CRYOCONITE PANS ON SNOWBALL EARTH: VAST SUPRAGLACIAL HABITATS FOR CRYOGENIAN BIOTIC EVOLUTION (Invited Presentation)

Combined U-Pb and Re-Os dating and cap-carbonate geochemistry imply that the oceans were covered by sea glaciers from 717 to 659 Ma (Sturtian), and again from ~645 to 635 Ma (Marinoan). Yet, micropaleontology, sterane biomarkers in bitumens and oils, and molecular clocks indicate that certain crown-group eukaryotic algae and protistans evolved before the Sturtian, and that a metazoan crown-group (demosponges) evolved before the Marinoan. Sterane biomarkers suggest further that a switch from red to green algae as the dominant eukaryotic primary producers occurred during the brief nonglacial interlude. Where on Snowball Earth did these clades survive and evolve? Recent atmosphere–ice sheet GCM results imply that Snowball continents remain partly ice-free at all CO₂ levels, and that tropical ice-sheet margins migrate under precessional forcing. This suggests a dust flux to the sea glacier comparable to that off present Antarctica, generated by glacial action and katabatic winds. Sea-glacier flow will advect most of the dust to the equatorial ablation zone, with accumulation rates of order 4-40 m Myr^-1. At first, when CO₂ is low, dust is not retained on the cold surface but is recycled back to the firn by wind. As ablative surfaces warm, dust starts to accumulate, creating cryoconite holes—clumps of dark dust suffused with organic matter that sink to an equilibrium depth and generate meltwater by Solar absorbtion. Modern cryoconite holes are inhabited by cyanobacteria, green algae, fungi, protists and certain metazoans. On Snowball Earth, the ablation surface will saturate with cryconite, creating interconnected pans of meltwater up to 60 million km² in potential area. Excess meltwater production generates drainages that flush into the subglacial ocean through moulins. They cleanse the ice surface of cryoconite, creating a stabilizing feedback. Why basal metazoans are stenohaline is paradoxical—cryoconite pans were fresh to brackish and the subglacial ocean was hypersaline. Flushing conduits were trophically attractive to filter feeders and had intermediate but fluctuating salinity. The severest bottlenecks were Snowball onsets, when ablative surfaces were too cold for dust retention. Eukaryotic survival may then have relied on hydrothermal areas and thin sea ice near dark snow-free land areas.