by Julia Rosen Thursday, May 14, 2015
Between about 720 million and 635 million years ago, Earth suffered two big chills. During these “Snowball Earth” episodes, geologists think the world’s oceans froze over and glaciers spilled from tropical coastlines. Scientists have previously suggested that intense volcanism, unleashed by the disintegration of the supercontinent Rodinia, plunged Earth into these global glaciations. New research now lends support to this so-called fire-and-ice hypothesis.
The idea of an ice-covered Earth faced tremendous skepticism when it was first proposed in 1992, but after decades of investigation, the idea is now well accepted if not well understood. “This is the biggest event in climate change in Earth history, and yet there is not any compelling explanation for its initiation,” says Francis Macdonald, a geologist at Harvard University and co-author of the new research.
Scientists have proposed various hypotheses over the years, most of which try to explain how massive amounts of carbon could have been removed from the atmosphere, reducing the greenhouse effect and plunging Earth into a deep freeze. But so far, most have failed to adequately explain the timing and magnitude of large excursions in the carbon isotopic composition of seawater, which precede both Snowball Earth episodes in the geologic record.
The fire-and-ice hypothesis, however, has faced a different problem — the data needed to test it simply didn’t exist. First articulated about 10 years ago, it holds that as plate tectonics ripped Rodinia apart, flood basalts gushed from fractures in the crust. Because the supercontinent straddled the equator, these basalts would have been pummeled by tropical rains, which would have weathered the rocks and driven chemical reactions that could have drawn down atmospheric carbon dioxide concentrations and cooled the planet.
Geologists already know that extensive flood basalts called large igneous provinces (LIPs) accompanied the demise of Rodinia. Most notably the vast Franklin LIP blanketed much of modern Canada and Siberia between 730 million and 710 million years ago. But they didn’t know what, if anything, these LIPs had to do with initiating Snowball Earth conditions.
“You can have basalts all over the place, but if they are not actually weathering, they are not going to do anything in terms of consuming carbon dioxide,” says Galen Halverson, a geologist at McGill University who presented the new results at the annual meeting of the American Geophysical Union in San Francisco in December. “The way to see if the weathering of these basalts potentially played a large role in regulating climate is to look at the sedimentary record,” he says.
So, Halverson, along with his doctoral student, Grant Cox, and colleagues spent the last decade measuring the chemistry of Snowball Earth-aged mudstones from around the world. In particular, neodymium isotopes reveal how much basalt — which contains a greater proportion of the heavier neodymium isotope — was being weathered compared to rocks that make up older continental crust, like granites, which have fewer heavy isotopes.
In samples from former marine basins around the world, Halverson’s team found heavier neodymium isotope ratios leading into the glaciations, suggesting that intense basalt weathering on adjacent landmasses probably drove cooling. Neodymium values remained high through the first Snowball Earth event, suggesting prolonged weathering; then they declined, probably after glaciers had eroded most of the flood basalts away. Measurements of strontium isotopes, which provide a more global picture of basalt weathering, tell a similar story.
Overall, these data provide the first physical evidence of the fire-and-ice idea, Halverson says. But, he adds, there’s still a nagging problem: For most of Earth’s history, climate and weathering have acted together to hold the planet within a narrow range of habitable conditions. In hot climates, weathering speeds up, drawing down carbon dioxide and cooling the planet; when climate cools, weathering slows down, allowing more carbon dioxide (which is continuously released by volcanoes) to build up in the atmosphere and warm the planet. So why did this stabilizing mechanism fail so spectacularly during Snowball Earth?
The answer, Halverson says, has to do with phosphorus, which all organisms need to live. It’s typically scarce in natural environments, but basalts contain plenty of the element. As LIPs weathered, Halverson says, they would have flooded the marine environment with the nutrient, spurring productivity. This likely led to large phytoplankton blooms, which took up carbon as they grew near the ocean’s surface and sequestered it in organic matter on the seafloor upon their deaths. In addition, related increases in ocean anoxia may have allowed seawater itself to absorb more carbon dioxide gas.
“This potentially gives another lever to suck down carbon dioxide,” Macdonald says, and it could have helped overwhelm the climate-weathering mechanism that would normally prevent runaway cooling.
“This gives us a broad idea for the background for this cooling,” he says, but there are still questions about the rate at which Earth descended into glaciation and exactly what caused the climate to change. “Did it happen rapidly, associated with the emplacement of the Franklin LIP, or was there something else?”
Perhaps most importantly, “whatever [the trigger] was, it happened twice,” Macdonald notes. The first major glaciation, which lasted from 717 million to 660 million years ago, was followed by another short-lived glaciation about 640 million years ago. Although the basalts appear to have mostly eroded away by then, the weathering of freshly exposed continental rocks (which also consumes carbon dioxide) could have tipped an already cool climate back over the edge, Halverson says, but his team is still working on this.
Another outstanding question is why LIPs that formed at other times in Earth’s history did not cause global glaciations. Huiming Bao, a geologist at Louisiana State University who was not involved in the work, says the Snowball Earth events may have resulted from a combination of factors converging “at the right place and right time.” Bao says Halverson’s data are comprehensive and impressive, and lend credence to the fire-and-ice hypothesis. However, he says, “numbers are important,” adding that he is eager to see calculations of how much carbon dioxide the basalts and phosphorus could consume, and how fast.
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