5.1 Relationship between local climate and drip water hydrogeochemistry
During the monitoring period, the drip water hydrogeochemical properties exhibited seasonal variations in the cave that were synchronized with the changes between the rainy and dry seasons in the study area (Fig. 3). During the rainy season, the EC, Ca2+ and HCO3− concentrations, and water temperature increased gradually with increasing temperature and precipitation, while the pH decreased. CO2 in the soil layer overlying the cave increased because of plant root respiration and intensified microbial activity during the rainy season. The average soil CO2 concentration at 30 cm was 5774 ppm and the average soil CO2 concentration at 60 cm was 8384 ppm overlying the cave (Ding et al. 2019). The main water source for the cave was the vertical infiltration of meteoric water (Wu et al. 2018). Therefore, the precipitation transported a large amount of CO2 into the cave with increased precipitation during the rainy season and caused a decrease in the pH. Therefore, this result indicates that the overlying soil CO2 concentrations had a significant effect on the pH of the drip water (Baldini et al. 2006; Frisia et al. 2011; Wu et al. 2015). This enhanced the dissolution of the carbonate bedrock, which increased the Ca2+ concentrations in the drip water (SpÖtl et al. 2005). High concentrations of soil CO2 were transported into the cave by precipitation, which increased the cave air CO2 concentrations (Fig. 4). Although the LF Cave was affected by the ventilation effect during the rainy season, the overlying soil CO2 concentrations were dozens times that of atmospheric CO2. Accordingly, soil CO2 was the main factor that affected cave CO2 during the rainy season.
During the dry season, the EC, Ca2+ and HCO3− concentrations, and water temperature decreased gradually with decreasing temperature and precipitation, while the pH increased. The average soil CO2 concentrations at 30 and 60 cm were 3288 ppm and 4856 ppm, respectively. Soil CO2 concentrations were lower than those in the rainy season, which was affected by plant root respiration and weaker microbial activity. With the decrease in precipitation, the dissolution capacity and amount of seepage water formed weakened and decreased, resulting in decreased Ca2+ and HCO3− concentrations in the drip water (Tooth et al. 2003). Due to decreased soil CO2 concentrations and weak seepage water migration capacity, atmospheric CO2 was the main factor affecting cave CO2 during the dry season.
The cave drip water temperature, EC, and Ca2+ and HCO3− concentrations increased slowly during the rainy season. This could be attributed to a mixture of seepage water and “old water” in the epikarst. The infiltrating water transported more CO2, which resulted in the enhancement of water–rock interactions during the transition from the dry season to the rainy season. However, due to infiltrating water mixing with “old water” in the epikarst, the drip water hydrogeochemistry was thereby susceptible to the mixing process. This implies that precipitation and runoff pathways will affect the degree of the hydrogeochemical response to external climate changes (Ban et al. 2008; Wu et al. 2015; Faimon et al. 2016). The cave water temperature, EC, and Ca2+ and HCO3− concentrations all decreased rapidly during the dry season. Precipitation also decreased sharply during the dry season, and precipitation was less than 8 mm during October 2016, 2017, and 2019. Lower precipitation resulted in less water seepage into bedrock pipes or fissures, which is the only recharge water source for the LF Cave. The dissolution of the surrounding rock was reduced, resulting in decreased Ca2+ concentrations in the drip water. Therefore, Ca2+ of drip water differences between the rainy and dry seasons can be used as a proxy for interpreting variations in precipitation. Drip water Ca2+ and HCO3− concentrations can indicate changes in precipitation and temperature, but their ranges are synergistically influenced by water–rock interactions, mixing, runoff pathway, and other factors. Consequently, drip water hydrogeochemistry responds to changes in precipitation and CO2 in the overlying soil. This provides further evidence that a regional climate signal could be recorded by drip water hydrogeochemistry in a ventilated cave and can be used to reconstruct rainy and dry seasonal variations.
