SNOWBALL EARTH
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June 8, 06: Ancient relatives of today's plants and animals may have survived Earth's oldest, longest winter, when the planet was covered in a deep sheet of ice.
. . Scientists refer to this chilly period as "Snowball Earth", which first occurred more than two billion years ago. Some computer models suggest the planet was encased in a shell of ice at least 700 meters thick.
. . Inside rocks collected near Elliot Lake in Ontario, Canada—rocks older than Snowball Earth—scientists found oil trapped in water droplets, in the crevices of rock crystals. Most oil is a residue of tiny organisms that once lived in the ocean. The oil contains biomarkers, or molecular fossils, that scientists can use to determine what types of life went into making it. The Ontario oil held answers to age-old questions about when oxygen-producing bacteria and multi-cellular living organisms, called eukaryotes, first appeared on Earth.
. . Evidence in the oil suggests eukaryotes and bacteria were alive and kicking 100 million years before the planet froze over. The finding fits with a study last year that concluded bacteria actually caused the first snowball scenario by producing oxygen that destroyed a greenhouse blanket of methane in the atmosphere.
. . If they could live through the toughest, coldest models of Snowball Earth, such hardy microbes could probably make a living on the frozen moons of Jupiter.
. . Geologist Mark McMenamin of Mount Holyoke College: "I don't think this tells us whether Snowball Earth was severe or not, because I think eukaryotes would have survived either way."
. . Similar claims were made in 2004 when a team led by Luann Becker of the University of California reported that a crater off the northwest coast of Australia showed evidence of a large meteor impact at the time of the early extinction. That team relied heavily on core samples provided by an oil company drilling in the region as evidence for its findings.
. . Von Frese acknowledged his discovery lacks such hard evidence. He said he wanted to visit Antarctica to hunt for rocks at the base of the ice along the coast that could be dated. "The strongest evidence would be rocks from the event, including meteorite fragments. ... There are at least 20 impact craters this size or larger on the moon, so it is not surprising to find one here", he said. "The active geology of the Earth likely scrubbed its surface clean of many more."
. . Approximately 100 million years ago, Australia split from Gondwana and began drifting north away from what is now Antarctica, pushed by the expansion of a rift valley into the eastern Indian Ocean, von Frese said. The rift cuts directly through the crater, so the impact of the meteor may have helped the rift to form, he said.
Aug 1, 05: Earth has been through many cold spells since its birth 4.5 billion years ago. Scientists say some drastic episodes froze the planet all the way to the equator. Yet these "snowball Earth" scenarios expose a gaping lack of understanding: What caused them? Lowly bacteria, according to a new study.
. . In the first and worst snowball episode, 2.3 billion years ago, bacteria suddenly developed the ability to break down water and release oxygen. The influx of oxygen destroyed methane in the atmosphere, which had acted as a blanket to keep the planet warm. In modelling the scenario, the scientists say Earth's exact position from the Sun is the only thing that saved the planet from a permanent deep-freeze. And, they caution, it could happen again.
. . Before the first snowball event, the Sun was only 85 percent as bright as now. But the planet was temperate, much like today. Scientists believe that's because the atmosphere was loaded with methane, a greenhouse gas. A regular old Ice Age set in, and glaciers advanced to middle-latitudes as they would many times in geologic history. When the glaciers retreated back toward the poles, they scoured the land and released abundant nutrients into the oceans. There were no plants or animals back then. The cyanobacteria, with their newly developed ability to make oxygen, fed off the fresh flow of nutrients, the thinking goes, and their numbers exploded. "Their greater range should have allowed the cyanobacteria to come to dominate life on Earth quickly and start releasing large amounts of oxygen", said study team member Robert Kopp, a Caltech graduate student.
. . Computer modeling shows that most of the atmospheric methane may have been destroyed within 100,000 years, certainly within a several million years. Methane is far more insulating than carbon dioxide, another greenhouse gas.
. . Global temperatures plummeted to minus 50 C. Ice at the equator was a mile thick. Most organisms died. Biology clung to hydrothermal vents or survived underground, Kopp and his colleagues say. Even today, life has shown itself to be incredibly resilient, eating rocks, swimming in boiling water and enduring thousands of years in the deep freeze.
. . Then evolution pulled another trick, the scientists figure. Some of the organisms that did survive adapted to breathe oxygen, now that there was a lot of it.
Eventually, the scientists say, the changed biology and chemistry caused carbon dioxide to build up enough to generate another greenhouse period. Temperatures climbed to perhaps 50 C (122 F) around the globe, evidence indicates.
. . "It was a close call to a planetary destruction", says Kopp's supervising professor, Joe Kirschvink. "If Earth had been a bit further from the Sun, the temperature at the poles could have dropped enough to freeze the carbon dioxide into dry ice, robbing us of this greenhouse escape from snowball Earth."
. . "We haven't had a snowball in the past 630 million years, and because the Sun is warmer now it may be harder to get into the right condition", Kirschvink said. "But if it ever happens, all life on Earth would likely be destroyed. We could probably get out only by becoming a runaway greenhouse planet like Venus."
Feb 9, 05: Our planet may have frozen over in the past as it drifted though giant dust clouds in space. The result of the dust-bath would have been an almost complete overcoat of ice for the world, according to a new theory --a 'snowball Earth' event in which ice sheets keep growing until they cover almost the entire globe.
. . But the idea does not persuade some geologists. One points out that there seem to have been dramatic changes in the Earth's carbon cycle up to a million years before known snowball Earth events, which the dust-cloud hypothesis is at a loss to explain.
