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Geez! I go to the quarry for ONE day and miss nearly missed the debate ;-)
I dont't think that anyone including myself believes that the taxa in
question all died out at the exact same stratigraphic horizon. Many of the
extinctions occurred _within_ centimeters or a meter or so of the boundary
clay. Now depending on the deposition rate of the sediment involved, a few cm
can be nearly instantaneous eg, the "Three Meter Gap" at Hell Creek and Bug
Creek has been pared down to less than 1 meter. Here the fluvial mechanism is
prominant and a one <1m gap becomes insignificant since even this much
sediment can be deposited (or scoured) in a single flood. Marine sequences
are obviously just as complex. The Littoral and Bathyal zones could be
expected to respond dilfferently to perturbation by similar factors.
> As for the Ir it's
>distribution is very curious. There are multiple Ir anomalies in many
>marine sections (e.g., Braggs, Brazos River) where the fluvial >mechanism
>Tom alludes to is not operable.
I would not expect to find Iridium at every K-T site known ( not every known
K-T sequence has microtektites or spherules associated with them either)
simply because the total global Ir input could not have been isotropically
distributed to all parts of the globe. The concentration of Ir per unit area
would probably be a function of the distance from the impact and
geological/meteorlogical factors that were extant at the time of input and
can just as easily have wiped out the Ir signature. The Ir could also have
leached out of some places creating multiple signatures, obscured signatures
and possibly no signature.
> Even more troubling is the fact that
>prominent Ir spikes show up in biostratigraphically incomplete >successions
>(e.g., Gubbio, Stvens Klint) whereas in some biostratigraphically >complete
>sections (e.g., Miller's Ferry) no Ir anomaly has been found. I don't >see
>how these data can simply be ignored.
No they should'nt be ignored. All sequences need to be satisfactorily
explained exclusive of the impact scenario then applied to the scenario if
relavent.
The "completeness" of each sequence is also a matter of debate. There are
those who regard many of the marine sequences as disconformable caused by
hiatuses (diastems). Reworking caused by bioturbation has also been used by
both sides to explain some of the discrepancies.
>These observations, >.coupled with the
>fact that instantaneous faunal turnovers only coincide with Ir >anomalies in
>the biostratigraphically incomplete sequences, should tell us that >the
>story is a bit more complicated than that a big rock fell out of the >sky
>and everything died (somehow), except for those things that didn't.
Assuming that this statement is correct for the moment, then does not the
"coincidence" of a faunal turnover, Ir anomaly and incomplete stratigraphic
sequences also hint at something catastrophic since the phenomenon is global
in nature?
Or do we start from scratch with the question; Was it a gradual, step-wise or
mass extinction?
What I would like to see published is a complete up -to -date correlation
chart or book of all known K-T sites worldwide that gives detailed geologic,
stratigraphic, paleomagnetic, paleontologic (sensu lato) , and isotopic etc.
data that everyone can refer to for debates such as this. I cant't keep the
stats on every site in my head and in the correct order and we have not even
talked about the isotopic aspect of the K-T. Where are those geochemists when
you need them :-)
Regards,
Thomas R. Lipka
Paleontological/Geological Studies
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