Fossil wood from the Miocene of Myanmar: application to the reconstruction of monsoonal paleoenvironments

In South Asia, monsoon intensity is significantly controlled by orogenic barriers (Tibetan plateau and Himalaya) that block dry, cold air in winter and hot, moisted air in summer, causing heavy rainfall. The history of the South Asian monsoon on geological time is poorly constrained but likely linked to regional topographic history, including to the uplift history of the Himalayas and the Indo-Burman Ranges, at the eastern edge of the South Asian monsoonal domain. This study provides a detailed description of fossil wood specimens recovered in fluvial deposits of the Natma Formation (Early Miocene, Myanmar) to reconstruct the Miocene vegetation in the South Asian monsoonal domain and document the regional paleoclimatic history. Fossil wood specimens are then compared with Eocene fossil wood and modern wood from the same region. The flora of the Natma Formation include several specimens of Fabaceae and Dipterocarpaceae, as well as Burseraceae and Cupressaceae. These specimens can be grouped into three types of forested environments, 1) tropical wet evergreen forests, 2) lowland forests with high seasonality, and 3) deltaic plain forest environments. We propose that the combination of specimens from these three different ecosystems reflects input from different upstream areas of the Miocene drainage. Thought the Miocene vegetation display ecotones similar to the onces found in the Eocene and modern vegetation, the presence of tropical wet evergreen forests, likely in the uppermost part of the drainage area, indicates overall wetter conditions than during these two time periods. Our results thus suggest strong monsoons during the early Miocene, possibly linked to an episode of regional uplift.