The profound glaciations of the Neoproterozoic Cryogenian period (ca. 850–544 Ma) represent an extreme climatic mode when, it is claimed, Earth was fully or almost completely covered with ice for millions of years. We show that the geochemistry and mineralogy of fine-grained Neoproterozoic sedimentary rocks in Oman are best explained by climatic oscillations that drove variations in the intensity of chemical weathering on contemporary land surfaces. The cold climate modes of the Cryogenian were therefore cyclical, punctuated with well-defined warm-humid interglacial periods. The hydrological cycle and the routing of sediment were active throughout the glacial epoch, which requires substantial open ocean water. This reconstruction represents a significantly different target for numerical climate models at this critical time in the evolution of Earth's biosphere.

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