. . Alexander Pavlov, of the University of Colorado at Boulder, and his colleagues counter that their climate-cooling mechanism is almost inevitable, however. They say that on at least two occasions in the past 2 billion years, the Solar System must have passed through clouds of dust thick enough to cause a snowball Earth. They think it is possible that two such ultracold episodes, 600 million and 750 million years ago, might have been triggered in this way.
. . Snowball Earth events are much more severe than normal ice ages. They occur through a runaway process in which growing ice sheets reflect ever more sunlight back into space, resulting in further cooling and more ice. Eventually, the ice advances from the Poles virtually all the way to the Equator, trapping the planet in a deep freeze.
Pavlov and colleagues have calculated how much of this dust might be captured by Earth's gravitational field, filling the atmosphere with dust. Dust particles reflect sunlight, but they let Earth's heat out into space. In other words, they act as the precise opposite of greenhouse gases, cooling the planet.
Mar 17, 04: It happened not once but at least twice, between 800 million and 550 million years ago, in extraordinary climate events that have become known as "Snowball Earth". Some speculate that the Sun abruptly cooled for a while or that the Earth tilted on its axis or experienced an orbital blip that dramatically reduced solar warmth.
. . A new study by French scientists throws light on a little-explored theory --how tectonic wrenches that ripped apart the Earth's land surface provoked the reverse of the greenhouse effect.
. . At the time, all the Earth's future continents were glommed together in a super-continent dubbed Gondwana, an entity so vast that rainfall, brought by winds from the oceans, failed to travel far inland. When Gondwana pulled apart, breaking up into smaller pieces that eventually formed today's continents, rainfall patterns changed dramatically.
. . Rain tumbled over basalt rocks, freshly spewed from vast volcanic eruptions. That initiated a well-known reaction between the water and calcium silicate, in which carbon dioxide (CO2) molecules are taken from the air and sequestered in calcium carbonate, which is then washed down to the seas.
. . The weathering of silicate rocks sucked CO2 out of the air, thus leading to a catastrophic cooling. before the super-continent broke up around 800 million years ago, C02 concentrations were around 1,830 parts per million. 50 million years later, the picture is greatly different. CO2 levels drop to 510 ppm.
. . Then albedo adds to the problem. When more of the planet is covered with ice and snow, the temperature falls faster, because white surfaces reflect the Sun's rays rather than capture them.
. . Eventually, volcanoes added enough CO2 (that wasn't being uptaken, w/o plants), and the episode ended.
============ from NewScientist.com. 2002?
About 700 million years ago, in the Neoproterozoic era, life was simple and single-celled.
A growing number of scientists believe that, after billions of years of comfortable existence, Earth suddenly plunged into an extreme winter. Last year, evidence emerged that is winning over the sceptics.
Soviet climate modeller M. I. Budyko noticed a curious effect. In his models, ice spreading far from the poles triggered a runaway reaction. Ice reflects sunlight, cooling the Earth and allowing more ice to grow. At a certain point, when the ice covers enough of the planet's surface, this feedback becomes unstoppable, and the whole planet freezes up in just a few decades. But other modellers spotted a fatal flaw in this scenario: once Earth was frozen, it would be stuck.
The white ice would go on reflecting sunlight, cooling the planet and leaving it forever frozen. The modellers' dilemma: how did a frozen Earth ever thaw? The answer, he realized, lies with volcanoes, which perpetually belch out CO2. On a frozen Earth, there would be no evaporation, no rain and no chemical weathering of rock. Volcanic CO2 would simply build up in the atmosphere, enhancing the greenhouse effect.
A staggering amount of CO2 --around 350 times the amount we have today-- would be needed to trap enough heat in the atmosphere to melt Snowball Earth's shiny white coat. The build-up would take millions of years. "In a few hundred years, you went from the coldest climate the Earth has ever experienced to the warmest ever."
The spread of glacial deposits across the globe, the evidence that at least some of them were formed close to the equator, the carbon isotopes, the iron formations and the capped carbonates.... The snowball accounts for them all. "Everything fell into place."
Glacial deposits elsewhere on the globe suggest there could have been up to five such cycles --though it is hard to know for sure.
. . The last one finished around 575 million years ago, and that's a very significant point in Earth's history. Though life made its debut 3.5 billion years earlier, it had till then been nothing but simple, single-celled creatures --bacteria and algae.
But immediately after the last of the snowballs came the Ediacara --strange organisms that may well have been the first multicelled animals. And after the Ediacara came the Cambrian explosion and the birth of every phylum that exists today. Why did complex life suddenly spring into being?
Kirschvink has found evidence for an earlier global glaciation at another of Earth's conspicuous biological moments --the one, around 2.5 billion years ago, when the first photosynthesising organisms and oxygen appeared. What could have triggered this earlier glaciation? Before then, says Kasting, there would have been plenty of methane around, as well as small amounts of CO2.
. . Acting in tandem, these two greenhouse gases warm the atmosphere very effectively. "They absorb in different spectral regions and plug up the holes", says Kasting. But if the methane was oxidized to create CO2, a hole would have appeared in the spectrum, allowing radiation to leak out, and the temperature to plummet.
. . This ice blanket would cut off chemical weathering of the rocks, reduce the amount of CO2 being locked into carbonate rocks and so free up more CO2 to warm the Earth. Soon this enhanced greenhouse effect would match the cooling effect of the ice, and the glaciers would stop spreading. Zero sum.
. . But if the continents were clustered around the equator when the cooling began, the polar icecaps would grow on water, not land. There would be nothing to stop the chemical weathering. Ice could march uninterrupted towards the equator and the whole effect would snowball.
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