State of the planet· The columns of time

Marinoan Snowball Earth

651 Ma → 635 Ma

From 651 to 635 million years ago, the Marinoan glaciation froze the planet a second time, far more briefly than the first, and its end defines a period boundary.

Second global glaciation of the Cryogenian, markedly briefer than the first, and the only glaciation whose end defines a period boundary.

What the evidence shows

The band covers sixteen million years, a width that expresses uncertainty about its onset and not the duration of the ice: dating the glacial deposits themselves gives a glaciation of at least four million years, against fifty-six for the Sturtian. Its end, by contrast, is the best characterised in this part of the record: within a few metres of rock, the ice disappears and a warm sea deposits its carbonates directly on the glacial debris. That end, around 635 million years, defines the base of the Ediacaran, which makes it the only glaciation on the chart to define a period boundary. The first indisputable animals appear in the world that follows it, closely enough in time for the relation to be suggested without being demonstrable.

Bounds and conventions

Its onset is poorly constrained, somewhere after 654.5 million years; its end is among the best dated limits in the Precambrian, fixed near 635 million years by two independent methods that agree, a rhenium-osmium age in North America and uranium-lead ages in China, Australia and Namibia. Agreement across four continents on the same layer establishes that the layer is global.