Orbital forcing of the eastern equatorial Pacific thermocline in the late Pleistocene

Eastern equatorial Pacific thermocline depth is an important component of the tropical Pacific's coupled ocean-atmosphere system and varies across El Niño and La Niña events. To investigate the influence of orbital forcing, we present a paleo-record of thermocline depth throughout the last large Pleistocene climate cycle, derived from downcore measurements of the oxygen isotope composition of multiple species of planktic foraminifera that calcified at different depths throughout the upper water column. We find that the eastern equatorial Pacific thermocline varied between deeper and shallower mean states, deepening when insolation at the equator in boreal late summer/early fall decreased and shoaling when this seasonal insolation increased. Here, we show a close phase relationship between precession-paced increases in boreal late summer/early fall equatorial insolation and thermocline shoaling, which suggests that heating of the tropical Pacific during months of maximum zonal sea surface temperature and thermocline depth asymmetry was a key influence on its mean state. • Foraminifera stable isotope records were used to reconstruct thermocline depth. • Eastern equatorial Pacific thermocline depth varied on precession timescales. • Thermocline depth was in phase with local boreal late summer/early fall insolation. • Eastern equatorial Pacific was directly orbitally forced in the Pleistocene.