THE NEOPROTEROZOIC ROCK RECORD: TESTING THE NATURE OF EXTREME CLIMATE CHANGE IN DEEP TIME

Every so often in Earth history events conspire to construct truly extraordinary periods of geological change; the Neoproterozoic was one such time. In particular, severe climatic extremes (globally frozen oceans and ultra-greenhouses) are envisaged to have repeatedly disrupted the Earth System and are invoked as a potential driving mechanism for biospheric evolution. Such hypotheses are vivid and grab public attention, but are they correct? Here we show examples of Neoproterozoic successions from Namibia, Scotland and Death Valley, California, which indicate that climate change was severe, but not catastrophic. Importantly, these rocks were deposited in low-latitudes and display evidence for freeze-thaw cycles on a number of time scales, including during deposition of banded-iron formation (BIF). This indicates that low-latitude climate was not characterised by a simple, unidirectional long-term refrigeration of Earth. Likewise, utilisation of high-resolution d¹³C datasets reveal two facts: (1) that presumed coeval carbonates that cap the glacigenic successions display a wide range in d13C values; and (2) carbonates units below the erosive base of the glacigenic rocks record a decline in d¹³C (as noted by previous workers) but, in more complete basinal sections, a return towards heavier values occurs prior to the initial glacial deposits. These data show that biogeochemically mediated conditions were regionally variable, both before and in the immediate aftermath of Neoproterozoic glaciations. Thus, no objective evidence exists at present to support the concept of a globally frozen Snowball Earth. Instead, it appears that, although the Neoproterozoic was marked by severe climates, the Earth System remained functional and did not undergo repeated, radically nonuniformitarian collapses.