Any viable explanation for snowball events must also explain why they are rare. Extensive low-latitude glaciation occurred only near the beginning (2.45-2.22 Ga) and the end (0.73-0.58 Ga) of the Proterozoic eon (Fig. 12). The former include the Makganyene glacials in South Africa (Evans et al., 1997; Tsikos and Moore, 1998; Kirschvink et al., 2000), three discrete Huronian glaciations in central Canada (Williams and Schmidt, 1997; Schmidt and Williams, 1999), and equivalents in Wyoming and Finland (Ojakangas, 1988). The Makganyene and Huronian glacials have similar age brackets, >2.22 and <2.45 Ga (see references in Williams and Schmidt, 1997; Bekker et al., 2001). The Makganeyene glaciation was preceded by carbonates with unusually high δ¹³C values, similar to LNGD (Bekker et al., 2001), and succeeded by a major Fe-Mn sedimentary formation (Tsikos and Moore, 1998; Kirschvink et al., 2000). The middle Huronian and equivalent glacial units have cap carbonates (the only carbonate in the 14-km-thick Huronian succession) with negative δ¹³C (Veizer et al., 1992; Bekker et al., 2001), and the ultimate Huronian glaciation was followed by an intense lateritic weathering regime (Young, 1973). Remarkably, there is virtually no evidence for large ice sheets at any paleolatitude during the 1.5 billion years in between the two snowball eras (Fig. 12). Two things need to be explained (Schrag et al., 2002): what special conditions set those two eras apart (allowing snowball events to occur)?, and what triggered the individual snowball events?

The length of both eras, 100-200 myr, is of the time scale of global paleogeographic reorganizations due to plate tectonics. Kirschvink (1992) proposed that a preponderance of land masses in the middle and low latitudes set the stage for snowball events, consistent with existing paleomagnetic data for LNGD (Fig. 1). This is broadly in line with global reconstructions for 750 Ma (Fig. 7), prior to the "Sturtian" (Table 1) glaciation(s), but "Varanger-Marinoan" (Table 1) paleogeography is quite uncertain, primarily because of poor age control. Reconstructions in which land masses are centered on the (south) pole (e.g., Hyde et al., 2000; Baum and Crowley, 2001; Crowley et al., 2001) hinge on Laurentia's migration to high latitudes by 577 Ma (Torsvik et al., 1996), after the last snowball event (Table 1), a migration which could have ended the snowball era rather than being a precondition for it (Hyde et al., 2000). Placing continents tangent to the tropics results in a colder Earth for a variety of reasons. It raises the surface albedo in areas of minimal cloud cover (Kirschvink, 1992) and it enhances silicate weathering unless tectonic activity is mysteriously absent there (Marshall et al., 1988; Worsley and Kidder, 1991; Schrag et al., 2002). Weathering rates are greater on small continents because they are wetter than large ones. The Neoproterozoic snowball era was a period of continental dispersal, involving the breakup of supercontinent Rodinia and the aggregation of megacontinent Gondwana (Hoffman, 1999). Polar ice caps may reduce global weathering rates and mitigate against snowball events if large land areas exist at high latitudes (Schrag et al., 2002), as in the Phanerozoic. An absence of high-latitude continents and a preponderance of tropical and subtropical land masses would be an unusual situation in Earth history, and might give rise to polar sea-ice caps large enough to threaten runaway ice-albedo feedback (Kirschvink, 1992; Schrag et al., 2002).

Clues regarding the triggering of individual snowball events may be found in pre-glacial δ¹³C records (Fig. 8,9). Snowball events generally follow long stages (>107 years) of high δ¹³C (>5‰ PDB in shallow marine carbonates) (Kaufman et al., 1997; Walter et al., 2000; Brasier and Shields, 2000; Halverson et al., 2002). This likely signifies that organic matter was disproportionately represented in the global carbon burial flux (forg≥0.4). We speculate (Schrag et al., 2002) that a preponderance of middle and low latitude continents would produce this effect. Nutrient transport patterns will cause marine biological productivity to be focused in the congested middle and low latitudes where high rates of organic production will drive some basins anoxic (Schrag et al., 2002). Anoxic bottom waters cause high C:P ratios in the organic burial flux (Van Cappellen and Ingall, 1994; Colman and Holland, 2000), which greatly increases phosphorus availability for recycling, which in turn allows high sustained rates of organic production and burial.

Spectacular declines in δ¹³C (Fig. 8,9) have been discovered directly beneath the younger LNGD in northwest Canada (Narbonne and Aitken, 1995), northwest Namibia (Hoffman et al., 1998b; Halverson et al., 2002), South Australia (Walter et al., 2000; McKirdy et al., 2001) and northwest China (Xiao et al., 2001), and also beneath the older LNGD in Scotland (Brasier and Shields, 2000) and northeast Svalbard (Halverson and Maloof, 2001). The time scale of the isotopic anomaly in Namibia is estimated to be on the order of 0.5 myr (Halverson et al., 2002). We argue that this anomaly is most likely the result of a sustained release of 12C-enriched carbon, possibly methane generated in the organic-rich sediments previously deposited (Halverson et al., 2002; Schrag et al., 2002). A prolonged release of methane into the atmosphere, engendered by the unusual continental distribution, would not only drive down marine δ¹³C but might also destabilize the climate. Because atmospheric methane is not equilibrated with the ocean, as is CO₂, any substantial dependence on methane greenhouse could counter-intuitively trigger a snowball event if the methane supply was interrupted for any reason (Pavlov et al., 2000; Schrag et al., 2002). These ideas are highly speculative and it remains to be seen if any or all snowball events were initiated in this way. Perhaps each event was triggered differently, a possibility that highlights the glaring uncertainty regarding the number and correlation of LNGD (Kaufman et al., 1997; Kennedy et al., 1998; Grey and Corkeron, 1998; Saylor et al., 1998; Prave, 1999b; Walter et al., 2000; Brasier and Shields, 2000; Corsetti and Kaufman, in press).