Early History of the Asian monsoons: A Paleogene perspective

The modern Asian monsoonal systems are currently stated to originate between 25 and 22 million years (Myr) ago following a crucial step of uplift of the Tibetan-Himalayan highs. However, documentation of Asian climate in older records remains poor, and its response to enhanced greenhouse conditions such as seen in the Eocene period (55-34 Myr ago) is unknown due to the paucity of well-dated records. This study reports on late Eocene climate records revealing marked monsoon-like patterns in rainfall and wind south and north of the Tibetan-Himalayan orogen. This is first indicated by low oxygen isotope values with strong seasonality in gastropod shells and mammal teeth from Myanmar, indicating a modern-like monsoonal rainfall in the Eocene Bengal Bay. These results are confirmed by growth lines in coeval primate jaws, pronounced cracking and swelling features in paleosols, and fossil woods similar to modern forests that only grow in environments with strong monsoonal influence. In addition to our results from Myanmar, we extend the aeolian sedimentary record of the Chinese Loess Plateau at its western margin to include the red mudstones of the Xining Basin, which were deposited between 40 to 33 Myr ago based on detailed magnetostratigraphy. These deposits indicate that inner Asian desertification was significant in the late Eocene and that winter monsoonal storms were already active. Furthermore, alternating saline lake gypsum deposits and red mudstones layers on orbital time scales indicate marked wet and dry periods and remind of the loess-paleosol alternations of the Chinese Loess Plateau which are attributed to periods of orbitally forced variability in summer monsoonal intensity. Eocene-like climate simulations support modern-like Eocene monsoonal rainfall and show that a reinforced hydrological cycle responding to enhanced greenhouse conditions counterbalanced the negative effect of lower Asian relief at this time. Data and models furthermore confirm weakening of the monsoons with decreasing pCO2 through the Eocene-Oligocene Transition and the fall into the icehouse 34 Myr ago. Regional forcing by the uplift of Himalayan-Tibet has a clear impact on monsoon strength in our (and other) studies, but global forcing from variations of the atmospheric pCO2 should be equally regarded as one of the governing parameters contributing to the singular strength of the Asian monsoons. As a result, the impact of later uplift events on the onset of monsoonal circulation may be significantly over-estimated.