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

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 δ13CDIC, 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 (δ13CDIC) is likely driven by seasonal changes in temperature controlling biological processes (vegetation and microbial soil activity) resulting in minimum δ13CDIC 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.

Exploring soluble and colloidally transported trace elements in stalagmites: The strontium-yttrium connection

“…Following observations by Fairchild et al (2010), Baldini et al (2012) and drip water monitoring in La Vallina (Kost et al, 2022; Fig. S4), we propose that drip water Y delivery (3) is much smaller than the variations seen in the stalagmite record, and this effect is therefore subordinate to crystallization dynamics.…”

“…An Y delivery is estimated for each drip site, amounting to $0.7-5700 lg yr À1 using average drip rates and drip water Y concentrations from January and February 2021 (Table 2, Fig. S4; see Kost et al, 2022). Yttrium deposition in the stalagmites is calculated from growth rate (lm yr À1 ), Y concentration (lg g À1 ) and calcite density (g lm À3 ), and amounts to 10 À8 to 10 À11 lg lm À2 yr À1 .…”

“…Meanwhile, linking fossil stalagmites to drip water data requires certain assumptions about composition and stability if trace element interpretations are to be made. The present study relies on drip water monitoring data provided by Kost et al (2022) from La Vallina cave. These data demonstrate seasonality and variability in drip water trace elements (Fig.…”

“…5). Drip water monitoring data suggest relatively minor seasonal changes (<15%) in drip water Sr/Ca (Kost et al, 2022; Fig. S4), so we assume that larger variations (factor of 2) in stalagmite Sr/Ca are driven by a combination of growth rate and PCP.…”

“…Fig. 4), while drip water Y concentrations typically vary by one order of magnitude (Kost et al, 2022;Fig. S4).…”