BIOLOGICALLY ENHANCED WEATHERING AS A CAUSE FOR NEOPROTEROZOIC SNOWBALL EARTH

The Cryogenian Period (717-635 Ma), or “Snowball Earth”, was an unusually cool period of Earth history when glaciers extended to low latitudes. Past ideas on causes of this widespread glaciation include increased consumption of atmospheric carbon dioxide by silicate weathering due to continental drift into tropical paleolatitudes, or by voluminous, easily-weathered volcanic tuffs. Alternatively, carbon sequestration from the atmosphere may have been intensified by advances in biomass on land or at sea. These hypotheses are tested with Tonian paleosols of the Johnnys Creek Formation (785-717 Ma) in Ellery Creek, central Australia, and of the Chuar Group (776-729 Ma) in Nankoweap Creek, Arizona. Paleomagnetic studies indicate little drift from a paleolatitude of 26.2^o during accumulation of the Johnnys Creek Formation but drift from 64 to 10^oN during deposition of the Chuar Group. Neither section has abundant easily weathered volcanic ash, but there are local basalt flows in Australia. In both Australia and Arizona, paleoproductivity of the paleosols increases up section, as estimated in ppm respired soil CO₂ from depth in paleosols to gypsic (By) and then calcic (Bk) horizons. Deepening and intensification of soil respiration reflects greater terrestrial carbon sequestration, and increased chemical weathering up section, drawing down atmospheric CO₂. Comparable transition from gypsic to calcic soils in modern deserts reflects change from cyanobacterial-gypsic to fungal-algal calcic soil microbiomes. This is supported by discovery of fungal and other microfossils in the Arizona paleosols and possible vendobiont megafossils in the Australian paleosols. Snowball Earth glaciation may have been induced by evolutionary advances to eukaryotic and multicellular life on land, in the same way as Ordovician glaciation was induced by land plants, Permo-Carboniferous glaciation by trees, and Pleistocene glaciation by grasslands.