Abstract: Background, aim, and scope The Cenozoic evolution of the aridification of the Asian interior has attracted major research interest in recent years. However, the history of aridification during the Eocene has been neglected and the forcing mechanism remains controversial. Xining Basin, located on the northeastern margin of the Tibetan Plateau, contains a thick sequence of Cenozoic fine grained sedimentary sequences which have been dated using high-resolution paleomagnetic studies. Thus, Xining Basin is well suited for studying the history of regional aridification and its forcing mechanisms. In this study, we selected two parallel early Cenozoic sections in the Xining Basin spanning the interval from ~43 Ma to ~35 Ma, for analysis of the stable isotope composition of total organic carbon (δ13 CTOC). Our aim was to reconstruct the history of aridification during the Eocene. Materials and methods The mudstone was sampled at intervals of 1—2 m with 96 samples being obtained from the East Xining Section, and 94 samples from the Shuiwan Section. The samples were pretreated with 2 mol/L HCl to remove carbonate and the stable carbon isotope composition f the resulting CO2 was measured on a Thermo Delta V mass spectrometer interfaced with a Flash EA 1112 elemental analyzer. Results The δ13 CTOC records of the two parallel sections in Xining Basin both exhibit an abrupt ~2.5‰ increase at ~39 Ma. Discussion The distribution of n-alkanes and the δ13 CTOC values indicate the organic matter is mainly derived from terrestrial C3 plants; provenance of the sediments remained constant. Thus, the increase of δ13 CTOC at ~39 Ma in Xining Basin can be mainly attributed to environmental factors that have influenced the carbon isotopic composition of terrestrial C3 plants. The concentration and carbon isotopic values of atmospheric CO2 can be excluded from the list of potential causal factors. Precipitation and temperature both have a negative relationship with the carbon isotopic composition of terrestrial C3 plants, the effect of the former is stronger. Notably, there was no abrupt decrease in global temperature at ~39 Ma and therefore, a decrease in precipitation was the principal cause of the carbon isotopic shift at ~39 Ma in the Xining Basin. An aridification enhancement event at ~39 Ma is also recorded by other proxies, such as pollen and spore assemblages, from other sections in Xining Basin as well as Qaidam Basin. It is likely that the retreat of Paratethys Sea was the main factor responsible for the reduction in precipitation in Xining Basin, against the background of global cooling. Conclusions The stable carbon isotope records of the total organic fraction of the sediments of Xining Basin reveals an abrupt enhancement of aridification happened at ~39 Ma, which is confirmed by other proxy records from the region.. The retreat of Paratethys Sea was likely the principle cause, while global cooling played a less important role. Recommendations and perspectives The explanation for Asian inland aridification at ~39 Ma still remains controversial and further research is needed to determine the cause.
Keywords: Xining Basin; ~39 Ma; total organic carbon isotope; Asian inland aridification; retreat of Paratethys Sea