Mid-Holocene Monsoon Weakening: A major cause for societal change in the Indian subcontinent

A sediment core retrieved from a water depth of 250 m near the Andamans Forearc Basin (AFB), the Landfall Island, North Andaman, reflects a record of sediment provenance and monsoonal shift since the mid to late Holocene. The core represents radiocarbon ages ranging from 6,078 to 1,658 yrs BP (from~ 6,500 yrs BP to the present). The core is dominantly clayey silt with incursions of coarser components that occur around 6,000, 5,400, and 3,400 yrs BP. Grain size variation indicates a cyclic variation of wetter and drier conditions matching changes in the intensity of the Indian Summer Monsoon (ISM), which was at its greatest intensity around 6,400, 5,300, and 3,300-3,000 yrs BP. Geochemical parameters including higher CaCO₃ content, εNd, and ¹⁸O in Globigerinoides ruber are consistent with the long-term trend from cooler, wetter conditions to warmer, drier conditions at present. Chemical weathering intensity, which lags behind climate changes on land, shows a pulse of highly weathered sediment deposited at about 4,000 BP. Clay minerals represented by smectite, illite, kaolinite, and chlorite in varying amounts indicate high kaolinite content and K/C ratio specify intense Southwest Monsoon (SWM) and stronger bedrock weathering in the hinterland (~6,500–5,400 years BP). Incidence of smectite (48.82 to 25.09 %) and chlorite/illite (C/I) ratio (0.56 to 0.28) indicate an overall weakened southwest monsoon since 6,000 to 2,000 years BP with a brief incursion of extremely reduced SWM around 4,400 to 4,200 years BP. This is corroborated by the oxygen isotope on G. ruber that indicates a significant shift in the isotopic values ~4,300 years BP (−3.39‰), indicating a weak SWM. Fluctuations in the intensity of SWM are also observed for 2,000 years to the present. Sandy sediment was supplied from the Andaman Islands, Irrawaddy, and the Salween sea. Since the Mid Holocene period, longer periods of aridification and shorter periods of wetter conditions increased in the region after approximately 4,300 yrs BP. A correlation of monsoonal events using the Godhavari marine sediment core (Ponton et al.,2012)  and our data is noted that Bronze Age Harappan urbanism flourished since 4,500 yrs BP along the river banks in the western region of the present semi-arid Desert and the Deccan owing to intensified rain-fed agriculture. Since approximately 3,900 yrs ago, the total settled area and many settlement sizes declined, abandoned, and a significant shift in site numbers and density towards the southeast and west is recorded. During the Iron Age, after ca. 3,200 yrs BP, adaptation to semi-arid conditions in western Rajasthan, central and south India appears to have been well established with a significant number of sites in areas receiving <500 mm of rainfall. Weak monsoon precipitation led to conditions adverse to both inundation and rain-based farming and encouraged pastoralism. Monsoonal-fed rivers were active during the short-wet periods and gradually dried or became seasonal, affecting habitability along their courses.