Large temperature drop 33 million years ago produced cooler, but not necessarily dryer, environs in Central North America
(Photo by Joseph V. Labolito / Temple University)
The largest shift in climate since the time of the dinosaurs 65 million years ago may have produced environments across Central North America different than what has been commonly perceived and accepted, according to a recent study co-authored by Dennis Terry of Temple’s Geology Department.
Terry and his collaborators, Alessandro Zanazzi and Matthew J. Kohn from the University of South Carolina and Bruce J. MacFadden from the University of Florida, published the results of their study, “Large temperature drop across the Eocene-Oligocene transition in central North America,” in the Feb. 8 issue of journal Nature (www.nature.com).
“The object of the study was to collect fossil bones across a particular boundary within geologic time called the Eocene-Oligocene, about 33 million years ago, and to analyze carbon and oxygen isotopes within the bones and teeth in order to see how that data can be used to interpret the way the climates and environments changed across this period from Greenhouse to Icehouse conditions,” said Terry, an associate professor of geology in the College of Science and Technology.
As part of the National Science Foundation-funded study, the researchers examined fossil samples taken from Toadstool Park in Nebraska, Badlands National Park in South Dakota and Torrington Quarry in Wyoming. Fossil tooth samples were also provided by the University of Florida (UF), the Denver Museum of Nature and Science, and the American Museum of Natural History. Fossils sampled from UF were collected on public lands under the auspices of a U.S. Forest Service permit or on private lands with the permission of the leaseholders.
Terry said that geologists and scientists have long believed that this period produced a worldwide shift from a hot and humid climate to a cooler and dryer environment. By looking at the fossilized teeth and bones of horses, oreodonts and other plant-eating animals, the researchers found a climate change of approximately 8 degrees Celsius (about15 degrees F).
“Fossil mammals are archives of ancient information,” said co-author MacFadden, a vertebrate paleontologist at the University of Florida. “Their teeth are like little time capsules that allow us to analyze chemicals captured millions of years ago within the animals’ skeletons.”
“A combined analysis of the isotope composition of bones and teeth is a new approach to studying this boundary in time,” added Zanazzi, a doctoral student in geology at the University of South Carolina and lead author of the study. “Tooth enamel has very low porosity and low organic matter, so it maintains the isotopic composition of when it was formed.”
“We know we have an overall shift that went from very hot and humid to at least a cooler climate within the area where we did our study,” said Terry. “Everybody thought it also got dryer; that it got more arid across this boundary or geologic time period.”
But the chemical data retrieved from the teeth and bones doesn’t support the concept that it got much dryer in this part of the United States 33 million years ago, he said. “That is really different from what everyone has been thinking, based on the study of sediments and interpretations of animals that didn’t survive during this time. So it’s making us all rethink what we thought we understood about these various proxies that people use for environmental interpretation.”
A proxy, Terry explained, is something that is influenced by environmental or climatic conditions.
“For example, when I study my ancient soils and my fossil landscapes, there are certain chemical and mineral proxies preserved inside of those ancient soils that tell me it was this type of climate or that type of environment,” he said. “So different types of proxies would be like the fossil record, the vertebrates themselves. The geo-chemical records of carbon and oxygen that are preserved in the bones and teeth are also a proxy.”
Terry said that scientists and geologists try to bring all the proxies in together, and hopefully they all agree or point to the same finding.
“What we’re seeing in this study is some of the proxies are not in agreement, and that’s very interesting,” he said. “In my mind, I’m wondering, ‘Are we sure about all we know about this boundary or time period?’”
Another reason Terry and his colleagues believe their findings are important is because researchers have been studying the marine transition of the Eocene-Oligocene period for decades.
“Not as much attention has been put into the non-marine (or terrestrial) sections of this transition when it happened,” he said. “What we think might be happening as well is a lag of almost 400,000 years between the responses of the oceans to the time that it finally makes its presence known or felt within the ecosystems on land.”
That presents scientists with another big question, said Terry: What is the dynamic link between oceanic and continental climate systems, and how do they work and feed off each other?
“It’s these periods of transition or climate change that we’re trying to understand because if we understand the nature of how they change, the speed over which they change in the geologic past, hopefully they’ll help us to understand what is happening to us today and in the future,” Terry said.
“By understanding where we’ve been and what has happened, we can hopefully understand what is going to happen in the future,” he added. “So even though 33 million years ago there were no people around, it helps us to understand the dynamics of the planet.”
— Preston M. Moretz