CONTROLS ON THE OXYGEN ISOTOPIC VARIABILITY OF METEORIC PRECIPITATION, DRIP WATER, AND CALCITE DEPOSIT AT BAOJINGGONG CAVE, CHINA

Chinese speleothem oxygen isotopic composition (δ¹⁸O_s) has been used to interpret variations of East Asian summer monsoon (EASM) history based on assumptions i) the oxygen isotopic composition of meteoric precipitation (δ¹⁸O_p) reflects the local summer precipitation or EASM intensity; ii) the oxygen isotopic composition of drip waters (δ¹⁸O_w) follows changes of δ¹⁸O_p; iii) the oxygen isotopic composition of calcite deposit (δ¹⁸O_c) is under equilibrium fractionation with δ¹⁸O_w. Therefore, an understanding of processes that control the δ¹⁸O variability among meteoric precipitation, drip water, and its corresponding calcite deposit is essential for proper interpretation of speleothem δ¹⁸O.

 A nearly 3-year-long (May 2011- Jan 2014) on-site monitoring program has been carried out with monthly sampling at Baojinggong Cave, South China. Main controls on the δ¹⁸O_p, δ¹⁸O_w, and δ¹⁸O_c at Baojinggong Cave are:

i) The “Circulation effect” of δ¹⁸O_p

Although the δ¹⁸O_p at Baojinggong cave shows a seasonal trend with low values in the summer season and high values in the winter season, the δ¹⁸O_p does not show significant amount effect at seasonal or inter-annual scale at Baojinggong Cave. The δ¹⁸O_p is relatively high if the short-distance water vapor is from the Pacific Ocean; whereas the δ¹⁸O_pvalue is relatively low if the long-distance water vapor is from the Indian Ocean.

ii) The “Evaporation effect” of δ¹⁸O_w

The δ¹⁸O against δD plots of drip waters are slightly below the corresponding Local Meteoric Water Line. The average δ¹⁸O_w at all drip sites are higher than the annually weighted δ¹⁸O_p. And the “peaks” of δ¹⁸O_w correspond with droughts above the cave or low humidity inside the cave. Therefore, seasonal changes of δ¹⁸O_w are caused by evaporation process in the vadose zone and inside the cave.

iii) The “Ventilation effect” of δ¹⁸O_c

CO₂ concentrations are higher in the summer season and lower in the winter season. Drip sites closer to cave entrances have lower CO₂ concentrations. Spatial variations of carbon and oxygen isotopic composition of the calcite deposit show positive correlations. δ¹⁸O_c values during enhanced ventilation periods are higher than those of calculated results based on δ¹⁸O_w and T under equilibrium fractionation.