NSFGEO-NERC Solving the enigma of the Miocene South Asian monsoon conundrum. An analog to our future

The evolution of the South Asian Monsoon represents the multi-sphere interactions that involve lithosphere (the uplift of Tibetan Plateau), atmosphere (monsoonal circulations), hydrosphere (hydrological cycle), and biosphere (e.g., terrestrial ecology evolution - C4-grassland expansion and ocean productivity). Therefore, the South Asian Monsoon research is directly related to the compelling, high-priority science questions, as identified in [A Vision for NSF Earth Sciences 2020-2030: Earth in Time (2020)], including Question 6 (What are the causes and consequences of topographic change?) Question 8 (What does Earth's past reveal about the dynamics of the climate system?) Question 9 (How is Earth's water cycle changing?) and Question 10 (How do biogeochemical cycles evolve?) (National Academies of Sciences and Medicine, 2020). Hence the study of the South Asian monsoon is exemplary in Earth System Science research.

Because of the nature of multi-sphere interactions, the South Asian Monsoon is manifest with multiple facets. It is characterized by the marked seasonal precipitation, high relevance to chemical weathering of silicate rocks and the efficiency of carbon burial, interannual reversal wind fields, ocean cooling, biogenic bloom, and expansion of oxygen minimum zone (e.g., Betzler et al., 2017; France-Lanord et al., 2016; Pandey et al., 2016). Decades of efforts have focused on characterizing those features of South Asian Monsoon; the accumulation of results has revolutionized our knowledge of South Asian Monsoon by offering long-term time series of evolutionary history from different perspectives to link the above phenomenon. However, due to the nature of multiple facets of the South Asian Monsoon, a vital issue arose, which reflects on the divergent observations and contradictory interpretations of the monsoon strength. Proxy studies from the ocean and terrestrial records give very opposite views on the South Asian monsoon: records of physical, chemical, and biological features of the North Indian Ocean reveal that the monsoon-induced upwelling has been strengthened between ca. 13 Ma and 8 Ma. The strengthening of upwelling is characterized by the rapid cooling of sea surface temperatures, the spike increases of cold water-dwelling foraminifera - the global bulloides, and the expansion of oxygen minimum zone; while terrestrial records reveal that the monsoonal precipitation became weaker, the climate became drier that drove the C4-grassland expansion, and chemical weathering lost its efficiency during the similar time interval.

We organized a US-UK team (with six PIs from four institutes). We proposed to use an integrated approach that combines the strength of organic geochemistry proxy and numerical modeling studies. We aim to reconstruct a long-term history of the South Asian monsoon since the mid-Miocene to characterize the regional precipitation pattern, the ocean production, and ocean circulation patterns reflecting changes in chemical and physical conditions. We will then establish and delineate the linkages between the uplift of the Tibetan Plateau, the evolution of monsoonal circulation, and ocean production in the context of numerical modeling simulations.