A Functional Thermocline Threshold for the Indonesian Seaway Restriction During the Late Miocene to Early Pliocene

The restriction of the Indonesian Seaway during the late Neogene fundamentally reorganized tropical ocean circulation and its associated heat distribution. However, the precise timing and vertical structure of this gateway evolution remain poorly constrained. In this study, we present continuous sea surface temperature (SST) and thermocline temperature (TWT) records from 6.0-3.8 Ma, reconstructed using foraminiferal Mg/Ca ratios from ODP Site 807 (Western Pacific) and Site 762 (Eastern Indian Ocean) at the inlet and outlet of Indonesian Throughflow (ITF). Our records reveal a pronounced and synchronous inter-basin thermal divergence between 5.2 and 4.9 Ma, marking a critical functional threshold in the seaway’s restriction. During this interval, the shoaling of gateway sills effectively blocked the exchange of Pacific thermocline water masses, forcing the ITF into a surface-intensified mode. The restricted thermocline export triggered heat accumulation in the Western Pacific Warm Pool, intensifying the Walker Circulation and altering the Indian Ocean’s thermal architecture. This underscores the sensitivity of tropical oceanography to thermocline-level gateway changes.