Carbon has two stable, naturally-occurring isotopes: 12C (98.89%) and13C (1.11%). Ratios of these isotopes are reported in ‰ relative to the standard VPDB (Vienna Pee Dee Belemnite). The d13C of the atmosphere is -7‰. During photosynthesis, the carbon that becomes fixed in plant tissue is significantly depleted in 13C relative to the atmosphere. There is a bimodal distribution in the d13C values of terrestrial plants resulting from differences in the photosynthetic reaction utilized by the plant. Most terrestrial plants are C3 plants and have d13C values ranging from -24 to -34 ‰. A second category of plants (C4 plants), composed of aquatic plants, desert plants, salt marsh plants, and tropical grasses, have d13C values ranging from -6 to -19 ‰ (Deines, 1980). An intermediate group (CAM plants) composed of algae and lichens has d13C values ranging from -12 to -23 ‰. The d13C of plants and organisms can provide useful information about sources of nutrients and food web relations, especially when combined with analyses of d15N and/or d34S.
The isotope fractionation in the system CO2- HCO3-CaCO3 results in calcite that is enriched in 13C by about 10‰ relative to CO2 at 20°C. Marine carbonate rocks typically have d13C values very close to PDB (ie, 0 ± 5 ‰). Lacustrine carbonates usually have lower d13C values due to incorporation of CO2 derived from the decay of plant material in soil. Carbonates formed by oxidation of biogenic CH4 (which is very depleted in 13C) also have light (more negative) d13C values, whereas carbonates formed in organic rich systems (where methanogenesis has reduced and removed significant portions of the total DIC into the gas phase) can have very positive values (> +20‰)
The d13C values of dissolved inorganic carbon (DIC) in subsurface waters are generally in the range of -5 to -25 ‰. The primary reactions that produce DIC are: (1) weathering of carbonate minerals by acidic rain or other strong acids; (2) weathering of silicate minerals by carbonic acid produced by the dissolution of biogenic soil CO2 by infiltrating rain water; and, (3) weathering of carbonate minerals by carbonic acid. The first and second reactions produce DIC identical in d13C to the composition of either the reacting carbonate or carbonic acid, respectively, and the third reaction produces DIC with a d13C value exactly intermediate between the compositions of the carbonate and the carbonic acid. Consequently, without further information, DIC produced solely by the third reaction is identical to DIC produced in equal amounts from the first and second reactions.
Under favorable conditions, carbon isotopes can be used to understand the biogeochemical reactions controlling alkalinity in watersheds (Mills, 1988; Kendall et al., 1992). If the d13C values of the reacting carbon-bearing species are known and the d13C of the stream DIC is determined, in theory one can calculate the relative contributions of these two sources of carbon to the production of stream or groundwater DIC and carbonate alkalinity. This assumes that: (1) there are no other sources or sinks for carbon, and (2) calcite dissolution occurs under closed-system conditions (Kendall et al., 1992). With additional chemical or isotopic information, the d13C values can be used to estimate proportions of DIC derived from the three reactions listed above.
Other processes that may complicate the interpretation of the d13C values of surface and subsurface waters include CO2 degassing, carbonate precipitation, exchange with atmospheric CO2, carbon uptake by aquatic organisms, methanogenesis, and methane oxidation. Correlation of variations in d13C with chemistry and other isotope tracers such as 87Sr/86Sr and14C may provide evidence that such processes are insignificant for a particular study. Carbon isotopes can also be useful tracers of the seasonal and discharge-related contributions of different hydrologic flowpaths to streamflow (Kendall et al., 1992). In many carbonate-poor catchments, waters along shallow flowpaths in the soil zone have characteristically light d13C values reflecting carbonic-acid weathering of silicates. Waters along deeper flowpaths within less weathered materials have intermediate d13C values characteristic of carbonic-acid weathering of carbonates (Bullen and Kendall, 1998).
The d13C values of organic materials can provide extremely useful information about sources of contaminants. One recent technological advance, compound-specific stable isotope geochemistry, allows the isotopic analysis of individual carbon and nitrogen "peaks" in substances separated by gas chromatography, combusted, and then carried into the mass spectrometer in a flow of helium (Macko, 1994). This method provides a much more distinctive signature of organic sources than the bulk isotopic composition usually measured. For example, the origins of hydrocarbons from oil spills in coastal regions or oil-field brines dumped into groundwater wells can potentially be identified by comparing the isotopic "chromatograms" of the pollutants and several likely sources for the hydrocarbons. Compound specific techniques can also be applied to food web studies where the lipids, carbohydrates, amino acids, etc., from different sources can be characterized isotopically, allowing more precise determinations of the contributions from different organisms to consumers.
A multi-isotope approach is almost always beneficial ("the more isotopes the merrier!"). Carbon-13 investigations are often supplemented by 14C data. Food web studies take advantage of the distinctive C, N, and S compositions of plants and consumers. Another recent example is the use the d13C and d37Cl of chlorinated solvents such as TCE and BTEX to identify the sources of the contaminants in groundwater, and the degradation reactions that may remediate the pollution plumes. One such study found that chlorinated solvents (TCE, PCA, and TCE) supplied by different manufacturers had distinctive C and Cl isotopic signatures (Aravena et al., 1996). This appears to be a very promising avenue of research, although the analysis of d37Cl is still technologically challenging.
Further information can be found in the following sections in Clark and Fritz (1997), Environmental Isotopes in Hydrology, (CRC Press):
Other information can be found in the chapter: "Tracing of Weathering Reactions and Water Flowpaths: A Multi-isotope Approach" by Bullen and Kendall (1998).
