SUPERCONTINENT PANNOTIA: A CAUTIONARY TALE

Following more than a decade during which the reality of Pannotia was widely accepted, the existence of this putative Ediacaran supercontinent has come into question. In the context of the supercontinent cycle, this change is of fundamental importance since the existence (or not) of Pannotia is central to the nature, duration and evolution of the cycle, and dictates the cycle’s geodynamic pathway from the breakup of Rodinia to the assembly of Pangea. Growing skepticism in the legitimacy of Pannotia is largely the result of advancing geochronology, which has raised the possibility that the supposed landmass had begun to break up well before it was fully assembled. Paleomagnetic data for the Ediacaran have been used to both support and refute the existence of Pannotia, but are notoriously equivocal with highly scattered datasets that are interpreted to reflect true polar wander and/or a non-uniformitarian magnetic field. Similarly, proxy signals for Ediacaran-Cambrian supercontinent assembly and breakup, although collectively compelling, can be individually challenged. Efforts to detect the mantle legacy expected of supercontinent amalgamation, however, are more telling, and support large-scale mantle upwelling in the wake of Pannotia assembly. Hence, irrespective of whether Pannotia was a supercontinent or not, its assembly appears to have influenced global mantle convection patterns in a manner consistent with one. The case against Pannotia would in that event be forced to concede that, in the wake of the Pan African cycle, a major change from mantle downwelling to mantle upwelling occurred in the absence of the amalgamation of a supercontinent. This raises the cardinal question as to whether a full-size supercontinent is needed to drive the supercontinent cycle from one iteration to the next and, if not, whether other such mantle turnovers are manifest in the geologic record; an outcome that would imply the cycle is misnamed.