THE SUPERCONTINENT CYCLE AND THE PROXY CASE FOR PANNOTIA

Disagreement about the existence of the Ediacaran supercontinent Pannotia centers on paleomagnetic data, which are permissive of a supercontinent but by no means conclusive, and absolute age constraints, which suggest that breakup may have started before the supercontinent was fully assembled. In the context of the supercontinent cycle, however, supercontinent assembly and breakup respectively mark the end of one cycle and the beginning of the next and can be recognized by the tectonic, climatic and biogeochemical trends that accompany them. Hence, the recognition of supercontinents need not rely solely on continental reconstructions, but can also exploit a variety of secular trends that accompany their amalgamation and breakup. While the age and paleogeographic constraints for the existence of Pannotia remain equivocal, the proxy signals of supercontinent assembly and breakup in the Ediacaran are unmistakable. These signals cannot be attributed to either the breakup of Rodinia or the assembly of Gondwana without ignoring either the assembly phase of Pan-African orogenesis and the profound changes in mantle circulation that accompany supercontinent amalgamation, or the fact that Gondwana is not a supercontinent from the standpoint of the supercontinent cycle because its breakup coincides with that of Pangea. The enigma of Pannotia also highlights the limitation of defining supercontinents solely on the basis of size, suggesting instead that, in the context of the supercontinent cycle, they need only be large enough to influence mantle circulation in such a way as to enable the cycle to repeat.