Highly resolved δ13C and trace element ratios of precisely dated stalagmite from northwestern China: Hydroclimate reconstruction during the last two millennia

Compared with the widely used stable oxygen isotope (δ18O), the climate response of the stable carbon isotope (δ13C) of speleothems in monsoonal China has received less attention, mainly because of its relatively complex controlling factors. Nevertheless, an increasing number of studies are suggesting that δ13C can represent hydroclimatic changes. Here, we present highly resolved continuous δ13C and trace element ratio records of a speleothem from Wanxiang Cave, northwestern China, spanning the last two millennia. Our results show that δ13C values oscillated around the mean value on centennial scales, indicating that the overlying vegetation types and soil humidity did not change significantly. High consistency was found between detrended δ13C and δ18O (δ13Cd and δ18Od) records, suggesting that soil moisture balance and vegetation density related to the regional hydrological cycle is an important factor controlling δ13C variations on multidecadal-centennial scales. The regional hydrological cycle is mainly controlled by summer precipitation induced by the Asian summer monsoon (ASM) and the stable prior calcite precipitation (PCP), which was inferred from trace element ratio (Mg/Ca, Sr/Ca, Ba/Ca) records. The δ13C and trace element ratio records, consistent with δ18O throughout the considered periods, are not only closely coupled with the intensity of the El Niño-Southern Oscillation and north-south shift of the Intertropical Convergence Zone but also respond to solar activity and temperature variations in the Northern Hemisphere. However, both δ13C and trace element ratios exhibited a persistent significantly decreasing trend in the last 150 years. Furthermore, the variation of δ18O during the Current Warm Period (AD. 1850–2000) suggests that anthropogenic emissions of atmospheric CO2 and aerosols rather than ASM caused an unusual increase of regional precipitation and depletion of atmospheric δ13C, which induced increased infiltration of atmospheric CO2 and available water into soil pores and caves without any obvious PCP effect. The abnormal changes in our speleothem δ13C are analogous to the oceanic 13C Suess effect, suggesting that stalagmite δ13C record is greatly affected by atmospheric CO2 concentration under global warming.