Carbon-14: The radioactive isotope of carbon, 14C, is continuously being produced by reaction of cosmic ray neutrons with 14N in the atmosphere and decays with a half-life of 5730 years. Prior to the inception of above- ground nuclear testing, all living organic matter had similar (+/- 10%) 14C values. 14C concentrations are usually reported as specific activities (disintegrations per minute per gram of carbon relative to a standard) or as percentages of modern 14C concentrations; occasionally they are expressed in permil. Under favorable conditions, 14C can be used to date carbon- bearing materials. However, because the "age" of a mixture of waters of different residence times is not very meaningful, the least ambiguous hydrologic use of 14C is as a tracer of carbon sources.
14C combined with d13C has been used to study the origin, transport, and fate of dissolved organic carbon (DOC) in streams and shallow groundwater in forested catchments (Schiff et al., 1990; Wassenaar et al., 1991; Aravena et al., 1992). Typically, DOC in groundwaters is composed of older carbon than surface waters, indicating extensive cycling of DOC. In addition, the 14C of DIC can be a valuable check on conclusions derived using 13C values. For example, if the d13C values suggest that the dominant source of carbon is from carbonic acid weathering of silicates, the 14C activities should be high, reflecting the young age of the soil CO2 (Schiff et al., 1990).
14C is probably the most important radio-isotopic tool for dating deep groundwater, despite serious problems related to potential interactions of DIC species with the carbonate minerals and possible contributions of 14C-free deep CO2. Such contributions are quite common in geothermal waters, which may have a very low 14C content even if they contain tritium. 14C is most useful for dating waters and organic material in the range of 1000 to 40,000 years. For age-dating waters, the best accuracy is obtained when the 14C data is used within the constraints of a geochemical reaction path model which accounts for the sources and sinks of carbon along the flowpath (Plummer et al., 1983; 1991).
Also in Clark and Fritz (1997):
Source of text: This review was assembled by Carol Kendall, Eric Caldwell and Dan Snyder, primarily from Kendall et al. (1995).
|•||Aravena, R., Schiff, S.L., Trumbore, S.E., Dillion, P.J. and Elgood, R. (1992). "Evaluating dissolved inorganic carbon cycling in a forested lake watershed using carbon isotopes", Radiocarbon, 34(3), 636-645.|
|•||Aravena, R., Frape, S.K., Moore, B.J., Van Warmerdam, and Drimmie, R.J., (1996). Use of environmental isotopes in organic contaminants research in groundwater systems, IAEA, Symposium on Isotopes in Water Resources Management, Vienna, 20-24 March, 1995.|
|•||Bullen, T.D., and Kendall, C. (1998), "Tracing of Weathering Reactions and Water Flowpaths: A Multi-isotope Approach" In C. Kendall and J. J. McDonnell (Eds.), Isotope Tracers in Catchment Hydrology, Elsevier, pp. 611-646.|
|•||Coplen, T.B.(1993) "Uses of Environmental Isotopes", In: W. M. Alley (Ed.), Regional Ground-Water Quality , Van Nostrand Reinhold, New York, pp. 227-254.|
|•||Deines, P., (1980). "The isotopic composition of reduced organic carbon." In: P. Fritz and J.Ch. Fontes (Eds.), Handbook of Environmental Isotope Geochemistry, Vol. 1. Elsevier, New York, pp. 329-406.|
|•||Faure, G. (1986). Principles of Isotope Geology, Second Edition. John Wiley & Sons, New York, 589 pp.|
|•||Kendall, C., Mast, M.A., and Rice, K.C. (1992). "Tracing watershed weathering reactions with d13C", In: Y.K. Kharaka and A.S. Maest (Eds.), Water- Rock Interaction, Proceedings of the 7th International Symposium, Park City, Utah, 13-18 July 1993, Balkema, Rotterdam, pp. 569-572.|
|•||Kendall, C., Sklash, M.G., Bullen, T.D. (1995). "Isotope Tracers of Water and Solute Sources in Catchments", In: Solute Modelling in Catchment Systems, John Wiley & Sons, New York, pp. 261- 303.|
|•||Macko, S.A., (1994). Compound-specific approaches using stable isotopes. In: K. Lajtha and R.M. Michener (Eds.), Stable Isotopes in Ecology and Environmental Science. Blackwell, Oxford, pp. 241-247.|
|•||Mills, A. L. (1988) "Variations in the dC-13 of stream bicarbonate: implications for sources of alkalinity", MS thesis, George Washington University, Washington DC, 160 pp.|
|•||Plummer, L.N., Parkhurst, D.L., and Thorstenson, D.C., (1983). Development of reaction models for ground-water systems. Geochim. et Cosmochim. Acta., 47: 665-686.|
|•||Plummer, L.N., Prestemon, E.C. and Parkhurst, D.L., (1991). An Interactive Code (NETPATH) for Modeling Net Geochemical Reactions along a Flow Path. USGS Water-Resources Inves. Rep. 91-4078, 227 pp.|
|•||Schiff, S. L., Aravena, R., Trumbore, S. E. and Dillon, P. J. (1990). "Dissolved organic carbon cycling in forested watersheds: a carbon isotope approach", Water Resour. Res., 26: 2949- 2957.|
|•||Wassenar, L. I., Aravena, R., Fritz, P. and Barker, J. F. (1991). "Controls on the transport and carbon isotopic composition of dissolved organic carbon in a shallow groundwater system, Central Ontario, Canada", Chem. Geol., 87: 39-57.|
|•||Fundamentals of Stable Isotope Geochemistry|