PRELUDE TO SNOWBALL EARTH

An outstanding question regarding Neoproterozoic ice ages is what conditions or events initiated the cooling that led to snowball Earth. Whereas a low-latitude distribution of continents likely set the stage for glaciation, a triggering mechanism must still be identified. If we assume that the carbon cycle and climate were intimately linked during this period, then any mechanism must be consistent with the pattern of marine d¹³C variation leading into glaciation.

In the Otavi Group of northwestern Namibia, a well documented pre-glacial excursion occurs in the Ombaatjie Fm. (below the Ghaub diamictite): d¹³C drops ca. + 6 to 8 to -5 to 7 per mil over a span of 30-40 m of section, after which it recovers to -2 to -3 per mil immediately below the sub-glacial surface. A very similar trend (but of slightly less magnitude) occurs in the Elbobreen Fm., below the older of two glacial deposits in the Polarisbreen group of northeastern Svalbard. In the Adelaide Fold-Thrust Belt in South Australia, d¹³C declines by 15 per mil through the Enorama and Trezona formations before recovering to ca. -3 per mil below the glacigenic Elatina Formation (McKirdy et al., 2001).

Documentation of this isotopic trend in three separate basins challenges the popular hypothesis that Neoproterozoic glaciations resulted from the drawdown of atmospheric CO₂ by a period of extensive organic carbon burial (as signaled by highly ¹³C-enriched interglacial carbonates). We propose that the pre-glacial d¹³C profile, which is analogous to but of greater magnitude and probably longer time scale than that spanning the Paleocene/Eocene boundary, is best explained by a protracted methane release. Global warming, which would result from an increase in greenhouse forcing, at first appears counterintuitive in light of impending glaciation. However, if the methane release persisted over a sufficient time scale for silicate weathering to respond, then atmospheric CO₂ would be drawn down, leaving the earth vulnerable to a greenhouse gas crisis if the methane flux ceased.