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We report the results of a stable isotope study of a late Pliocene fauna recently discovered in the Kunlun Mountain Pass area (similar to 4700 m above sea level) on the northern Tibetan Plateau. The delta C-13 values of enamel samples from modern herbivores from the Kunlun Pass Basin range from -14.8 to -10.6%. with a mean of -12.0 +/- 0.7 parts per thousand, indicating pure C3 diets consistent with the current dominance of C3 vegetation in the area. In contrast, enamel samples from fossil herbivores yielded delta C-13 values of -5.4 parts per thousand to - 10.2 parts per thousand (with a mean of -7.9+/-1.3 parts per thousand), significantly higher than those of modern herbivores in the area. The higher delta C-13 values indicate that these ancient herbivores, unlike their modern counterparts, had a variety of diets ranging from pure C3 to mixed C3/C4 vegetation. The local ecosystems in the Kunlun Pass area in the late Pliocene likely included grasslands that had small amounts of C4 grasses. The delta O-18 values of enamel from large herbivores shifted to higher values after the late Pliocene, indicating a significant change in the delta O-18 of local meteoric water. We estimate that there has been approximately 3.2 parts per thousand increase in annual delta O-18 values of meteoric water since similar to 2-3 Ma, most likely driven by changes in the regional hydrological cycle possibly as a result of tectonic and climate change. The delta O-18 values of fossil fish teeth/bones and gastropod shells, along with abundance of aquatic plants and other invertebrate fossils, clearly indicate that the Kunlun Pass Basin once had plenty of water and was occupied by a freshwater lake in the late Pliocene. Our isotope data from both terrestrial and aquatic fossils suggest that the Kunlun Pass Basin was a hospitable place with a much warmer and wetter climate in the late Pliocene, very different from today’s rock desert and cold steppe environments. The mean annual temperature in the late Pliocene estimated from the delta O-18 of fossil bone carbonate and paleo-water was about 10+/-8 degrees C, much higher than the present-day mean annual temperature in the basin. If valid, the estimated temperature change would imply that the elevation of the basin has increased by similar to 2700+/-1600 m since similar to 2-3 Ma. (C) 2008 Elsevier B.V. All rights reserved.