Dinosaur-Killing Firestorm Theory Questioned

SPACE.com -- Dinosaur-Killing Firestorm Theory Questioned
http://www.space.com/scienceastronom...-wildfire.html

New research challenges the idea that the asteroid impact that killed
the dinosaurs also sparked a global firestorm.

Scientists modeled the effect that sand-sized droplets of liquefied rock
from the impact had on atmospheric temperature. The asteroid is thought
to have gouged out the Chicxulub crater on the Yucatán Peninsula in Mexico.

It was previously thought that the falling spherules, as the tiny rocks
are called, heated up the atmosphere by several degrees for up to 20
minutes — hot enough and long enough to cause whole forests to
spontaneously burst into flames.

As evidence for this, scientists pointed to what appears to be
carbon-rich soot from burned trees discovered in the thin band of debris
dating back to the impact some 65 million years ago, a shift in geologic
time called the K-T boundary.

A new theory

But a new computer model, detailed in a recent issue of the journal
Geology, suggests that the first barrage of falling spherules coalesced
into a descending opaque cloud about 40 miles (70 km) above the Earth's
surface, shielding our planet (and the dinosaurs) from the heat of
spherules raining down from above.

"As more and more spherules are injected into the upper atmosphere, the
cloud of settling spherules becomes thicker and denser," study team
member Tamara Goldin of the University of Vienna told SPACE.com.

"So previously entered spherules help to shield the ground of some
fraction of the thermal radiation from the subsequently entering spherules."

This "self-shielding" may have prevented global wildfires and limited
other environmental effects from the impact, Goldin said.

The Earth's atmosphere likely did heat up, Goldin said, but the
temperature increase may not have been as dramatic or as long-lasting as
previously estimated.

"If you were on the ground, it would feel at the maximum like you're
under a broiler in your oven," she said. "It would not be very
comfortable, but it would not be instant immolation."

Burning oil

So if burning forests didn't create the K-T boundary soot, what did?

Some scientists have suggested the soot might have been caused by the
burning of petroleum in the rocks of the Chicxulub impact site when the
asteroid struck. "We know it's a pretty oil-rich area today," Goldin said.

She added that even if the Chicxulub impact didn't cause trees to catch
fire worldwide, it almost certainly triggered other environmental
catastrophes that contributed to the dinosaurs' demise, such as global
dimming and acid rain.

"Just because we didn't have a global firestorm doesn't mean that
Chicxulub is not the cause of the [dinosaurs'] extinction," she said.