| The Chicxulub Debate; what next?
Gerta Keller
The debate, Chicxulub the Non-Smoking Gun? was sponsored by the Geological
Society of London and kicked off with a provocative article here and in
Geoscientist (November 2003), with debate comments published on this Web
site.
Dr Ted Nield was the instigator, and the heart and soul of this debate.
He and his colleagues at www.geolsoc.org.uk and Geoscientist deserve the
highest praise for devising a forum to encourage open discussion of such
controversial issues, both with this debate and the earlier Mantle Plumes
debate.
The Chicxulub controversy has resulted in a lively on-line debate, with
the result that no readers will ever again view the K/T impact theory
as a "done deal". For the very first time the debate allowed contrary
evidence to the popular impact theory to be publicised, and it has raised
the level of awareness among Earth scientists and the general public as
a whole. I have experienced even six year-olds who now question the popular
impact theory.
This is a singular success, regardless of which side of the debate you
are on. It allows us now to move forward with the search for the real
reasons for the K/T mass extinction, whether single or multiple impacts,
major volcanism and climate change, or a coincidence of impacts and volcanism.
The Chicxulub Debate was conceived in response to new results from NE
Mexico and the Chicxulub crater core Yaxcopoil-1 by Keller, Adatte and
Stinnesbeck, showing evidence that indicates this impact predates the
Cretaceous/Tertiary boundary and was not the cause for the mass extinction.
The debate centred on three questions:
* What is the age of the Chicxulub crater?
* Can the clastic deposits in NE Mexico be interpreted as impact-tsunami
deposits?
* Are there multiple impacts?
These queries directly question the commonly accepted Alvarez KT impact
theory and Smit et al.'s impact-tsunami theory.
We presented data from impact spherule deposits in NE Mexico and from
the crater core Yax-1 that indicate that the Chicxulub impact predates
the K/T boundary by about 300Ka and therefore was not related to the mass
extinction. We presented evidence of bioturbation throughout the clastic
deposits that effectively rule out deposition via impact tsunami over
a period of just hours or days. And we presented evidence of multiple
impacts, including the pre-K/T Chicxulub and the K/T boundary impacts.
Readers' comments were generally supportive. The only serious challenge
came from Jan Smit in three lively and lengthy contributions defending
the impact-tsunami theory and a K/T age for the Chicxulub impact. Smit's
defence rested on denying the evidence of bioturbation, claiming widespread
slumping caused impact ejecta to be folded into late Maastrichtian sediments,
and claiming that late Maastrichtian foraminifera identified from the
laminated limestone above the impact breccia in the Chicxulub core Yax-1
are just dolomite rhomb crystals.
What next?
Clearly, this debate did not change Smit's mind. Our findings are revolutionary;
if we are right they require rethinking the way we view mass extinctions
and particularly the biotic effects of impacts. To move the science of
mass extinctions and the biotic effects of impacts and volcanism forward,
we need to shed previous prejudices and assumptions.
Smit's proposal to conduct a blind test on the Yax-1 core samples will
not solve the controversy. Blind tests in the past have been total failures
with at best inconclusive results (e.g. Ir anomaly at Gubbio, El Kef mass
extinction). They are poor substitutes for real in-depth scientific investigations.
The controversy also does not rest on the presence or absence of microfossils
in the Yax-1 core, but also on the magnetostratigraphy, sedimentology
and mineralogy that provide no evidence for chaotic high-energy backwash
and crater infill. Moreover, the controversy does not rest upon the Yax-1
core. What is even more important are the many outcrops in NE Mexico with
multiple spherule layers interbedded in late Maastrichtian marls and the
bioturbation within the so-called impact tsunami deposits of Smit and
others. These sections provide conclusive evidence that Chicxulub predates
the K/T boundary impact.
Ultimately, it is the reproducibility of these mineralogic, paleomagnetic,
biostratigraphic and geochemical results that will determine who is right.
Science is not done by consensus; what is relevant are reproducible results.
To this end the Yax-1 samples and the K/T sections of NE Mexico should
be made available to investigators willing to conduct the research.
Field Trip to NE Mexico
We propose to lead a field trip to the NE Mexico sections to show all
interested parties the evidence we have presented in this debate and to
let them draw their own conclusions. This field trip would also facilitate
collection of samples for further studies by interested parties. Ideally,
such a field trip could be an extension of this debate &endash; Could
it perhaps be sponsored by the Geological Society of London?
Regrets
I have two regrets I about this debate. First, it got narrowly stuck
with evidence from NE Mexico and the Chicxulub crater and never broadened
to the wider question of spherules in the Danian of southern Mexico, Guatemala,
Belize and Haiti.
Second, the debate never touched the question of the mass extinction,
or the biotic effects of impacts and volcanism. However, this is a topic
worthy of a Debate of its own, with wide interest and a guaranteed multitude
of participants across all fields of palaeontology.
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