RESPONSE OF LATE ORDOVICIAN GLOBAL OCEAN CIRCULATION AND OCEAN TEMPERATURE TO CHANGES IN SEA LEVEL, CONTINENTAL DRIFT, AND ATMOSPHERIC PCO2

We performed computer experiments using MOM2 to simulate global ocean circulation during two stages in the Late Ordovician (Caradocian, ~454 Ma; Ashgillian, ~446 Ma) under a range of atmospheric pCO₂ values (8-18x pre-industrial atmospheric level) and two stages of sea level (high and low).

Our results show that sea level change is the most important factor controlling the Late Ordovician surface ocean circulation. In particular, exposed shelf areas during low sea level narrowed and strengthened the Iapetus Current. Changes in paleogeography had only a minor effect on the surface circulation pattern. The most notable change occurred in the Iapetus Ocean: northward movement of Baltica in the Ashgillian narrowed the Iapetus Ocean, which led to a slower Iapetus Current. Atmospheric pCO₂ variations had almost no effect on the global ocean circulation pattern. The long-term cooling trend during the Late Ordovician and the associated development of temperate water conditions on equatorial North America can be explained by progressive cooling of the global ocean in response to lower levels of atmospheric pCO₂, coupled with sea level and paleogeographic changes.  However, despite its insignificance to the overall ocean circulation in the deep-ocean, atmospheric pCO₂ substantially affected surface and deep-water temperatures.  This is consistent with the interpretation that global cooling during the Late Ordovician resulted from a drawdown of pCO₂ due to the deposition of organic matter or by increased silicate weathering. 

Our simulations also suggest that meridional overturning was the main mechanism of poleward ocean heat transport during the Late Ordovician.  The response of the meridional overturning to changes in paleogeography, atmospheric pCO₂, and sea level is stronger than the response of the surface circulation to these perturbations.  In all simulations, a drop in sea level led to a reduction in poleward ocean heat transport.  A possible implication is that a positive feedback was responsible for enhanced global cooling caused by pre-glaciation sea level drop during the Late Ordovician.