Transmission characteristics and the factors influencing stable oxygen isotopes in precipitation, soil water, and drip water in Remi Cave, Western Hunan, China

Understanding the modern transport processes of δ18O between different waters in cave systems can provide a basis for the paleoenvironmental interpretation of records of stalagmite δ18O. We conducted monthly in-situ monitoring of local precipitation, soil water at three different soil depths, five drip water sites and environmental parameters in Remi cave, Western Hunan Province, Central China, from December 2020 to January 2024. During the monitoring period, due to differences in geological structure and ventilation effect at different locations within the cave, cave air temperature and relative humidity gradually remained constant from the cave entrance to the deep parts of the cave. On intra-annual timescales, precipitation δ18Op were depleted in the wet season and enriched in the dry season, which were mainly controlled by changes in moisture sources, upstream convective activity, and rainout effects. With the increasing thickness of the soil layer above the cave, soil water δ18Osoil gradually became negative and leveled off, possibly because shallow soil water responded sensitively to local year-round precipitation, accompanied by surface evaporation. While, deep soil water with more negative δ18O, may be mainly replenished by heavy precipitation during the wet season, but the effect of water retained in the soil from preceding years couldn’t be excluded and would be needed to verify by conducting more monitoring studies on the variation of δ18Osoil covering the long period. The mean drip water δ18Od values at sites D1 and D2 near the cave entrance were heavier than that of shallow soil water and precipitation, possibly due to evaporation from the thin overlying soil layer, together with ventilation effect. There were seasonal variations of δ18Od values at site D5 in the deep parts of the cave, which may be associated with the seasonal variation of drip rate at this site. Due to the smoothing effect of soil layer and epikarst zone, there were no seasonality in δ18Od values at sites D3 and D4, in the middle and deep parts of the cave. The mean δ18Od at these two sites were lighter than amount-weighted annual mean δ18Op, and close to the deep δ18Osoil and the wet-season amount-weighted mean δ18Op values. This suggests that the thick soil layer overlying the cave raises the precipitation threshold, causing these sites to be mainly recharged by heavy rainfall in the wet season. Therefore, the δ18O of stalagmites growing under these drip sites may mainly inherit the wet-season δ18Op signal. The short monitoring period of this study (three years) and the lack of modern calcite δ18O data, mean that caution is need in interpreting stalagmite δ18O on a timescale longer than that of this study.