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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