Did a Giant Asteroid Strike End the Deep Freeze Known as the Early “Snowball Earth?”
An artist’s impression of a "Snowball Earth." Imagine a world without liquid water, just solid ice in all directions. It would certainly not be a place that most life forms would like to live. And yet our planet has gone through several frozen periods, in which a runaway climate effect led to global, or near global, ice cover. The last of these so-called "Snowball Earth" glaciations ended around 635 million years ago when complex life was just starting to develop. Early models showed that once ice reached tropical latitudes, a positive feedback loop would take hold, in which ice cover would lead to lower temperatures, which would add more ice cover, which would lower temperatures even more. This runaway effect would presumably continue until the entire planet froze over, with even the oceans covered with as much as a kilometer thick layer of ice. This so-called hard “Snowball Earth" would lock the planet into an eternal winter with no apparent way to escape from such a deep freeze. Indeed, scientists have had a hard time explaining how a hard snowball could ever thaw. One proposal is that volcanic activity releases greenhouse gases that eventually warm the planet back up. The amount of carbon dioxide (CO2) needed might be several hundred times higher than what our atmosphere contains now. The other method might be a giant asteroid strike. (Image credit: NASA)
Did a Giant Asteroid Strike End the Deep Freeze Known as the Early “Snowball Earth?”
A number of Snowball Earth events occurred during the Proterozoic Era (543 million to 2.5 billion years ago) -- A time when only very primitive organisms inhabited the planet and oxygen levels were considerably lower than today. The aptly named Snowball Earth represents some of the most bizarre climate conditions ever experienced by the Earth, with strong evidence for ice sheets in equatorial locations. The evidence comes from sedimentary layers with a mixture of fine particles and large boulders, that look just like the rocks deposited by modern glaciers. These layers are found at locations known to be near the poles, which is not surprising, but also at those that are thought to have been near the equator, which is really unusual. This apparent global distribution of layers is why the events are termed "Snowball Earth." The main snowball events occurred about 2.22 billion years ago, 710 million years ago, and 640 million years ago.
So how cold was the Snowball Earth? If, as it appears, the planet was covered by ice from pole to pole, all of the sun’s radiation would be reflected back to space and temperatures must have been frigid. Models suggest that the global average temperature was about -50oC and the temperature at the equator would be similar to that at the poles today, about -20oC. With these conditions, most parts of the planet would have been under about 1 km of ice. With no ability to retain heat, it is hard to imagine how Earth recovered from a Snowball event, but surprisingly the end of the glacial events appear to have been extraordinarily abrupt.
Curtin University scientists in Australia have discovered that Earth’s oldest asteroid strike occurred at Yarrabubba, in Western Australia, and coincided with the end of the first global deep freeze known as a “Snowball Earth.”
The research used isotopic analysis of minerals to calculate the precise age of the Yarrabubba crater for the first time, putting it at 2.229 billion years old, making it 200 million years older than the next oldest impact.
Dr Timmons Erickson, from Curtin’s School of Earth and Planetary Sciences and NASA’s Johnson Space Center, together with a team including Professor Chris Kirkland, Associate Professor Nicholas Timms, and Senior Research Fellow Dr Aaron Cavosie, all from Curtin’s School of Earth and Planetary Sciences, analyzed the minerals zircon and monazite that were “shock recrystallized” by the asteroid strike, at the base of the eroded crater to determine the exact age of Yarrabubba.
The team inferred that the impact may have occurred into an ice-covered landscape, vaporized a large volume of ice into the atmosphere, and produced a 70km diameter crater in the rocks beneath.
Professor Kirkland said the timing raised the possibility that the Earth’s oldest asteroid impact may have helped lift the planet out of a deep freeze.
“Yarrabubba, which sits between Sandstone and Meekatharra in central Western Australia, had been recognized as an impact structure for many years, but its age wasn’t well determined,” Professor Kirkland said.
“Now we know the Yarrabubba crater was made right at the end of what’s commonly referred to as the early Snowball Earth, a time when the atmosphere and oceans were evolving and becoming more oxygenated and when rocks deposited on many continents recorded glacial conditions.”
Associate Professor Nicholas Timms noted the precise coincidence between the Yarrabubba impact and the disappearance of glacial deposits.
“The age of the Yarrabubba impact matches the demise of a series of ancient glaciations. After the impact, glacial deposits are absent in the rock record for 400 million years. This twist of fate suggests that the large meteorite impact may have influenced global climate,” Associate Professor Timms said.
“Numerical modeling further supports the connection between the effects of large impacts into ice and global climate change. Calculations indicated that an impact into an ice-covered continent could have sent half a trillion tons of water vapor, an important greenhouse gas, into the atmosphere. This finding raises the question whether this impact may have tipped the scales enough to end glacial conditions.”
Dr. Aaron Cavosie said the Yarrabubba study may have potentially significant implications for future impact crater discoveries.
“Our findings highlight that acquiring precise ages of known craters is important. This one sat in plain sight for nearly two decades before its significance was realized. Yarrabubba is about half the age of the Earth and it raises the question of whether all older impact craters have been eroded or if they are still out there waiting to be discovered,” Dr Cavosie said.
Snowball Earth Glaciation Dramatically Changed the Carbon Cycle
For insight into what can happen when the Earth's carbon cycle is altered -- a cause and consequence of climate change -- scientists can look to an event that occurred some 720 million years ago.
