Stable Carbon and Oxygen Isotopes in Cetaceans and in Terrestrial Herbivores as Indicators of Diet, Movement and Environment

Stable oxygen and carbon isotopic compositions of modern and fossil cetacean teeth and bones were analyzed to examine if and how they reflect the diet, habitat preference and migratory patterns of modern whales and to reconstruct the oxygen isotopic compositions of ancient ocean waters in the Mio-Pliocene. In addition, as a separate project, terrestrial mammal teeth and bone samples from Yushe Basin in North China were analyzed to examine long-term changes in diets and environments of mammals in the area over the past 6-7 million years to understand the development of C4 ecosystems in North China and the effects of Tibetan uplift on regional climate and ecosystems. The major results are as follows: (1) The oxygen isotopic compositions of phosphate (δ18Op) in teeth and bones from 5 different modern cetacean species including sperm whale, pygmy sperm whale, short-finned pilot whale, killer whale and Cuvier's beaked whale were analyzed. The δ18Op values range from 16.7 / to 21.3 /, averaging 19.6±1.0 / (n=89), for tooth samples, and from 15.5 / to 19.7 /, with an average of 19.0±0.9 / (n=47), for ear bone samples. The new δ18Op data, along with data compiled from literature, were used in conjunction with modern ocean δ18Osw data to examine the relationships between δ18Op and δ18Osw, yielding the following equations: Teeth: δ18Op= 19.173 + 0.9296 δ18Osw R2 = 0.8723 Bones: δ18Op = 18.109 + 0.8436 δ18Osw R2 = 0.86 The new equations, when applied to fossil teeth and bones yielded more reasonable estimates of ancient ocean water δ18O values than the original equation given in Yoshida and Miyazaki (1991). Intra-tooth isotopic variations were observed within individual teeth. Among the selected species, the killer whale (O. orca) displayed the most depleted δ18Op values (~18 /) and the largest intra-tooth δ18O variation (2.8 /), reflecting its habitat preference and migratory behavior. (2) Stable carbon and oxygen isotopic compositions of teeth and ear bones from 19 modern whales representing 4 different toothed whale species of the sperm whale, pygmy sperm whale, short-finned pilot whale and killer whale were analyzed. While tooth-δ13C values range from -8.2 / to -12.7 /, averaging -10.5±1.2 / (n=87), ear bone δ13C values range from -9.9 / to -11.8 /, averaging -10.7±0.6 / (n=19). The δ13C and δ18O values are generally consistent with their known diets and habitats. Synchronous positive shifts in the δ13C chronological profiles of all teeth from short-finned pilot whales (G. macrorhynchus) from Florida might indicate a shift in diet and/or habitat from coastal areas to offshore areas and feeding on relatively high δ13C preys. Structural carbonate in bioapatite appears to be enriched in δ13>C by ~3 / relative to collagen and by ~8 / relative to muscles. This suggests a carbon isotopic offset of ~9 / due to biochemical fractionation between structural carbonate in bioapatite and the diet. The δ13C data reveal that the diet-δ13C values of killer whales were about -20 - -21 /, consistent with a fish-based diet. The diet-δ13C values of the sperm whale, short-finned pilot whale and pygmy sperm whale were ~ -18- -19 /, in agreement with a diet consisting of both cephalopod and fish. (3) Stable carbon and oxygen isotope ratios of 310 enamel samples from a diverse group of herbivorous mammals including Equidae, Rhinocerotidae, Bovidae, Rodentia and Ochotonidae from Yushe Basin, North China were analyzed. 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 from Rodentia and Ochotonidae families vary from –11.9 / to -7.6 /, with a mean of -9.7±1.1 / (n=16), indicating that theses small mammals were feeding predominantly on C3 plants. 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 the 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 biomass at cooler and/or wetter times.