Polar front shift and atmospheric CO2 during the glacial maximum of the Early Paleozoic Icehouse

Abstract : Our new data address the paradox of Late Ordovician glaciation under supposedly high pCO2 (8 to 22X PAL: preindustrial atmospheric level). The paleobiogeographical distribution of chitinozoan (¡°mixed layer¡±) marine zooplankton biotopes for the Hirnantian glacial maximum (440 Ma) are reconstructed and compared to those from the Sandbian (460 Ma): They demonstrate a steeper latitudinal temperature gradient and an equatorwards shift of the Polar Front through time from 55¡ã¨C70¡ã S to ¡"40¡ã S. These changes are comparable to those during Pleistocene interglacial-glacial cycles. In comparison with the Pleistocene, we hypothesize a significant decline in mean global temperature from the Sandbian to Hirnantian, proportional with a fall in pCO2 from a modeled Sandbian level of ~8 X PAL to ~ 5 X PAL during the Hirnantian. Our data suggest that a compression of midlatitudinal biotopes and ecospace in response to the developing glaciation was a likely cause of the end-Ordovician mass extinction.