Distinct development of South Asian monsoon in the northwestern and the north-northeastern Indian subcontinent since the Miocene

In the present-day context, South Asian monsoon has a characteristic core monsoon zone that is defined to lie along the NW Indian subcontinent and Indo-Gangetic Plain while the fringe monsoon zone lies in the NW Indian subcontinent. The spatial divergence of monsoon evolution in these regions is currently understudied. Our work is aimed at investigating the development of South Asian monsoon in these regions during the Miocene. We analyzed the variation in clay mineral assemblages and major element composition as weathering proxies together with grain size distribution to create a continental chemical weathering record from two Ocean Drilling Program (ODP) sites, ODP Site 717 in the distal Bengal Fan and ODP Site 730 in the western Arabian Sea. Our results indicate that at ODP Site 730 the chemical index of alteration (CIA) values range between 40 to 60 and at ODP Site 717 they range between 60 to 70, suggesting that overall chemical alteration is higher in NE Indian subcontinent. At ODP Site 730, between 14.5 Ma and 9.5 Ma there is a decreasing trend in CIA values. Similarly a trend of decreasing smectite/ illite+chlorite values is observed during this period which indicates a reduced seasonality effect. This suggests that in the NW Indian subcontinent at the end of Mid-Miocene Climatic Optimum (MMCO),  monsoon was stronger and had significant seasonal variations. As cooling progressed, the monsoon became weaker and more tropical. Since 9.5 Ma, this weak, tropical monsoon has persisted in the region. At ODP site 717, from 9 Ma to 7 Ma, smectite/illite+chlorite values are close to zero and CIA values are between 60 and 65, implying that the monsoon precipitation was not very seasonal yet wetter causing significant chemical alteration in the northeastern Indian subcontinent. However, after 7 Ma, the CIA values show large fluctuations at a sub-million year timescale and smectite production rises significantly suggesting a transition to more seasonal monsoon. The results demonstrate that during the late Miocene, a distinct development of South Asian monsoon is seen in these two regions where the NW region became arid and experienced less seasonal variability in rainfall intensity and the NE region that transitioned from tropical wet monsoon to more seasonal precipitation with variability in monsoon strength. Our study provides a systematic understanding of variability within the South Asian monsoon system and is useful for assessing other paleoenvironment proxies from the region.