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Re: The dinosaurs did not die in fire, from the latest Geology
Phil Bigelow wrote:
> I would venture a wild guess that the local source of soot would "swamp
> out" the global soot's signature.
While I agree with you in general, that may be true only in areas of
local deposition (for example low, flat areas within the flood plain,
and the inside of river bends). The reason being that other areas
within the basin would be subject to severe erosion due to the massive,
recurrent rains and the newly barren soil. If so, the local soot and
initial iridium would move downstream, and be partially replaced by
later global soot deposited after the initial rainfall surge and/or
after partial regrowth of vegetation.
>(although such a discrimination of soot types would be a
> worthy scientific study in its own right).
> The only environments that could be expected to preserve a reasonably
> intact iridium signature are lakes and stagnant water environments.
> Unfortunately, these two environments are rare (accounting for maybe 2-5%
> of the total volume of the Formation.
I agree. I think we're making much the same point.
>....., rather than providing the vertical distance
> from the base of the Formation. That's because the iridium "layer" is
> always incomplete.
As you would expect it to be.
> The author(s) know this. It doesn't make their results
> worthless, but it opens the door for reinterpretation by another worker.
Additional work and alternate interpretations add to the total body of
knowledge. They are useful, even if wrong -- though more useful if
> Two phenomena are in play here:
> - Chronostratigraphic resolution...
> - Stratigraphic integrity....
> Regarding the first phenomenon, I think its safe to say that if a global
> wild fire occurred, then both the air fall iridium and the air fall soot
> were deposited *geolgically* simultaneously.
I don't. As a bit of an oversimplification, much of the iridium was
deorbited, from rather selective trajectories. The dispersion as it
reached the upper atmosphere would not be expected to be identical with
the soot deposits which first entered the atmosphere from relatively
near the surface. A 'different strokes for different folks' sort of
> we can't distinguish a few days of Cretaceous time from a few years of
> Cretaceous time. So "simultaneously" is used rather loosely here.
Yes, though probably with a few rare exceptions. For example, I suspect
the eastern seaboard landslide occured rather rapidly, with lower,
larger sediments being redeposited in a matter of hours and/or days
rather than years.
> Regarding the second phenomenon: I am making an assumption (which may or
> may not be true) that if the iridium-laden dust wasn't removed by
> erosion, then the intermixed/overlying air fall soot (if any soot was
> originally present) couldn't have been removed by erosion either. In
> other words, if subsequent flooding removed the iridium layer, then it
> also would have removed the soot.
That makes the assumption that the iridium detritus wasn't removed by
initial runoff (not flooding) prior to the deposition of soot. I'm not
sure that that is always a safe assumption.
> A somewhat sloppy analogy can be found
> in the kitchen. Mix wheat flour with corn flour. Then try to figure out
> how to remove one of the two components from the mix. It ain't easy, and
> I assume that Nature couldn't do a good job of separating them either.
Here's one technique. Deposit a substantial fraction of the corn
flour. Wash it off with a hose. Then deposit a mix of corn and wheat
flour. Then measure. Presto, corn flour is underrepresented at the
original site. Nature can seperate with the 'first flush' because the
the mechanism for initiating the iridium deposits is not identical with
the mechanism for distributing the soot. Note that I'm not saying that
this is what happened in every case, only that it is the sort of thing
that needs to be considered.
> BTW: neither iridium nor soot is soluble in water (even in highly acidic
> water), so dissolution is not a consideration.
I think we're both talking about suspended solids, not dissolved solids.
> Of course, none of the above comments apply to _in situ_ charcoal
> (charred roots, stumps, etc.). There is still so much that we don't know
> about the boundary zone that its ridiculous.
Too true. That's why I like to see these studies and discussions. I
wonder if the timing of the contrary opinion in one of the 'popular'
science magazines was accidental? :-)