Neoproterozoic glacial palaeolatitudes: a global update

New stratigraphic, geochronological and palaeomagnetic constraints allow updates to be made to a synthesis of Neoproterozoic glacial palaeolatitudes, including modifications to some reliability estimates. The overall pattern of a Neoproterozoic climatic paradox persists: there is an abundance of tropical palaeolatitudes and near to complete absence of glaciogenic deposits demonstrably laid down between latitudes of 608 and 908. In addition to 12 units with palaeolatitude estimates that are somewhat reliable, estimates with moderate to high reliability now include Konnarock (less than 108 from the palaeo-equator), Elatina, Rapitan, Mechum River, Grand Conglomerat (10–208), Upper Tindir, Puga (20–308), Nantuo, Gaskiers (30–408) and Walsh (40–508). Among these, Elatina, Upper Tindir and Nantuo are considered to have the highest reliability, all with estimates of low to moderate palaeolatitude. The Elatina result stems from sedimentary rocks with quantitative correction of inclination-shallowing effects, and the Upper Tindir result stems from data collected from igneous rocks that are precisely coeval with the glacial deposits. Despite continuing debate on the global character of Neoproterozoic ice ages, their pan-glacial extent (ice extending to low latitude in a low-obliquity world) is well demonstrated. Palaeomagnetism of glaciogenic deposits has provided a quantitative basis for hypotheses of extreme climatic shifts in the late Neoproterozoic Era. The most recent global compilations of palaeomagnetic depositional latitudes for Proterozoic ice ages indicate a dominant mode near the palaeo-equator (Evans 2000, 2003), with no robust palaeo-polar deposits yet discovered. Such results could therefore support either the Snowball Earth (Kirschvink 1992) or the high-obliquity (Williams 1993) hypotheses for Precambrian ice ages, but would appear to reject the uniformitarian comparison to polar/temperate-restricted Phanerozoic glaciogenic deposits (Evans 2000). Hoffman (2009) has suggested that Neoproterozoic ice ages represent a globally all-encompassing ‘pan-glacial’ state of the Earth’s climate system, fundamentally distinct from either partially ice-covered ‘glacial–interglacial’ or ice-free ‘nonglacial’ palaeoclimates experienced during the Phanerozoic Eon. Several reviews of stratigraphic (Halverson et al. 2005, 2007), sedimentological (Hoffman & Schrag 2002; Eyles & Januszczak 2004; Fairchild & Kennedy 2007; Hoffman et al. 2007; Allen & Etienne 2008; Hoffman 2009) and palaeomagnetic (Trindade & Macouin 2007; Hoffman & Li 2009) data sets pertaining to Neoproterozoic ice ages have appeared recently. These discussions of glacial deposits owe much to the pioneering synthesis of Hambrey & Harland (1981), but the recent reviews arrive at differing conclusions regarding the extent and severity of Neoproterozoic ice ages. In particular, the study by Eyles & Januszczak (2004) is commonly cited as a palaeogeographic alternative that avoids the need for nonuniformitarian processes to account for the advance of widespread continental ice sheets into tropical palaeolatitudes. It becomes useful, then, to review the global evidence for or against low-latitude glaciation. Herein, we reassess Neoproterozoic glacial palaeolatitudes in light of new stratigraphic, geochronological and palaeomagnetic data obtained within the last decade, providing the first comprehensive update and revision of the palaeographic analyses of Evans (2000).