Period· The columns of time

Cryogenian

720 Ma → 635 Ma

Twice, ice reached sea level at every latitude, tropics included; between the two, algae took the open ocean from bacteria.

What the evidence shows

Its name means born of ice, and it holds the two most extreme glaciations in the planet's history. They are not alike: the Sturtian runs some 56 million years, the Marinoan at least four, and both leave glacial debris at tropical latitudes, under ice thick enough to rest on the seabed. Each ends the same way, abruptly, under a few metres of pale dolostone lying straight on the rubble with no weathered surface in between, because there was no time for one to form. In the narrow warm gap that separates them, from 659 to 645 million years ago, the molecular fossils of algae rise from near-absence to dominance: eukaryotic algae unseat the cyanobacteria, until then the only primary producers at sea, and the richer food webs that follow will feed the first large animals.

Bounds and conventions

This is one of the few Precambrian periods whose base was set to catch an event, the onset of the Sturtian glaciation, rather than fixed at an arbitrary round figure, and it therefore moves with the dating of the Franklin volcanism, now placed at 720 to 717 million years. How completely the ocean froze is still argued, between a hard snowball and a slushball keeping open water in the tropics; ash beds dated inside the Marinoan deposits show that such open water did exist, at least at times, four million years before the glaciation ended.

Sources

1. Cohen, K. M., Finney, S. C., Gibbard, P. L., Fan, J.-X. (2013). The ICS International Chronostratigraphic Chart. Episodes 36(3), 199–204, mise à jour continue, voir Cohen et al. (2025), Episodes.
2. Hoffman et al. (2017), Science Advances · Snowball Earth climate dynamics and Cryogenian geology-geobiology
3. Prave et al. (2016), Geology · Duration and nature of the end-Cryogenian (Marinoan) glaciation
4. Brocks et al. (2017), Nature · The rise of algae in Cryogenian oceans and the emergence of animals
5. Thomas et al. (2024), Nature Communications · Three-stage formation of cap carbonates after Marinoan snowball glaciation consistent with depositional timescales and geochemistry