Evolution of nutricline dynamics in the equatorial Pacific during the late Pliocene

The tropics have played a central role in modulating Earth's climate throughout the Plio-Pleistocene, with tropical productivity fluctuations a key mechanism in the operation of the global carbon cycle and linkage of high-and low-latitude climates. Published records of tropical sea surface temperatures (SSTs) during the Plio-Pleistocene appear to vary primarily in tune with high-latitude climate on both orbital and secular timescales. However, contemporaneous changes in equatorial primary productivity are less well constrained, particularly at sites where climate is not dominated by upwelling or monsoon systems. Furthermore, the role of thermocline dynamics (tilt and mean depth changes) in forcing SST and productivity on orbital timescales remains uncertain. Here we report new, high-resolution calcareous nannofossil records from two Ocean Drilling Program sites in the western and eastern equatorial Pacific during marine isotope stages 95-101, about 2400-2600 kyr ago. Our records of paleoproductivity and nutricline depth reveal synchronous, large-amplitude glacial-interglacial productivity variations at both ends of the equatorial Pacific indicating (1) remote (high-latitude) forcing of primary productivity and (2) no primary role for east-west tilting of the equatorial Pacific thermocline, with important implications regarding the operation of El Nino-like dynamics in the Pliocene Pacific. Instead, the paleoproductivity variations and phase relationships that we document suggest the interaction of two mechanisms operating on obliquity timescales: a bottom-up forcing transmitted via the upwelling of high-latitude source waters in conjunction with the top-down forcing of atmospheric greenhouse gases.