Relationship of seasonal variations in drip water δ¹³C_DIC, δ¹⁸O and trace elements with surface and physical cave conditions of La Vallina Cave, NW Spain

Abstract. Cave monitoring studies clarify the climatic, surface vegetation, and karst processes affecting the cave system and lay the foundation to interpreting geochemical stalagmite records. Here we report monitoring of cave air, bedrock chemistry, and drip water δ¹³C_DIC, δ¹⁸O and δD as well as 16 trace elements covering a full annual cycle spanning 16 months between November 2019 and March 2021 in La Vallina cave in the Northwest Iberian Peninsula. While decreased rainfall and increased evapotranspiration in summer months lead to a strong reduction in drip rates, there is little seasonal variation of δ¹⁸O and δD in a given drip, likely reflecting discrete moderately- to well-mixed karst water reservoirs. Small differences in δ¹⁸O and δD between drip sites are attributed to variable evaporation intensity and/or transit times. The dissolved inorganic carbon composition of drip water (δ¹³C_DIC) is likely driven by seasonal changes in temperature controlling biological processes (vegetation and microbial soil activity) resulting in minimum δ¹³C_DIC in summer and autumn months. Increased bedrock dissolution due to higher soil pCO₂ in summer and autumn results in increased trace element concentrations of congruently dissolved elements. Cave air measurements indicate seasonal ventilation (winter) and stagnation (summer) of cave air. The opposite effects of reduced cave air pCO₂, seasonally variable biological activity and increased drip rate limit the extent of seasonal variation of degassing and prior calcite precipitation (PCP) supported by trace elements (Sr/Ca-index). Estimated stalagmite growth rates using monitoring data suggest calcite precipitation is restricted to certain seasons (summer and winter) at certain locations within the cave, which has important implications on proxy interpretation of stalagmite records.