This is a multidisciplinary, international collaborative project to study Cenozoic vertebrate fossils and paleoenvironments of the Tibetan Plateau. The uplift and growth of the Tibetan Plateau have been linked to regional and global climatic and environmental changes during the past 20 million years. Progressively cooler and drier conditions, possibly linked to the development of East Asian winter monsoon, seem to be a long-term trend and left deep marks on biological communities. Our preliminary explorations in Zanda, Kunlun Pass, and Qaidam basins in Tibet, funded by a prior NSF grant, have resulted in a tantalizing collection of fossil vertebrates including mammals, fishes, and turtles. Pliocene fishes from the Qaidam Basin are highly sensitive to the drying of Tibet and fossil mammals from the Zanda Basin suggest the origin of Ice Age megafauna elements from high plateau. Collectively these fossil assemblages hint at the emergence of an endemic fauna sometime in the late Miocene to Pliocene (12-2.6 million years ago). This new grant will allow us to greatly expand our research program that strives to gain a detailed understanding of zoogeographic barriers and environmental volatility resulting from the growth of the high plateau and how these profound changes have affected mammals in that region. An international team of paleontologists, structural and sedimentary geologists, stable isotope geochemists, and paleomagnetists, all experienced in fieldwork in Tibet, will attempt to achieve several objectives: (1) systematic collections of fossil mammals and fishes in these basins; (2) establishment of a regional chronologic framework based on the integration of biostratigraphy and magnetostratigraphy; (3) reconstruction of depositional environments and basin history and their relationships to the uplift of surrounding mountains and biologic events; (4) studies of the plateau drainage systems based on genealogical relationships of fossil fishes; (5) analyses of stable isotopes to explore changes in the diets of mammalian herbivores, plant communities, and regional climate and hydrological cycle; and (6) explorations of environmental implications related to the rising of the Tibet based on information of faunal successions, sedimentation, and stable isotopes. Results of our paleontological exploration are likely transformative because the poorly explored Tibetan Plateau yields unique fossils that are instructive about environmental changes and adaptations, zoogeographic barriers, and Ice Age megafauna origins. Fossil vertebrates fill a crucial missing piece of the puzzle in the current debate on past biological world and paleoenvironments in the region as the Plateau elevated to a critical height to form a formidable barrier for animal migration, to forge new drainage systems, and to fundamentally affect the climate of the northern hemisphere during the late Cenozoic. These new data are expected to afford testing of several hypotheses regarding systematic relationships of several unique mammals restricted to the plateau, timing and rate of growth of the plateau, environmental implications of the tectonic upheaval, and biogeographic consequences for mammals, fish and plants
This project was to study fossil mammals in the Tibetan region, to reconstruct their diets and environments, and to explore the potential linkage among tectonic, climatic and biotic changes. Three graduate students and one undergraduate student were involved in various aspects of the project, gaining valuable field and/or laboratory research experience. The project has resulted in 8 peer-reviewed publications, contributing significantly to our understanding of the biostratigraphy, mammalian evolution, and environmental change in the Tibetan region. Below is a brief summary of the major results from this project. Our team uncovered a mid-Pliocene fauna containing a new species of woolly rhino in the Zanda Basin at the foothill of the Himalaya in southwestern Tibetan Plateau. Careful studies of the fossil and associated materials reveal that the new Tibetan rhino, 3.7 million years old (middle Pliocene), is much older and more primitive than its Ice Age descendants in Europe and Asia. The results suggest that the harsh winters of the rising Tibetan Plateau could well have provided the initial step towards cold-adaptation for several subsequently successful members of the late Pleistocene mammoth fauna. This finding was reported in the journal Science (Deng et al., 2011). Detailed isotope analyses of this mid-Pliocene fauna from the Zanda Basin reveal that these ancient mammals, like modern herbivores in the area, fed primarily on C3 vegetation and lived in an environment dominated by C3 plants. This suggests that the area had reached high elevations (>2.5 kilometers) by at least the mid-Pliocene. Our data also suggest a shift in climate to much drier conditions after ~4-3 million years ago. Paleo-temperature estimates derived from two different temperature proxies suggest that the paleo-elevation of the Zanda Basin in the mid-Pliocene (~4-3 million years ago) was similar to or slightly lower than its present-day elevation, which is consistent with the inference from the carbon isotope data and with fossil evidence. These findings were reported in the journal Earth and Planetary Science Letters (Wang et al., 2013) and the journal PNAS (Deng et al., 2012). Our high-resolution isotope data from the Gyirong Basin in the central Himalaya reveal major changes in drainage systems and depositional settings at ~7.2, ~5.5 and ~3.2 million years ago. These environmental changes appear to be driven by regional-scale tectonics. The oxygen isotope record also reveals alternating wet and dry climates with periodicities of about 24,000 and 100,000 years that were likely controlled by orbital forcing. The oxygen isotope data suggest that the Gyirong Basin had much warmer temperatures about 7 million years ago than today, which is consistent with fossil mammalian and pollen assemblages and sediment clay mineralogy as well as carbon isotope data from fossil tooth enamel samples and ancient soil carbonates. The estimated temperature difference would require the study area to have been raised by about 2-2.5 kilometers since 7 million years ago to its current elevation of 4-5 kilometers above sea level. These results were reported in Journal of Asian Earth Sciences, Special Issue on Asian Climate and Tectonics (Wang et al., 2012). Our initial data from several fossil localities in Yunnan Province show that unlike the Siwalik fauna in the Indian subcontinent, mammals in Yunnan on the southeast side of the Himalayan-Tibetan Plateau lived in an environment dominated by dense forests until ~3-4 million years ago. Nonetheless, the carbon isotope data from both ancient soil carbonates and fossil mammalian tooth enamel samples indicate that C4 grasses were present in the Yuanmou region from about 8 to 3.5 million years ago, likely in greatly dispersed, small patches of open habitats where the forest canopy was broken or on flood plains, and the C4 biomass increased significantly since then. The data indicate a general drying of the local climate over time and a change from a largely dense-forest environment at ~8 million years ago to a more open environment with a mosaic of forests and grasslands after 3-4 million years ago in the Yuanmou region. These initial results were published in Journal of Asian Earth Sciences, Special Issue on Asian Climate and Tectonics (Biasatti et al., 2012). Our isotope data from the Qaidam Basin in northern Tibetan Plateau suggest increased aridity after the early Pliocene. This environmental change is likely due to a combined effect of regional tectonic changes, which resulted in a more effective moisture barrier preventing moisture from the Indian Ocean and Bay of Bengal from entering the basin, and global cooling. This work was published in the journal Earth and Planetary Science Letters (Zhang et al., 2012).
