GSA Connects 2021 in Portland, Oregon

Paper No. 206-1
Presentation Time: 8:05 AM


DOCTOR, Daniel, U.S. Geological Survey, Florence Bascom Geoscience Center, 12201 Sunrise Valley Drive, MS 926A, Reston, VA 20192

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.