Understanding the modern transport processes of δO between different waters in cave systems can provide a basis for the paleoenvironmental interpretation of records of stalagmite δO. 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 δO 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 δO 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 δO, 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 δO covering the long period. The mean δO values of drip water 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 δO 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 δO values at sites D3 and D4, in the middle and deep parts of the cave. The mean δO at these two sites were lighter than amount-weighted annual mean δO, and close to the deep δO and the wet-season amount-weighted mean δO 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 δO of stalagmites growing under these drip sites may mainly inherit the wet-season δO signal. The short monitoring period of this study (three years) and the lack of modern calcite δO data, mean that caution is need in interpreting stalagmite δO on a timescale longer than that of this study.

中文翻译：

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湘西热米洞降水、土壤水和滴水中稳定氧同位素传输特征及影响因素

了解洞穴系统中不同水域之间δ18O的现代传输过程可以为石笋δ18O记录的古环境解释提供基础。 2020年12月至2024年1月，我们对湖南西部热米洞当地降水量、3个不同土层深度的土壤水、5个滴水点以及环境参数进行了月度原位监测。监测期间，由于由于洞内不同地点地质结构和通风效果的差异，从洞口到洞深处，洞内空气温度和相对湿度逐渐保持恒定。年内时间尺度上，降水δ18O在雨季减少，在旱季增加，主要受水汽来源变化、上游对流活动和降雨影响的控制。随着洞穴上方土层厚度的增加，土壤水δO逐渐变为负值并趋于平稳，可能是因为浅层土壤水对当地常年降水反应敏感，并伴有地表蒸发。而负δ18O较多的深层土壤水，可能主要由雨季强降水补充，但不能排除往年土壤中滞留水的影响，需要通过更多的监测研究来验证δ18O 的长期变化。洞口附近D1和D2点的滴水平均δ18O值比浅层土壤水和降水重，可能是由于上覆土层薄层的蒸发和通风作用所致。洞穴深处D5点δ18O值存在季节变化，这可能与该点滴水率的季节变化有关。由于土层和表层岩溶带的平滑作用，洞穴中部和深处的D3和D4点δ18O值不存在季节性。这两个站点的平均 δ18O 值均小于年平均 δ18O 量加权值，接近于深部 δ18O 值和雨季量加权平均 δ18O 值。这表明洞穴上方的厚土层提高了降水阈值，导致这些地点主要由雨季的强降雨补给。因此，这些滴水点下生长的石笋的δ18O可能主要继承了雨季的δ18O信号。本研究的监测周期较短（三年）且缺乏现代方解石 δ18O 数据，这意味着在比本研究更长的时间尺度上解释石笋 δ18O 值时需要谨慎。