AN OCEANIC GATEWAY HYPOTHESIS FOR THE LATE PALEOZOIC ICE AGE

The major period of ice sheet advance during the Late Paleozoic Ice Age appears to have occurred in the Chesterian (late Visean-Namurian) based on evidence from Gondwanan tillites, and sequence stratigraphic and chemostratigraphic analysis in Euramerica. This advance followed a latest Devonian and early Mississippian (Kinderhookian-Meramecian) period characterized by minor and discontinuous glaciers, mainly known from South American basins. Based on this pattern of ice buildup, it would appear that gradual drawdown of atmospheric carbon dioxide cooled the Earth to a critical point, but a source of high-latitude moisture was lacking until the Chesterian. The relative importance of oceanic circulation changes related to gateway closure versus carbon dioxide drawdown in part relates to the interpretation and biostratigraphic dating of carbon isotope trends in Carboniferous carbonates. Carbon isotope values in European successions show a dramatic increase to a Phanerozoic maximum of ~+6‰ across the mid-Carboniferous boundary, which has been interpreted to reflect a period of globally enhanced organic carbon burial and carbon dioxide drawdown associated with the widespread Pennsylvanian coal basins. However, new data from the Arrow Canyon Range in Nevada do not show an equally large increase at this time and raises questions concerning the true magnitude of organic carbon burial. This micrite-based curve is in good agreement with the biostratigraphically-calibrated brachiopod calcite curve of Mii et al. (1999) from the midcontinent of North America. An alternative hypothesis for the European shift is linked to the closing of the oceanic gateway between Euramerica and Gondwana associated with the assembly of Pangea. This gateway closure increased the transport of heat and moisture to high southern latitudes of Gondwana, resulting in widespread glacial advance and eustatic drop. The formation of the land bridge also may have had profound biological consequences, triggering widespread marine extinctions and leading to the development of a distinct Tethyan faunal realm in the Chesterian.