Latitudinal Heterogeneity in South Asian Monsoon Changes Since the Last Glacial Maximum Revealed by Ganga–Brahmaputra Vegetation Record

Abstract

Future changes in South Asian climate are expected to amplify flooding and drought, yet the spatial and temporal variability of the South Asian monsoon (SAM) under different climate regimes remains poorly constrained. Here, we present a new marine pollen record from the Bay of Bengal (BoB), informing on the vegetation of the Ganges‐Brahmaputra (G‐B) River basin whose outlet via the lower Meghna lies near 21.3°–21.8°N, and compare it with bulk and leaf‐wax δ ¹³ C and δD, and planktonic foraminiferal δ ¹⁸ O records, all from the same site. The G‐B basin was characterized by an open landscape with sparse subtropical trees during Marine Isotope Stage (MIS) 2 indicating drier and colder conditions. This is in sharp contrast with the development of tropical forests during MIS 1. During the Holocene, changes in the forest composition, characterized by a progressive increase in dry tropical trees relative to moist taxa, showed large‐scale drying, with more pronounced winter dryness after 8 ka. The comparison with the pollen records located off Mahanadi (20°N) and Godavari (18°N) catchments, both within the Indian core monsoon zone reveals that the composition of the forests differs strongly, showing a persistent north–south hydroclimate gradient. At orbital time scales, vegetation records from the western BoB support that the SAM variability is intrinsically linked to the ITCZ migration and largely affected by the Northern hemisphere ice‐sheets and insolation forcing; this is particularly significant for the Holocene. However, the spatial heterogeneity of hydroclimate conditions suggests additional modulation by Himalayan orography, and vegetation through moisture recycling.