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 (δ¹³C_d and δ¹⁸O_d) 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.