Period· The columns of time

Cryogenian

720 Ma → 635 Ma

The Cryogenian, from 720 to 635 million years ago, is the period of the two most extensive glaciations in the planet's history, during which eukaryotic algae took over primary production in the open ocean.

Period of the two most extensive glaciations in the history of the planet, in which eukaryotic algae displace bacteria in the primary production of the open ocean.

What the evidence shows

Its name means born of ice, and it carries the two most extreme glaciations in the record. They are not comparable: the Sturtian extends over some 56 million years, the Marinoan over at least 4, and both leave, in tropical latitudes, debris deposited by ice thick enough to rest on the seabed. Each ends abruptly, beneath a few metres of pale dolostone laid directly on the debris with no weathered surface between, no time having elapsed for weathering to occur. In the temperate interval separating them, 659 to 645 million years, the molecular fossils of algae pass from near-absence to predominance: eukaryotic algae displace the cyanobacteria, until then the sole primary producers at sea, and the richer food chains that result will support the first large animals.

Bounds and conventions

This is one of the few Precambrian periods whose base rests on an event, the onset of the Sturtian glaciation, rather than on a round number; it therefore moves with the dating of Franklin volcanism, now placed between 720 and 717 million years. The degree of ocean glaciation remains debated, between a wholly covered Earth and one retaining open water in the tropics; dated ash beds within the Marinoan deposits establish that such open water existed, at least intermittently, four million years before the glaciation ended.