Relationship of seasonal variations in drip water d13CDIC, d18O, and trace elements with surface and physical cave conditions of La Vallina cave, NW Spain

Cave-monitoring studies clarify the climatic, surface vegetation, and karst processes affecting the cave system and lay the foundation for interpreting geochemical stalagmite records. Here we report the monitoring of cave air, bedrock chemistry, and drip water d(13)C(DIC), d(18)O, and dD, as well as 16 trace elements, covering a full annual cycle spanning the 16 months between November 2019 and March 2021 in La Vallina cave in the northwestern Iberian Peninsula. While decreased rainfall and increased evapotranspiration in the summer months lead to a strong reduction in drip rates, there is little seasonal variation in d(18)O and dD in a given drip, likely reflecting the discrete moderately mixed to well-mixed karst water reservoirs. Small differences in d(18)O and dD between drip sites are attributed to variable evaporation intensity and/or transit times. The carbon isotope signature of the dissolved inorganic carbon of drip water (d(13)C(DIC)) is likely driven by seasonal changes in the temperature controlling biological processes (vegetation and microbial soil activity), resulting in minimum d(13)C(DIC) in summer and autumn months. Increased bedrock dissolution due to higher soil pCO(2) in summer and autumn results in increased trace element concentrations of congruently dissolved elements. Cave air measurements (pCO(2), d(13)C(air), and temperature) indicate the seasonal ventilation (winter) and stagnation (summer) of cave air. The opposite effects of reduced cave air pCO(2), seasonally variable biological activity, and increased drip rate limit the extent of the seasonal variation in degassing and prior calcite precipitation (PCP) supported by trace elements (Sr/Ca index). Estimated stalagmite growth rates using monitoring data suggest biannual phases of potential calcite precipitation in summer and winter and growth cessation during spring and autumn, depending on cave and drip water conditions and the location within the cave, which has important implications for the proxy interpretation of stalagmite records.

Automated summary

Cave-monitoring studies are crucial for understanding the factors influencing cave systems and interpreting stalagmite records. This study conducted a comprehensive monitoring of cave air, bedrock chemistry, and drip water in La Vallina cave over a 16-month period. The results reveal seasonal variations in drip rates, stable isotopes, trace elements, and carbon isotope signatures, which are driven by factors such as rainfall, evapotranspiration, biological processes, and bedrock dissolution. The findings also provide insights into the seasonal ventilation and stagnation of cave air, as well as the growth patterns of stalagmites.