Western Pacific thermocline structure and the Pacific marine Intertropical Convergence Zone during the Last Glacial Maximum

Paleoclimate proxy evidence suggests a southward shift of the Intertropical Convergence Zone (ITCZ) during times of Northern Hemisphere cooling, including the Last Glacial Maximum, 19-23 ka before present. However, evidence for movement over the Pacific has mainly been limited to precipitation reconstructions near the continents, and the position of the Pacific marine ITCZ is less well constrained. In this study, we address this problem by taking advantage of the fact that the upper ocean density structure reflects the overlying wind field. We reconstruct changes in the upper ocean density structure during the LGM using oxygen isotope measurements on the planktonic foraminifera G. ruber and G. tumida in a transect of sediment cores from the Western Tropical Pacific. The data suggests a ridge in the thermocline just north of the present-day ITCZ persists for at least part of the LGM, and a structure in the Southern Hemisphere that differs from today. The reconstructed structure is consistent with that produced in a General Circulation Model with both a Northern and Southern Hemisphere ITCZ.

► Planktonic foraminiferal δ18O is used to reconstruct thermocline depth in the western Pacific. ► The Holocene and LGM thermoclines depths are compared. ► We find tropical thermocline ridges on both sides of the equator during the LGM. ► This suggests a Southern Hemisphere ITCZ (or more zonal SPCZ).