Reconstructing thermocline depth in the south Atlantic subtropical gyre during the Pliocene-Pleistocene

Abstract

During the early Pliocene, 3-5 million years ago (Ma), global average temperatures were 3^-°C greater and sea level was ~35 meters higher than today while, climate forcings such as pC0 2 were effectively the same. A current hypothesis is that a globally shoaling thermocline at ~3 Ma contributed to the climate transition to the cooler Pleistocene. This study presents records of subsurface temperature and salinity from Ocean Drilling Program (ODP) site 1264, located in the south Atlantic subtropical gyre. Mg/Ca-derived temperature estimates from G. tumida and G. crassaformis, show no long-term trend over the last 4 Ma. This implies that the observed trend towards lighter 1 8 6 Ocaicite data is due to a freshening of the subsurface. These results are consistent with G. sacculifer surface 6 Ocaicite and Mg/Ca-derived temperature records from this site and are inconsistent with the hypothesis that the thermocline shoaled globally over the last 4 Ma. We conclude that increased Agulhas leakage during the Pliocene brought relatively salty water to ODP site 1264, and that a decrease in Agulhas leakage is responsible for the freshening of the water through the last 4 Ma.