The buzz over the causes of the mass extinction 65 million years ago, at the
Cretaceous-Tertiary (K/T) boundary, is getting louder. In the past 20 years,
many geologists have accepted the idea that a massive asteroid, or some other
large projectile, crashed off the Yucatán Peninsula in Mexico and triggered
the demise of the dinosaurs and 75 percent of life on Earth through ensuing
climate changes. Now, a paleontology graduate student has found evidence in
the survival of tiny honeybees that could be another sting to that theory.
Although the impact theory has come under scrutiny in recent years, many scientists have proposed that in the aftermath of the K/T impact, dust and debris swirled in the atmosphere for about 10 years, choking out sunlight, halting photosynthesis and cooling the planet. Models of this "impact winter" have estimated that global temperatures plummeted 7 to 12 degrees Celsius (13 to 22 degrees Fahrenheit), freezing out ecosystems and exterminating three out of every four species at the K/T boundary. In the oceans, it may have taken up to 2,000 years for temperatures to return to previous levels.
Jacqueline Kozisek, a graduate student in vertebrate paleontology at the University of New Orleans, recognized that there were not many specifics of temperature change from the models, so she went looking for K/T survivors that might give some clues about environmental conditions following the impact. She focused on tropical honeybees because they have modern descendents with which to compare them, and their very survival meant extreme temperature changes could not have dominated the entire planet.
Based on fossil records, bees first evolved during the Late Cretaceous. While many species were omnivorous, the tropical honeybee, Cretotrigona prisca, relied mainly on nectar from flowering plants and never stored any food. Specimens captured in amber show few changes between then and now. After the giant impact 65 million years ago, the only way the tropical bees could have survived was if the tropics and the flowers did too.
"The tropics are such a fragile environment," Kozisek says, and these ecosystems would have been the ones most easily affected by the impact and a long, dark and cold impact winter. Fossil specimens support that the tropical honeybee endured the collision, and not just in an isolated refuge. Based on environmental requirements for tropical bees today, temperatures could not have dropped more than 7 degrees Celsius after the impact, says Kozisek, who presented her results at the the Geological Society of America's annual meeting in November.
Matthew Huber of Purdue University, who has co-authored a paper on the impact winter, says that such local variations among greater global temperature changes make sense. "It is quite conceivable, based on the modeling that I did," Huber says, "that bees living near the coasts in the tropics would experience significantly above-freezing temperatures for several years during even a total impact winter."
"I'm definitely not trying to say a nuclear [impact] winter didn't happen," Kozisek says. She hopes other scientists will perform similar analyses of K/T survivors to help "flesh out" the specific consequences of the impact winter and create a better picture of post-impact environmental conditions.
"We need more studies such as this to constrain the magnitude of and severity of the global cooling episode," Huber says. Similar research should continue to clear the haze of the K/T extinction.
Geotimes contributing writer
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