IS IT POSSIBLE TO RECONSTRUCT RATES OF OCEANIC OVERTURNING CIRCULATION DURING THE EARLY PALEOGENE?

The modern mode of oceanic meridional overturning circulation (MOC) is responsible for a significant contribution of poleward heat transport, and changes in the pattern and strength of the MOC potentially played a large role in past climate variations. The Late Cretaceous – Early Paleogene greenhouse climate interval provides a unique opportunity to investigate the relationships among the mode of MOC, poleward heat transport, and oceanic thermal structure.

Neodymium (Nd) isotope data from Pacific Ocean deep-sea drill cores and Fe-Mn crusts spanning the interval ~70 to 30 Mya reveal a geographic gradient in deep-water mass composition with high values in the north and low values in the south. This pattern supports convection in both the South and North Pacific, with mixing in the low latitudes. Nd isotope data also suggest that the MOC in the Pacific was separate from that in the Atlantic. Fully coupled GCM simulations produce this mode of MOC, with Pacific deep waters “aging” from high to low latitudes. Ocean-only GCM simulations employing a range of boundary conditions reproduce the Nd isotope gradient. The best model-data match results from imposing strong vertical mixing within the water column, and the condition of strong vertical mixing results in enhanced oceanic heat transport. Thus poleward oceanic heat transport may have contributed significantly to warm high latitude sea surface temperatures.

To further refine the MOC reconstruction, we generated Pb isotope data from Pacific drill sites. The seawater Pb isotopic composition recorded in the subtropical North Pacific and subtropical South Pacific suggests that the dissolved Pb isotopic composition at the seafloor in both regions was dominated by dissolution of dust and/or ash that accumulated in the regions. However, Nd isotope values recorded by the same samples indicate the signature of North Pacific Deep Water (NPDW) at DSDP Site 464 and South Pacific Deep Water (SPDW) at DSDP Site 596, suggesting that the “age” of NPDW and SPDW had exceeded the residence time of Pb (~<200 years) by the time each water mass reached the respective subtropical regions. These deep-water aging estimates of ~>200 years are consistent with ages from the fully-coupled GCM simulations, suggesting that pairing the Nd and Pb tracers may provide qualitative rates of deep-water circulation.