Why was there a Neoproterozoic Snowball Earth?

Abstract

Snowball Earth is a nickname for the Cryogenian Period, 717 to 635 million years ago, when glaciers extended well into tropical latitudes. Such extreme planetary chilling has been blamed on increased carbon sequestration by life in the sea, by chemical weathering of volcanic rocks, and by life on land. Discovery of pre-Cryogenian paleosols containing a diverse eukaryotic soil microbiome including fungi and testate amoebae suggests that glacial expansion was caused by increased carbon consumption due to evolutionary advances in life on land. Microfossils in paleosols of the 776–729 Ma Chuar Group of Arizona include cyanobacterial microfossils (Siphonophycus typicum and Polytrichoides linearis), fungal hyphae (Tortotubus protuberans and Mucorites ripheicus), fungal vesicles (Palaeoglomus strotheri), and testate amoebae (Cycliocyrillium torquatum, Trigonocyrillium horodyskii, Melanocyrillium hexadiadema, Bonneia dachrychares, and Carabinia granosa). Also found were fragments of enigmatic lichen-like, multicellular thalli described here as Porterra dehlerae gen. et sp. nov. A few fossils by themselves did not change the world, but Chuar and other paleosols reveal transition from low productivity Gypsids to high productivity Calcids. Increased biomass, soil respiration, and nutrient depletion documented from chemical analyses of paleosols accelerated silicate weathering and carbon consumption, diminishing CO2 greenhouse, and ushering in Snowball Earth.