Carbon stable isotope

CONTENTS
INTRODUCTION
STABLE ISOTOPES
CARBON STABLE ISOTOPE
IMPORTANCE OF CARBON STABLE ISOTOPE
APPLICATIONS OF CARBON STABLE ISOTOPE
CONCLUSION
REFERENCE

INTRODUCTION
ISOTOPE
Atoms of the same atomic number but different atomic
weights are called isotopes.
Such atom which have the same proton number(Z) but
different values of neutron number(N) are called
isotopes.
Isotopes are atoms of the same chemical elements.They
have very similar chemical properties and differ only in
their masses.
Isotopes occupy the same position in the periodic table.
Eg:Carbon,hydrogen,etc
Isotopes can be divided in to stable and
unstable(radioactive) species.

STABLE ISOTOPES
Elements can exist in both stable and unstable (radioactive)
forms.
Many isotopes that are classed as stable (i.e. no radioactivity
has been observed for them) are predicted to have extremely
long half-lives.
The term “stable” is a relative one,it depends on the
detection limits of radioactive decay times.
some stable isotopes also show abundance variations in the
earth as a result of decay from long-lived radioactive
nuclides. These decay-products are termed radiogenic
isotopes.
Eg: Carbon(¹²C,¹³C ), Sulfur(³²S, ³³S, ³⁴S, ³⁶S)

CARBON
Carbon is one of the most abundant element in the
universe and is the basis for the existence of life on the
earth.
 It occurs in the earth as a trace element.
Carbon content of crust and mantle probably lis in the
range of several hundred parts per million.
Carbon occurs in the reduced form in organic compounds
and coal.
Also occurs in the oxidised state primarly as
carbon dioxide, carbonates. In addition, it is found as
native element in the form of graphite and diamond.
Carbon (Z=6) has two stable isotopes
¹²C =98.89% , ¹³C=1.11%

Cont…
The ¹²C/¹³C ratios of about 90:1 is roughly constant in all
natural materials.
The isotopes of carbon are fractionated by a variety of
natural processes, including photosynthesis and isotope
exchange reaction among carbon compounds.
Phosynthesis=enrichment of ¹²C
Isotope exchange reaction=enrichment of ¹³C
The study of the natural variation in the relative
abundance of the stable carbon isotopes is an important
tool in any geological investigations.
The isotopic composition of carbon is expressed in terms
of the delta notation
δ ¹³C ={(¹²C/¹³C )sample-(¹²C/¹³C std)}×1000
(¹²C/¹³C std)

Carbon in modern Biosphere and in Sediment
Photosynthesis is a complex process by means of which
atmospheric CO₂ is incorporated into green plants.
6 CO₂ +6H₂O C₆H₁₂O ₆ +6O₂→
In the coarse of photosynthesis,the carbon that is in fixed in
the plant tissue is significantly enriched in ¹²C relative to the
atmospheric CO₂,whose δ ¹³C value is 7.0‰.
In terrestrial plants δ ¹³C value is 24-34 ‰.
In aquatic plants, desert and salt marsh plants, and tropical
grasses δ ¹³C value is 6-19 ‰.
Algae and lichens δ ¹³C value is 12-23 ‰.

Cont...
Organic matter in recent sediments has δ ¹³C value is
10-30‰.(this sediments is used to distinguish between
sedimentary rocks of marine origin and those deposited in
terrestrial, sedimentary transport)

Fossil fuels
 The carbon contained in fossil fuels is strongly enriched in
¹²C, which is consistent with the view that these materials
were derived from biogenic materials.
 The δ ¹³C value of coal average about 25 ‰ relative to the
PDB standard.
 The δ ¹³C value of petroleum range from 18-34‰. In general
petroleum appears to be enriched in ¹²C by about 10‰
relative to modern aquatic plants.
 The δ ¹³C value of methane varied from 40-70 ‰ relative to
PDB.
 The burning of fossil fuels increased the CO₂ content and has
caused a measurable decrease in its δ ¹³C value.(also

Sedimentary rocks of Precambrian age
One of the most important discoveries in the Earth science
during the 1960 was that sedimentary rocks of Precambrian
age contain evidence for the existence of life in the ocean. This
conclusion is generally based on the significant enrichment of
the reduced carbon in ¹²C similar to that found in modern
sediments.
Eg:sedimentary rocks of Witwatersrand.
The δ ¹³C value become less negative with increasing
metamorphic grade. The enrichment in ¹³C result from the
formation of methane by thermal cracking of kerogen.

Marine and Nonmarine carbonates
Carbon isotopes are fractionated in the system CO₂(gas)-
carbonate ions(aqueous)-CaCO₃(solid) in such a way that
calcite is enriched in ¹³C by about 10 ‰ at 20°c relative to CO₂
gas.
The δ ¹³C values of marine carbonates are close to zero relative
to PDB and do not vary appreciably with age.But freshwater
carbonates are enriched in ¹²C compared to marine carbonates
and have more variable δ ¹³C values.
The δ ¹³C values in mollusks is +4.2-1.7‰(marine)
 The δ ¹³C values in mollusks is 0.6-15.2‰(freshwater)
The δ ¹³C values in carbanaceous condrites is range from +58.6
- +64.4‰.
It helps to solve the problem of the origin of precambrian iron
formations.

