VARIATION OF δ18O IN THE METEORIC PRECIPITATION, DRIP WATER, AND CALCITE DEPOSIT IN MIAODONG CAVE, LIAONING PROVINCE AND ITS IMPLICATION FOR PALAEOCLIMATIC RECONSTRUCTION

Miaodong Cave is located in northeastern China, a northern margin region of the East Asian Summer Monsoon (EASM). Sub-monthly in-situ monitoring of δ¹⁸O of meteoric precipitation, drip-water, and calcite deposition were carried out at Miaodong Cave, Liaoning province, through two hydrological years (from January 2012 to December 2013) to determine seasonal variability and mechanisms of oxygen isotope transfer in Miaodong Cave.

Oxygen isotope ratio in precipitation (δ¹⁸O_p, VSMOW) exhibits significant seasonal trend, with most negative values in winter snow (~-13.5‰), less negative values in summer rainfall (~-9.6‰), and highest values in spring and autumn rainfall. Annually weighted average value of δ¹⁸O_p is about -9.8‰. The local Meteoric Water Line (LMWL) has smaller slope, which is different from Global Meteoric Water Line (GMWL). This trend is similar to Shihua Cave in northern China but different from caves in southern China.

δ¹⁸O in drip water (δ¹⁸O_d, VSMOW) at three drip sites (MD1, MD3 and MD4) shows extremely constant values at both seasonal and spatial scales with a mean value of -9.6‰ and minor inter-annual variability (about 0.2 ‰ negative in 2013 than in 2012). δ¹⁸O_d ranges between annually weighted average of precipitation and weighted average of rainfall (from April to October). The δD-δ¹⁸O_d in drip water is plotted in a medium position of the linear regression line of rainfall δ¹⁸O_d, which is different from that of snowfall. This implies that drip water δ¹⁸O_d corresponds well with rainfall δ¹⁸O_p and snowfall contributes much less to the drip water.

The variability of δ¹⁸O in calcite deposit of drip water (δ¹⁸O_c) exhibits site-dependent values. At a site with moderate drip rate (MD1), the δ¹⁸O_c is relatively constant, similar to those of feeding drip water. On the contrary, the δ¹⁸O_c shows seasonal trend at drip site MD3, with slower drip rate (less than 2 drops per minute). δ¹⁸O_c at site MD3 is ~0.4‰ heavier in dry season than that in rainy reason. In addition, δ¹⁸O_c of MD3 is slightly (~0.3‰) heavier than that of MD1. This discrepant δ¹⁸O_c values between the two drip sites maybe caused by dynamic fractionations due to much different drip rates.