Early Palaeogene temperature evolution of the southwest Pacific Ocean

Relative to the present day, meridional temperature gradients in the Early Eocene age (similar to 56-53 Myr ago) were unusually low, with slightly warmer equatorial regions(1) but with much warmer subtropical Arctic(2) and mid-latitude(3) climates. By the end of the Eocene epoch (similar to 34 Myr ago), the first major Antarctic ice sheets had appeared(4,5), suggesting that major cooling had taken place. Yet the global transition into this icehouse climate remains poorly constrained, as only a few temperature records are available portraying the Cenozoic climatic evolution of the high southern latitudes. Here we present a uniquely continuous and chronostratigraphically well-calibrated TEX86 record of sea surface temperature (SST) from an ocean sediment core in the East Tasman Plateau (palaeolatitude similar to 65 degrees S). We show that southwest Pacific SSTs rose above presentday tropical values (to similar to 34 degrees C) during the Early Eocene age (similar to 53 Myr ago) and had gradually decreased to about 21 degrees C by the early Late Eocene age (similar to 36 Myr ago). Our results imply that there was almost no latitudinal SST gradient between subequatorial and subpolar regions during the Early Eocene age (55-50 Myr ago). Thereafter, the latitudinal gradient markedly increased. In theory, if Eocene cooling was largely driven by a decrease in atmospheric greenhouse gas concentration(6), additional processes are required to explain the relative stability of tropical SSTs given that there was more significant cooling at higher latitudes.