Periodic Table--Carbon

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):

• Carbonates
• The Evolution of Carbonates in Groundwaters
• ¹³C in the Carbonate System

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):

14C Dating with DOC

Source of text: This review was assembled by Carol Kendall, Eric Caldwell and Dan Snyder, primarily from Kendall et al. (1995).