Geochemical and hydrological controls on aragonite versus calcite precipitation in speleothems and paleoclimatic implications

Speleothem mineralogy (aragonite vs. calcite) and geochemical signatures are governed by complex hydrological and geochemical processes. However, previous studies have predominantly focused on geochemical constraints, whereas the role of cave hydrological processes in regulating carbonate polymorphism has received far less attention. Furthermore, long-term in situ monitoring of the environmental conditions controlling aragonite and calcite precipitation in natural cave settings remains limited. Here, we combine three years of cave monitoring in Shennong Cave, southeastern China, with glass-plate observations and a global dripwater compilation to show that carbonate polymorphism is governed by threshold effects in Mg/Ca–calcite saturation index (SI_cc) space, and that precipitation-driven prior carbonate precipitation (PCarbP) favors aragonite precipitation by increasing Mg/Ca while maintaining relatively low SI_cc. At Mg/Ca values ≥ 1000 mmol/mol, Mg/Ca defines the threshold for aragonite occurrence and crystal elongation, whereas pH and SI_cc are most closely associated with depositional proportion and crystal habit, respectively, with aragonite evolving from slender needles to shorter, thicker bundles as supersaturation increases. At aragonite-precipitating sites, PCarbP during wetter periods drives spatial partitioning of carbonate polymorphs on glass plates, with calcite precipitating at the drip-impact point and aragonite toward the margins, providing direct modern-process evidence for calcite–aragonite fabrics in speleothems. Monitoring results further show that aragonite- and calcite-precipitating sites differ in their hydrological setting, the extent of PCarbP, and the magnitude of Mg/Ca fluctuations. At the seasonal scale, reduced effective infiltration and intensified in-cave degassing drive substantial Mg/Ca increases at aragonite-precipitating sites, whereas lower soil and epikarst pCO₂ primarily enhances PCarbP at calcite-precipitating sites. At the interannual scale, dripwater Mg/Ca declined from approximately 2000 to 500 mmol/mol from 2022 and 2025, broadly tracking the long-term increase in regional precipitation. Overall, this study demonstrates how cave hydrology and PCarbP control carbonate polymorphism in cave systems, and indicates that dripwater Mg/Ca reflects different hydroclimatic controls across seasonal and interannual timescales, improving the interpretation of speleothem mineralogy and geochemical proxies in paleoclimate studies.