Rock layers recording the extreme chemical changes in ocean water when Earth emerged from the Marinoan glaciation 635 million years ago. The transition from frozen planet to hothouse happened so rapidly that the evidence is compressed into a few meters of rock.
From icehouse to hothouse
In the Three Gorges area of Hubei Province, rock outcrops tell a dramatic story of planetary climate change. These layers document the end of the Marinoan glaciation around 635 million years ago, one of Earth's most severe ice ages. During this "Snowball Earth" event, ice sheets may have stretched from the poles to the equator, covering the globe for millions of years. The geological evidence for this deep freeze is found in a layer called the Nantuo Formation, a type of rock known as diamictite. This formation is a chaotic mixture of boulders, pebbles, and mud, characteristic of deposits left behind by glaciers. Its presence here confirms that this region, like much of the planet, was once buried under ice.
The story frozen in the Nantuo Formation ends abruptly. Sitting directly on top of the glacial diamictite is a completely different type of rock: a thick layer of dolostone and limestone known as a "cap carbonate." This boundary marks one of the most rapid and extreme warming events in Earth's history. The transition from a global icehouse to a super-greenhouse climate is captured in just a few vertical meters of this rock sequence. These cap carbonates, part of the Doushantuo Formation, could only have formed in warm waters, indicating a sudden and shocking climatic shift from frigid to tropical conditions.
A chemical fingerprint of climate change
The cap carbonates of the Doushantuo Formation are a geochemical archive of the post-glacial world. During the long ice age, volcanoes continued to release carbon dioxide, but with the oceans sealed by ice, the gas built up in the atmosphere to extreme levels—perhaps 350 times higher than today. This created an intense greenhouse effect that eventually triggered a rapid, planet-wide meltdown. The ensuing hothouse climate caused torrential, acidic rain that intensely weathered the newly exposed continents.
This accelerated weathering washed enormous quantities of dissolved minerals, particularly calcium and bicarbonate, into the oceans. The seawater became massively oversaturated with these minerals, causing them to precipitate out and quickly form the thick layers of carbonate rock seen today. This process drew down the atmospheric CO2 over thousands of years. The rock layers contain unusual chemical signatures that verify this story, including a sharp depletion in Carbon-13 isotopes and the presence of barium sulfate, or barite. These features record the chemistry, recording the unique conditions of the seawater as the planet violently rebalanced its climate. Within these same layers, researchers also find some of the earliest fossils of complex multicellular life, suggesting the nutrient runoff from this great thaw may have fueled a biological revolution.
💡Fun Facts
The term "Snowball Earth" was first coined in 1992 by Caltech geologist Joseph Kirschvink.
The intense weathering after the meltdown was so severe it might have triggered a global deluge of acid rain that lasted for hundreds of thousands of years.
Some cap carbonates contain giant wave ripples, evidence of the violent storms and sea-level rise that characterized the hothouse aftermath.
Recent evidence from black shales in Hubei suggests that some patches of open, habitable water may have existed, making it more of a "Slushball Earth."