Stable carbon and oxygen isotopic evidence for Late Cenozoic environmental change in Northern China
Abstract Stable carbon and oxygen isotope ratios of 311 enamel samples from a diverse group of herbivorous mammals including Equidae, Rhinocerotidae, Bovidae, Rodentia and Ochotonidaewere were analyzed in order to reconstruct the Late Cenozoic history of vegetation and environmental change in the Yushe Basin in North China. The δ13C values of bulk and serial enamel samples from large mammals show a wide range of variation from − 13.3‰ to 1.4‰, with a mean of − 7.4‰ ± 3.5‰ (n = 294). This indicates that large herbivorous mammals in the area had a variety of diets since 6.5 Ma, ranging from pure C3 to mixed C3–C4 and pure C4 diets. In contrast, the δ13C values of small mammals vary from − 11.9‰ to − 7.6‰, with a mean of − 9.7 ± 1.1‰ (n = 17), indicating that rodents and ochotonids were feeding mostly on C3 plants. Variations in δ13C values within and between species reflect the variations in the habitat and the vegetation consumed by the animals. In general, horses had higher amounts of C4 grasses in their diets than other contemporary taxa such as bovids, rhinos, rodents and deer, suggesting that horses exploited more open habitats such as grasslands while deer, rhinos and rodents may have preferred more C3 vegetation, which is more indicative of forested environments. The carbon isotope data show that C4 grasses have been an important component of horses' diets and of local ecosystems since ~ 6.5 Ma, confirming that the “late Miocene C4 expansion” occurred in North China as it did in Africa, Indian subcontinent and the Americas. This supports a global factor as a main driver of the late Miocene C4 expansion. The combined carbon and oxygen isotope data reveal major shifts in climate to drier and/or warmer conditions after ~ 5.8, ~ 4.1, ~ 3.3, and ~ 2.5 Ma, and significant shifts to relatively wetter and/or cooler conditions after ~ 6.4, ~ 5, ~ 3.5 Ma. The shifts to drier and/or warmer climate after ~ 5.8 Ma and ~ 2.5 Ma coincide with two major fauna turnover events. Intra-tooth δ13C and δ18O values are negatively correlated within individual modern teeth and some fossil teeth, displaying the characteristic pattern of the summer monsoon regime and confirming a strong monsoon influence in the area since at least the early Pliocene. The data also suggest that the C4 abundance in the area has fluctuated over the past 6.5 Ma in response to changes in climate, with more C4 grasses during warmer and/or drier periods and a reduced C4 component at cooler and/or wetter times.