SOME CONSIDERATIONS ON THE USE OF STABLE CARBON ISOTOPES AS TRACERS IN KARST AQUIFERS (Invited Presentation)

The stable isotopic composition (δ¹³C) of dissolved inorganic carbon (DIC) has long been used to document changes in carbonate chemistry along flow paths in natural systems. In karst, δ¹³C of DIC is useful to infer mixing and relative residence times of water, as well as to track processes related to carbon dioxide (CO₂) dynamics and carbon cycling. Several key concepts should be considered when applying carbon isotopes as tracers in karst systems:

1) Most groundwaters are supersaturated with respect to CO₂, thus the assumption of equilibrium conditions when modeling carbonate chemistry has limited validity.

2) Outgassing of CO₂ is a key process governing the δ¹³C of DIC. Due to an activation energy barrier, dissolved CO₂ must be first converted to bicarbonate (HCO₃^-) before outgassing. This causes a large kinetic isotopic fractionation, a process often overlooked in studies of carbonate chemical evolution. Carbonic anhydrase can facilitate the conversion, and this enzyme in water will impact overall CO₂ solubility and the δ¹³C values of DIC.

3) Temperature exerts a primary control on CO₂ solubility thus influencing changes in δ¹³C of DIC, especially in surface waters. Although calcite solubility thermodynamically decreases with increasing temperature, soil CO₂ production tends to increase with increasing temperature in temperate climates; therefore, carbonate weathering in these zones can be expected to increase overall with future warming.

4) In-situ aqueous pCO₂ measurements are becoming common with advances in instrumentation. Direct measurements of pCO₂ may be preferable instead of pH for carbon speciation in natural waters since field-deployed pH meters often show significant instrumental drift and need to be replaced after a short period. Total DIC, pCO₂, and temperature are sufficient to speciate DIC in a water sample without a pH measurement.

5) Additional geochemical parameters such as major ions, stable water isotopes, and other dissolved gases (e.g., nitrogen, argon, and oxygen) are useful to understand water-rock interaction, mixing between end-member groundwater components and CO₂ dynamics in karst.

Several examples illustrating the use of δ¹³C of DIC as a tracer in karst systems will be presented, ranging from regional spring flow, to tufa deposition, to cave drip waters.