ORBITALLY FORCED CLIMATIC FLUCTUATIONS IN SNOWBALL EARTH: COMPELLING EVIDENCE FROM A DATA-MODEL STUDY

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-lived arid periglacial phase; (2) multiple switches between subglacial, glaciolacustrine, carbonate lacustrine and non-glacial terrestrial deposition; and (3) rapid deglaciation. Oxygen and sulfur isotopes in carbonate facies indicate persistently high pCO₂.

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, surface temperature of ice free regions in low latitudes overcomes the albedo effect, and deglaciation is initiated. The results show that the Snowball Earth model can be reconciled with evidence for prolonged, pulsed glacial deposition. Although the main, longest phase of the glacial epoch appears to have been cold and arid with a limited hydrological cycle, deglaciation was not a simple switch from icehouse to greenhouse but was characterized by a climate transition during which glacial cycles could be forced by Milankovitch orbital variations.