The late Pliocene Benguela upwelling status revisited by means of multiple temperature proxies

As compared to the late Pleistocene, Alkenone-based sea surface temperature (SST) in the Benguela region revealed relatively warm and stable SST recorded between approximate to 3.5 and 2.0 Ma, and coincide with a period of increasing biological productivity as revealed by increasing deposition of biogenic opal. We assess how the hydrological patterns recorded in SST proxies are embedded in the geological record by performing a proxy-proxy comparison. We used Laser-Ablation Inductively Coupled Plasma-Mass Spectrometry to measure the Mg/Ca on the planktonic foraminifera species Globigerina bulloides, allowing in situ measurements of Mg/Ca on individual foraminiferal tests. Mg/Ca-derived temperatures provide much colder temperatures than alkenone-derived SST by up to 10 degrees C. We build a scenario involving contrasting sensitivities of paleothermometers upon the annual cycle, namely alkenones preferentially capturing SST when the surface ocean is warmer than the mean-annual average SST, and G. bulloides capturing SST when upwelling intensifies. Multichamber analysis also suggests that G. bulloides migrates below the sea surface while calcifying its last chambers prior to gametogenesis, allowing the extraction of both surface and subsurface temperature from Mg/Ca measured on different chambers. The range of temperatures recorded between our multiple SST proxies is supported by the range of temperatures simulated with a general circulation model when different seasons, different water depth and different orbital configurations occurring during the late Pliocene are considered. A greater seasonal cycle in SST during the Pliocene can account for alkenone and Mg/Ca-derived temperature contrast, pointing to a radically different mode of upwelling activity in the Benguela region compared to today.