Testing for oceanographic controls on Neogene TEX86 temperatures in the Eastern Equatorial Atlantic

Abstract

The TEX86 proxy is a frequently used tool in paleoclimate studies. While TEX86 temperatures are often interpreted as sea surface temperatures (SSTs), the oceanographic controls on TEX86 are still highly debated. The input of non-locally produced isoGDGTs, enhanced activity of methanotrophic archaea, the variable production depth of isoGDGTs and/or enhanced upwelling may alter the TEX86 signal that is ultimately retrieved from the sedimentary archive. Here, Late Miocene to Early Pliocene TEX86^H temperatures from the eastern equatorial Atlantic are reconstructed and tested for oceanographic controls using CCSM4/CESM1 model output. TEX86^H values are indicative of a more than 10 °C decrease in SST from the Late Miocene to Early Pliocene, larger than can be expected from surface water cooling alone. An abrupt fall in SST (3 °C) around 5.0 million years (Ma) marks a change in the signature of TEX86. From the model output, I find contradictory evidence for the reinforcement of coastal upwelling at this time. While both proxy data and model output indicate a deepening and weakening of the thermocline and a strengthening of the countercurrent system, the models show no sign of excessive surface water cooling as would be expected with the onset of strong upwelling. This may suggest upwelling is not the major factor controlling TEX86 temperatures in the eastern equatorial Atlantic. Other processes, such as archaeal community changes, instead may underlie the change in the TEX86 signal.