Global Tectonic Setting and Climate of the Late Neoproterozoic: A Climate-Geochemical Coupled Study

Abstract

Whereas the snowball Earth hypothesis seems to account for most of the major fea tures of the Neoproterozoic glacial records, the causes that drove the Earth into a snowball state remain largely open to debate. Most of the mechanisms leading to the initiation of a snowball Earth are based on the existence of the unusual preponder ance of land masses in the tropics. However, the time of the youngest Neoprotero zoic glaciation is characterised by a rather widely distributed geography from low-to-high latitudes. In the absence of reliable knowledge of Neoproterozoic topog raphy, two series of coupled ocean-atmosphere climate model simulations were car ried out with a Late Neoproterozoic paleogeography (580 Ma) and solar luminosity reduced by 6% relative to today, the first one with flat continents and the second one with mountain ranges that mimic the Pan-African Orogen occurring at this time. Those climatic simulations coupled to the long-term carbon cycle have allowed to better constrain the atmospheric pC0 2 and the associated climate by the time of the youngest late Proterozoic glaciation. The Pan-African Orogen runs result in a snow accumulation pattern compatible with a regional-scale glaciation more less exten sive while the no relief runs do not succeed in initiating any glaciation. These results could give additional support to the inferences from many authors that some of the glacial deposits originally attributed to a snowball-like glaciation could in fact be the consequence of a more localised glaciation due to the important orogen occurring at the end of the Neoproterozoic.

Climates of the Earth and Cryosphere Evolution

“…Certainly, the most documented 'snowball Earth' episodes are those of the Neoproterozoic Era. They consist of three main glaciations: the two oldest ones, the Sturtian (715 Ma) and the Marinoan (635 Ma), are supposed to be global (Hoffman et al 1998;Hoffman and Schrag 2002), whereas the third and most recent one, the Rapitan glaciation (550 Ma), appears to be essentially regional and linked to the Appalachian uplift (Donnadieu et al 2004b). Such global glaciations result from a 'rapid' decrease in the atmospheric CO 2 content.…”