How did I miss seeing
that talk in the program?! At the 2014 Geological Society of America meeting
Sunday, I made a point to go to Gerta Keller’s talk on her research on the end-Cretaceous
extinction (she has long advocated that Deccan trap volcanism is the cause, not
Chicxulub impact), but luckily heard the last half of Cynthia Belcher’s preceding
talk “Cause or consequence? Wildfires at the Triassic-Jurassic and
Cretaceous-Paleogene boundaries”
(https://gsa.confex.com/gsa/2014AM/webprogram/Paper246260.html). I was actually more interested in
Belcher’s talk due my background in coal petrology, doctoral research on
Triassic-Jurassic eastern US rift basins, and USGS Mendenhall post-doc research
(2006-08) on the Chesapeake Bay impact crater.
For
the end-Triassic, Belcher concluded, based on amounts of fossil charcoal, that subsequent
increase in wildfire was a consequence of the change in vegetation after the
extinction (http://www.nature.com/ngeo/journal/v3/n6/abs/ngeo871.html). Her study was in Greenland. My dissertation
research on changes within and among orbitally-driven 20,000-year lacustrine
sedimentary cycles in the earliest Jurassic of the Newark basin (New Jersey)
showed differences in amount of fusinite (fossil charcoal) between cycles that
could be attributed to the cyclic climate variability during that time period.
Thinking of change across the Tr-J boundary, these Jurassic lake cycle differences
could possibly mask any notable extinction-related fossil charcoal variation. I
did not sample Triassic sediments since most of the immediately underlying Newark
Triassic is red, therefore organically barren, and is overmature; regrettably,
southern US early Mesozoic basins like the Richmond and Taylorsville basins are
missing the Jurassic.
For
the K/Pg (end-Cretaceous) boundary, Belcher called any wildfire due to the
meteor impact a “one-and-out” event (I think that is the term she used) that did
not promote environmental change. To lay my cards on the table, I have long
been a fan of Andrew Scott, one of Belcher’s doctorate advisers, and his coal
petrographic studies of fossil charcoal in coals across the K/Pg boundary that
show NO evidence of a giant impact-related wildfire or increase in wildfire activity.
Mostly due to Scott’s research, I have not been convinced, despite impact
modeling studies, that the atmosphere caught on fire during Chicxulub impactor
entry enough to burn vegetation or fry dinosaurs. Suggestions that the worldwide
presence of soot indicates a global K/Pg impact-related wildfire is negated by
modern studies that show soot from large wildfires can circle the globe in less
than a month.
Various
organic compounds, like polycyclic aromatic hydrocarbons (PAH), are also geochemical
indicators of combustion and can be used to identify paleo-wildfire events.
Geochemistry is a powerful tool, but being a petrologist, I see microscopy and
geochemistry as partners in research. Sometimes one really has to look at the
rock to understand the geochemical context. Both have trade-offs: geochemistry
can be quick but expensive, while traditional light microscopy is economical
but time-consuming.
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