Miocene lake evolution in the western Qaidam Basin, northern Tibetan Plateau: Evidence from aquatic-plant pollen

Qaidam Basin existed as a paleolake at least as early as the Oligocene, which was quite different from the present-day desert landscape. The paleolake in the basin evolved from its greatest extent and reduced in size during the Miocene, however, the evolution of the lake during this period remains unclear. In this study, we obtain a high-resolution aquatic-plant pollen record from the early to late Miocene (-18.7--6.5Ma) in a long and continuous drilling core, SG-4, mainly composed of lacustrine mudstone, taken from the western Qaidam Basin. The results are analyzed to systematically understand the Miocene lake evolution in the Qaidam Basin. Our results show that the concentration of submerged-plant pollen overall decreased in stages at -14.4 Ma and -8.6 Ma, and that there was a corresponding increase, also in stages, in the concentration of emergent-plant pollen at -14.4 Ma and -8.6 Ma. Combining these findings with available climatic and sedimentological records, we infer that the Miocene lake shrank, and the climate became more arid, in a step-wise manner across the western Qaidam Basin at -14.4 Ma and -8.6 Ma. We propose that global cooling was the main driver for these changes during the earlier stage, and that tectonic activity may have intensified the changes in the later stage.