Version du 10 janvier 2017 à 14:01

Cryoconite ponds on Snowball Earth: implications for Cryogenian organic productivity, atmospheric oxygenation and microbial evolution Paul F. Hoffman, Department of Earth & Planetary Sciences, Harvard University (Emeritus) School of Earth & Ocean Sciences, University of Victoria (Adjunct) Body fossils indicate that some eukaryotic crown-group autotrophs and heterotrophs evolved before the 58-Myr Sturtian (717-659 Ma) low-latitude glaciation and accordingly survived it. Biomarkers imply that demosponges evolved before the subsequent Marinoan (645-635 Ma) low-latitude glaciation. However, geological observations pertaining to both glaciations are most consilient with coupled GCM climate simulations if it is assumed that the world ocean was covered by a thick dynamic ice shelf (‘sea glacier’) from pole to pole. Where on a frozen planet did eukaryotes and metazoans survive? Did the evolutionary experience impart a genetic imprint on modern organisms? Geology and GCMs agree that while all continents bore ice sheets when the ocean was frozen, areas of bare ground (and low albedo) always existed in the equatorial zone where net ablation occurred (due to the effect on the annual mean Hadley circulation of the low thermal inertia of the solid surface). These ice-free areas were sources of dust, produced by glacial trituration. Dust and volcanic ash was continually advected to the ablation zone of the sea glacier, where it accumulated on the ice surface as cryoconite (ice dust). Cryoconite absorbs solar energy and sinks to an equilibrium depth (<1.0 m) in ice, while maintaining a column of meltwater, capped nightly at low latitude by clear ice. Even with conservative dust fluxes, cryoconite accumulation would have saturated the ablation surface of a Snowball Earth, resulting in networks of meltwater ponds over ~12% of global surface area. Such ponds were habitable by cyanobacteria, fungi, green algae, protozoans and metazoans adapted for freshwater. Flushing of excess meltwater through moulins created a stabilizing climate feedback by removing cryoconite and raising the surface albedo. Modern cryoconite contains ~10 wt % organic matter (mostly extracellular cyanobacterial polysaccharide), so cryoconite removal by meltwater flushing was also a means of burying organic matter and producing atmospheric O2. A small positive net balance in O2 production over consumption by oxidation of reduced volcanic gases could have generated large surpluses given the duration of Snowball Earth events. Fossil biomarkers suggest that green algae replaced red algae as the dominant eukaryotic primary producers after the Sturtian glaciation, and molecular-divergence chronology allows for major groups of modern marine planktonic cyanobacteria to be derived from inhabitants of cryoconite ponds on a Cryogenian Snowball Earth.