The Eocene-Oligocene palynological record from the Xining Basin (Tibetan Plateau, NW China) as evidence for Asian palaeoenvironments and regional change

Abstract

The Eocene–Oligocene transition (EOT) is marked by global cooling trend that coincided with rapid growth of the Antarctic ice sheet and a drop in atmospheric carbon dioxide levels. This trend is particularly well registered in the oceanographic record, but complete continental sequences comprising this transition are sparse. Even rarer are palynological records of this period of climatic change. Here we report on a palynological study of playa lake deposits situated in the Xining Sub-basin (Tibetan Plateau, NW China). In this basin the EOT was identified by a regional lithological change in the red bed / gypsum alternation and was precisely dated through palaeomagnetostratigraphy. This sedimentary sequence also proved rich in pollen. High abundances (and many varieties) of Ephedra and Nitraria, taxa typical for arid vegetation types, characterize the palynology of the Xining Sub-basin. Subtle fluctuations of these taxa throughout the sequence suggest that the local vegetation alternated from desert type (Ephedra dominated) to - slightly more humid- steppe type (Nitraria dominated) and back. The palynological results of the studied section fit well in the wider Chinese context during the EOT when a broad arid belt crossed China from East to West. By Neogene times the arid zone was restricted to NW China (Sun & Wang, 2005) and palynological assemblages were no longer Ephedra and Nitraria dominated. A surprising increase in high altitude distal pollen influx (Pinaceae) occurs just below the E-O boundary and is considered to be evidence for climatic cooling and/or increased topography in the Tibetan Plateau during the Late Eocene.