Was 'Snowball Earth' a global event? Study delivers best proof yet

The study, led by the University of Colorado Boulder, is a coup for proponents of a long-standing theory known as Snowball Earth. It posits that from about 720 to 635 million years ago, and for reasons that are still unclear, a runaway chain of events radically altered the planet's climate. Temperatures plummeted, and ice sheets that may have been several miles thick crept over every inch of Earth's surface.

"This study presents the first physical evidence that Snowball Earth reached the heart of continents at the equator," said Liam Courtney-Davies, lead author of the new study and a postdoctoral researcher in the Department of Geological Sciences at CU Boulder.

The team will publish its findings in the Proceedings of the National Academy of Sciences. Co-authors include Rebecca Flowers, professor of geological sciences at CU Boulder, and researchers from Colorado College, the University of California, Santa Barbara and University of California, Berkeley.

The study zeroes in on the Front Range of Colorado's Rocky Mountains. Here, a series of rocks nicknamed the Tavakaiv, or "Tava," sandstones hold clues to this frigid period in Earth's past, Courtney-Davies said.

The researchers used a dating technique called laser ablation mass spectrometry, which zaps minerals with lasers to release some of the atoms inside. They showed that these rocks had been forced underground between about 690 to 660 million years ago -- in all likelihood from the weight of huge glaciers pressing down above them.

Courtney-Davies added that the study will help scientists understand a critical phase in not just the planet's geologic history but also the history of life on Earth. The first multicellular organisms may have emerged in oceans immediately after Snowball Earth thawed.

"You have the climate evolving, and you have life evolving with it. All of these things happened during Snowball Earth upheaval," he said. "We have to better characterize this entire time period to understand how we and the planet evolved together."

Searching for snow

The term "Snowball Earth" dates back to a paper published in 1992 by American geologist Joseph Kirschvink.

Despite decades of research, however, scientists are yet to agree whether the entire globe actually froze. Geologists, for example, have discovered the fingerprints of thick ice from this time period along ancient coastal areas, but not within the interior of continents close to the equator.

Which is where Colorado enters the picture. At the time, the region didn't sit at the northern latitudes where it does today. Instead, Colorado rested over the equator as a landlocked part of the ancient supercontinent Laurentia.

If glaciers formed here, scientists believe, then they could have formed anywhere.

Going deep

The search for that missing piece of the puzzle brought Courtney-Davies and his colleagues to the Tava sandstones. Today, these features poke up from the ground in a few locations along Colorado's Front Range, most notably around Pikes Peak. To the untrained eye, they might seem like ordinary-looking yellow-brown rocks running in vertical bands less than an inch to many feet wide.

But for geologists, these features have an unusual history. They likely began as sands at the surface of Colorado at some point in the past. But then forces pushed them underground -- like claws digging into the Earth's crust.

"These are classic geological features called injectites that often form below some ice sheets, including in modern-day Antarctica," Courtney-Davies said.

He wanted to find out if the Tava sandstones were also connected to ice sheets. To do that, the researchers calculated the ages of mineral veins that sliced through those features. They collected tiny samples of the minerals, which are rich in iron oxide (essentially, rust), then hit them with a laser. In the process, the minerals released small quantities of the radioactive element uranium. Because uranium atoms decay into lead at a constant rate, the team could use them as a sort of timekeeper for the planet's rocks.

It was a Eureka moment: The group's findings suggest that the Tava sandstone had been pushed underground at the time of Snowball Earth. The group suspects that, at the time, thick ice sheets formed over Colorado, exposing the sands to intense pressures. Eventually, and with nowhere else to go, they pushed down into the bedrock below.

"We're excited that we had the opportunity to unravel the story of the only Snowball Earth deposits that have so far been identified in Colorado," Flowers said.

The researchers aren't done yet: If such features formed in Colorado during Snowball Earth, they probably formed in other spots around North America, too, Courtney-Davies said:

"We want to get the word out so that others try and find these features and help us build a more complete picture of Snowball Earth."