Northward migration of the maximum Indian summer monsoon precipitation during the early–mid-Holocene: Evidence from sporopollen in the Andaman Sea

The Indian summer monsoon (ISM) significantly influences the evolution of the natural environment and the development of human societies in South Asia, however, the spatiotemporal nature of the maximum ISM precipitation remains uncertain. We present the first sedimentary sporopollen record from core ADM-C1 in the southern Andaman Sea, covering the past 11.2 ka to better understand the evolutionary history of the ISM. We use the percentage values of tropical–subtropical broad-leaved tree pollen to indicate regional precipitation. These pollens reached their highest abundances during ∼11–9 cal ka BP, indicating warmest and wettest climatic conditions in the southern Andaman Sea region. The subsequent retreat of the tropical–subtropical broad-leaved forest could reflect a gradual decline in ISM. By comparing numerous ISM precipitation records, we divided these records into two patterns: the timing of maximum ISM precipitation of Pattern 1 records during the early-Holocene (11–9 cal ka BP) and that of Pattern 2 records after 9 cal ka BP. The shift in the maximum ISM precipitation from Pattern 1 to Pattern 2 reflects a northward migration during the early–mid-Holocene, probably induced by the northward migration of the mean position of the Intertropical Convergence Zone (ITCZ). Furthermore, the ∼1.8 ka temporal lag between Patterns 1 and 2 could have resulted from the relatively weak Atlantic Meridional Overturning Circulation, which might have hindered the ITCZ from migrating northward before 9 cal ka BP.