Evolution of Indian Summer Monsoon Across the Mid‐Pleistocene Transition: Evidence From Planktic Foraminifera Records at IODP 353 Site U1448 in the Andaman Sea

Abstract The Indian summer monsoon (ISM) is crucial in interhemispheric heat and moisture transport, significantly impacting South Asia's climate and environment. However, long‐term ISM variability remains poorly understood. We reconstruct paleosalinity and paleoproductivity over the past 1.45 myrs using planktonic foraminiferal assemblages from sediment cores of Site U1448 drilled off the SW Andaman Sea. A comparison with Site U1446 in the Bay of Bengal (BoB) reveals a clear shift in the salinity across the mid‐Pleistocene transition (MPT), with larger salinity difference between the two sites during the pre‐MPT than the post‐MPT time. We speculate that the larger salinity difference may be attributed to a more northerly position of the Intertropical Convergence Zone driven by the interhemispheric thermal gradients. Prior to the MPT, the paleoproductivity parameters of U1448 all suggest a relatively weak ISM activity. After the MPT, a new pattern of variation in productivity and salinity gradient is developed along with an overall intensified regime of ISM in the region. This reorganization at ∼0.9 Ma reflects a pronounced regional hydroclimatic threshold amidst the broader MPT. There exists regional heterogeneity in obliquity signals displayed by the change in the salinity difference between the Andaman Sea and the BoB, which underscores the importance of localized monsoon–climate interactions in the northeastern Indian Ocean. We suggest that the significant reorganization of the ISM across the MPT reflect complex interplay among monsoon system, external forcing and internal feedback, and ocean–atmosphere dynamics in the region.