Seven-hundred million years ago, Earth was a frozen, white sphere, its rocky surface buried kilometers under ice. Despite the barren landscape, the evolution of complex life in the oceans was about to pick up steam. New research published this week in Geology suggests that the two realms were more connected than previously thought.
As massive glaciers scratched and scarred Earth’s rocky surface, they freed less-common minerals, which were later flushed into the seas as the ice melted into giant glacial rivers. These minerals in turn may have spurred nutrient cycling in the oceans, boosting the metabolism of microbial life.
“In retrospect, I’m surprised it took [researchers] so long to go and do a study like this,” says Galen Halverson, a stratigrapher at McGill University who was not involved in the work. “It fits with what we understand” about the glaciated Earth.
Reading the rocks
Earth didn’t just go through one deep freeze; it experienced two, separated by about 15 million years of thawing in the Neoproterozoic. Chris Kirkland, a geochronologist at Curtin University in Australia, and his collaborators wanted to study the rock record from this freeze-thaw period, called the Cryogenic, to better understand the widespread environmental changes that occurred over this time.
The researchers turned to a vast swath of rock called the Dalradian Supergroup located in present-day Scotland and Ireland. Formed about 800 million years ago on the eastern shore of a past continent called Laurentia, its rocks would have been covered in ice during the so-called “snowball Earths.”
The team collected samples of sandstones, which are rocks comprised of many different individual grains that provide a “fingerprint of where that rock has come from,” says Kirkland. Many of those grains contain zircon, a mineral resistant to erosion, making it stable through time. Zircons also incorporate tiny amounts of the element uranium into their crystal structure, which decays into lead at a known rate and can therefore be used to precisely date the age of the minerals. While zircons from this time period have not yet been studied, Halverson notes that previous studies of snowball Earth have tracked the presence of iron, neodymium, and osmium in the geologic record.
After bringing tens of kilograms of sandstones back to the lab, the team crushed them to separate out hundreds of individual grains. Then, they used tweezers to image grains in an electron microscope. Finally, they fired lasers into the crystal structures of the grains to release their constituent elements, which were then sent into a mass spectrometer to determine the ratio of uranium to lead. Using this ratio to calculate the ages of crystals within different layers of ancient rock, the researchers determined how the rock grew or eroded over time.
During the years of the snowball Earths, the researchers found signs that older, uranium-containing rocks were being broken down. They hypothesized that, as giant glaciers inched across Earth, they etched deep into the ground like a bulldozer, grinding down into deeper rocks and minerals that were not as prevalent on the surface. Then, when temperatures rose, the glaciers melted into powerful streams that dissolved some of the material in the loose rocks and minerals and flushed it into the ocean, causing a spike in oceanic nutrients.
While scientists had already known that the Neoproterozoic ocean became enriched in minerals, the prevailing hypothesis was that this was the product of an oxygenating planet. As early cyanobacteria produced increasingly massive amounts of oxygen, the make-up of the atmosphere changed, along with the gasses present in the ocean. After reacting with this oxygen, some dissolved elements precipitated out of the seawater, becoming available for life.
Now, says Kirkland, the research shows that “you’re changing atmospheric conditions and oxidation states, but you’re also delivering more material to the oceans.” In other words, both the air and land were changing the ocean.
Life is complex
But when new minerals made their way to the water, what did they actually do? Cycle throughout the bottom of the ocean, delivering new elements to previously barren locations and providing energy for microbial life. At the end of the Cryogenic, these early lifeforms appear to have gotten gradually more complex, paving the way for the first known multicellular life in the ensuing Ediacaran.
“Any time there’s a really radical environmental shift, we know that’s an interesting time for evolution,” says Chris Kempes, a theoretical biophysicist at the Sante Fe Institute who was not involved in the research. For example, when temperatures drop or less sunlight is available, organisms’ speed and metabolic rates generally slow down, creating new pressures on life, Kempes’ research has found. Halverson thinks the extreme habitats that life had to endure during the snowballs played more of a role in shaping evolution than the nutrient flushes from glaciers.
Even so, studies like Kirkland’s that try to understand how nutrients and energy availability changed throughout history are “the key to understanding when and why there are major evolutionary transitions,” Kempes says.
To determine what other minerals may have been key players in the ancient oceans, Kirkland hopes to look at rocks called apatites, which contain oxygen and other elements like strontium and phosphorus. However, these break down much easier than zircon-rich rocks, meaning they are less stable through long stretches of time.
Though the global changes of the Cryogenic happened eons ago, Kirkland sees parallels with the wide-scale climate changes of today. “The atmosphere, the land, and the oceans are all interconnected,” he says. “Understanding these [ancient] cycles gives us information about how more modern cycles on the planet may work.”
Geology, 2025. DOI: 10.1130/G52887.1
Hannah Richter is a freelance science journalist and graduate of MIT’s Graduate Program in Science Writing. She primarily covers environmental science and astronomy.