Gradual Warming of Western Pacific Warm Pool Due To Weakening of Indonesian Throughflow During the Late Mid‐Pleistocene Transition

Abstract The nature and cause(s) of the Mid‐Pleistocene Transition (MPT) remain enigmatic. Notably, the role of tropical Pacific oceanographic changes during the late MPT (∼0.9–0.6 Ma) remains understudied. Here, we generated ∼1.4‐Myr‐long Mg/Ca and δ 18 O records for the planktonic foraminifer Trilobatus sacculifer from IODP Site U1490 to reconstruct sea‐surface temperature and regional seawater δ 18 O (ice‐volume‐corrected, a proxy for salinity) variation in the Western Pacific Warm Pool (WPWP). We find that the WPWP was characterized by a gradual warming and slight freshening of surface waters at ∼0.9–0.6 Ma, caused by the Indonesian Throughflow weakening and consequent accumulation of oceanic heat. These effects were likely driven by sea‐level fall due to global icesheet expansion and/or Australian monsoonal intensification. The gradual warming signal was subsequently propagated eastward via strengthened North Equatorial Countercurrent, which may have acted as a positive climatic feedback, contributing to the expansion of the North American icesheet during the late MPT.