A foraminifera-derived reconstruction of the Eastern Equatorial Pacific thermocline between 70,000 and 150,000 years ago

The depth of the thermocline across the equatorial Pacific is a key determinant of the mean oceanographic and climatic state of the region. The thermoclinethe depth interval between warm surface water and cooler subsurface water where temperature decreases most rapidly with depthis shallower in the Eastern Equatorial Pacific (EEP) and deeper in the west, resulting in sea surface temperature and pressure gradients along the equator. During an El Nino event, the east warms, the easterly trade winds weaken, and the EEP thermocline deepens. This phenomenon has climatic implications around the world, as El Nino events cause perturbations in global atmospheric circulation. Tracking past changes in the depth of the EEP thermocline during different global climate scenarios can provide insights into the important influences on tropical dynamics and feedback systems. In this study, we focused on Ocean Drilling Program (ODP) Site 1240 (2921 m; 0.02N, 86.46W). ODP Site 1240 is located within the EEP cold tongue, where strong upwelling occurs in the present. We created records of foraminiferal 18O from the marine sediment core at ODP Site 1240 by analyzing multiple planktic speciesG. tumida, P. obliquiloculata, G. hexagonus, T. sacculifer, and G. menardii, which dwell at different depths in surface to subsurface waters. We reconstructed temperature profiles of the water column at this site, using 18O of these foraminifera that live throughout the upper 500 meters as a proxy. These profiles were used to characterize the depth of the thermocline and the mean state of the EEP between 70 to 150 kyr before present. This period encompasses the termination of Marine Isotope Stage 6a glacial stage, as well as the penultimate deglaciationa period of rapid sea level rise and ice melt approximately 130,000 years ago, and Marine Isotope Stage 5ea particularly pronounced interglacial period. Our record over this period will offer unique insights into the role of tropical dynamics during times of dramatic climate change and will allow us to investigate the forcings that drive the system.