Hope this sheds some light on the issues raised by Bjorn:
By the early 1980s, Walter and Luis Alvarez (sometimes working with Frank Asaro) had discovered charcoal and iridium in the K/T boundary at Gubio, Italy. They suspected an impact crater as the source, but did not have a clue as to the location. They published their theory with not much evidence other than the Gubio layer. Most of the dating work on the K/T boundary in the 1980s and in previous decades was done by sedimentary petrologists, paleontologists and paleobotonists using fossil evidence and strata correlation's to estimate when the K/T extinction event took place (and over what duration). These estimates generated a wide array of dates and durations for the K/T event. The Alvarez' were considered renegades with radical ideas by most geologists and paleontologists in the early 1980s.
In 1991, Hildebrand, Penfield, Kring, et al, published their first paper on the Chicxulub crater. They sited gravity and magnetic anomoly data and compiled stratigraphic profiles of the crater that they mapped from core drillings. The Alvarez' appeared to be vindicated and K/T impact crater research became respectable. Many, many scientists joined the search for additional evidence. Some who had been secretly working on Chicxulub went public. The Hildebrand/Penfield team's work meant that for the first time there was a major heating event (the impact) that must have left heated, altered rocks and minerals which could be radiometrically dated. REFERENCE:Hildebrand, et al. GEOLOGY. v. 19. pp. 867-871. 1991.
Two months later, Izett, from he USGS Denver office, published the first radiometric date for the Chicxulub event using 40Ar-39Ar analysis. He dated the Haitian tektites at 64.5 +/- 0.1 Ma. REFERENCE: Izett. JOURNAL OF GEOPHYSICAL RESEARCH. v. 96, no. E4. pp. 20,879-20,905. 1991.
All of the subsequent radioisotope dating by other scientists also clustered around the similar ages considering +/- error of the techniques employed. These dates were far more precise than the earlier work by the sedimentary petrologist and paleo scientists. The fact that the new radiometric dating correlates so well is compelling evidence that Chicxulub did have a global impact.
Unfortunately, a lost of popular books and magazines (encyclopedias, Popular Science, etc.) do not keep up on the current scientific literature. Many still use the older, imprecise dating from the 1980s. Scientific American has had many excellent, easy-reading and accurate articles on this entire issue -- I would recommend these for those wanting an accurate overview of the scientific research on Chicxulub.
BJORN SORHEIM WROTE:
First of all, I must say that I believe that the Chicxulub impact theory is the
best explanation. Such a large impact would naturally produce volcanic
side-effects
at the center, so such features would not prove or disprove either theory.
But there is a small detail that I wonder how came to be, because it seems
to be wrong.
At 17:08 28.02.98 -0800, Steve Excell wrote:
>Potassium feldspar grains shocked and melted to glass spheres and
distributed spatially in greater population closest to the crater site.
Dated 64.98 million years +/- 50,000 years ago.
^^
>Yellow glassy tektites in bedrock formation on Haiti, other Carribean
islands and Gulf Coast that are classic crater ejecta. The richness in Ca-S
match the Ca-S rich target rocks near Chixulub. Absolute age of tektites:
65.03 +/- 0.1 Ma.
^^
Gee, this is really impressing! Just 0.02/0.03 off the 65 Ma K/T boundary!
This must
be the conclusive evidence!
Or is it? Of course NOT. I have seen the K/T boudary set to 66 Ma in some
credible sources. Britannica Online have the K/T boudary for the moment at
66.4 Ma. So those values cited are 1.400.000 years off!
It almost would seem that the researchers (childishly?) have believed that
the K/T
boundary was forever fixed at 65.00 Ma B.P., and therefore have 'adjusted'?
their
readings to fall nicely into the right interval. Of course it don't. The values
change quite much with time as better methods of absolute age determination
comes
into use. The time of the boundary is fixed (and NOT _determined_ by anyone)
to what
happened in the geologic record, e.g. the irridium layer. Thereafter an
absolute age
determination is done, which is very much dependent on the quality of the
method.
The values would never fall exactly at an xx.00 value (at least only by
sheer luck),
but it would indeed if some scientific committee decided to place it there.
Unfortunately
that is not how it works in geology.
It seems to me that these values almost disprove the impact theory, if not
sommeone
can explain why they are so far off the present established value of the K/T
boundary.
Regards,
Bjørn
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