Deep ocean circulation through the Icehouse World

Climate change since the Pliocene "greenhouse" world has been in the direction cooler temperatures, increasing ice sheet presence and higher amplitude climate fluctuations. While a steady decrease in atmospheric CO2 concentrations is thought to be a main control on this trend, other factors, such as ocean circulation, may have been instrumental in driving final transitions between climate states. For example, the final closure of the Central American Seaway (CAS) is believed to have influenced Pliocene climate through its impact on Atlantic meridional overturning circulation. The effect of CAS closure on deep-ocean circulation, however, has been poorly tested. We present new, orbitally tuned, oxygen and carbon isotope records from ODP Site 1264 (2.5km depth) in the SE Atlantic, which document changes in deep ocean circulation over the past ~5myrs. This site is currently bathed in upper North Atlantic Deep Water (uNADW). The new d13C record fails to show the predicted trend of enhanced deep-ocean ventilation across the final stages of CAS closure. Comparison with other records from the Atlantic suggests that controls, other than increased NADW formation, are needed to explain the d13C differences through time. Potential factors may include changes to preformed d13C values at sites of deep water formation or locally changing water mass distribution patterns.