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 (>
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
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):
Dating with DOC
Source of text: This review was assembled by
Carol Kendall, Eric Caldwell and Dan Snyder, primarily from Kendall
et al. (1995).
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||Aravena, R., Frape, S.K., Moore, B.J., Van Warmerdam,
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