Geochemical responses of cave drip water to vegetation restoration

Abstract

The transmission of stable isotopes and elements/ions from the outside to the inside of a cave and their incorporation into drip water can involve numerous biogeochemical processes. To understand how the original signals of stable isotopes and elements/ions are modified by these processes, integrated studies of the interactions between vegetation, soil, epikarst, and caves are required. We conducted a multi-year monitoring study of the vegetation biomass, tree breast-height diameter, PCO2 in soil air and cave air, δ13C in soil air, stable oxygen isotope in rainwater and drip water, and stable carbon isotope and elements/ions concentrations in drip water in Shawan Cave system, southwest China. The main results were as follows: (1) The evaporation effect weakened and the transpiration effect strengthened outsides the cave as vegetation improved, thus leading to a year-by-year increasing trend in the δ18O value of drip water. This indicates that changes in vegetation may have been another potential factor influencing the interannual variation of the δ18O value of drip water. (2) The CO2 concentration and δ13C value in soil air increased and decreased, respectively, with vegetation restoration, which caused the interannual variation in the dissolved inorganic carbon isotope (δ13CDIC) value of drip water during the autumn and winter to exhibit a year-by-year decreasing trend. (3) The variations in the elements/ions concentrations of drip water were affected by vegetation uptake, vegetation transpiration, and water–rock interactions. It is inferred that the interannual variation in the elements/ions concentrations of drip water responded to vegetation restoration. (4) A conceptual model demonstrated that the three response modes of drip water δ18O value, δ13CDIC value, and elements/ions to variations in vegetation. Overall, this study highlights the responses of the interannual changes in δ13CDIC, δ18O, and elements/ions of drip water to vegetation restoration, which contributes critical insights into the paleoenvironmental interpretation of proxies of speleothems.