Neoproterozoic glaciation in the Earth System

The Neoproterozoic contains severe glacial intervals (750–580 Ma) including two extending to low palaeomagnetic latitudes. Paucity of radiometric dates indicates the need for chronostratigraphic tools. Whereas the marine Sr/Sr signatures show a steady rise, C fluctuates, the most reproducible variations being negative signatures in carbonate caps to glacial units, but more diagenetic work is needed. Four conceptual models for the icehouse conditions are contrasted: Zipper-Rift Earth (diachronous glaciation related to continental rift margins), High-tilt Earth (high-obliquity and preferential low-latitude glaciation), Snowball Earth (extreme glaciation related to runaway ice–albedo feedback) and Slushball Earth (coexistence of unfrozen oceans and sea-level glaciers in the tropics). Climate models readily simulate runaway glaciation, but the Earth may not be able to recover from it. The Slushball state requires more extensive modelling. Biogeochemical models highlight the lack of CO2 buffering in the Neoproterozoic and the likely transition from a methaneto a CO2-dominated climate system. Relevant processes include tropical weathering of volcanic provinces, and new land biotas stimulating both clay mineral formation and P delivery to the oceans, facilitating organic C burial. Hence a step change in the Earth System was probably both facilitated by organisms and responsible for moderating Phanerozoic climate. The foundation of the Geological Society in 1807 followed closely on James Hutton’s field demonstration in 1785 of the reality of deep (geological) time (Craig 1997). Hutton took pleasure in contemplating the physiology of the planet, which he regarded as a ‘compound system of things, forming together one whole living world’ (McIntyre & McKirdy 2001). Now planetary physiology is back on the agenda, following the thinking of the Society’s 2006 Wollaston medallist James Lovelock (Lovelock 1988). Earth System Science represents the mainstream scientific effort equivalent to Lovelock’s Gaia theory, but without a search for an underlying purpose (Lovelock 2006). Such concerns with global cycling and feedbacks, and in particular Paul Hoffman’s vigorous advocacy of the Snowball Earth hypothesis (Hoffman et al. 1998; Hoffman & Schrag 2000, 2002), have generated excitement in the field of study of Precambrian glaciation (Walker 2003). More soberly, Earth System approaches have provided a medium by which to develop Hutton’s gift of testing conjectures in historical science by making verifiable predictions. For newcomers to the complex Neoproterozoic literature, we have tried to emphasize the more firmly accepted information and highlight those differences in interpretation that are still current. We illustrate that age-uncertainties are still significant in permitting different interpretations of the stratigraphic record, and hence stimulating vigorous debate, but also that carbonate rocks reveal vital information on global elemental cycling and climate. Three mutually exclusive viewpoints on the extent and significance of Neoproterozoic glaciation (Snowball Earth, Hightilt Earth and Zipper-Rift Earth) are contrasted with a more pragmatic synthesis of the most extreme glacial state, Slushball Earth (Fig. 1). We show how quantitative climatic and biogeochemical models constrain the state of the Earth System, and suggest the types of research that will reduce the uncertainties.