5.2 Differences between seasonal and persistent drip water responses
As shown in Fig. 3, EC, Ca2+, and HCO3− concentrations, water temperature, and the pH of the seasonal drip water (LF-1) and persistent drip water (LF-4 and LF-9) sites responded to regional climatic changes during the monitoring period, but had significantly different ranges of change between the sites. This result reflects the migration process, thickness, and water retention of the epikarst, which play important roles in initiating the drip water response (Genty et al. 1998; Guo et al. 2015). Field monitoring results indicate that LF-1 dripped during the rainy season, but stopped dripping during the dry season. Moreover, drip water did not respond to precipitation early in the rainy season when precipitation is preferentially absorbed by the unsaturated soil layer (Kogovek 2007; Nathan et al. 2011). When the soil moisture content in the replenishment area was saturated, seasonal drip water hydrogeochemistry responded to regional precipitation. The hydrogeochemical properties of the seasonal drip water increased with the rapid inflow of mixed water, then decreased due to the dilution effect of increased precipitation during the rainy season. The sharp variations in EC and Ca2+ and HCO3− concentrations indicate that dense fissures and large fractures in the overlying bedrock generated high permeability (Guo et al. 2015). The LF-4 and LF-9 hydrogeochemical properties increased during the rainy season and decreased during the dry season, water dripped continuously, and modern carbonate deposits formed throughout the year. This may be caused by sparse fissures and small fractures in the overlying bedrock that generated low permeability and thick epikarst. The hydrogeochemical properties of seasonal drip water first increased and then decreased during the rainy season, but these values continuously and slowly increased in persistent drip water (Figs. 3 and 5). This difference resulted from differing thicknesses of the overlying bedrock and the flow path at each drip site. Consequently, the hydrogeochemistry of the different types of drip water was affected by hydrological conditions and had different responses to regional climate (Roberts et al. 1998; Treble et al. 2003; Fairchild et al. 2000; Gregory et al. 2009). This yielded different hydrogeochemical values (Mcdonald et al. 2007), but did not change the response of the drip water to regional climate.
In addition, LF-1 can record regional climate information for the entire year using hydrogeochemistry and modern carbonate deposits in high precipitation year (e.g., 2015). However, this site can only record regional climate information during the rainy season in seasonally dry years (e.g., 2016, 2017, and 2019). Accordingly, speleothems from this site would merely record years with heavy precipitation or rainy seasons on annual and seasonal scales that were formed by seasonal drip water. Persistent drip water can record regional climate information throughout the year. However, there were differences in the response times and variation amplitudes between the two persistent drip water sites caused by differences in hydrodynamic processes, water–soil interactions, water–rock interaction times, and material sources. This indicates that different drip waters record different climate information in the same cave. If only a single speleothem from this cave was used to reconstruct the paleoclimate, a significant amount of uncertainty would be introduced to quantitative calculations.
5.3 Local drought and extreme precipitation events
An ENSO event led to an abnormal increase in precipitation during the dry season in Guilin from November 2015 to January 2016 (Wu et al. 2020). In other years, precipitation in the dry season decreased significantly, especially in October. The abnormal changes in precipitation produced responses in the hydrogeochemical properties of the drip water in the LF Cave. The Ca2+ and HCO3− concentrations and EC of the drip water had increasing trends that were affected by extreme precipitation events (Fig. 3) The increased Ca2+ concentrations ranged from 4 to 72 mg/L, the HCO3− concentrations ranged from 0.2 to 3.6 mmol/L, and EC ranged from 3 to 301 µS/cm. The pH and water temperature of the drip water both decreased over time. The pH decrease ranged from 0.12 to 0.28, and the the decreasing of water temperature ranged from 1.1°C to 1.8°C. The pH value decreased because of the infiltration of large amounts of precipitation, which dissolved soil CO2 and generated HCO3−. Moreover, this water can dissolve the carbonate bedrock and may have influenced the Ca2+ of drip water increase (Wu et al. 2018). The drip water temperature decreased owing to precipitation derived from subsurface water percolating into the cave during extreme precipitation events (Guo et al., 2019). Therefore, the hydrogeochemistry of drip water can respond to extreme precipitation events.
The EC and Ca2+ and HCO3− concentrations of seasonal drip water changed more drastically due to extreme precipitation events than persistent drip water, but the durations of these changes were shorter than those in persistent drip water. Moreover, seasonal drip water stopped dripping during seasonal drought events, while persistent drip water continued to drip. This indicates that the overlying bedrock at the seasonal drip water sites was highly permeable and had an active connection with the surface that led to hydrogeochemistry that was very sensitive to variations in precipitation. During extreme precipitation events, the hydrogeochemical values (EC, Ca2+, and HCO3− concentrations) of both types of drip water were greater than the maximum values observed during the other rainy seasons in the study period. Therefore, the hydrogeochemistry at these sites was controlled by the amount of precipitation during this time. The alternating dry and rainy seasons could be determined using the chemistry of the drip water. However, extreme precipitation events may obscure dry season information in drip water chemistry when such events occur.
In addition, persistent drip water recorded not only extreme precipitation events (November 2015), but also seasonal drought events (October 2016, 2017, and 2019). The hydrogeochemistry of the persistent drip water did not exhibit abnormal decreases during seasonal drought events, and the amplitudes of the two drip water types were different. This mainly contributes to the difference in the retention time of seeping water (Musgrove et al. 2004; Liu et al. 2004). Therefore, monitoring multiple drip water sites in the same cave and analyzing their responses to climate is crucial for understanding this type of variability in climate records.