ISOTOPIC EVIDENCE FOR LATE CENOZOIC ENVIRONMENTAL CHANGE IN SW TIBET

Fossils are important archives of paleoenvironments. Here we use stable carbon and oxygen isotopes preserved in fossil mammalian teeth and fossil shells to reconstruct ancient diets, vegetation and temperatures in the Mid-Pliocene in a high elevation basin - the Zanda (Zhada) Basin in southwestern Tibet. The δ¹³C values of fossil enamel samples show that these ancient mammals, just like modern herbivores in the area, fed primarily on C₃ vegetation and lived in an environment dominated by C₃ plants. Taking into account the changes in the δ¹³C of atmospheric CO₂ in the past, the enamel-δ¹³C values suggest that the average δ¹³C value of C₃ vegetation in the Zanda Basin in the Mid-Pliocene was ~2‰ lower than that of the C₃ biomass in the basin today, implying a reduction in annual precipitation by about 200-300 mm in the area since then. The enamel-δ¹⁸O values of Pliocene obligate drinkers are lower than those of their modern counterparts, most likely indicating a shift in climate to much drier conditions after ~3-4 Ma, consistent with the δ¹³C data. Paleo-temperature estimates derived from the carbonate “clumped” isotope thermometer for the Mid-Pliocene Zanda Basin are higher than the present-day mean annual temperature in the area. After accounting for late Cenozoic global temperature change, these paleo-temperature estimates suggest that the paleo-elevation of the Zanda Basin in the Mid-Pliocene was similar to or slightly (less than ~1 km) lower than its present-day elevation. The results provide new insights into the environmental and tectonic evolution of the area from the Mid-Pliocene to the present.