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Slower cooling in Oregon

Fifty million years ago, Oregon was a balmy, tropical coastal environment filled with shallow bays and beaches, boasting lush tropical flora and marine fauna. Then around 33.7 million years ago, something abruptly changed, and the area quickly grew colder, with many plants and animals becoming extinct, or so scientists have thought. New research, however, is offering a different explanation for the changes: that the climate slowly cooled over 6 million years as a result of evolving grasslands pulling carbon dioxide out of the air and locking the carbon into the soil.

The transition at 33.7 million years ago between the Eocene (55 to 33.7 million years ago) and the Oligocene (33.7 to 23.8 million years ago) has long been thought of as a turning point between the warmer greenhouse climate of the Eocene and the cooler modern climate. Just why the change occurred has remained shrouded in mystery, however.

Some scientists believe the cooling was caused by the continental drift of Australia and South America away from Antarctica, isolating the continent, creating the circum-Antarctic ocean current and encouraging ice sheet growth. Others believe that carbon dioxide was drawn out of the atmosphere by mountain building and subsequent weathering. Still others suggest that a barrage of meteorite impacts around the transition caused global climate change.

Greg Retallack, a geologist at the University of Oregon, and colleagues suggest that instead, the evolution of extensive grasslands created a new carbon sink that sequestered carbon dioxide to cool the atmosphere. It is an “interesting hypothesis that is worth throwing out there,” says Gary Smith, a geologist at the University of New Mexico.

As reported in the July/August GSA Bulletin, Retallack and co-authors recalibrated the extinction and climate change record by assigning new radiometric dates to thick volcanic ash-flow tuffs from the end of the Eocene and the early Oligocene in the Eugene, Ore., area. They then matched the dates to the unique fossil evidence — a “fantastically well-dated 3-kilometer-thick sequence of marine and non-marine rocks,” Retallack says. The sequence left a sequential fossil record of the diversity of floral and faunal life spanning the entire climatic transition period.

From 46 million years ago through 35 to 34 million years ago, tropical plant and animal fossils reveal a steady rise in species diversity, the authors write. But after 33.4 million years ago, 300,000 years after the Eocene-Oligocene boundary, these species began going extinct, according to Retallack’s team’s dates. Terrestrial plants and animals began to take over, including vast fields of grasslands, which continued expanding until 29 million years ago.

As the grasslands evolved, Retallack says, they began sucking carbon dioxide out of the atmosphere and increasing the amount of light reflected from the ground into space, further propagating the cooling. “The evolution of cyanobacteria and trees altering our air has been known to happen in other periods in geologic history, and I think it happened at the Eocene-Oligocene transition,” he says. Grassland evolution also explains a lot of changes in the oceans at the time, he says.

“It’s definitely possible,” says Linda Ivany, a geologist at Syracuse University in New York. One reason that scientists may have believed the climate change was more abrupt in the past, she explains, is that the impacts and mountain-building events would have kept the environment relatively warm while the grasslands evolved and sequestered carbon dioxide. Then when the “post-apocalyptic” environmental effects of the impacts died down, the cooling would have seemed extreme and sudden. “There are a lot of questions still to be answered here,” she says, but it is an exciting scenario to consider.

Ivany says that researchers need to examine independent records of temperature and to figure out how to measure carbon dioxide levels in the ancient atmosphere. Furthermore, Smith says, researchers need to examine the dental records of vertebrates from this time period. If grasslands were indeed extensive, he says, dietary adaptations should be apparent in fossilized horse teeth, for example. Retallack and colleagues are already looking at vertebrate and plant fossils to figure out the exact size, location and timing of these grasslands.

Megan Sever

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