Explaining the dramatic changes on a global scale following the world’s final “Snowball Earth” event
A person looks at carbonate rocks in southern China in 2019. New research offers a new explanation for the dramatic global environmental changes that led to the formation of carbonate rocks. Photo by Yaron Liu
One of the most dramatic climatic events in Earth’s history was the “Snowball Earth” event that occurred hundreds of millions of years ago, when almost the entire planet was covered in ice 0.6 miles (1 kilometer) thick.
Such “Snowball Earth” events have only happened a few times before, and they do not occur regularly, lasting millions or tens of millions of years and followed by dramatic warming events that are poorly understood.
A new study 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 study, published in Nature Communications, focuses on ancient rocks called “cap carbonates,” which are thought to have formed as glaciers melted. These rocks preserve clues about Earth’s atmosphere and oceans from about 640 million years ago, much older than those recorded in ice cores and tree rings.
“Cap carbonates contain information about important properties of Earth’s atmosphere and oceans, such as changing atmospheric carbon dioxide concentrations and ocean acidity,” said lead author Trent Thomas, a doctoral student in Earth and space sciences at the University of Washington. “Our theory tells us how these properties may have changed during and after Snowball Earth.”
Cap carbonate rocks are layered limestone or dolomite rocks with a unique chemical composition that are currently found in over 50 locations around the world, including Death Valley, Namibia, Siberia, Ireland and Australia. These rocks are thought to have formed when ice sheets surrounding the Earth melted, causing dramatic changes in the chemical composition of the atmosphere and oceans, depositing this unique type of sediment on the ocean floor.
These are called “caps” because they sit on top of glacial deposits left behind after Snowball Earth, and “carbonates” because limestone and dolomite are both rocks that contain carbon. Understanding their formation helps explain carbon cycling during periods of dramatic climate change. This new study, which models environmental change, also offers hints about the evolution of life on Earth and why more complex life forms followed after the last Snowball Earth.
These panels present a new theory of the three stages that ended the Snowball Earth event. In the first stage, thick ice sheets separated much of the atmosphere and ocean. In the second stage, freshwater flowed into the ocean and mixed with meltwater floating on the ocean’s surface. In the final stage, ocean mixing resumed, allowing exchange between the atmosphere, upper ocean, and deep ocean. Credit: Thomas et al./Nature Communications
“For more than 2 billion years leading up to Snowball Earth, life on Earth was simple: microbes, algae and other small aquatic organisms,” said David Catling, professor of Earth and space sciences at the University of Washington and lead author of the paper.
“In fact, the billion years leading up to Snowball Earth are called the ‘boring billion’ because very little happened during that time. Then two Snowball Earth events occurred, and shortly after that, animals appeared in the fossil record.”
The new paper provides a framework for how those last two facts are related.
The study modeled the chemistry and geology of Snowball Earth during three stages: At its peak, a thick layer of ice surrounded the planet and reflected sunlight, but some open water allowed ocean-atmosphere interaction, while frigid ocean water continued to react with the ocean floor.
Eventually, carbon dioxide built up in the atmosphere and trapped enough solar energy to warm the planet and melt the ice. This caused rain to fall on the planet and freshwater to flow into the oceans, adding to the layer of glacial meltwater floating on top of the denser, saltier water. This layered ocean slowed ocean circulation. Ocean churning then intensified, and mixing of the atmosphere, upper ocean, and deep ocean resumed.
“We predict that as Earth recovers from the Snowball Period, we will see big changes in the environment, some of which affected ocean temperature, acidity and circulation. Now that we know these changes, we can more confidently work out how they affected life on Earth,” Thomas said.
Future research will explore how the host of life that may have survived the chaos of Snowball Earth and its aftermath evolved into the more complex life forms that followed soon after.
Further information: Trent B. Thomas et al., “Three-phase formation of post-Marinoan glacial cap carbonates is consistent with depositional timescales and geochemistry,” Nature Communications (2024). DOI: 10.1038/s41467-024-51412-8
Courtesy of University of Washington
Source: Dramatic global changes explained after the world’s final “Snowball Earth” event (September 19, 2024) Retrieved September 19, 2024 from https://phys.org/news/2024-09-planetwide-world-snowball-earth-event.html
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