Coupled Ocean-atmospheric forcing on Indian Summer Monsoon variability during the middle Holocene: Insights from the Core Monsoon Zone speleothem record

A high resolution ²³⁰Th–dated δ¹⁸O record of stalagmite from the Mahadev cave (MG-1) implies an overall decreasing trend in the strength of the Indian summer monsoon (ISM) since ∼8.30 to 4.90 k year before the Present (kyr BP with respect to 1950 CE). The δ¹⁸O values show a pronounced increase since ∼5.6 kyr BP which continued until 4.90 kyr BP, which suggests weakening of the Indian monsoon and corresponds with an ocean wide decrease in tropical Indian Ocean sea surface temperature (SST). Mahadev cave is a part of the karst system located in the Jagdalpur region of Chhattisgarh in central India, within the core monsoon zone (CMZ). The ISM intensity during the middle Holocene follows the decreasing trend in the Northern Hemisphere summer insolation, corresponding with the southward migration of the Inter Tropical Convergence Zone (ITCZ). Furthermore, the MG-1 δ¹⁸O record reveals a climatic teleconnection between North Atlantic climate and ISM intensity, with a weak ISM during the two Bond events 4 and 5a in the middle Holocene. The decrease in the tropical Indian Ocean SST weakened the land-sea thermal contrast and reduced moisture supply to the Indian subcontinent, leading to a weak ISM. Our findings highlight the modulation of the ISM by coupled ocean-atmospheric factors during the middle Holocene. The δ¹⁸O–δ¹³C coherence at different scales suggests a close relation between ISM intensity and vegetation pattern in the CMZ during the studied time frame.