Early Pliocene deepening of the tropical Atlantic thermocline

Abstract. The tropical thermocline may have played a crucial role in maintaining weaker sea surface temperature gradients during the early Pliocene and in the onset of late Pliocene northern hemisphere glaciation. Whereas the Pliocene Pacific thermocline evolution is well documented, complete records of Pliocene tropical Atlantic thermocline depths are limited to the Caribbean region. Here, we use the oxygen isotope gradient between surface to subsurface dwelling planktic foraminifera from Ocean Drilling Program Site 959 in the eastern equatorial Atlantic to track vertical changes in thermocline depth over the course of the Pliocene. This record shows that eastern equatorial Atlantic thermocline depth varied substantially during the early Pliocene, before finally deepening abruptly around 4.5 Ma to remain relatively stable until at least 2.8 Ma. Eastern equatorial Atlantic and Caribbean records are almost identical, suggesting a common control on the sudden step-wise thermocline deepening across the basin, in contrast to previous assumptions. The Pliocene evolution of the tropical Atlantic thermocline differs is remarkably from the Pacific, which is characterized by gradual basin-wide shoaling. It remains unclear what mechanisms were involved in the dichotomous thermocline evolutions. Whereas Central American Seaway closure may have shoaled the Pacific thermocline, it is not yet understood if and how this process may have deepened the Atlantic thermocline. A divergent evolution of temperatures of the source regions may explain the opposite thermocline developments observed, possibly amplified by a positive feedback loop involving tropical cyclone intensity.