The carbon isotope geochemistry of a small groundwater system in northeastern Ontario

Abstract

The carbon isotopic composition (•8C and •4C) of the inorganic carbon dissolved in the waters of a small, largely unconfined aquifer in unconsolidated sediments on the Canadian Shield has been investigated. Three principal carbon sources are recognized: soil COa, rock carbonate, and biogenic CO:. The average •8C value of the soil COa is close to -21.0 4-1.5%, and present-day '4C activities of the soil COa vary between 130 and 162% modern •4C. Very minor amounts (< 1.0%) of carbonate minerals (6'aC = -0.6%o) are present within the aquifer and react with this soil CO: to produce a dissolved inorganic carbon (DIC) with 'aC activities which are as much as 50% below the initial activities of the soil COa. The third carbon source, a biogenic CO:, could be detected only indirectly, and its presence is primarily deduced from the occurrence of methane in the deeper parts of these aquifers. The large isotope fractionation which occurs during bacterial coproduction of CO: with this methane results, however, in a •aC-rich CO•. and thus a DIC with high •'aC values. Values of •'•C as high as +11%o have been measured in groundwaters on the Canadian Shield. Since the origin of the destroyed organic matter is not yet known, no assessment of the importance of this CO: for the •aC contents of the DIC is possible. models which are based on utilization of these ions as indicators for such processes. A Fortran IV computer program attached to the water analyses treatment program (WATEQF) by Plumruer et al. [1976] has been presented by Reardon and Fritz [1978]. This program considers corrections based on initial versus final carbonate, simple carbonate, dissolution carbonate plus gypsum dissolution with and without ion exchange processes, and dissolution-precipitation processes as they may occur during the incongruent dissolution of dolomites. Chemical and isotope analyses are compared to model calculations for both chemistry and carbon 13. 1059 1971)', Vienna, 1969-1971. Kiiley, R. W. D., Carbon geochemistry of an unconfined aquifer-lake system on the Canadian Shield, M.Sc. thesis, Univ. of Waterloo, Waterloo, Ont., 1977. McNaughton, D. M., A hydrogeological, geochemical and isotopic study of an uncontaminated flow system, Perch Lake Basin, Ontario, M.Sc. thesis, Univ. of Waterloo, Waterloo, Ont., 1975. Mock, W. G., The dissolution-exchange model for dating groundwater with carbon-14, Interpretation of Environmental Isotope and Hydrochemical Data in Groundwater Hydrology, STI Publ. 429, pp. 213-225, lnt. At. Energy Agency, Vienna, 1976. Park, R., and S. Epstein, Carbon isotope fractionation during photosynthesis, Geochim. Cosmochim. Acta, 21, 110-126, 1960. Parsons, P. J., Movement of radioactive wastes through soils, l, Soil and groundwater investigations in lower Perch Lake basin, Rep. AECL 1038, At. Energy of Can. Ltd., Chalk River, Ont., 1960. Reardon, E. J., and P. Fritz, Computer modelling of groundwater and •4C isotope compositions, J. Hydrol., 36, 201-224, 1978. Rightmire, C. T., and B. B. Hanshaw, Relationship between the carb...

Calculation of Past Dead Carbon Proportion and Variability by the Comparison of AMS 14C and Tims U/TH Ages on Two Holocene Stalagmites

“…Apparently, the main factor that controls the calcite o13C is soil b13C, and reasonable changes in temperature (±3 °C, which is much higher than the Holocene temperature variation) and in dcp do not significantly change the calcite 613C. For Vil-stml b, the theoretical calcite b13C agrees well with measured b13C for a mean temperature of 13 °C and a soil CO2 b13C of -20.5%0, which is in the range of accepted values -20%o to -22%o for soil under C3 vegetation (Fritz et al 1978; Dorr and Munnich 1986 etc.). However, for the Han-stmlb stalagmite, the calculated calcite b13C can fit with the measured values only if we use a very high soil CO2 b13C (-17.8%0).…”

The hydromagnesite playas of Atlin, British Columbia, Canada: A biogeochemical model for CO2 sequestration

“…However, based on a mixing model of the two dominant carbon sources in groundwater, soil CO 2 (δ 13 C ≈ −25‰) and bedrock magnesite (δ 13 C ≈ −4.0‰), the groundwater DIC (δ 13 C ≈−6.4‰) only contains ∼12% modern carbon from soil CO 2 (Fritz et al, 1978;Landmeyer and Stone, 1995). This value closely corresponds to the radiometric carbon datum of 11.3% modern carbon.…”

Geochemical and isotopic evidence of a groundwater source in the Corral Canyon meadow complex, central Nevada, USA

“…The observed decrease in υ 13 C DIC with increase in DIC indicates a source of lighter isotopic CO 2 input in the waters (e.g. Fritz et al, 1978;Wassenar et al, 1991). The higher DIC and enriched 13 C of DIC in seep S-7 may result from carbonate dissolution from groundwater flowing in fractures within the bedrock.…”