Monday, February 12, 2007

 

Study shows largest North America climate change in 65 million years

The largest climate change in central North America since the age of the dinosaurs 65 million years ago, a temperature drop of nearly 15 degrees Fahrenheit, is documented within the fossilized teeth of horses and other plant-eating mammals, a new study reveals.

[Image: Mesohippus]

Mesohippus (Evolution Research: John Latter / Jorolat)

The overwhelming majority of previous climate-change studies on the 400,000-year transition from the Eocene to the Oligocene epochs, about 33.5 million years ago, focus on marine environments, but University of Florida vertebrate paleontologist Bruce MacFadden and his colleagues turned their attention to fossils from the Great Plains.

The study were published online on February 7 in the journal Nature and appeared in the February 8 print edition.

"If a temperature change of this magnitude occurred today, Florida would have weather similar to Washington, D.C., or even farther north," said MacFadden, a curator at the Florida Museum of Natural History.

The Eocene-to-Oligocene transition (or Eocene/Oligocene Boundary) is known in the fossil record as the Grande Coupure, the "Big Cut" in French, because it marks a massive extinction of life in both marine and land environments. Scientists believe the drop in temperature was likely due to changes in oceanic currents, MacFadden said.

"Fossil mammals are archives of ancient information," MacFadden said. "Their teeth are like little time capsules that allow us to analyze chemicals captured millions of years ago within the animals' skeletons."

MacFadden said 49 of the 68 fossil teeth analyzed came from the Florida Museum's vertebrate paleontology collection. Researchers analyzed oxygen and carbon isotopes in the preserved teeth and bones of primitive fossil horses and a primitive cloven-hoofed mammal called an oreodont. Isotopes are atoms of naturally occurring elements, characterized by varying numbers of neutrons but constant numbers of protons. Oxygen isotopes act as thermometers, telling scientists at what temperature they were formed; and carbon isotopes act as barometers, revealing relative humidity.

"A combined analysis of the isotope composition of bones and teeth is a new approach to studying this boundary in time," said Alessandro Zanazzi, a doctoral student in geology at the University of South Carolina and lead author of the paper. "Tooth enamel has very low porosity and low organic matter, so it maintains the isotopic composition of when it was formed."

Donald R. Prothero, a professor of geology at Occidental College and an expert on the Eocene-to-Oligocene transition, said, "We have long known that there were some dramatic climatic changes in the earliest Oligocene based on the record of marine plankton and isotopes. But we didn't know how much change there was in degrees, although the plant changes suggested it was indeed about 15 degrees."

Prothero also said gaps in the fossil record from Nebraska may have prevented researchers from obtaining detailed temperature data, and he hopes further studies will be completed at other sites such as Wyoming.

Source: University of Florida February 7, 2007 (Adapted).

[Evolution]

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Based on the journal Nature paper:

Large temperature drop across the Eocene-Oligocene transition in central North America
Alessandro Zanazzi, Matthew J. Kohn, Bruce J. MacFadden and Dennis O. Terry
doi:10.1038/nature05551

The Eocene-Oligocene transition towards a cool climate (approx33.5 million years ago) was one of the most pronounced climate events during the Cenozoic era. The marine record of this transition has been extensively studied. However, significantly less research has focused on continental climate change at the time, yielding partly inconsistent results on the magnitude and timing of the changes. Here we use a combination of in vivo stable isotope compositions of fossil tooth enamel with diagenetic stable isotope compositions of fossil bone to derive a high-resolution (about 40,000 years) continental temperature record for the Eocene-Oligocene transition. We find a large drop in mean annual temperature of 8.2 plus or minus 3.1 degrees C over about 400,000 years, the possibility of a small increase in temperature seasonality, and no resolvable change in aridity across the transition. The large change in mean annual temperature, exceeding changes in sea surface temperatures at comparable latitudes and possibly delayed in time with respect to marine changes by up to 400,000 years, explains the faunal turnover for gastropods, amphibians and reptiles, whereas most mammals in the region were unaffected. Our results are in agreement with modelling studies that attribute the climate cooling at the Eocene-Oligocene transition to a significant drop in atmospheric carbon dioxide concentrations.

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