More than 600 million years ago, the planet was frozen from pole to pole, covered in half-kilometer-thick ice sheets that darkened every ocean. How sea life clung on during Snowball Earth, as this inhospitable period is known, has long been a mystery.
A new study bolsters the idea that the global glaciation wasn’t all encompassing. Geochemical evidence from ancient rocks suggests zones of open ocean may have been present north of the Tropic of Cancer, a region that was previously considered too cold to host life during this period. “There’s a habitable zone,” says Shuhai Xiao, a geobiologist at the Virginia Polytechnic Institute and State Universityand co-author of the new work. And it’s “perhaps wider than previously thought.”
Other scientists aren’t yet convinced, however. Climate simulations have trouble creating even an ice-free equator during Snowball Earth periods. The possibility that there was uncovered ocean outside the tropics “makes this thing really difficult to swallow,” says Paul Hoffman, a geologist at Harvard University who pioneered the Snowball Earth hypothesis.
Climate models since the 1960s have shown how planetary deep freezes can arise from a simple feedback loop. When temperatures drop, Earth’s ice caps expand, reflecting sunlight and creating further cooling. If the ice manages to creep to roughly 30° to 40° latitude—about where North Africa and the continental United States are today—the global climate enters a runaway freezing cycle and glaciers end up covering the entire planet within a few hundred years.
The geological record indicates Earth has experienced at least two such periods. The most recent one is known as the Marinoan Ice Age, between 654 million and 635 million years ago. Life was limited to the oceans and large creatures had yet to evolve, but fossils show that microscopic eukaryotes such as algae lived before and after the episode. Such organisms require sunlight and open water, Xiao says. “You have to envision some sort of refuge where these algae can survive.”
Previous work has shown that storm-driven currents may have been openly circulating in shallow seas during the era, although much of the evidence remains ambiguous. To study conditions during this period, Xiao and his colleagues examined a thin layer of dark shale found in Shennnongjia National Forest in south China that dates to the Marinoan. The researchers believe the shale originated from mineral-rich ocean mud that was laid down at midlatitudes, perhaps between 30° and 40° north.
Xiao and his colleagues found the shale was not only chock full of various algal fossils, but also nitrogen compounds. A thick ice sheet would have cut the oceans off from the atmosphere, preventing oxygen from entering their waters. But the nitrogen compounds were found at levels similar to those in modern-day oceans, suggesting oxygenated waters where nitrogen and oxygen were able to freely interact.
The results indicate that the area where the shale came from might not have been capped with ice and was instead an abode where photosynthetic organisms could thrive, the team reports today in Nature Communications. If true, it would require climate modelers to tweak their simulations and find ways to sustain an open ocean over a long period of time, Xiao says.
The results jibe with multiple lines of evidence from other studies, which indicate clement conditions at similar latitudes during the ice age, says Carol Dehler, a geologist at Utah State University who wasn’t involved in the work.
But Hoffman still believes it’s likelier that the fossils were from microscopic algae that hid out in tiny, shallow pools of freshwater atop glaciers. Today, such pools appear in polar ice sheets and provide a refuge for cold-adapted microbes.
All modern algae are descended from freshwater species, Hoffman says, suggesting that those in the seas were wiped out during Snowball Earth periods and had to re-evolve afterward to return to the ocean. “I don’t see the survival as being a problem,” he adds, “nor does this paper solve it.”
It’s also possible that all the researchers are at least a little right. There could have been a span in time where even the equator froze over and living creatures were forced into the glacial pools. But this interval might not have endured for the entirety of the Marinoan.
Geologists know the dark shale layer came from the Marinoan, but precisely when it formed within the ice age is unclear. Perhaps the rocks are a witness to the era’s waning days, a time when ice sheets had already begun to retreat, Dehler says. Xiao and his colleagues agree with this prospect.
“We can’t get too used to any one model,” Dehler says. “We need to be really open minded, because I think we’re going to find a lot of new data will come out in the future.”
Source: Science Mag