Multi-Timescale Variations of ¹⁸O-¹³C in Stalagmites: Insights Into Isotopic Disequilibrium and Human Activities

The reasons for covariations of speleothem delta C-13 and delta O-18 remain controversial, primarily due to the limited high-resolution stalagmite records that can be compared with meteorological data. This study presents delta C-13 and delta O-18 records of two coeval, annually laminated stalagmites (TS9701 and TS9501) spanning the past similar to 2000 years from Shihua cave, Beijing, northern China. The low correlation between stalagmite TS9701 records, with annual resolution, and local annual precipitation as well as mean annual temperature on interannual to decadal scales indicates that the positive covariations of delta O-18 and delta C-13 in TS9701 are partly attributed to kinetic isotope effects caused by rapid CO2 degassing. Isotopic disequilibrium between HCO3(aq)- and drip water, induced by prior calcite precipitation on cave ceiling and stalactite surface, is another potential contributing factor. delta O-18 and delta C-13 exhibits distinct patterns on multidecadal to millennial timescales. delta O-18 records show notable centennial variability, aligning with El Ni & ntilde;o-Southern Oscillation cycles. In contrast, delta C-13 profiles reveal a decreasing trend during the first similar to 750 years, followed by an increasing trend. Prior to 1588 AD, variations in delta C-13 broadly correspond to changes in warm season temperature and/or moisture on centennial scale. Both delta C-13 records show an abrupt enrichment between 1588 and 1654 AD. Historical documents indicate that this anomaly is likely attributed to coal mining and resultant deforestation around Shihua cave during late-Ming and early-Qing Dynasties. In summary, while isotopic disequilibrium can cause high-frequency covariations of speleothem delta O-18 and delta C-13, it does not erase the imprints of climate changes and human activities on multidecadal to millennial timescales.