Geoscience Reference
In-Depth Information
13
C/
12
Canalysis
Briefsummaryof
δ
Carbonconsistsoftwostableisotopeswithatomicmassesof12and13.Theheavierisotopemakesup1.1%
of naturally occurring carbon (Nier 1950). Variations in the ratio of
13
Cto
12
C are usually expressed as per
mil(partsperthousandor
‰
)deviationsfromtheratiointheViennaPeeDeeBelemnite(V-PDB)standard.
Theoriginalsamplewasusedupsomeyearsago,butaVienna-basedlaboratorycalibratedanewreference
sample to the original, giving rise to the widespread use of the term Vienna PeeDee Belemnite standard,
abbreviatedtoV-PDB.Thechangein
13
C/
12
Cisgivenbytheequation:
13
C
12
C
13
C
12
C
[
(
/
)
Sample
−
(
/
)
VPDB
]
13
C
12
C
δ
/
=
×
1000‰
13
C
12
C
(
/
)
VPDB
13
C/
12
Cscale.Samplesenrichedin
13
Crel-
ative to the standard have positive values and those depleted in
13
Chavenegativevalues(forexample,
see Figure 3.4, below). In terrestrial ecosystems, atmospheric CO
2
is used in photosynthesis, and ter-
restrial C
3
photosynthetic carbon (see section 3.3.11) has a
The(
13
C/
12
C)
V-PDB
standardalwayshasavalueofzeroonthe
δ
δ
13
C value of around
−
27
‰
.Conversely,
13
C value of around −13
‰
. The marine situation is more
terrestrial C
4
photosynthetic carbon has a
δ
complex.
TheV-PDBstandardisaspeciicgeological formation that contains the remainsof cephalopod mollusc
speciesofthesubclassBelemnoideafromtheMesozoicera.ThestratacomefromthePeeDeeFormationin
SouthCarolina,USA.Thismaterialhasahighabsolute
13
C/
12
Cratio(0.0112372),andwasestablishedasa
13
C (V-PDB) value of zero. Use of this standard gives most natural material a negative
13
C value. Because
δ
13
C(V-PDB)valuesarenegative(althoughsome,likeinthelateOrdovician,arepositive),theequa-
tionresearcherssometimesuseis:
most
δ
[
13
C
12
C
13
C
12
C
(
/
)
Sample
−
(
/
)
VPDB
]
13
C
12
C
δ
/
=
−
1
×
1000‰
13
C
12
C
(
/
)
VPDB
The existence of this alternative equation means that it is important, if you are conducting your own
δ
13
Canalysisandrelatingthoseresultstothoseofothers,toreadtheMethodssectionofacademicpapers
(ortheMethodsappendix)toseehowtheresearchershaveused
δ
13
Canalysis.Fortunatelyformorecasual
readers of academic papers, most simply show a graph denoting a change in
δ
13
C across some period of
geologicaltime.
The isotopic compositions of carbonate carbon and organic carbon leaving the oceans by burial in sed-
iments are enriched and depleted, respectively, in
13
C compared with most carbon in the carbon cycle. On
longtimescalesthesizesandisotopiccompositionsofthesemajorpoolsinthecarboncyclemustbalance
tomatchthebulkcarbonisotopiccomposition.Onshortertimescales,however,therecanbereadjustment
of isotopic compositions that are a response to changes in the ratio in which organic and carbonate forms
are buried. Essentially, if the rate of burial of (
13
C-depleted) organic carbon increases then
12
C is removed
from the oceans disproportionately quickly compared with
13
C. In response to this,
13
C will accumulate in
alloceancarbonpools(i.e.their
13
Cvaluewillincrease)untileithertherateoforganiccarbonburialfalls
or a new isotopic equilibrium is reached (with a higher rate of organic carbon burial but having a higher
δ
δ
13
Cvaluesofmarinecarbonatesandorganiccarbonovergeological
time can be used to estimate the fractions of carbon that are buried as carbon ultimately from an organic
source(photosyntheticallyderived,suchascarbonfromdecayingplants,orcarbonatefromcarbondioxide
13
C).Consequently,changesinthe
δ
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