South Asian monsoon induced weathering and erosion inputs into the Bay of Bengal during the Miocene : Evidence from clay radiogenic isotope compositions

The South Asian monsoon (SAM) is a major component of the global climate system and influences the lives of more than a billion people. Despite its importance, it remains difficult to predict. Thus, it is crucial to understand how the monsoon behaved in the past under different boundary conditions such as a warmer world with reduced continental ice cover as we may experience in the near future and to apply this knowledge for the improvement of future projections. Furthermore, feedbacks which operate between the various forcing mechanisms of the monsoon, such as tectonics and climate as well as the associated changes in weathering and erosion regimes in the catchment areas of the SAM, need to be better understood on tectonic to orbital timescales. To disentangle these different controls, geochemical records for International Ocean Discovery Program Site U1443 in the southern Bay of Bengal were produced that cover the middle to late Miocene (15.8-5 Ma). Radiogenic strontium, neodymium, hafnium and lead isotope compositions of detrital clays are used to infer changes in the provenance of the sediments supplied to the Ninetyeast Ridge and the prevailing weathering regime in the catchment of the Bay of Bengal. The study of orbital-resolution intervals of the middle Miocene (15.8-9.5 Ma) as well as of a 100 kyr resolution record of the late Miocene (9-5 Ma) allow tectonic and climatic forcing of changes recorded in the detrital clay radiogenic isotope record to be distinguished based on timing. The individual contributions from different erosional sources overall remained remarkably constant during the middle to late Miocene, despite major tectonic reorganizations in the Himalayas. The major contributing lithologies were the High Himalayan Crystalline, the Tethyan Sedimentary Series and the Indo-Burman Ranges. However, on orbital timescales, the high-resolution data from the five middle Miocene intervals show marked fluctuations of the radiogenic Sr, Nd and Pb isotope systems. Interestingly, their variability decreased markedly at 13.5 Ma after the middle Miocene global cooling step. This change is attributed to a major restriction on the supply of High Himalayan erosion products due to climatically-induced migration of the peak precipitation area towards the frontal domains of the Himalayas and the Indo-Burman Ranges. These transient orbital-scale fluctuations of the radiogenic isotope signals were mainly triggered by climatically-driven changes in the balance of source contributions. The presence of a significant 30 kyr periodicity in the Nd isotope record suggests that changes of the location of peak monsoon precipitation between higher and lower elevations influencing weathering regimes on land exerted a major control on those short-term fluctuations. At 7.3 Ma, Nd and Pb isotope compositions indicate a switch in provenance of the clays transported to the Ninetyeast Ridge and contributions from the Irrawaddy River became more dominant. This change happened at the same time as the decline in global benthic δ13C which was associated with the expansion of grasslands on land. Global cooling as well as increasing aridity likely resulted in a shift of the areas of highest precipitation to the Indo-Burman Ranges. An increase in Sr isotope compositions between 6 and 5 Ma was decoupled from Nd and Pb isotope signatures and likely indicates enhanced chemical weathering intensities in the catchments of the Ganges-Brahmaputra and Irrawaddy Rivers. A possible mechanism responsible for increased chemical weathering may have been the expansion of grassland which generated thick, easily weatherable soils as well as increased exposure of continental shelves. To better disentangle the silicate weathering intensity signal from changes in source provenance, coupled Nd and Hf isotope compositions were used to infer changes in silicate weathering intensities. Deviations of the Hf isotope compositions from the array defined by global clays (∆εHf clay) exhibited some of the highest values measured to date suggesting the prevalence of extreme chemical weathering intensity under tropical warm and wet climate conditions during the Miocene. Secondary clays incorporated highly radiogenic Hf from weathering solutions that obtained their signatures through the dissolution of labile mineral phases carrying those radiogenic Hf signatures. The extremely high ∆εHf clay values were likely generated by a combination of highly weathered sediments and substantial erosion in tropical environments with intense seasonal precipitation, as is the case for seasonal monsoon rains on the floodplains surrounding the Bay of Bengal.