MODERN-LIKE INDIAN MONSOONS IN THE EOCENE: EVIDENCE FROM THE SEDIMENTARY SERIES OF MYANMAR
This study reports on well-dated Middle to Upper Eocene climate records in Myanmar, revealing marked monsoon-like patterns in rainfall and related seasonality in the Proto-Bengal Bay. This is first indicated by low oxygen isotope values with strong seasonality in fossil 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.
To understand the environmental context in which those monsoons occurred, we run climate simulations with late Eocene, greenhouse boundary conditions. Despite a less expansive and lower Tibetan Plateau in our paleogeographical reconstructions, the resulting simulations display strong Asian monsoons with summer and annual rainfall amounts close to present-day values in Myanmar. These strong monsoons imply that a reinforced hydrological cycle responding to enhanced greenhouse conditions counterbalanced the negative effect of less developed Asian relief at this time. The simulations finally show a substantial reduction of the monsoonal intensity as the pCO2 decreased through the Eocene-Oligocene Transition, 34 million years ago. They thus highlight the significant impact of the pCO2 variations and related global climatic events on the monsoons and minor the influence of tectonic uplifts on the past evolution of the Asian climate.