Pakistan mammal tooth stable isotopes show paleoclimatic and paleoenvironmental changes since the early Oligocene

Abstract

This research presents an isotopic study of three large mammal groups (elephantoid proboscideans; hyracodontid and rhinocerotid rhinocerotoids; equids of the Hipparion-group) from the Bugti Hills sequence (eastern Balochistan, Pakistan) combined with published data from the Miocene Siwalik group (northern-central Pakistan). These two exceptional fossil records from Pakistan provide an unambiguous source of biotic information from the early Oligocene up to the late Miocene. The δ 13C C and δ 18O C results from the same herbivore groups show several significant variations in environmental and climatic conditions throughout the ~ 30 to 7.5 Myr period in this region. First, from ~ 30 to 22 Myr, the environment is characterized by a relatively dry but dense forest existing under a temperate to subtropical climate, with a more closed tropical environment at 23 Myr. Oligocene δ 18O of the terrestrial herbivore teeth agrees with the marine foraminifera δ 18O and Mg/Ca trends, suggesting that the Pakistan region was affected by global climate rather than regional conditions. After 22 Myr, the δ 18O C mammal enamel results show a long term decreasing trend until 15 Myr toward a wetter environment with the appearance of tropical forests, in response to the probable onset of the Asian monsoon. This is a period of high tectonic activity with the India–Asia collision leading to uplift of the Tibetan Plateau and shrinkage of the Epicontinental Paratethys Sea. This activity modified the land-sea distribution and the thermal contrast between ocean and continent, the driving force of the monsoon climate. The middle Miocene environment in Pakistan experienced moist conditions under an intense monsoon system with relatively closed rainforest. After 12 Myr, the δ 18O C enamel values record a positive shift until 9.3 Myr toward a drier environment with an increase of seasonality as suggested by higher δ 18O C variability after 9.3 Myr. These late Miocene drier climatic conditions and the Asian monsoon intensification could potentially favour the C 4 photosynthetic pathway and hence the development of grassland to the detriment of subtropical forests, resulting in the higher enamel δ 13C values recorded after 9.3 Myr.