Speaker: Paul F. Hoffman, Department of Earth and Planetary Sciences, Harvard University and School of Earth and Ocean Sciences, University of Victoria (British Columbia)

The Earth was almost entirely covered by grounded and floating glaciers for most of the Cryogenian period (717 to 635 Ma). The glaciation was interrupted by a nonglacial intermission between about 665 and 650 Ma. The older and longer (~52 myr) ‘Sturtian’ glaciation was accompanied by deposition of sedimentary Fe and Fe-Mn ores, and postglacial ‘cap’ carbonate formed only after complete deglaciation. Iron isotope profiles point to a ferrous subglacial ocean with oxic plumes of meteoric glacial meltwater. Ice-sheet interiors and outlets were warm-base because of low accumulation rates. The cap carbonate of the younger and briefer (<15 myr) ‘Marinoan’ glaciation records nearly the full deglaciation as well as the postglacial aftermath. Boron, carbon, triple-oxygen and calcium isotope profiles are consistent with extreme atmospheric CO2 concentrations and ocean acidification during Marinoan glaciation, and severe temperature changes and transient ocean stratification during deglaciation. The occurrence of wave-generated bedforms within glacial sequences in some areas is compatible with the co-occurrence of open waters and tidewater glaciers during deglaciation. Geological and geochemical observations favour a pair of CO2 hysteresis loops coupled to reversible planetary ice-cap albedo instability. The cause of the Cryogenian glacial disasters appears to have been the migration of supercontinent Rodinia to the paleoequator, its intratropical fragmentation, and the eruption of a 2200-km-diameter flood-basalt plateau at ~720 Ma, squarely across the paleoequator on the windward (now Arctic) margin of Laurentia. A third recurrence was averted by true polar wander, which rotated the girdle of continents far out of the equatorial plane. As global climate states, Cryogenian glaciations and their greenhouse aftermaths are unique in the last 2 gyr for which there is an extensive sedimentary record.

Paul Hoffman speaker was born in Toronto and studied geology at McMaster and Johns Hopkins universities. As a Research Scientist with the Geological Survey of Canada, he mapped Paleoproterozoic sedimentary basins and orogenic belts in the northwest of the Canadian Shield, synthesized the development of cratonic North America in terms of plate tectonics, and speculated on the recurrence of supercontinents. Since moving to Harvard University, he and his students have studied Neoproterozoic marine carbonate successions globally, providing support for the Snowball Earth theory of Cryogenian glaciation. He has received major awards from leading national and international scientific organizations in Europe and North America. Retired, he now lives and writes in Victoria, British Columbia.

Attendance is free and all are welcome but space will be limited so you are advised to come early.