Data from a Princeton University-led team of geologists in 2010 suggests that an episode called "Snowball Earth," which is believed to have covered the continents and oceans in a thick sheet of ice, produced a dramatic change in the carbon cycle. This change in the carbon cycle, in turn, may have triggered future ice ages.
Pinpointing the causes and effects of the extreme shift in the way carbon moved through the oceans, the biosphere and the atmosphere -- the magnitude of which has not been observed at any other time in Earth history -- is important for understanding just how much Earth's climate can change and how the planet responds to such disturbances.
The researchers put forth a hypothesis to explain how changes to Earth's surface, wrought by the glaciers of the Neoproterozoic Era, could have created the anomaly in carbon cycling.
"The Neoproterozoic Era was the time in Earth history when the amount of oxygen rose to levels that allowed for the evolution of animals, so understanding changes to the carbon cycle and the dynamics of the Earth surface at the time is an important pursuit," said Princeton graduate student Nicholas Swanson-Hysell at the time of the study.
The Neoproterozoic era, which lasted from 1,000 million years ago to 542 million years ago, is divided into three distinct periods, beginning with the Tonian, extending through the Cryogenian and ending with the Ediacaran. The Cryogenian period is notable in Earth history for the extensive and repeated ice ages that took place, beginning with the massive Sturtian glaciation at the start of the period. This marked the first ice age on Earth in roughly 1.5 billion years, which is an unusually long time span between glaciations. Since the Cryogenian, Earth has endured an ice age about once every 100 to 200 million years.
The "Snowball Earth" theory suggests that the Sturtian glaciation was global in scope, literally encasing the planet in ice, which could have wreaked havoc on the normal functioning of the carbon cycle. While the theory is controversial and the extent of the deep freeze is under investigation, research team member Adam Maloof suggested that glaciers reached the equator some 716.5 million years ago, providing further evidence to support the existence of a Cryogenian "Snowball Earth."
In the research, Swanson-Hysell, Maloof, and their collaborators collected samples of limestone from Central and South Australia dating back to the Tonian and Cryogenian periods. Using a technique known as isotope analysis to learn how the carbon cycle worked in ancient times, the team pieced together clues that are hidden in the atomic composition of the carbon found in inorganic limestone sediment and ancient organic material. In addition, the geologists recorded where the samples were found in the rock layers to determine crucial information about the relative age of the samples and the environmental conditions under which they formed.
Their results documented a peculiar and large shift in the carbon cycle based on analyses of samples obtained from tropical limestone sediments known as the Trezona Formation, which dates to the end of the Cryogenian period approximately 650 million years ago and was deposited between "Snowball Earth" events.
"The disturbance we're seeing in the Neoproterozoic carbon cycle is larger by several orders of magnitude than anything we could cause today, even if we were to burn all the fossil fuels on the planet at once," said Maloof, an assistant professor of geosciences at Princeton.
Previous data from the Ediacaran period at the end of the Neoproterozoic era have shown a similar perturbation to the carbon cycle. In 2003, Massachusetts Institute of Technology geophysicist Daniel Rothman suggested that a buildup of a huge pool of organic carbon in the ocean could have led to the observed disturbance.
The perturbation studied by the Princeton researchers shows this same behavior during an event that was roughly 25 percent larger and 100 million years older than the previously recognized disturbance. The team also documented that the carbon cycle was not operating in this bizarre fashion 800 million years ago prior to the first Neoproterozoic glaciations, constraining in time the onset of such behavior and linking it to the proposed "Snowball Earth" event.
"The new carbon isotopic data shows a whopping… downshift in the isotopic composition of carbonate, possibly the largest single isotopic change in Earth history, while the isotopic composition of organic carbon is invariant," said Rothman, who was not part of the research team. "The co-occurrence of such signals is enigmatic, suggesting that the carbon cycle during this period behaved fundamentally differently than it does today."
Building on Rothman's framework, the Princeton-led geologists set out to explain how an ice-covered globe in the early Cryogenian period could have prompted the accumulation of massive amounts of organic carbon in the ocean, leading to the observed disturbance to the carbon cycle later in the period.
According to their proposed hypothesis, the passage of the Sturtian glaciers across continental surfaces would have removed the weathered material and debris, which had accumulated in the 1.5 billion years since the preceding ice age. When the glaciers receded, this would have exposed vast amounts of bedrock to the carbon dioxide in the atmosphere for weathering, freeing up nutrients in the rock for delivery into the oceans.
This process would have generated a relatively large influx of iron into the oceans, which could have interrupted biomechanisms used by marine bacteria during the Tonian to process (that is eat) the organic carbon in the water and convert it into carbon dioxide and other dissolved inorganic carbon compounds. If the organic carbon was not eaten by bacteria, it would have accumulated into a massive oceanic reservoir and resulted in the strange carbon cycle of the Cryogenian and early Ediacaran.
The interaction of carbon dioxide with the continental surfaces during the weathering process also would have removed some of the carbon dioxide from the atmosphere, lowering the global temperatures and creating conditions conducive to the series of glacial events that were observed throughout the Cryogenian.
According to Rothman's hypothesis, over millions of years the levels of oceanic and atmospheric oxygen would have grown as a consequence of the altered carbon cycle, ultimately leading to the oxidation of the large reservoir of organic carbon, removing the extra organic carbon from the oceans and returning the carbon cycle to a steady state more similar to how it functions today. Increased levels of oxygen in the atmosphere also would have provided the conditions that were necessary for the explosive diversification of animal life at the end of the Neoproterozoic and into the Cambrian Period.
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