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Re: K-Pg extinction global firestorms



The Robertson et al. calculations, which find global firestorms, were
apparently contradicted by computer simulations by Schulte et al.(review:
5 Mar. 2010. The Chicxulub asteroid impact and mass extinction at the
Cretaceous-Paleogene boundary. Science. 327(5970):1214-8. doi:
10.1126/science.1177265.):

"Detailed multiphase flow models suggest that the atmospheric reentry of
the ejecta spherules may have caused a global pulse of increased thermal
radiation at the ground (42). Such a thermal pulse is below the lower
limits of woody biomass ignition, in agreement with studies yielding no
evidence for widespread large wildfires at the K-Pg boundary (43), with a
possible exception for the Gulf of Mexico region close to the impact site
[(9) and references therein]. However, the modeled level of radiation is
expected to have resulted in thermal damage to the biosphere even if the
maximum radiation intensity was only sustained for a few minutes."

In general, that is my problem with the Robertson et al. hypothesis, it is
TOO deadly to explain the survival pattern. They had a great idea in
realizing that everything that survived could have sheltered, and I am
looking forward to the new paper to see what new stuff they've come up
with. But if any unsheltered animals anywhere in the world would have been
burned to death by the incandescent, broiler - like, heat of the
atmosphere, and there were no refugia, I don't think ratites, other
palaeognaths, and other neornithines could have survived. Birds don't
really use shelters except when they are nesting, and basal ones usually
nest in simple scrapes. Nocturnal birds like Steatornis nest in caves, and
Apteryx shelters in burrows or crevices, but theses behaviors are most
likely derived. Moreover, if there were no refugia I don't think we would
have seen the forests of New Zealand come back so completely -- all the
species of podocarps and all angiosperms except two species that went
extinct. If it was a fire, buried tree seeds should have sprouted the next
year and we'd see no fern spike. The recovery took thousands of years,
like trees spreading from isolated refuges and recolonizing continents in
a pioneer succession.

Also, the converse of their hypothesis is not true. Animals that could
shelter went extinct in droves, like ammonites and waterbirds like
hesperornithiforms, ichthyornithiforms, and lakeside enantiornithines like
those reported by Dyke et al. (2012). Even pterosaurs appear to have laid
soft shelled eggs in damp sediment, like turtles, according to Lu et al.
(2011).

A paper I just read is:

Barreda et al. (2012) Cretaceous/Paleogene Floral Turnover in
Patagonia: Drop in Diversity, Low Extinction, and a Classopollis
Spike. PLoS ONE 7(12): e52455. doi:10.1371/journal.pone.0052455.

In it the authors acknowledge less than ideal resolution in the Patagonian
fossils, but they find no fern spike. They find a drop in diversity and
about 10% of plant species go extinct (compared to 17-30% in North
America), but the early Danian flora here is a lot more diverse than in
North America. Remarkably they report a Danian rebound of Mesozoic,
Cheirolepidiaceous, pollen Classopolis. The authors suggest that this
Mesozoic survivor may have been the dominant pioneer plant in the
disturbed habitat. They also note that this supports previous hypotheses
that high southern latitudes were refugia during the K-T crisis.

We know that there is less ejecta found the further we get from Chicxulub,
so it follows that sites very distant would have had lower heat fluxes.
Birds that persisted there (in Gondwana) could have recolonized the
planet. Indeed, that is where ratites are still confined today.

 



Jason Brougham
Senior Principal Preparator
American Museum of Natural History
jaseb@amnh.org
(212) 496 3544





On 3/27/13 5:14 PM, "Ben Creisler" <bcreisler@gmail.com> wrote:

>From: Ben Creisler
>bcreisler@gmail.com
>
>
>A new paper of interest:
>
>Douglas S. Robertson, William M. Lewis, Peter M. Sheehan & Owen B. Toon
>(2013)
>K-Pg extinction: Reevaluation of the heat-fire hypothesis.
>Journal of Geophysical Research: Biogeosciences (advance online
>publication)
>DOI: 10.1002/jgrg.20018
>http://onlinelibrary.wiley.com/doi/10.1002/jgrg.20018/abstract
>
>
>The global debris layer created by the end-Cretaceous impact at
>Chicxulub contained enough soot to indicate that the entire
>terrestrial biosphere had burned. Preliminary modeling showed that the
>reentry of ejecta would have caused a global infrared (IR) pulse
>sufficient to ignite global fires within a few hours of the Chicxulub
>impact. This heat pulse and subsequent fires explain the terrestrial
>survival patterns in the earliest Paleocene, because all the surviving
>species were plausibly able to take shelter from heat and fire
>underground or in water. However, new models of the global IR heat
>pulse as well as the absence of charcoal and the presence of
>noncharred organic matter have been said to be inconsistent with the
>idea of global fires that could have caused the extinctions. It was
>suggested that the soot in the debris layer originated from the impact
>site itself because the morphology of the soot, the chain length of
>polycyclic aromatic hydrocarbons, and the presence of carbon
>cenospheres were said to be inconsistent with burning the terrestrial
>biosphere. These assertions either are incorrect or have alternate
>explanations that are consistent with global firestorms. We show that
>the apparent charcoal depletion in the Cretaceous-Paleogene layer has
>been misinterpreted due to the failure to correct properly for
>sediment deposition rates. We also show that the mass of soot
>potentially released from the impact site is far too low to supply the
>observed soot. However, global firestorms are consistent with both
>data and physical modeling.
>
>===
>
>Press release:
>http://www.colorado.edu/news/releases/2013/03/27/cu-study-provides-new-evi
>dence-ancient-asteroid-caused-global-firestorm