[M-IS17] Paleoclimatology and paleoceanography

[MIS17-P14] cave air monitoring and oxygen isotopic variation in drip water at Inazumi Cave, Oita, Japan

*Tatsuro Shindoh¹ (1.Graduate school of Earth and Planetary Science, Kyoto University)

Keywords:stalagmite, δ18O, paleoclimatology

One year cave air monitoring and isotopic analysis of drip water were conducted at Inazumi Cave, Oita, Japan from February 2014 to February 2015 to interpret the behavior of drip water’s δ¹⁸O. The monitoring and sampling water were done every month. For the cave air monitoring, cave air temperature (T-_{cave air}), relative humidity (RH) and CO₂ (pCO_{2-cave air}) were measured. For sampling drip water, two different sampling methods were performed at two different drip sites, SS2 and SS3. At SS2, the drip water before and after hitting on handrails were sampled and named as SS2U, M2, M3 and L sequentially. At SS3, the water before contacting with cave air (WBCA) and the ones after contacting with cave air were sampled and named as SS3U and L. Meteoric water around Inazumi cave was also collected every month. For isotopic analysis, δ¹⁸O of drip water and meteoric water around the cave was measured.
As the result, T_{-cave air} and RH were very stable through a whole year (16.2℃ for T_{-cave air} and 100% for RH) while pCO_{2-cave air} showed distinct seasonal variation: higher in summer and lower in winter. The δ¹⁸O of meteoric water showed seasonal trend and the one on August showed the lowest value (-13.23‰) and March was the highest (-4.68‰). The δ¹⁸O of the drip water at SS2, even though the drip water hit on the handrails, showed little sequential variations and very stable at all sampling points through a whole year (-8.00, -8.00, -8.01 and -7. 98 ‰, p<0.001 for SS2U, M2, M3 and L, respectively). Likewise, at SS3, WBCA, SS3U and SS3L showed stable δ¹⁸O (-8.04, -8.05 and -8.05 p<0.001 for WBCA, SS3U and SS3L respectively). δ¹⁸O of the drip water at SS2 and SS3 was plotted on/ close to the Meteoric Water Line made by the δ¹⁸O of meteoric water around Inazumi Cave, indicating that the drip water at Inazumi cave was originated from meteoric water around Inazumi cave and that the δ¹⁸O of meteoric water was mixed well in the host carbonate rock and homogenized δ¹⁸O of drip water was produced, offsetting seasonal variations of drip water’s δ¹⁸O. Besides, the mean δ¹⁸O of precipitation from May to October was the closet to the mean δ¹⁸O of drip water at SS2 and SS3, indicating that the δ¹⁸O of precipitation on August, showing the lowest δ¹⁸O, probably contributes to the overall mean δ¹⁸O of drip water.
Stalagmite farming is going to be initiated in the future to interpret how the δ¹⁸O of CaCO₃ is changed and fractionated from the one of drip water.