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The name describes its appearance from outer space — a glistening white ball. The ice surface is mostly coated with frost and tiny ice crystals that settled out of the cold dry air, which is far below freezing everywhere. Gale-force winds howl in low latitudes. Beneath the floating ice shelf, a dark and briny ocean is continually stirred by tides and turbulent eddies generated by geothermal heat slowly entering from the ocean floor. What first tipped off geologists that this could have happened?
Geologists were struggling to understand what they saw in the geologic record — that not too long before the first appearance of complex life, there was unmistakable evidence of glaciation even in the warmest areas of the Earth. Geologists had a very difficult time understanding how this was possible.
The deposits that glaciers leave behind are very distinctive. They look like cement that has been dumped out of a cement truck. These Snowball ice sheets would have flowed from the continents out onto the ocean, so we have a lot of deposits that formed in the marine environment where you get what are known as dropstones: pebbles or boulders that are out of place.
Very often, you see structures related to the impact, as if the stone was somehow dropped and then plunked into the underlying sediment. Example of a glacial dropstone from Namibia, in rocks that date to the second Snowball Earth. The stone was likely carried and dropped by a floating ice shelf, and when it plunked into seafloor sediment below, that sediment folded around it. Penny shown for scale. How did you get involved in studying this hypothesis? I had known about the hypothesis since even before I was interested in working on the problem myself.
Joe Kirschvink at Caltech told me about it a few months after he had the idea in , but he never did anything more with it at that time. I liked it because I like ideas, but there was a credibility gap, so before our work, the hypothesis was dormant.
We had to have climate models to see what actually happens under Snowball conditions, and that modeling, developed later, has been extremely important. My main contribution was making the case that it was a credible scientific hypothesis by arguing, from different disciplines within geoscience, that there was a lot of geological evidence consistent with the predictions.
Very often, the problem with new ideas is not that they are wrong, but that they are incomplete. What triggered these events? There are a number of factors that contributed, and I think it is useful to look at this in two ways.
First of all, what was the general condition that made for a colder climate and therefore made the Earth more susceptible to this runaway ice growth phenomenon? And then what was the immediate trigger that tipped it over the edge? The carbonate rocks rest directly above glacial deposits from the second Snowball Earth event. This juxtaposition of carbonates — which form only in warm parts of the ocean — and glacial rocks supports the theory that ice covered the entire planet during the Snowball Earth episodes.
When the Snowball events occurred, the supercontinent Rodinia was in the process of breaking up. A supercontinent is a state in which all of the continents are clustered together in one group. The reason why people think there is a connection there is that the breakup of a supercontinent would increase rainfall in the continental areas, and that would increase the weathering of crustal rocks.
The weathering of rocks actually consumes carbon dioxide, so that would lead to less carbon dioxide in the atmosphere and therefore a colder climate. As for what actually caused the immediate trigger, attention has focused in recent years on a sequence of very large volcanic eruptions that occurred in what is now the high arctic of Canada.
These eruptions occurred around million and million years ago. When you get fire fountains — lava that comes out of one place over a period of weeks or months — you get a strong thermal upwelling in the atmosphere from the heating effect of that lava.
These upwellings can loft sulfur aerosols into the stratosphere where they hang around for a significant amount of time. These sulfur gas particles reflect incoming solar radiation and have a strong cooling effect.
What did life on Earth look like at the time, and how did it change as a consequence of these events? There were certainly bacteria and there were also algae and unicellular primitive animals, or protists. There is also evidence that the first multicellular animals originated at this time, probably something like sponges. Why is a matter of speculation: There are a number of ideas on this, but they are difficult to test. One idea is that on Snowball Earth, ecosystems may have been more isolated from one another and this might be a situation that would be helpful for evolving new forms of life, and particularly forms of life that are altruistic — ones with cells that find that there is an advantage in working together rather than working individually.
So more isolation of different ecosystems might have allowed certain ecosystems that had a higher proportion of these multicellular altruists to establish a foothold. Some scientists think that the conditions of Snowball Earth changed life in the oceans — leading to the rise of more complex algae large cells over cyanobacteria small cells , as depicted in this illustration.
That, in turn, may have helped set the stage for the evolution of multicellular life. How was the Snowball theory received by other geologists? Suggestions or feedback? Previous image Next image. Scientists have considered multiple scenarios for what may have tipped the planet into each ice age.
The findings may also apply to the search for life on other planets. Researchers have been keen on finding exoplanets within the habitable zone — a distance from their star that would be within a temperature range that could support life. The new study suggests that these planets, like Earth, could also ice over temporarily if their climate changes abruptly.
Even if they lie within a habitable zone, Earth-like planets may be more susceptible to global ice ages than previously thought. As more ice covers the globe, the planet becomes more reflective, or higher in albedo, which further cools the surface for more ice to expand.
Eventually, if the ice reaches a certain extent, this becomes a runaway process, resulting in a global glaciation. When the planet is not covered in ice, levels of carbon dioxide in the atmosphere are somewhat controlled by the weathering of rocks and minerals.
When the planet is covered in ice, weathering is vastly reduced, so that carbon dioxide builds up in the atmosphere, creating a greenhouse effect that eventually thaws the planet out of its ice age. Scientists generally agree that the formation of Snowball Earths has something to do with the balance between incoming sunlight, the ice-albedo feedback, and the global carbon cycle. The researchers were able to tune each of these parameters to observe which conditions generated a Snowball Earth.
Ultimately, they found that a planet was more likely to freeze over if incoming solar radiation decreased quickly, at a rate that was faster than a critical rate, rather than to a critical threshold, or particular level of sunlight.
Nevertheless, Arnscheidt estimates that the Earth would have to experience about a 2 percent drop in incoming sunlight over a period of about 10, years to tip into a global ice age.
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