Rapid melt after last global ice age turned snowball Earth into a giant slushy

Scientists have found the first geochemical evidence of the slushy planet – the period when high carbon dioxide levels led to rapid melting after “snowball Earth”.

More than 700 million years ago, ice covered our planet even reaching the equator.

This snowball phase lasted nearly 70 million years, known as the Sturtian glaciation (717–660 million years ago). This was quickly followed by another global ice age called the Marinoan glaciation from about 655–632 million years ago.

“A quarter of the ocean was frozen due to extremely low carbon dioxide levels,” says Virginia Tech geologist Shuhai Xiao, senior author of the new research published in the Proceedings of the National Academy of Sciences journal.

The snowball emerged due to a runaway “albedo effect”. This is when built up ice reflects sunlight, further lowering global temperatures.

But the snowball created its own demise through a chain reaction.

When the oceans froze over, the water cycle locked up. There was no evaporation and very little rain or snow. This meant a massive drop in the processes which carbon dioxide is consumed through chemical weathering when rocks erode and disintegrate.

Increased carbon dioxide in the atmosphere had the opposite outcome to albedo – it led to a greenhouse effect, trapping heat.

“It was just a matter of time until the carbon dioxide levels were high enough to break the pattern of ice,” Xiao says. “When it ended, it probably ended catastrophically.”

Global temperatures soared and the ice caps receded. Over just 10 million years, average global temperatures went from –45 to 48°C.

Xiao’s team analysed geochemical signatures of carbonate rocks that formed as the global ice age was ending. The relative abundance of different lithium isotopes (lithium atoms with different numbers of neutrons in their nucleus) showed a stronger signature of freshwater in rocks formed under nearshore waters, as opposed to those formed offshore which would have been beneath the deep, salty sea.

It shows that Earth’s climate became drippy and soupy during this period rather than the ice melting and remixing with seawater at the same time.

They refer to this “slushy planet” as the “plumeworld ocean” era due to plumes of fresh glacial meltwater that would have inundated the very salty seas, sitting on top of the denser saline water.

It’s believed that snowball Earth was a kind of “reset button” for life on Earth which led to the emergence of the first large multicellular organisms in the Ediacaran period (635–541 million years ago).

The researchers say the findings give insight into the resiliency of life under extremes of heat, ice and slushy.