Changes in the surface and subsurface temperature across Glacial-Interglacial transitions in the Indonesian Throughflow over the past 150-kyr: A perspective from the northern Makassar Strait

The Indonesian Throughflow (ITF) is a key component of the Indo-Pacific Warm Pool (IPWP) and global ocean circulation, regulating the transfer of heat and freshwater from the Pacific to the Indian Ocean. Here we reconstruct surface and thermocline hydrographic variability of the ITF over the past 150 kyr using paired δ¹⁸O and Mg/Ca records from Globigerinoides ruber, Pulleniatina obliquiloculata, and Neogloboquadrina dutertrei in sediment core MD10-3334 (0.22°N, 119.30°E; 1169 m water depth) from the northern Makassar Strait. The records reveal pronounced glacial–interglacial variability in upper-ocean thermal structure and ITF dynamics. During the last two deglaciations (~27 and ~144 ka), upper thermocline warming preceded sea-surface warming resulting in a smaller vertical temperature gradient (i.e., deepening of the thermocline), which suggests an increase in La Niña–like mean state conditions associated with enhanced heat accumulation in the IPWP and intensified subsurface ITF transport. In contrast, the mid-Holocene and early Marine Isotope Stage (MIS) 5e are characterized by a progressive cooling of thermocline temperatures (increasing vertical temperature gradients), suggesting a reduced subsurface heat transport, consistent with a weakened ITF. These changes likely reflect adjustments in zonal and/or monsoonal wind-driven circulation and upper-ocean stratification linked to orbital-scale shifts in ITCZ position, sea level, and seasonal insolation. Our results suggest changes in the vertical temperature gradients in the Makassar Strait that we interpret as a measure of ITF strength and highlight the critical role of subsurface processes in modulating tropical Indo-Pacific climate variability across glacial–interglacial timescales.