New research on how ozone affects plants and their reproduction may be the
key to figuring out what happened to trigger Earth’s largest extinction
event, which occurred around 250 million years ago. The new findings may also
prove useful for determining atmospheric conditions long ago.
Plants
such as this one from South Georgia Island in South America, beneath the ozone
hole, create more pigment to protect their spores when bombarded with extra
ultraviolet light. Large volcanic eruptions could enhance ozone holes, and leave
evidence in spores in the fossil record. Courtesy of Barry Lomax.
The end of the Permian saw the biggest wipeout of life forms of all time, with
the disappearance of 90 percent of all species before the beginning of the Triassic.
The extinction roughly coincided with the eruption of the Siberian Traps, a
voluminous outpouring of volcanic materials that emitted carbon dioxide and
other gases.
One angle of proof to pin extinctions on a volcanic eruption “is just showing
that the timing is right. Once you’ve got that established,” says
Paul Wignall of the University of Leeds in the United Kingdom, the remaining
angle is “how did it do it?”
The carbon dioxide from the massive eruption would have contributed to global
warming at the time; the halogens released, such as chlorine and fluorine, would
have damaged the ozone layer. A large ozone hole would allow more ultraviolet
(UV) radiation to reach Earth’s surface — ultimately affecting land
plants and animals.
The effects of such UV exposure could be extreme. Using present-day examples
with an eye to the past, Barry Lomax, a research associate at the University
of Sheffield in the United Kingdom, and his co-workers studied plants on South
Georgia Island, located east of South America’s tip, beneath the present-day
ozone hole. As presented on Aug. 10, in Calgary, Alberta, at the joint meeting
on Earth System Processes held by the Geological Society of America and the
Geological Association of Canada, they found that modern relatives of prehistoric
plants, Lycopodium annotinum and Selaginella selaginoides, react to excess UV rays by changing key proteins that protect
spores from damage.
The modern plants showed “a marked increase in UV-screening pigments,”
up to a threefold increase that matched increased ozone exposure, says David
Beerling, who leads the research program. The plants “clearly have responded
consistently to this change in the stratospheric ozone layer” with changes
to “some of their component molecules,” Beerling says. But the key
will be finding whether the proteins that triggered the pigment changes can
survive in the fossil record (pigments do not). “Samples are being run
as we speak,” he says, from fossils preserved from the Permian-Triassic
boundary.
Direct evidence of ozone depletion “is very hard to find” in the fossil
record, but if the pigment-changing molecules survive, they would be an apt
biomarker, says Wignall, who co-convened the meeting session. The team’s
idea is “a novel approach, and they’re only halfway there,” he
says.
Lomax and his co-workers cite recent reports of twisted and mutated spores from
similar plants at the end of the Permian; such changes could be attributable
to UV effects. Other researchers have pointed out that it is difficult to prove
whether plants living at the time actually mutated to extinction, instead of
surviving the UV bombardment. Still, the changes in spore proteins could be
a possible proxy for determining atmospheric conditions for other “deep
time” periods as well, says Jeff Kiehl of National Center for Atmospheric
Research in Boulder, Colo. The spores would serve as proxies in the same way
that fossils of small single-celled phytoplankton in the oceans provide proxies
for ocean chemistry and temperature.
Naomi Lubick
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