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 latitudes than during the Pleistocene. Deglaciation and breakup of the supercontinent were followed by the first appearance/expansion of metazoans. Herein we report results from a seasonal climate model that clarify 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 depends on choice of paleogeographic reconstruction. If the supercontinent was located entirely in low latitudes, the freezing line changes would not be enough to trigger glaciation on land. However, the luminosity changes are much more important if the supercontinent extended into midlatitudes (approximately 50-degrees paleolatitude). Such a configuration has extensive summer snowcover and would provide a ''seed'' area 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. Such a response could reconcile models and geologic data, but the reconciliation would critically depend on a more precise definition of low latitude 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 midlatitude supercontinent into lower latitudes. The deglaciation could have been associated with a sea level rise of as much as 250-300 m, creating a much larger area for habitat occupation by benthic biota. Although more work is required on this topic, our results could explain both glaciation and deglaciation, with the explanation critically dependent on choice of paleogeographic reconstruction and more precise descriptions of late Precambrian ice sheet locations.