Odd Observations at the K/T Boundary

From
Date
Reply-To
I can only underscore Jere's point that what we need to advance the overall
K-T debate are testable hypotheses that relate specific extinction
mechanisms to the data of the fossil record.  I'll be taking a look a Jim
Pospichal's paper, but as of now I can't understand how bioturbation (a
process that disturbs the differentiation between layers of sediment) can
generate distinct, large-scale anomalies at different stratigraphic
horizons.  But even if a viable mechanism of this sort was available, does
that mean that it would be is the only possible way of accounting for these
observations?  Of course not.  We know that Ir is very mobile in the
sediment and that it can be chemically concentrated at the base of shale
layers and at REDOX boundaries (see Wang et al., 1993 [Geology] Sawlowicz,
1993 [P3]).  Moreover, the fact that Ir anomalies appear in demonstrably
incomplete sequences suggests to me that chemical concentration is more
likely.

As I see it there are two different questions here that are getting mixed
together.  First, there is the question of whether or not a bolide collided
with the Earth at the end of the Cretaceous.  Then, there is the question
of whether this collision (if it took place) perturbed the environment in
such a way as to account for the observed extinction record.  The answer to
the second question does not necessarily follow from the answer to the
first.

The Ir discussion plays an important role in both questions because it
originally provided evidence for the identification and the timing of the
impact (the latter of which is necessary to test extinction causation
hypotheses).  I'm perfectly happy with the idea that the Ir itself was
ultimately derived from an impact event.  But the fact that it moves around
in the sediment (by whatever means) compromises its utility as a reliable
indicator of the timing of the event(s).  Not all Ir anomalies are
synchronous.  There is often a pronounced discrepancy between the level if
Ir concentration and the level of enrichment of other (less ambiguous)
types of impact debris (e.g., shocked quartz, glass spherules) that can
range from a few mm to several meters.  These observations have led some to
believe that there may have been multiple K-T impacts - a scenario that
might also account for the multiple Ir anomalies, while others (e.g., Wang,
et al., 1993, Sawlowicz, 1993) have pretty much given up trying to
interpret Ir anomalies as high-resolution evidence for the timing of an
impact.  Interestingly, Donovan et al. (1988) showed that the multiple
anomalies in the Braggs section correlated to parasequences boundaries,
once again suggesting a long Ir residence time in the environment and an
inorganic control on its concentration.

Finally, should we be suspicious of the coincidence of a putative impact
event and large numbers of extinctions at the end of the Cretaceous?
Certainly something major happened in the Maastrichtian-Paleocene
transition and, as I've argued in several recent papers, I have no problem
with calling it a mass extinction. The impact(s) would be strongly
implicated on circumstantial evidence alone if no substantial environmental
changes were taking place in the time interval leading up to the event.
Unfortunately, this is not the case.  During that time one of the largest
and fastest marine regressions in the entire Cretaceous was in full swing.
Recent high resolution isotopic evidence also shows that the climate was
flipping back and forth between hot and cold phases (not ice ages mind you,
but cold by Cretaceous standards).  We know that these types of
environmental changes have been responsible for biotic extinctions at other
points in Earth history.  The problem (perhaps an intractable one) is going
to be identifying which species succumbed to "normal" environmental
changes, which extinctions were uniquely associated with an impact event,
and which were the product of both.  The sensationalistic seduction of
explaining everything by a big fireworks display is very tempting (and it
makes excellent media copy). But good science can't be fit into sound bites
and it's going to take a lot of hard interdisciplinary work before we get
this part of Earth history properly sorted out.  To paraphrase Mark Twain,
"Rumors of a solution to the K-T extinction mystery have been greatly
exaggerated."


Norm MacLeod




----------------------------------------------------------------------------
Norman MacLeod
Senior Scientific Officer
N.MacLeod@nhm.ac.uk (Internet)
N.MacLeod@uk.ac.nhm (Janet)

Address: Dept. of Palaeontology, The Natural History Museum,
         Cromwell Road, London, SW7 5BD

Office Phone: 071-938-9006
Dept. FAX:  071-938-9277
----------------------------------------------------------------------------