If someone could have seen the Earth from space some 630 million years ago, they would have found a large white ball. Neither seas nor continents were visible, covered by a thick layer of ice half a kilometer thick that stretched almost uniformly from pole to pole. With an average temperature of 50 degrees below zero, those global ice remained for more than ten million years, giving rise to a phenomenon that has been repeated at least twice in the history of the planet. It is not for nothing that geologists refer to these events as ‘Snowball Earth’, and the one 630 million years ago was the most recent known.
But how could he resist life in those conditions? The answer has, until now, been a mystery to scientists.
A new study led by researchers at the Wuhan University of Geosciences in China, and just published in ‘Nature Communications’ introduces, however, the idea that that great global glaciation may not have covered the entire surface of the planet.
Geochemical evidence from ancient rocks, indeed, suggests that vast areas of open sea could have become ice-free and persisted north of the Tropic of Cancer, a region previously considered too cold to support life during this period. “There was a habitable zone,” says Shuhai Xiao, a geobiologist at Virginia State University and co-author of the new paper. And it was probably “wider than thought.”
reasonable doubts
Other scientists are not so clear. Climate simulations, in fact, have serious problems modeling an ice-free equator during Snowball Earth periods. Introducing the possibility that there was a part of the ocean exposed “makes this really hard to swallow,” says Paul Hoffman, a geologist at Harvard University and a pioneer of the Snowball Earth hypothesis.
Since the late 1960s, climate models have shown that ‘planetary freezes’ are possible from a simple feedback loop. When temperatures drop, the polar caps expand, reflecting sunlight and creating more cooling. If the ice manages to extend to about 30° or 40° latitude, roughly where North Africa is today, the global climate enters a runaway freeze cycle and glaciers end up covering the entire planet in a few hundred years.
Two Snowball Earths
The geological record indicates that the Earth has experienced at least two of these icy periods. The most recent is known as the Marinoan Ice Age, between 654 million and 635 million years ago. Life was then confined to the oceans and large creatures had yet to evolve, but the fossils clearly show that microscopic eukaryotes, such as algae, lived both before and after the event. Such organisms require sunlight and open water, Xiao says. Therefore, “we must imagine some kind of refuge where these algae can survive.”
Previous studies have already shown that storm-driven currents may have flowed openly in shallow seas during that frigid era, though much of the evidence remains equivocal. To study conditions during this period, Xiao and her colleagues examined a thin layer of dark slate found in southern China’s Shennongjia National Forest that dates back to the Marinoan Ice Age. The researchers believe that shale layer originated from mineral-rich ocean mud that was deposited in mid-latitudes, perhaps between 30° and 40° north.
But Xiao and his team found that the shale was not only teeming with fossil algae, but also nitrogenous compounds. A thick layer of ice would have isolated the oceans from the atmosphere, preventing oxygen from entering their waters. But nitrogen compounds were found at levels similar to those in modern oceans, suggesting waters that were well-oxygenated and in which nitrogen and oxygen could freely interact.
These results suggest that the area from which the shale originated might not have been covered in ice and thus be a safe abode in which photosynthetic organisms could thrive. If true, climate simulations would need to be changed to figure out how to maintain an open ocean during those long periods of global ice.
Opponents of the idea, such as Hoffman, continue to think that it is far more likely that the microscopic algae fossils took refuge in small, shallow pools of freshwater formed directly on top of the glaciers. Today, these pools appear in the polar ice caps and provide shelter for cold-adapted microbes. According to this researcher, all modern algae descend from freshwater species, which suggests that those that lived in the seas were eliminated during the Snowball Earth periods and had to evolve again later to return to the ocean. “I do not see survival as a problem -sentence Hofman- nor does this article solve it”.
Although an intermediate solution would also be possible, in which everyone is partly right. In fact, there might have been a time span where even the equator froze over and living creatures were forced into the glacial pools. But that interval may not have lasted the entire ice age.
Geologists know that the dark shale layer comes from the Marinoan, but it’s not clear exactly when it formed within that ice age. Perhaps the rocks bear witness to the last days of the era, a time when the ice sheets had already begun to retreat, a perspective Xiao and his colleagues agree with.