Seasonal variations of cave dripwater hydrogeochemical parameters and δ13CDIC in the subtropical monsoon region and links to regional hydroclimate

Responses of underground air and drip water geochemistry to meteorological factors: A multi-parameter approach in the Rull Cave (Spain)

Our research aims to assess the complex interactions between the elements that constitute and influence a cave system through the analysis of an extensive dataset of climatic and environmental parameters (222Rn, CO2, drip rates, chemical composition, and environmental isotopes) measured in air, water, and solid in the Rull Cave (southeastern Spain). Of particular importance is understanding the effect of rainfall and temperature on water and gas transport through the epikarst and the involved processes. Our results show that the cave gaseous concentration patterns do not only depend on the temperature-caused movement of air masses, but they can also be affected by abundant rainfall. The δ18O and δD composition of cave water also relies on such precipitations for the effective transfer of the rainfall signal into the cave, which can take between 3 and 7 days. The elemental ratios (Sr/Ca and Mg/Ca) show high responsiveness to the water drip rate, hinting that enhanced prior calcite precipitation (PCP) occurs at slower drip rates. Despite this, and regardless of drip rates, calcite saturation indices follow a seasonal variation pattern inversely proportional to the cave air CO2 concentration, while δ13C-DIC is proportional. Our results show how the interlinkage between these multiple components defines the dynamics of the atmosphere-soil-cave system. Cave monitoring is then essential to understand the karstic vadose zone, which is highly sensitive to climatic influence and its changes.

Sources and transport of CO2 in the karst system of Jiguan Cave, Funiu Mountains, China

Conveyance and modification of carbon-isotope signals within the karst system remain difficult to constrain, due to the complexity of interactions between multiple components, including precipitation, bedrock, soil, atmosphere, and biota. Cave monitoring is thus critical to understanding both their transport in the karst system and dependence on local hydroclimatic conditions. Jiguan Cave, located in Funiu Mountain in central China, is representative of karst tourist caves with relatively thin epikarst zone. We conducted a comprehensive monitoring program of cave climate from 2018 to 2021 and measured δ13C during 2021 in monthly and heavy-rainfall samples of soil CO2, cave CO2, cave water (drip water and underground river), and underground river outlet. Our results demonstrate synchronous variations between CO2 concentration and δ13CCO2 in both soil and cave air on seasonal time scales. Cave pCO2 and carbon-isotope composition further exhibited a high sensitivity to human respiration with fluctuations of ~2000-3000 ppm within 4 days during the cave closure period in July 2021 without tourists. 13C-depleted isotopic signal of cave air in summer is the mixture of human respiration and soil CO2 which varies as a function of regional hydrological conditions of the summer monsoon during the rainy season with high temperatures and humidity. However, respired CO2 from the overlying soil was expected to be the only principal source of the cave CO2 when the anthropogenic CO2 source was removed. The high seasonal amplitude of cave air δ13CCO2 reflects ventilation dynamics, which leads to a prominent contribution from the external atmosphere during winter. Intriguingly, although the δ13C signal reflects complex vertical processes in the vertical karst profile, a heavy summer rainfall event was related to anomalously high δ13C values of cave water that can be utilized to interpret rainfall intensity and regional hydroclimate.

Sulfur-oxygen isotope analysis of SO42- sources in cave dripwater and their influence on the karst carbon cycle

Sulfuric acid is involved in carbonate rock dissolution in karst areas and affects regional hydrogeochemical and carbon cycling processes. Sulfate sources and carbonate weathering at dripwater points of different hydrological types also differ significantly. Therefore, in this paper, three dripwater points (SQ2#, PB and MH6#) of different hydrological types in Dafeng Cave and Mahuang Cave were selected and systematically analyzed. The results show that: (1) ions in the cave dripwater mainly originated from rock weathering, and the water chemistry types were HCO3·Ca-Mg and HCO3-SO4·Ca-Mg. (2) Sulfur and oxygen isotopes reveal that sulfate in the cave water of Shuanghe Cave mainly came from a mixture of soil sulfate and chemical fertilizers. (3) The Simmr model shows that the main sources of sulfate at each dripwater point were natural sources, such as soil sulfate and gypsum dissolution. The natural sources contributed more than 80% of the sulfate in the cave water and were less affected by anthropogenic activities. (4) Carbonate weathering by sulfuric acid (CSW) can accelerate rock weathering and the prior calcite precipitation effect, involving regional CO2 exchange through fissure pipes. The forward model results show that CSW reduced the karst carbon sinks at SQ2#, PB and MH6# by about 19.44%, 23.88% and 12.74%, respectively. Therefore, the impact cannot be ignored in assessing carbon source and sink processes in karst areas.