Earth Sciences Lecture: Paul Hoffman (Harvard University)

Geological observations informed by climate dynamics imply that the oceans were 99.9% covered by light-blocking ice shelves during two discrete, self-reversing, Snowball episodes spanning a combined 60−70 Myr of the Cryogenian Period (720−635 Ma). Optically-thin sea ice occupying 0.1% of ocean area has been invoked to account for fossil marine phototrophs, including macroscopic multicellular eukaryotes (of different taxa) before and after each Snowball episode. Ecosystem relocation is a scenario not requiring thin marine ice. Assume that long before Cryogenian Snowballs, diverse supra- and periglacial biomes were established in polar−alpine regions. At Snowball onsets, those biomes migrated in step with the ice margins to the equatorial zone of net sublimation. There they prospered and evolved, their habitat areas expanded and seasonality reduced. Nutrients were supplied by dust (loess) derived from cozonal ablative lands, where surface winds were strong. When each Snowball finally ended, those biomes were mostly inundated by the meltwater-dominated and rapidly-warming lid of a nutrient-rich but depauperate ocean. Some taxa returned to the mountaintops while others restocked the oceans. This scenario is consistent with the phylogenetically inferred freshwater ancestry of modern marine primary producers, the two-fold division of all green plants, and Cryogenian gene reduction that limits the meridional range of the picocyanobacterium Prochlorococcus. It implies that the extant surface biosphere evolved from a polar−alpine subset of pre-Cryogenian biotic diversity.