The monsoon system is the primary driver of agricultural production, ecosystems, and societal development across the Indian subcontinent. However, monsoon’s seasonal response to Holocene climate perturbations remains poorly constrained. The boundary between Northgrippian and Meghalayan ages, defined as the 4.2 ka event, is widely associated with Indian Summer Monsoon (ISM) failure, regional aridification, and the collapse of many civilizations. However, its precise timing, structure, and impact on monsoon seasonality, marine productivity, and carbon burial remains inadequately documented owing to the lack of decadal resolution marine records. Here, for the first time, we present a decadal resolution multiproxy marine record from the southwestern Bay of Bengal, integrating geochemical and planktic foraminiferal proxies to reconstruct the oceanographic response to the 4.2 ka event. We report a two-step decrease in carbon burial, beginning at 4.65 ± 0.17 ka and persisting until 3.97 ± 0.18 ka, spanning ~680 years. The event evolved through two phases, with Phase 1 (4.65–4.45 ka) characterized by a stronger but gradually weakened ISM, a moderate strengthening of the Indian Winter Monsoon (IWM), and enhanced terrestrial organic matter input, followed by Phase 2 (4.45–3.97 ka) marked by a pronounced and sustained ISM weakening and intensified IWM. We propose that the Atlantic Meridional Overturning Circulation triggered the 4.2 ka event, with a complex interaction of Inter Tropical Convergence Zone migration, El Niño-Southern Oscillation variability, and insolation changes operating across different timescales. Our findings establish a mechanistic link between altered monsoon seasonality and reduced carbon burial during the 4.2 ka event, highlighting how a climatically sensitive ocean basin can respond to short-term climatic perturbations.