EARTH × CLIMATE

The Layer That Ends a Snowball

~635 million years ago

Proterozoic · Neoproterozoic · Ediacaran

On every continent that keeps the rocks, a few metres of pale dolostone lie straight on glacial rubble with no weathered surface between them: nothing happened in the interval, because there was no interval. Carbonate came out of the water fast and in quantity, because the sea had been loaded with alkalinity: some of it made on the seafloor under the ice, the rest delivered by rivers stripping continents that had just been uncovered.

Why it matters

This one contact is the physical evidence that a global glaciation ends the way the theory says it must: carbon dioxide builds up under the ice until the greenhouse overwhelms the reflectivity, and then everything happens at once. How hot the aftermath was, and for how long, is where the models part company. The newest of them make it warm but moderate, and shorter than the hundreds of thousands of years long assumed.

Dating & uncertainty

The layer sits directly on glacial debris, so its base is the end of the Marinoan glaciation to within the resolution of the rock. How long it took to pile up is not settled: estimates run from a few thousand years to several hundred thousand, and the models that fit the chemistry disagree with those that fit the thickness.

Sources

1. Thomas et al. (2024), Nature Communications · Three-stage formation of cap carbonates after Marinoan snowball glaciation consistent with depositional timescales and geochemistry
2. Ramme et al. (2024), Nature Communications · Moderate greenhouse climate and rapid carbonate formation after Marinoan snowball Earth
3. Zhang et al. (2024), Palaeogeography, Palaeoclimatology, Palaeoecology · Tracing the Neoproterozoic Marinoan Snowball Earth thaw from deep sea to coastal waters

Earth history as one day

20:38:353 h 21 min before midnight

if Earth’s 4.54 billion years were compressed into a single day