Response of Late Ordovician paleoceanography to changes in sea level, continental drift, and atmospheric pCO ₂: Potential causes for long-term cooling and glaciation

We performed sensitivity experiments using an ocean general circulation model at two stages of the Late Ordovician (Caradoc, ∼454 Ma; Ashgill, ∼446 Ma) under a range of atmospheric pCO ₂ values (8-18× PAL; pre-industrial atmospheric level) at high and low sea level. The model results indicate that the long-term cooling trend during the Late Ordovician can be explained by progressive cooling of the global ocean in response to falling levels of atmospheric pCO ₂, sea level change, and paleogeographic change. These results also explain the occurrence of low latitude cool-water carbonates in North America. In all simulations, a drop in sea level led to a reduction in poleward ocean heat transport. This indicates a possible positive feedback that could have enhanced global cooling in response to sea level drop during the Late Ordovician. Alterations in poleward ocean heat transport linked to changes of atmospheric pCO ₂ also indicate that there is a threshold of 10× PAL, above which ocean current change cannot be responsible for glaciation in the Late Ordovician. Continental drift could explain the observed global cooling trend in the Late Ordovician through a combined poleward ocean heat transport feedback and increased ice-albedo effect if atmospheric pCO ₂ was low during the entire Late Ordovician. The model results further indicate that the response of meridional overturning to changes in paleogeography, atmospheric pCO ₂, and sea level is stronger than the response of surface circulation to these perturbations. Because the overturning circulation is so strong, meridional overturning was the dominant mechanism for described changes in heat transport in the Late Ordovician.