GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 252-9
Presentation Time: 9:00 AM-6:30 PM

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


TIAN, Lijun1, TAN, Ming2, DUAN, Wuhui2 and GAO, Yongli3, (1)Department of Geological Sciences, University of Texas at San Antonio, San Antonio, TX 78249; Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China, (2)Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China, (3)Center for Water Resources, Department of Geological Sciences, University of Texas at San Antonio, San Antonio, TX 78249, tianlj87@foxmail.com

Chinese speleothem oxygen isotopic composition (δ18Os) has been used to interpret variations of East Asian summer monsoon (EASM) history based on assumptions i) the oxygen isotopic composition of meteoric precipitation (δ18Op) reflects the local summer precipitation or EASM intensity; ii) the oxygen isotopic composition of drip waters (δ18Ow) follows changes of δ18Op; iii) the oxygen isotopic composition of calcite deposit (δ18Oc) is under equilibrium fractionation with δ18Ow. 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 δ18Op, δ18Ow, and δ18Oc at Baojinggong Cave are:

i) The “Circulation effect” of δ18Op

Although the δ18Op at Baojinggong cave shows a seasonal trend with low values in the summer season and high values in the winter season, the δ18Op does not show significant amount effect at seasonal or inter-annual scale at Baojinggong Cave. The δ18Op is relatively high if the short-distance water vapor is from the Pacific Ocean; whereas the δ18Opvalue is relatively low if the long-distance water vapor is from the Indian Ocean.

ii) The “Evaporation effect” of δ18Ow

The δ18O against δD plots of drip waters are slightly below the corresponding Local Meteoric Water Line. The average δ18Ow at all drip sites are higher than the annually weighted δ18Op. And the “peaks” of δ18Ow correspond with droughts above the cave or low humidity inside the cave. Therefore, seasonal changes of δ18Ow are caused by evaporation process in the vadose zone and inside the cave.

iii) The “Ventilation effect” of δ18Oc

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. δ18Oc values during enhanced ventilation periods are higher than those of calculated results based on δ18Ow and T under equilibrium fractionation.