[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

the rules of dating



>Wow!  You are erroneously assuming that I am looking at DNA that does not
change at a constant rate.  Have you ever heard of molecular clocks? Clock
genes? 

How can you be certain that your dna changes at the given rate?
There are no dating methods that directly date fossils in the time range of
which you speak.  Good dating mathods for fossil material 
only exists for the Quaternary.  There, the dating methods are 
fraught with problems.  If you have been relying on $^{14}$C dates
for your calibration (assuming that you are calibrating - which I have
not seen you demonstrate), then there is a vast literature on problem
$^{14}$C applications.  Note also that it is limited to periods less
than 50 ka (actually more like 40 ka, when you consider the 
contamination problems).  The only other method that dates fossils
is the ESR dating method.  This one currently only has a precision
ranging at $\pm$ 5-8 \% at the best of times.  
This method has a range that varies from 10-50 ka on the young end
and 1-6 Ma on the old end.   Some Quaternarists would claim that
AAR (amino acid racemization) provides good absolute dates, but
most geochronologists would not agree.  At best AAR provides 
a reliable relative age, except for the rare case of ratite egg shells,
where it works very well. It has only been tested in a few localities,
and the range seems to be perhaps 500 ka.  U series methods 
($^{230}$Th/$^{234}$U,
$^{231}$Pa/$^{235}$U, and a couple of others) date corals very nicely,
but again the time range is limited to 500 ka with the best of techniques,
and more often 250-300 ka because isotopic exchange occurs that destroys
the assumptions needed to make the date.

For older time periods, you must date the fossils by correlation.  By that
I mean that you date layer X which relates in some fashion to layer Y where
you find the fossils.   In some areas where volcanic eruptions are common,
you may get good correlations that easily defensible, but any correlation
adds to the uncertainty in the dates!  Therefore, if you have 200 $\pm$
5 for layer X, and 220 $\pm$ 8 for layer Z, then for layer Y between 
X and Z, at best you have 200 < Y < 220, but more likely anything
like this 195 < Y < 228 is possible. 
For most sites with fossils, the connections are much more tenuous, and the
associated enlargement in errors (i.e. the error propogation) concomittantly
larger. 

None of the references you list include anyone in the list of authors who
appears to be a geochronologist, which leads me back to my original challenge:
prove that your dates for the species that define your molecular clocks
are correctly dated.

If your clocks are constructed using modern species (and all the early work
in this field followed that paradigm), then your "dated" rates are
uncalibrated completely.  Even when you claim that all species are
equally evolved from organisms 4.0 Ga, that does not prove that their
equally separated, nor that the gene evolves at a uniform rate.
That can only be proven by calibrating the separations.  To do that you
MUST use fossil species at some point.  Otherwise, you are arguing circularly.

If you are using fossil species to date
your important calibration points, demonstrate that they have been correctly
dated.  Give us the fossil species you have used and the methods (and
refs to the published work) by which they have been dated.

Until you do this, no geologist worth their salt is going to believe
your numbers, because they are unfounded in geological reality.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Bonnie Blackwell,                               bonn@qcvaxa.acc.qc.edu
Dept of Geology,                                (718) 997-3332
Queens College, City University of New York,    fax:  997-3349
Flushing, NY 11367-1597