Evidence from the British Isles suggests that during 3 intervals in the early Palaeozoic conditions approximating to those of a non-uniformitarian ‘greenhouse (G)’ state Earth were realized: middle–early Upper Cambrian (St. Davids–Merioneth ages, ~30 Ma), mid-Ordovician (Llandeilo–Caradoc ages, ~32 Ma) and early Silurian (Llandovery–early Wenlock ages, ~12 Ma). During these times sea levels were high and ocean waters below the zone of surface overturning, poorly aerated and, at times, euxinic; pelagic biotas (graptoloids) were diverse, and in aerated portions of the highly productive shelf seas, benthic biotas (brachiopods) flourished. Opposing conditions, marking a well oxygenated (‘O’ state) Earth, occurred to varying degrees in the intervening intervals: early Cambrian (Comley age, ~20 Ma), late Cambrian–mid-Ordovician (Tremadoc–late Llandeilo ages, ~46 Ma), late Ordovician (Ashgill age, ~4 Ma) and mid to late Silurian (post early Wenlock age, >15 Ma). Within the constraints of the present data, perturbations in the early Palaeozoic marine realm do not appear to follow the same regular periodicity as in the Mesozoic and Caenozoic. Among a number of likely causes for this, the greater tendency for the early Palaeozoic seas to become anoxic seems the most important.

First Page Preview

First page PDF preview
You do not currently have access to this article.