Facies indicators and geochemical proxies of early Palaeozoic global climate cooling suggest episodes of fluctuating glacio-eustasy and severe cold or glaciation from the Mid–Late Cambrian to Silurian (c. 85 myr), with a mean frequency of 2.6 myr. Long-period orbital time series predicted through the Phanerozoic are used to generate sine waves to test against these data; the null hypothesis of no orbital influence is rejected with a high confidence level. Cooling episodes appear most frequent through the Late Ordovician leading up to the Hirnantian glacial maximum, but even ‘greenhouse’ intervals of the Early–Mid-Ordovician and early Silurian provide evidence for periodic cooling episodes.

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