Stable isotopes of carbon dioxide in soil gas over massive sulfide mineralization at Crandon, Wisconsin

Stable isotope ratios of oxygen and carbon were determined for CO₂ in soil gas in the vicinity of the massive sulfide deposit at Crandon, Wisconsin with the objective of determining the source of anomalously high CO₂ concentrations detected previously by McCarthy et al. (1986). Values of δ¹³C in soil gas CO₂ from depths between 0.5 and 1.0 m were found to range from −12.68‰ to −20.03‰ (PDB). Organic carbon from the uppermost meter of soil has δ¹³C between −24.1 and −25.8‰ (PDB), indicating derivation from plant species with the C₃ (Calvin) type of photosynthetic pathway. Microbial decomposition of the organic carbon and root respiration from C₃ and C₄ (Hatch-Slack) plants, together with atmospheric CO₂ are the likely sources of carbon in soil gas CO₂. Values of δ¹⁸O in soil-gas CO₂ range from 32 to 38‰ (SMOW). These δ¹⁸O values are intermediate between that calculated for CO₂ gas in isotopic equilibrium with local groundwaters and that for atmospheric CO₂. The δ¹⁸O data indicate that atmospheric CO₂ has been incorporated by mixing or diffusion. Any CO₂ generated by microbial oxidation of organic matter has equilibrated its oxygen isotopes with the local groundwaters.The isotopic composition of soil-gas CO₂ taken from directly above the massive sulfide deposit was not distinguishable from that of background samples taken 1 to 2 km away. No enrichment of the δ¹³C value of soil-gas CO₂ was observed, contrary to what would be expected if the anomalous CO₂ were derived from the dissolution of Proterozoic marine limestone country rock or of Paleozoic limestone clasts in glacial till. Therefore, it is inferred that root respiration and decay of C₃ plant material were responsible for most CO₂ generation both in the vicinity of the massive sulfide and in the “background” area, on the occasion of our sampling. Interpretation of our data is complicated by the effects of rainfall, which significantly reduced the magnitude of the CO₂ anomaly. Therefore, we cannot rule out the possible mechanism of carbonate dissolution driven by pyrite oxidation, as proposed by Lovell et al. (1983) and McCarthy et al. (1986). Further work is needed on seasonal and daily variations of CO₂ concentrations and stable isotope ratios in various hydrogeologic and ecologic settings so that more effective sampling strategies can be developed for mineral exploration using soil gases.