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