Some of the most dramatic climate events in our planet’s history are “Snowball Earth” events that occurred hundreds of millions of years ago, when almost the entire planet was encased in ice up to 1 kilometer thick.
These ‘Snowball Earth’ events have only happened a handful of times and do not occur in regular cycles. Each phase lasts for millions or tens of millions of years and is followed by dramatic warming, but the details of these transitions are poorly understood.
New research from the University of Washington provides a more complete picture of how the last Snowball Earth ended, and suggests why it preceded a dramatic expansion of life on Earth, including the emergence of the first animals.
The research was recently published in Nature communication focuses on ancient rocks known as ‘cap carbonates’, which are thought to have formed when glacial ice thawed. These rocks hold clues to Earth’s atmosphere and oceans some 640 million years ago, much earlier than what ice cores or tree rings can record.
“Cap carbonates contain information about important properties of Earth’s atmosphere and ocean, such as changing levels of carbon dioxide in the air, or the acidity of the ocean,” said lead author Trent Thomas, a UW doctoral student in earth and space sciences . “Our theory now shows how these properties changed during and after Snowball Earth.”
Cap carbonates are layered limestone or dolomite rocks with a distinct chemical composition and are today found in more than 50 locations worldwide, including Death Valley, Namibia, Siberia, Ireland and Australia. These rocks are believed to have formed when the ice sheets that encircled the Earth melted, causing dramatic changes in the chemistry of the atmosphere and ocean and depositing this unique type of sediment on the ocean floor.
They are called ‘caps’ because they are the caps above glacial deposits left behind after Snowball Earth, and ‘carbonates’ because both limestone and dolomite are carbonaceous rocks. Understanding their formation helps explain the carbon cycle during periods of dramatic climate change. The new study, which models environmental changes, also provides hints about the evolution of life on Earth and why more complex life forms followed the last Snowball Earth.
“Life on Earth was simple — in the form of microbes, algae, or other small aquatic organisms — for more than 2 billion years prior to Snowball Earth,” says senior author David Catling, professor of earth and space sciences at the UW. ‘In fact, the billion years leading up to Snowball Earth are called the ‘boring billion’ because so little happened. Then two events took place on Snowball Earth. And soon after, animals appear in the fossil record.”
The new article provides a framework for how the last two facts can be linked together.
The study modeled the chemistry and geology during three phases of Snowball Earth. First, the thick ice surrounding the planet reflected sunlight during Snowball Earth’s peak, but some areas of open water allowed exchanges between the ocean and the atmosphere. Meanwhile, icy seawater continued to react with the ocean floor.
Eventually, carbon dioxide built up in the atmosphere to the point where it trapped enough solar energy to raise Earth’s temperature and melt the ice. This allowed rain to reach the Earth and fresh water to flow into the ocean to join a layer of glacial meltwater that floated over the denser, salty ocean water. This layered ocean slowed down ocean circulation. Later, ocean churning increased and mixing between the atmosphere, upper ocean, and deep ocean resumed.
“We predict major environmental changes as Earth recovered from the snowball period, some of which affected ocean temperature, acidity and circulation. Now that we know these changes, we can more confidently figure out how they affect life on Earth,” Thomas said.
Future research will explore how parts of life that survived the tumult of Snowball Earth and its aftermath might have evolved into the more complex life forms that followed soon after.
The research was funded by the National Science Foundation and NASA, in part through a NASA Astrobiology Program grant to the UW’s Virtual Planetary Laboratory.
