CHANGES IN PALEOGEOGRAPHY AND SEA LEVEL AS DRIVERS FOR GLOBAL COOLING DURING THE LATE ORDOVICIAN

The glaciation at the end of the Ordovician has attracted attention not only because of its association with a mass extinction, but also because of its short duration and its underlying causes.  Recent research has focused mainly on the drawdown of atmospheric pCO₂ in order to explain global cooling during this time period. However, our results from atmospheric general circulation model (AGCM) simulations indicate that paleogeographic evolution and sea level change alone might have pushed the global climate over a threshold toward glaciation.

In order to determine the importance of paleogeographic changes (e.g. southward movement of Gondwana) and different shoreline positions (high and low sea level) on the timing of the glaciation, we performed sensitivity experiments with an AGCM (GENESIS v2.0) on two stages of the Late Ordovician (Caradocian, ~454 Ma; Ashgillian, ~446 Ma) under a range of atmospheric pCO₂ levels (18x, 15x, 10x, and 8x PAL).  The model results indicate that under the prescribed boundary conditions, the paleogeographic changes coupled with an ice-albedo feedback were essential pre-conditioning events for glaciation.  For pCO₂ levels greater than 10x PAL, the Caradocian experiments yielded higher annual mean temperatures than Ashgillian experiments and remained virtually free of extensive snow cover and global mean temperatures stayed above freezing with a high sea level.  However, with a lower sea level and thus an increased land area, global mean temperatures drop significantly for both paleogeographies and glaciation can occur also for Caradocian simulations.  The combination of the southward movement of Gondwana and sea level change could have resulted in breaching a threshold for the initiation of the glaciation.