Orbitally Forced Climatic Fluctuations in Snowball Earth: Compelling Evidence from a Data-Model Study

Abstract:

The Snowball Earth model provides a powerful conceptual framework for understanding the causes and demise of the extreme global glaciations that occurred during the Neoproterozoic, and can be refined through creative interactions between geological, geochemical and modeling studies. We present unequivocal evidence for oscillating glacier extent and varying hydrological conditions during the Marinoan, the second major Cryogenian glaciation (~650-635 Ma), from the Wilsonbreen Formation of NE Svalbard. Extensive exposures record (1) a possibly long arid periglacial phase; (2) multiple switches between subglacial, glaciolacustrine, carbonate lacustrine and non-glacial terrestrial deposition; and (3) rapid deglaciation. Oxygen and sulfur isotopes of sulfate in carbonate facies indicate persistently high pCO₂ consistent with deposition of the Formation on a 10⁵-year timescale.

Using an ice sheet model (GRISLI) coupled to a General Circulation Model (LMDz), the response of the continental ice-sheet to insolation changes (i.e. orbital forcing) and pCO₂ is investigated. We show that land-ice cover waxes and wanes over the course of a precession cycle for pCO₂ levels from 0.01 to 0.05 bar. Growth and retreat of ice reflect shifts in the balance between precipitation and evaporation (P-E), in response to migration of the tropical Hadley Cell. In cold conditions (<0.01bar), the weakness of the hydrological cycle makes the system insensitive to precession changes, explaining relatively stable ice-sheets during the earlier stages of the Snowball Earth. With 0.01<pCO₂<0.05bar, variations of the P-E pattern caused by insolation changes (orbital forcing) produces advances/retreats of the ice sheet in <10 kyr. With pCO₂>0.05bar, the albedo effect is overcome and air temperatures rise in ice free regions in low latitudes and deglaciation occurs. The results show that the Snowball Earth model can be reconciled with evidence for prolonged, pulsed glacial deposition. The main, longest phase of the glacial epoch appears to have been cold and arid with a limited hydrological cycle, but deglaciation was not a simple switch from icehouse to greenhouse. Instead, there was a climate transition during which glacial cycles could be forced by Milankovitch orbital variations.