A late Miocene ostracod record from the northeastern Tibetan Plateau

Lake Qinghai, the largest inland brackish lake in China, is located on the northeastern margin of the Tibetan Plateau and is affected by both the Asian Monsoon and Westerlies climate systems. It is very sensitive to climate and environmental changes. Moreover, its sediments accumulated over the Cenozoic Era and contain abundant information about past tectonics and environmental changes. Here we present a record of late Miocene ostracods from Lake Qinghai sediment cores. A taxonomic analysis of the ostracods yielded 7 families, 14 genera, and 31 species. Six distinct sediment facies were recognized and their corresponding paleolimnological and paleoenvironmental changes were inferred from the ostracod assemblages. The first appearance of ostracod shells at about 7.60Ma reflects a switch from an eolian sedimentation facies (ca. 8.10-7.60Ma) to a shallow lacustrine environment (ca. 7.60-5.85Ma). Ostracod shells were absent from ca. 5.85 to 4.64Ma in eolian sediments. A transition to a deep lacustrine facies began at ca. 4.64Ma, and ended at ca. 3.58Ma. Thus, Lake Qinghai existed in the late Miocene and became larger and deeper in the early Pliocene (ca. 4.64Ma). During the time interval ca. 3.58-0.70Ma, littoral lacustrine sedimentation occurred at the core sites. After ca. 0.70Ma, there were alternating shallow lacustrine and lakeshore environments at the sites, until ca. 0.12Ma. Changes in ostracod assemblages and their abundance during the intervals ca. 4.64-3.56Ma and ca. 0.70-0.12Ma indicate that the sedimentary environment fluctuated over short time scales. Given Lake Qinghai's sensitivity to environmental and tectonic changes, related to its special location, we concluded that changes in the relative and absolute abundances of ostracods reflect the evolution of the Asian monsoon-arid environment and Plateau growth, as well as global climate changes.