Diet and environment of a mid-Pliocene fauna from southwestern Himalaya: Paleo-elevation implications

A mid-Pliocene fauna (4.2–3.1 Ma) was recently uncovered in the Zanda (Zhada) Basin in the southwestern Himalaya, at an elevation of about 4200 m above sea level. These fossil materials provide a unique window for examining the linkage among tectonic, climatic and biotic changes. Here we report the results from isotopic analyses of this fauna and of modern herbivores and waters as well as paleo-temperature estimates from the Zanda Basin. The δ¹³C values of enamel samples from modern wild Tibetan asses, and domesticated horses, cows and goats in the area are −9.4±1.8‰, which indicate a diet comprising predominantly of C₃ plants and are consistent with the current dominance of C₃ vegetation in the region. The enamel-δ¹³C values of the fossil horses, rhinos, deer, and bovids are −9.6±0.8‰, indicating that these ancient mammals, like modern herbivores in the area, also fed primarily on C₃ vegetation and lived in an environment dominated by C₃ plants. The lack of significant C₄ plants in the basin suggests that the area had reached high elevations (>2.5 km) by at least the mid-Pliocene. Taking into account the changes in the δ¹³C of atmospheric CO₂ in the past, the enamel-δ¹³C values suggest that the average modern-equivalent δ¹³C value of C₃ vegetation in the Zanda Basin in the mid-Pliocene was ∼1–2‰ lower than that of the C₃ biomass in the basin today. This would imply a reduction in annual precipitation by about 200–400 mm in the area since then (assuming that the modern C₃δ¹³C–precipitation relationship applied to the past). Consistent with this inference from the δ¹³C data, the enamel-δ¹⁸O data show a significant shift to higher values after the mid-Pliocene, which also suggests a shift in climate to much drier conditions after ∼4–3 Ma. Paleo-temperature estimates derived from a fossil bone-based oxygen isotope temperature proxy as well as 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 cooling, 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, which is consistent with the inference from the δ¹³C data.