A research report by Daniel Perez-Pinedo (Instituut voor Biodiversiteit en Ecosysteem Dynamica, Universiteit van Amsterdam). The resulting masters thesis has been awarded the East-West Seed Graduation Prize for Plant Sciences 2020 of the The Royal Holland Society of Sciences and Humanities.

Introduction

The Eocene–Oligocene Transition (EOT) (~33.9Ma) is characterized by climate cooling and paleogeographic change and constitutes one of the most remarkable time intervals in the Cenozoic. This climate transition had a major impact on SE Asia, one of the most biodiverse areas and tectonic active regions in the world. However, terrestrial vegetation response in SE Asia across the EOT transition remains poorly understood.

The Cenozoic vegetation dynamics of Myanmar have been scarcely documented, yet are of great interest to unravel paleogeographic and paleoclimatic history. The family Sapotaceae is of particular concern since it has been a dominant component of SE Asian megathermal lowland wet forests ever since the early Eocene. However, their genera delimitation, origins, and direction of evolutionary change across the EOT remain uncertain. The Central Myanmar Basin (CMB; Myanmar) holds a sedimentary record ranging from upper Eocene – lower Oligocene, which is subdivided into the Yaw and Shwezetaw formations respectively. This sedimentary record presents an excellent opportunity to study the development of the SE Asian forests across the EOT.

Expedition

In this project I joined the Myanmar Paleoclimate and Geodynamics Research group which focuses on geodynamic, tectonic, paleoclimatic, and paleoenvironmental studies of the Burmese sedimentary archives over the last 120 Ma. The last MyaPGR expedition took place in January 2020. It was led by Dr. Alexis Licht and consisted of Dr. Pierrick Roperch and PhD candidate Jan Westerweel (both from Université de Rennes), Dr. Amy Gough (Royal Holloway, University of London), and myself (Universiteit van Amsterdam). The main goal of the expedition was to report changes in palyno-assemblages across the EOT revealed by the main pollen markers.

The expedition operated in the south of the CMB located at the southern edge of the eastern Himalayan orogen and the Indochinese margin (Figure 1). We traveled to the Minbu sub-basin and visited 6 geological sections of scientific interest where we sampled for pollen, biological markers, fossil wood, amber, and also collected samples in order to conduct paleomagnetic dating (Figure 2). In particular we focused on the Yaw and the Shwezettaw formations, ranging from the late Eocene to the Early Oligocene respectively.

The analysis

The samples were processed at the University of Amsterdam and pollen were examined under light microscopy (LM) and scanning electron microscopy (SEM). The palynological data were analyzed with a cluster analysis tool CONISS that is based on a broken stick model. We also applied ordination multivariate analysis such as Detrended correspondence analysis (DCA) and non-metric multidimensional scaling (NMDS). We observed differential palyno-assemblages resembling changing vegetation across the tropical Eocene into the cooler Oligocene. We also reported different depositional environments across the transition. We interpreted these changing trends to be derived from potential global cooling, increased aridity, and the progressive uplift of the Indo-Burman Ranges (IBR). The EOT reveals itself as one of the main drivers of paleoenvironmental dynamics leading to a pronounced terrestrial reorganization within terrestrial vegetation related to climatic and biogeographic dynamics in Myanmar, possibly extensive to SE Asia.

Learnings

Having had the opportunity of going to the tropics to conduct fieldwork has resulted in a greater personal engagement in the project as well as a better understanding of the topic. I acquired a better grounded in situ understanding of both the ecological context (vegetation, forest composition) and the geological context regarding lithology and sedimentology of the outcrops of our interest in the Minbu sub-basin. Finally, having had the opportunity of working as a junior member of a well-structured professional research team which consisted of both international and Burmese researchers was both enriching and professionally and culturally challenging.