also changed.
Evolutionary theory _about_ the fossils was what gave credence to the
published dates that were selected from the range of dates obtained by
radiometric and other dating methods.
Is this an isolated case? Not at all. In studies where a range of dates is
obtained, the ones in the right ball park are presented, giving the limits
of the correct age, and the anomalous ones (if mentioned at all) are
explained in terms of mixing of sediments, extra Argon retention, leaching,
and other very plausible processes. The explanations for anomalously old
or anomalously young ages may be quite accurate (and may appliy to the
accepted ages as well!). If no acceptable age is found, the results may
not be published at all.
My point is that the accepted ("non-anomalous") age is the age within the
timeframe set by evolutionary theory. So it is evolutionary theory that
dictates which radiometric results will be acceptable to publish as the age
for a fossil. If you want to accept this, you may. But there may be a
more sure timescale. I hope we all look very seriously at this and make
wise choices.
That's why I asked. :-)
Karen