Monday, October 20, 2014

Paleo-wildfires and extinctions at GSA 2014

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” (  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 ( 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 late Mesozoic basins like the Richmond and Taylorsville basin 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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