EFFECT OF DECREASED SOLAR LUMINOSITY ON LATE PRECAMBRIAN ICE EXTENT

The latest Precambrian (approximately 0.57 Ga) was marked by extensive glaciation on a supercontinent. Ice cover may have been in lower lati tudes than during the Pleistocene. Deglaciation and breakup of the sup ercontinent were followed by the first appearance/expansion of metazoa ns. Herein we report results from a seasonal climate model that clarif y some of the processes operating during this important time interval. We demonstrate that, because solar luminosity was about 6% less than present, the modeled snowline was approximately 15-degrees equatorward of its modeled Pleistocene limit. The significance of this response d epends on choice of paleogeographic reconstruction. If the supercontin ent was located entirely in low latitudes, the freezing line changes w ould not be enough to trigger glaciation on land. However, the luminos ity changes are much more important if the supercontinent extended int o midlatitudes (approximately 50-degrees paleolatitude). Such a config uration has extensive summer snowcover and would provide a ''seed'' ar ea for ice growth into lower latitudes. We postulate that if the large snowline changes we simulate were coupled to an ice sheet model, the ice margin could have reached to within 25-degrees of the equator. Suc h a response could reconcile models and geologic data, but the reconci liation would critically depend on a more precise definition of low la titude glaciation, that is, whether the ice was at 25-30-degrees or 0- degrees latitude. Additional simulations for one Precambrian/Cambrian boundary reconstruction (approximately 0.54 Ga) suggest that reduction in late Precambrian snow cover might simply reflect movement of a mid latitude supercontinent into lower latitudes. The deglaciation could h ave been associated with a sea level rise of as much as 250-300 m, cre ating a much larger area for habitat occupation by benthic biota. Alth ough more work is required on this topic, our results could explain bo th glaciation and deglaciation, with the explanation critically depend ent on choice of paleogeographic reconstruction and more precise descr iptions of late Precambrian ice sheet locations.