STRATIGRAPHIC CONSTRAINTS ON THE CHARACTER AND FREQUENCY OF EXTREME CLIMATE CHANGE DURING THE NEOPROTEROZOIC

An assessment of the geobiological consequences of extreme climate change during the Neoproterozoic depends on the timing of events and on the character of the climatic fluctuations. The distinctive physical characteristics and negative carbon isotopic signature of "cap carbonates" imply that major ice ages ended rapidly (~10³-10⁴ yr), with a global chemical oceanographic event not matched at glacial-interglacial timescales. The growth of ice sheets and development of glacially influenced depositional environments is likely to have been substantially more diachronous, and the local record of an ice age therefore incomplete.

For spans in which the Earth was only partially glaciated, the physical stratigraphic record of "non-glacial" deposits provides an independent semi-quantitative proxy for the magnitude of glacio-eustatic drawdown (and hence volume of continental ice sheets) and for the number of Neoproterozoic ice ages. Sequence boundaries are surfaces of subaerial degradation associated with such phenomena as valley incision, karst development, abrupt upward shoaling of marine facies, marine bypass and degradation and, in some cases, a reorganization of sediment dispersal patterns ("lowstand" sedimentation). They do not arise exclusively as a result of eustatic change, but they are an inevitable consequence of ice sheet formation. A mismatch between the distribution of sequence boundaries and carbon isotopic minima in terminal Proterozoic carbonate platforms of northern India and southern China casts doubt on the assumption that such minima necessarily relate to glaciation and cap carbonate deposition. However, intriguing stratigraphic similarities between these platforms permit the existence of small ice sheets elsewhere in post-Marinoan time.

The Sturtian glacial record in the western U.S. and the Marinoan glacial record in Australia are inconsistent with an expectation of the snowball Earth hypothesis Ð that the volume of ice sheets was greatest at the end of each ice age. Overall glacial retreat appears to have taken in the order of at least 10⁵-10⁶ yr. In Australia, cap carbonate deposition and glacio-eustatic rise are inferred to have lagged local glacio-isostatic rebound, consistent with the survival of small terrestrial ice caps at higher latitude through the cap carbonate event.