Change in South Asian Summer Monsoon During MIS7, an Interglacial Complex of the Middle Pleistocene With High Obliquity Forcing

Abstract

The South Asian summer monsoon is a major component of the global climate system, yet its response to varying boundary conditions remains debated. We present pollen data from IODP Site U1446 (Bay of Bengal), recording vegetation changes in India's Core Monsoon Zone (CMZ) between 260 and 190 ka, spanning Marine Isotopic Stage (MIS) 7. This interglacial complex is marked by exceptionally large changes in obliquity and precession resulting in two glacial terminations (TIII and TIIIa) and two interglacials (MIS 7e and 7c). Pollen data reveal alternations between semi‐arid/open savanna (MIS 8, 7d, 7b) and tropical forest (MIS 7e, 7c, 7a), reflecting orbital‐scale monsoon intensity fluctuations. As observed for other interglacials periods, monsoon rainfall intensifies during MIS 7e and 7a as precession‐driven northern hemisphere summer insolation increases. However, during MIS 7c, the intense humidity peak coincides with maximum obliquity, rather than precession minimum. We propose that under warmer conditions, both precession and high obliquity forcing enhance summer moisture transport in the CMZ. Conversely, reduced precipitation corresponds to periods of higher ice volume and lower CO ₂ levels, likely restricting the northward ITCZ migration and atmospheric moisture, although peak aridity coincides with minima in boreal summer insolation. Our results challenge studies attributing monsoon variability solely to precession or ice volume/CO ₂ , emphasizing instead the variable interactions of external and internal forcings under differing boundary conditions. Millennial‐scale variability also influenced vegetation and monsoon dynamics during TIII and TIIIa, modulating the monsoon response in South Asia to the forcings acting at the orbital‐scale.