Some of the most significant climatic occurrences in Earth’s history are known as “Snowball Earth” events. These events, which took place hundreds of millions of years ago, saw nearly the entire planet covered in ice reaching depths of up to 0.6 miles (1 kilometer). Recent studies have shed light on how the last Snowball Earth event came to an end and why it was followed by a substantial surge in life on the planet, including the appearance of the first animals.
Some of the most significant climatic occurrences in Earth’s history are known as “Snowball Earth” events. These events, which took place hundreds of millions of years ago, saw nearly the entire planet covered in ice reaching depths of up to 0.6 miles (1 kilometer).
Such “Snowball Earth” events have occurred only a few times and do not follow a predictable cycle. Each event can last for millions or even tens of millions of years, culminating in a significant warming phase, though the specifics of these shifts are not well understood.
New findings from the University of Washington offer a clearer understanding of how the final Snowball Earth event ended and suggest the reasons behind its link to a remarkable expansion of life on Earth, including the rise of the first animals.
The study, recently published in Nature Communications, examines ancient rocks referred to as “cap carbonates.” These rocks are believed to have formed during the melting of glacial ice and hold important information about Earth’s atmosphere and oceans around 640 million years ago—much earlier than what can be captured by ice cores or tree rings.
“Cap carbonates give insights into vital characteristics of Earth’s atmosphere and ocean, such as shifting levels of carbon dioxide and ocean acidity,” explained lead author Trent Thomas, a UW doctoral student in Earth and space sciences. “Our theory now clarifies how these factors evolved during and after the Snowball Earth period.”
Cap carbonates consist of layered limestone or dolomite rocks with a unique chemical composition, currently found in over 50 locations around the globe, including Death Valley, Namibia, Siberia, Ireland, and Australia. These formations are thought to have emerged as the extensive ice sheets melted, resulting in striking alterations in atmospheric and ocean chemistry while depositing this specialized sediment onto the ocean floor.
They are termed “caps” because they sit above the glacial deposits left behind after the Snowball Earth, and “carbonates” because both limestone and dolomite contain carbon. Understanding their formation is crucial for explaining the carbon cycle during periods of severe climate change. The new study, which models environmental shifts, also provides clues about the development of life on Earth and why more advanced organisms appeared after the last Snowball Earth event.
“For over 2 billion years leading to the Snowball Earth, life on Earth was quite primitive, mainly consisting of microbes, algae, and other tiny aquatic organisms,” noted senior author David Catling, a UW professor of Earth and space sciences. “The period immediately before the Snowball Earth is often described as the ‘boring billion’ due to its lack of significant developments. Following two Snowball Earth events, animals suddenly appeared in the fossil record.”
The latest paper presents a framework that connects these two observations.
The research modeled the chemistry and geology during three stages of the Snowball Earth phenomenon. Initially, at the height of the Snowball Earth event, thick ice surrounding the planet reflected sunlight, yet some regions of open water facilitated exchanges between the ocean and atmosphere. Meanwhile, icy seawater continued to interact with the ocean floor.
Eventually, an accumulation of carbon dioxide in the atmosphere generated enough heat to warm the planet and melt the ice. This allowed rainfall to return to the Earth and freshwater to flow into the ocean, mixing with layers of glacial meltwater that sat atop denser, salty ocean water. This stratified ocean slowed oceanic circulation. Eventually, the ocean began to churn again, leading to renewed mixing between the atmosphere, upper ocean, and deep ocean.
“We anticipate significant changes in the environment as Earth emerged from the Snowball era, many of which influenced ocean temperature, acidity, and circulation. Now that we understand these changes, we can better determine their impact on life on Earth,” said Thomas.
Future studies will investigate how small pockets of life that might have survived the challenges posed by Snowball Earth and its aftermath could have evolved into the more complex organisms that soon appeared thereafter.
This research received funding from the National Science Foundation and NASA, partially through a NASA Astrobiology Program grant for the University of Washington’s Virtual Planetary Laboratory.
