Tooth Enamel Oxygen and Carbon Isotope Variations in Modern Central Asian Horses: Development of a Calibration Database for the Interpretation of Stable Isotope Signals Preserved in Fossil Horse Remains from Archaeological Sites

Recent studies of early nomadic pastoralism within Central Asia have focused on the rich archaeological history of Mongolia and specifically the examination of so-called khirigsuur sites. These monumental Bronze Age (first and second millennium B.C.) structures consist of stone constructions that contain abundant horse remains (typically skulls) and occasionally human burials. Isotopic analysis of horse remains preserved within these sites may provide a wealth of paleoclimatic information. However, the accuracy of climate reconstructions from fossil teeth collected at archaeological sites in Mongolia is currently limited by insufficient knowledge of the relationship between local environmental conditions and tooth enamel isotope ratios as well as uncertainties attributed to intra-population variability. In this study we measured the oxygen and carbon isotope (δ18O and δ13C) values of enamel samples from modern horse teeth collected along a nearly 550 km latitudinal (northwest-southeast) transect within central Mongolia. Preliminary results suggest that modern tooth enamel records local environmental conditions, although δ18O and δ13C values are not always a direct proxy for climate and vegetation cover. The modern samples serve as a baseline for interpreting isotope signals preserved in fossil horses and thereby increase the accuracy of paleoclimatic reconstructions. Approximately one-hundred individuals were examined from highly varied geographic zones (forest steppe, grassland steppe, dry steppe, and semi-desert) and compared with the δ18O of meteoric waters and δ13C values of local vegetation. Oxygen isotope ratios of bulk enamel samples (spanning the entire vertical axis of tooth growth) parallel spatial trends observed in isotopic composition of meteoric waters and major north-south gradients in temperature and precipitation. Average δ13C values similarly increase with decreasing geographic latitude and likely correspond to documented increases in the relative proportion of C4 grass species. Intra-tooth δ18O variations reflect the pronounced seasonal cycle of precipitation and temperature observed annually in Mongolia. However, these intra-tooth isotopic variations exhibited significantly reduced amplitudes (relative to the isotopic composition of precipitation) and thereby indicate buffered water supplies. Calculated values for the δ18O of ingested water were also typically several per mil higher than the mean isotopic composition of corresponding precipitation, suggesting that horses either consumed evaporatively enriched waters and/or received higher proportions of dietary water from plants. The observed amplitude of δ13C variation within individual teeth (typically less than ~1%) suggests only minimal local seasonal differences in the relative proportion of C3 and C4 grasses consumed and/or minimal seasonal variation in the δ13C of grasses attributed to heat and water stress. Alternatively, the minimal seasonal changes observed within intra-tooth δ13C values may be the direct result of fodder provisioning (i.e., consistent year-round diets).