NEOGENE EVOLUTION OF ATLANTIC DEEP-OCEAN CIRCULATION FROM WALVIS RIDGE ODP SITES 1262 AND 1264

Ocean circulation acts as a moderator for global climate by transporting heat from low to the higher latitudes. During the warm greenhouse climate of the early Cenozoic, deep-water circulation in the Atlantic was characterized by downwelling in the Southern Ocean. Global cooling and tectonic alteration of ocean basins modified this mode of circulation to the modern bimodal system in which downwelling occurs in the North Atlantic (forming North Atlantic Deep Water – NADW) as well as in the Southern Ocean. Previous studies have suggested that the evolution to the modern mode occurred in two steps. The first step of deep-water convection has been identified as taking place in the Nordic Seas during the early Oligocene. The second step in the evolution of North Atlantic Deep Water formation was a resultof the addition of unradiogenic water from the Labrador Sea ~4 Ma.

Here we examine the Neogene evolution of the water column by reconstructing the water mass Neodymium isotopic composition at South Atlantic ODP Sites 1264 and 1262. These two sites form a depth-transect on Walvis Ridge, and the initial data suggests that the lower deep waters in the region (~4000m) were influenced by Labrador Sea convection earlier than the upper deep waters (~2500m). New Site 1262 data indicate the contribution of water mass from the Labrador Sea to the base of North Atlantic Deep Water occurred around 4.4 Ma. The influence on water mass from the Labrador Sea may have been a consequence of global cooling and the formation permanent ice sheets at the poles.