This document discusses using stable carbon isotope analysis to study phase transitions of soil carbonates during anaerobic biodegradation of petroleum hydrocarbons. It explains that carbonate minerals like calcite and dolomite are common in soils and can dissolve or form new minerals as conditions change. Stable carbon isotope ratios are measured to identify carbonate dissolution or addition. Laboratory experiments show that dissolving soil carbonates leads to re-formation of new carbonates during anaerobic degradation of hydrocarbons. This stable isotope method allows changes in the carbonate fraction of soil to be monitored.

1. Phase Transitions of Soil Carbonates during Anaerobic Biodegradation of
Petroleum Hydrocarbons – A δ13C Stable Isotope Method
Orlu, R.N.a
, Stewart, D.I.a
, Bottrell, S.H.b
a Institute of Public Health and Environmental Engineering, School of Civil Engineering,
b Earth Surface Science Institute, School of Earth and Environment,
University of Leeds, Leeds, United Kingdom, LS2 9JT.
Stable carbon isotopes in soil environments
The major elements associated with organic compounds include
carbon, hydrogen, nitrogen, oxygen and sulphur. Each of these
elements have at least two stable isotopes that can be distinguished
with the use of mass spectrometry.
Carbonates are often found as limestones on the Earth’s surface (Ehrlich,
1998). Carbonate minerals tend to be one of the most common groups of
non-silicate minerals and can be found in several rock types. Dolomite,
calcite and the iron carbonate siderite are the most common carbonate
minerals found in soil. The total carbonate content in soil represents the
organic and inorganic carbonate minerals consumed, produced and
mobilised by biotic and abiotic processes. This carbonate fraction is mostly
used in reference to the amount of carbonates in solid matter, however, a
substantial proportion rapidly turned over through a soil solution
ultimately becomes CO2 in gaseous phase (Manning, 2008). Generally,
organic carbon is differentiated into labile (rapid turnover) and recalcitrant
(slow turnover) pools, with differing chemical reactivity and residence
times. Inorganic carbon is composed of Ca- and to a lesser extent Fe- and
Mg-carbonates (Schlesinger, 1982). Carbonate minerals tend to be the most
reactive minerals in subsurface environments. Their occurrence in
geological formations is influenced by dissolution and precipitation (or
addition) reactions. Calcite (CaCO3), dolomite and siderite (FeCO3) are the
most common carbonate minerals, with calcite being the most reactive and
dolomite least reactive of the three minerals.
The isotope abundance of a compound may be indicated by an
isotopically light (negative δ) or heavy (positive δ) signal. In stable
isotope convention, a negative δ in an experimental observation is
indicative of the sample being depleted in the less common isotope
relative to the standard (usually Vienna Pee Dee Belemnite i.e. VPDB).
A positive δ is indicative of a sample in which the less common isotope
is in greater abundance compared to the standards (Schmidt et al.,
2004). For example, crude oil gives off a light isotope signature which,
in stable isotope sign convention, is denoted by numerical values along
the lower negative end of the δ13
C scale. Marine carbonates on the
other hand possess a heavier isotope signature in comparison denoted
by numerical values closer to the positive end of the δ13
C scale.
Soil carbonate transitions and stable isotope analysis
Bivariate δ13
C / carbonate-carbon profile illustrating the effects of soil carbonate
reactions on carbonate-carbon and δ13C (a’ - the incubated material following
carbonate dissolution; ‘b’ - the incubated material after addition of carbon from
respiration of 13
C-depleted organic carbon)
References
1. Ehrlich, H.L. 1998. Geomicrobiology: its significance for geology. Earth-Science Reviews. 45, pp.45-60
2. Sawyer, C.C., Parking, G.F. and McCarty, P L. 1994. Chemistry for environmental engineering. 4th Edition ed. New York: McGraw-Hill
3. Schlesinger, W.H. 1982. Carbon storage in the caliche of arid soils: A case study from Arizona. Soil Sci. 133, pp.247-255
4. Schmidt, T.C., Zwank, L., Elsner, M., Berg, M., Meckenstock, R.U. and Hardelein, S.B. 2004. Compound-specific stable isotope analysis of organic contaminants in natural environments: a critical review of the state of the art, prospects, and future challenges. Anal. Bioanal.
Chem. 378, pp.283-300
- 80 - 60 - 40 - 20 0
atmospheric CO2
marine HCO3
marine carbonate
groundwater HCO3
fresh-water carbonate
coal
oil
+20
13δ VPDB (‰)
biogenic methane
Stable carbon isotope composition of common carbon-containing organic
and inorganic materials
Mechanisms of soil carbonate phase transitions
Sample isotope ratios are, by standard procedure, evaluated initially relative
to a reference CO2 stream calibrated relative to the standard PDB carbonate
formation. Final sample δ13
C are subsequently reported relative to the
standard PDB with corrections made for 17
O contributions. Where stable
isotope analysis provides the δ13
C signature and mass of carbonate carbon in
analysed samples, carbonate dissolution will result in decreasing carbonate
concentration with little change in δ13
C between the starting and residual
carbonate while addition will lead to increased carbonate concentrations.
Addition of carbonates results in a change in δ13
C of the carbonate pool
towards the source of carbon.
Precipitation reactions are influenced by the alkalinity of the water medium
and the reactions of divalent cations. A solution that has become saturated
with a cation will have further cation contribution exceeding the rate at
which it is removed from solution. When this happens the solution will no
longer support the formation of carbonates. During anaerobic degradation
of hydrocarbons in subsurface soil environments, bicarbonate
(HCO$(&')
)
) released in solution increases the alkalinity of the solution which
results in an increase in hardness (Sawyer et al., 1994) and the precipitation
of new carbonates (carbonate addition). Therefore the dissolution of
carbonates will be followed by the re-addition of newly-formed carbonates.
In laboratory studies incorporating the use of anaerobically sealed
mesocoms, the dissolution of carbonates will lead to the re-addition of newly
formed carbonates in the containment vessel.
Isotope ratios are expressed in terms of δ13
C values, which are reported in per
mil and calculated relative to the standard Pee Dee Belemnite (PDB),
according to the relationship.
𝛿,$C (‰) =
R0&1234) 56789:8;:
R0<&=>&?>
x 10$
𝐹𝑒EF + HCO$(&')
)
+ 𝑂𝐻) → FeCO$(0) + HEO
CaCO$(0) + COE(&') + HEO → 𝐶𝑎(PQ)
EF
+ 2HCO$(&')
)
Carbonate dissolution
Carbonate addition