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- 27 November 2013 12:59 pm , Vol. 342 , #6162
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Did Plants Freeze the Planet?
1 February 2012 4:30 am
The first plants to colonize land didn't merely supply a dash of green to a drab landscape. They dramatically accelerated the natural breakdown of exposed rocks, according to a new study, drawing so much planet-warming carbon dioxide (CO2) from the atmosphere that they sent Earth's climate spiraling into a major ice age.
About 460 million years ago, the concentration of CO2 in the atmosphere ranged somewhere between 14 and 22 times the current level, and the average global temperature was about 5°C higher than it is now. Climate models suggest that widespread glaciations couldn't take place at that time unless CO2 levels dropped to about eight times what they are at present, says Tim Lenton, an earth scientist at the University of Exeter in the United Kingdom. (At the time, the sun was as much as 6% fainter than it is now, Lenton says, so the planet-warming effect of greenhouse gases wasn't as strong.)
Nevertheless, during a 10-million-year-period that started about 455 million years ago, Earth experienced two major glaciations. At the time, the supercontinent Gondwana sat atop or near the South Pole, much as Antarctica does today. At the height of the ice ages, much of the supercontinent, including areas that are now Africa and South America, was smothered with ice. These glaciations may have played a large role in mass extinctions of species that had previously thrived in the shallow seas surrounding landmasses.
Scientists have long considered the two cold spells, which came on abruptly, surprising, Lenton says. The chemical weathering of silicate rocks—reactions between exposed minerals and acidic rain or with oxygen and other gases in the atmosphere—would have naturally but slowly pulled carbon dioxide from the atmosphere, forming carbonate minerals that temporarily locked away the carbon, he notes. But current geochemical models suggest that that process couldn't have taken CO2 levels low enough to bring about the two ice ages, nor can it explain their sudden onset. "Explaining these glaciations has always been a problem," says Charles Wellman, a paleobotanist at the University of Sheffield in the United Kingdom, who was not involved in the new work.
Now, Lenton and his colleagues suggest that the evolution of land plants may have triggered the glaciations—and they've got lab data to back up the idea. In one set of tests, the team placed bits of common silicate rocks that had cooled from molten material, such as granite and andesite, in sealed beakers for 130 days with a modern-day species of moss believed to be similar to the first land plants, which didn't have the so-called vascular tissues that help circulate water throughout the plant. Such so-called nonvascular plants would have been able to exploit only the limited number of environments that were constantly damp, the researchers note. In the other set of beakers, the team placed rocks and water but no moss.
The presence of moss boosted the weathering of calcium from andesite by a factor of 3.6 and the weathering of magnesium from the same rock by a factor of 5.4, the researchers report online today in Nature Geoscience. When they plugged these figures into a model that simulated the presence of land plants over 15% of Earth's surface—the approximate percentage of wetland environments on continents today, Lenton says—for the interval between 475 million and 460 million years ago, the model predicted that CO2 levels at the end of that period would have dropped to about 8.4 times that seen today, enough to trigger a major glaciation.
In the team's lab tests, the moss also increased rates of weathering of iron and phosphorus from granite by 60 and 170 times, respectively. These extra nutrients would have boosted plant growth on land, but a considerable fraction would have flowed to the seas and fueled algal blooms in shallow waters surrounding the continents. That could explain two other anomalies from the era's geologic record, Lenton says: the large amounts of organic-rich shale that were deposited as nearshore sediments and the unusually high proportion of carbon-13 isotopes in the rocks. "It's a complex picture, but it all fits," Lenton says.
The moss the team used in its tests, Wellman suggests, is probably a reasonably good analog for the primitive plants of that era "and could have had a strong effect on climate. It increased the weathering rate rather definitely," he notes.
"I think they're perfectly right," says Gregory Retallack, a paleobotanist at the University of Oregon, Eugene. And these two ages likely aren't the only instance, he notes. The fossil record suggests that "whenever you have a major evolution of plants, you get a major cooling," Retallack says.
While the first of the glaciations that the team studied was probably triggered by nonvascular plants such as mosses and liverworts, the second ice age—the one that began around 445 million years ago—may have been brought on by the rise and spread of vascular plants. Spores from such plants, which aren't limited to permanently damp environments and therefore may have turned down Earth's thermostat even more than nonvascular plants did, show up in the fossil record about 450 million years ago. So, Retallack suggests, colonization of the land by vascular and nonvascular plants together may help explain the massive drawdown of CO2 that triggered the most recent and stronger of the two major glaciations that Lenton and his colleagues studied.