Researchers at the China University of Geosciences in Wuhan, China, have found concrete evidence of a partially open ocean during one of the most intense ice ages on Earth. Their research, which appeared this week in Nature Communications, would disprove the snowball Earth theory. He says that during the so-called Marinoan ice age – about 650 to 635 million years ago – the earth was completely covered with ice. What was the earth then? An ice cold slush puppy.
The Marinoan Ice Age occurs during the cryogenium, or “the birth of the ice.” In this, glaciers from the poles would have stretched thousands of kilometers towards the equator as long, sometimes two kilometers thick ice shelves. Not surprising, because temperatures at the poles could drop to -130 degrees Celsius. As a result, glaciers moved a little further and further like an up and down accordion over hundreds of thousands of years. Until almost the entire earth was frozen, making life almost impossible.
shaky theory
“But that’s where the theory is already starting to falter,” says Bas van de Schootbrugge, paleo-oceanographer at Utrecht University, who is not involved in the research. “Many algae and other macroorganisms now living in the sea arose before the earth became covered with ice. They wouldn’t survive such a thick ice shield at all.” The hypothesis: Marine ‘reservoirs’ existed where life could survive during these ice ages. But yes, where?
In the Nantuo Formation in southern China, the researchers found sediment deposited by glaciers during the Marinoan Ice Age. This rock formation was in the sea at the time. The researchers found fossils of photosynthesizing algae in small chunks of shale. Van de Schootbrugge: “That would mean that there were open spaces in that thick ice shield.” Earlier publications suggested the existence of an ice-free belt around the equator, but the formation studied was much further north 635 million years ago.
To investigate what the habitat of these macro-organisms looked like during the ice age, the sediment in the Nantuo formation was examined for nitrogen isotopes present. “When everything is under the ice, the ocean is low in oxygen because there is no longer any circulation,” explains Van de Schootbrugge. “The ice then works as a kind of lid.” The result: a relatively high concentration of light nitrogen isotopes (14N) in the sediment. However, the researchers found an extraordinary amount of heavy nitrogen isotopes (15N) in the sediment. So that doesn’t seem right. There is a complex explanation behind it: if life is present in the water, these organisms fish out the light nitrogen isotopes. They are easier to process. In an active cycle where organisms have access to light and oxygen, nitrogen in the form of ammonium (NH4+) converted to nitrate (NO3-). This is used by phytoplankton. In addition, the nitrate is converted, or denitrified, by bacteria into nitrogen (N2) that eventually ends up back in the atmosphere.
Melting image
Due to these two processes, relatively many heavy nitrogen isotopes remain in the sediment of the seabed. This is reflected in the Chinese formation that was studied here. Based on the ratio between the light and heavy nitrogen isotopes in the sediment, it can be seen whether there was an active nitrogen cycle, and therefore whether there was sea life during this ice age. The conclusion: “So there it was.”
It seems like an increasingly convincing story. Most of the earth was frozen, but there were iceless reservoirs not only around the equator, but also further north. According to the researchers, these reservoirs consisted of lakes surrounded by ice, or even a much wider ice-free belt that surrounded the Earth. This allowed eukaryotic organisms – organisms with a cell nucleus – to survive and after the Ice Age, in the Cambrian, to evolve into more complex life forms that can be found as fossils in rocks from that time. “This research now confirms that hypothesis with concrete evidence,” says Van de Schootbrugge. “The image of a completely frozen earth is melting away little by little.”