Carbonatites and Diamonds
The isotopic composition of carbon in the mantle of the
earth is difficult to determine because isotope
fractionation effects are important even at very high
temperature.
The δ ¹³Cvalues of carbonatites(-2.0 to-8.0‰) and
diamond (-2.0 to-10‰) exhibit significant variations either
because of isotopic heterogeneity of carbon in the mantle
or because of isotopic fractionation,or both.

Carbon in Igneous andVolcanic gases
The igneous rocks contain a variety of carbon compounds:
1)carbonate minerals and CO₂ gas in fluid or
gaseous inclusions
2)element carbon in the form of graphite and
diamond
3)mixture of organic molecules and carbides.
Oxidised carbon in igneous rocks has variable δ ¹³C
values(+2.9 to -18.2‰) and may have been deposited to a
large extent by circulating aqueous solutions after
crystallization.

Cont...
Reduced carbon in igneous rocks (-20 to -28‰)overlaps that of
biogenic carbon,but an inorganic origin for the reduced carbon
is possible.
Volcanic gases contain CO₂ and lesser concentration of CH₄
which appear to be in isotopic equilibrium and permit
temperature determinations.The methane may be produced by
reduction of CO₂ with hydrogen.The CO₂ is probably derived
from crustal rocks including limestones in subsurface.
Geochemical balance calculations indicate a δ ¹³C value of
-7‰ for average crustal carbon.

Marble and Graphite
When marine carbonates rocks are subjected to contact
metamorphism, the isotopic compositions of carbon as well as
oxygen are changed.
Contact metamorphism of carbonate rocks leads to a decrease
in δ ¹³C values and implies that CO₂ gas liberated by
recarbonation reactions is enriched in ¹³C.
However, δ ¹³C values of marbles generally do not vary
systematically with increasing grade of metamorphism.
Graphite is variably enriched in ¹²C compatible with a
biogenic source for the carbon.However,some graphites
containing “heavy” carbon maybe abiogenic in origin.
Coexisting calcite and graphite are not in isotopic equilibrium
in many cases.

Hydrothermal Ore deposits
The carbon content of hydrothermal ore deposits is
represented primarly by carbonates of
calcium,magnesium,iron,and manganese and by CO₂ and
CH₄ gas in fluid and gaseous inclusions in ore and gangue
minerals.
The principle source of carbon in hydrothermal fluid
include marine limestone (δ ¹³C=0‰) deep-seated or
average crustal source (δ ¹³C=-7‰) and organic compounds
of biogenic origin (δ ¹³C=-25‰).
The δ ¹³C values of CO₂ in fluid inclusion range from -4 to
-12‰ with respect to PDB. Most early formed carbonate
minerals in hydrothermal veins have δ ¹³C values in the
range from -6 to-9‰.

Cont…
The isotopic composition of carbon in hydrothermal
carbonates depend not only on the δ ¹³C of the total carbon
in the ore bearing fluid but also on the fugacity of oxygen,
the pH, the temperature, the ionic strength of the fluid ,and
on the total concentration of the carbon.
Changes in the abundance of CH₄ have a dramatic effect on
the δ ¹³C values of coexisting carbonate species because CH₄
is strongly enriched in ¹²C.

APPICATIONS OF CARBON STABLE
ISOTOPE
To determine the organic or inorganic origin of carbon
bearing materials in samples and in distinguishing
between marine and nonmarine deposits.
The isotopic composition of organic carbon(δ ¹³C value
-25‰) is different from that of sedimentary carbonate
carbon δ ¹³C value =0 ‰) and hence, it is possible to
distinguish between an organic and inorganic origin for
the carbon in sedimentary rocks.
Kumar et al (1982) observed that δ ¹³C ratio for
Precambrian graphite's (age˜2700-3000 Ma) from the
Dharwar Craton, India, vary from -16‰ to -41‰ vs. PDB
suggesting a biogenic origin of the graphitic carbon.

Cont…
The carbon isotopic work on diamond, carbonatites, and
kimberlite carbonates is of interest to the earth scientists
because these materials are thought to be samples of deep-
seated carbon (δ ¹³C value -2.0 to -10.0‰).
Carbon isotope have been recognized as a tool in
hydrocarbon exploration. And also to identify the maturity
of hydrocarbons trapped in an oil basin by isotopically
analyzing hydrocarbon gases released from shallow
drilling cutting mud samples.

CONCLUSION
The study of the natural variation in the relative
abundance of the stable carbon isotopes is an important
tool in any geological investigations.
Of all available material, carbonaceous chondrites hold the
best record of the isotopic carbon composition of the
primitive nebular material from which the solar system
condensed.
It helps to distinguish between an organic and inorganic
origin for the carbon in sedimentary rock.
Carbon isotopes are having enormous applications in
hydrocarbon exploration, works on diamond and
carbonatites, hydrothermal ore deposits, etc..

REFERENCE
Gunter Faure (1977),Principles of isotope geology,
John Wiley & sons publishing co,Pages:491-508
C.S.Pichamuthu (1985),Archaean geology, Oxford &
IBH publishing co,Pages:95-96
E.Jager & J.C.Hunziker(1979),Springer-Verlay Berlin
Heidelberg Newyork,Pages:73-75

Carbon stable isotope

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