Continental growth spurt 2.4B years ago brought snow, oxygen

Image shows a portion of a rewashed sand dune, depicting how sandstone forms similary to shale

Earth’s first snow may have fallen after a lot of land rose swiftly from the sea and set off dramatic changes on Earth 2.4 billion years ago, says UO geologist Ilya Bindeman.

That notion comes from research done on shale in Bindeman’s Stable Isotope Laboratory. Shale is the world’s most abundant sedimentary rock, and the lab used samples drawn from every continent.

Scientists looked at ratios of three common oxygen isotopes, or chemical signatures. They found archival-quality evidence from as far back as 3.5 billion years ago showing traces of rainwater that caused weathering of land.

Shale rocks are formed by the weathering of crust. Bindeman, a professor in the Department of Earth Sciences, initially began collecting shale samples while doing petroleum-related research.

"They tell you a lot about the exposure to air and light and precipitation,” he said. “The process of forming shale captures organic products and eventually helps to generate oil. Shales provide us with a continuous record of weathering."

Exposed mudrock
Previously submerged surfaces become exposed to weathering, leading to the accumulation of mudrocks and shales. In this scene, winter drainage at Fern Ridge reservoir west of Eugene exposes mudrocks, providing an example of how newly risen land is exposed to weathering forces.

Bindeman and his eight co-authors detected a major shift in the chemical makeup of 278 shale samples at the 2.4-billion-year mark. They detailed their conclusions in a paper in the May 24 issue of Nature.

Those changes began on a planet that was much hotter than today when the newly surfaced land rose rapidly and was exposed to weathering. Based on his own previous modeling and other studies, Bindeman said the total landmass of the planet 2.4 billion years ago may have reached about two-thirds of what is seen today.

How Earth's land elevations may have looked before and after the Great Oxygenation Event
Before and After: How Earth's land elevations may have looked before and after the Great Oxygenation Event (Courtesy of Ilya Bindeman)

The emergence of so much land changed the flow of atmospheric gases and other chemical and physical processes, primarily between 2.4 billion and 2.2 billion years ago, he said. It also happened amid large-scale changes in mantle dynamics.

"What we speculate is that once large continents emerged, light would have been reflected back into space and that would have initiated runaway glaciation," said Bindeman, a third-generation geologist who grew up in Russia. "Earth would have seen its first snowfall."

Chemical changes recorded in the rocks coincide with the theorized timing of land collisions that formed Earth's first supercontinent, Kenorland, and the planet’s first high-mountain ranges and plateaus. When the planet was much hotter, Bindeman said, such mountainous land could not be supported.

"Land rising from water changes the albedo of the planet,” he said. “Initially, Earth would have been dark blue with some white clouds when viewed from space. Early continents added to reflection.”

The rapid changes, the researchers noted, may have triggered what scientists call the Great Oxygenation Event, in which atmospheric changes brought significant amounts of free oxygen into the air.

Scientists have long believed that Earth experienced a gradual or stepwise emergence of land between 1.1 billion and 3.5 billion years ago. Bindeman’s study points to an age near the middle of that span.

The timing also coincides with the transition from the Archean Eon, when archaea and bacteria — simple, single-cell life forms — thrived in water, to the Proterozoic Eon, when more complex life forms, such as algae, plants and fungi, emerged.

UO co-authors with Bindeman, who was supported by the National Science Foundation, were doctoral student David O. Zakharov and research associate James Palandri, both in Bindeman's UO lab, and Gregory J. Retallack, a professor in the UO Department of Earth Sciences.

—By Jim Barlow, University Communications