Late Ordovician Ocean-Climate System and paleobiogeography

by Herrmann, Achim D.

iii The Ordovician was a time of extensive diversification and radiation of marine life. The end of the Ordovician is marked by a major mass extinction that is generally attributed to environmental perturbations associated with an extensive yet short-lived glaciation. The understanding of the climate dynamics during this crucial time period for the evolution of life is still fragmental. I used an atmospheric general circulation model (AGCM) and an ocean general circulation model (OGCM) to study the climate system in the Caradoc (~454 Ma) and the Ashgill (~545 Ma). Specifically, I investigated the response to changes in paleogeography, atmospheric pCO2, solar insolation cycles (obliquity), poleward ocean heat transport, and sea level. I also used a 3-dimensional ice sheet model to explore the necessary boundary conditions for ice sheet formation. The AGCM results indicate that, assuming that pCO2 did not fall below 8x PAL (a minimum value for this time period), a drop in pCO2 and the paleogeographic evolution can only be regarded as preconditioning factors in the glaciation. In order for ice sheets to form, other factors must have changed such as a drop in sea level from its generally high Late Ordovician levels and/or a reduction in poleward ocean heat transport. In all OGCM simulations, a drop in sea level led to a reduction in poleward ocean heat transport. This indicates a possible positive feedback that could have led to enhanced global cooling in response to pre-glaciation sea level drop. Continental drift could explain the observed global cooling trend in the Late Ordovician through a combined reduction in poleward ocean heat transport and increased ice-albedo effect. The ocean-climate system was also dominated by strong latitudinal temperature gradients and vigorous horizontal and vertical ocean circulation. Finally, I compared the paleobiogeography of different taxonomic groups to the results of the climate models. The spatial distribution of Caradocian marine organisms is consistent with climatic and oceanographic gradients inferred from coupled oceanclimate models. The paleobiogeographic data thus provide an important evaluation of the global ocean-climate models and lead to a more robust inference of the early Late Ordovician global ecosystem.