2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 106-7
Presentation Time: 9:30 AM


LICHT, Alexis1, LADANT, Jean-Baptiste2, FRANCE-LANORD, Christian3, DONNADIEU, Yannick4, LECUYER, Christophe5, BOURA, Anais6, DE FRANCESCHI, Dario6, DUPONT-NIVET, Guillaume7 and JAEGER, Jean-Jacques8, (1)Department of Geosciences, University of Arizona, Tucson, AZ 85721, (2)Laboratoire des Sciences du Climat et de l'Environnement (LSCE), L'Orme des Merisiers, Batiment 701, Gif-sur-Yvette, 91191, France, (3)Crpg, Vandoeuvre les Nancy, 54501, France, (4)Cnrs, Laboratoire des Sciences du Climat et de l'Environnement (LSCE), L'Orme des Merisiers, Batiment 701, Gif-sur-Yvette, 91191, France, (5)Laboratoire de Géologie de Lyon, Terre, Planètes, Environnement, Université de Lyon, Lyon, 69622, France, (6)Centre de Recherche sur la Paléodiversité et les Paléoenvironnements, UPMC, MNHN, CNRS, Paris, 75005, France, (7)Geosciences Rennes, Université de Rennes, Rennes, 35042, France, (8)Iphep, Université de Poitiers, Poitiers, 86000, France

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. Documentation of Asian climate in Paleogene 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 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.