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

南亚季风的演化代表了岩石圈(青藏高原隆升)、大气(季风环流)、水圈(水文循环)和生物圈(如陆地生态演化-C4-草原扩张和海洋生产力)的多圈相互作用。因此，南亚季风研究与[美国国家科学基金会地球科学展望2020-2030年：及时的地球(2020)]中确定的引人注目的高优先科学问题直接相关，包括问题6(地形变化的原因和后果是什么？)问题8(关于气候系统的动态，地球过去揭示了什么？)问题9(地球的水循环是如何改变的？)问题10(生物地球化学循环是如何演变的？)(美国国家科学院和医学科学院，2020年)。因此，南亚季风的研究在地球系统科学研究中具有典范意义。由于多球体相互作用的性质，南亚季风具有多面性。它的特点是显著的季节性降水、与硅酸盐岩石的化学风化和碳埋藏效率高度相关、年际逆转风场、海洋冷却、生物水华和氧气最小带的扩大(例如，Betzler等人，2017；France-Lanord等人，2016；Pandey等人，2016)。几十年来的努力侧重于描述南亚季风的这些特征；成果的积累从不同的角度提供了长期的演化历史序列，将上述现象联系起来，从而彻底改变了我们对南亚季风的认识。然而，由于南亚季风的多方面性质，出现了一个关键问题，反映了对季风强度的不同观察和相互矛盾的解释。来自海洋和陆地记录的代理研究对南亚季风给出了截然相反的观点：北印度洋的物理、化学和生物特征记录表明，季风引起的上升流在大约13 Ma至8 Ma之间得到了加强。上升流增强的特征是海面温度迅速冷却，冷水有孔虫-全球海泡的尖峰增加，氧最小带扩大；而陆地记录显示季风降水变弱，推动C4草原扩张的气候变得更加干燥，化学风化在类似的时间间隔内失去作用。我们组织了一个美英小组(由来自4个研究所的6个PI组成)。我们建议使用有机地球化学替代方法和数值模拟研究相结合的综合方法。我们的目标是重建中新世以来南亚季风的长期历史，以表征反映化学和物理条件变化的区域降水模式、海洋生产和海洋环流模式。然后，我们将在数值模拟的背景下建立和描绘青藏高原隆起、季风环流演变和海洋生产之间的联系。

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.