Biology Reference
In-Depth Information
Understanding that stable isotope analyses require comparison of isotopic values in
unknown samples to reference standards clarifies how results from the mass spectrometer
are calculated and reported. Taking carbon as an example, the following notation is used:
d
13
C
¼ð
R
sample
=
R
standard
1
Þ
1000
%
where R is the ratio of the heavier to the lighter isotope (e.g.,
13
C/
12
C) and the
d
(delta) nota-
tion expresses this ratio as parts per thousand (
, per mil). For all stable carbon isotopes, the
original standard material was a sample of marine limestone called the
Peedee belemnite
(PDB). Though PDB has been exhausted in its original form, other standards whose
d
13
C
values have been calibrated against it are readily available (
Hoefs, 2009
). A positive
d
13
C
value indicates that
13
C is enriched compared to the standard and depleted for
12
C, or as
is the case with human tissues, negative
d
13
C values are depleted for
13
C and enriched in
12
C(
Brown and Brown, 2011
).
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Stable Isotopes and Diet: Carbon and Nitrogen
In particular, skeletal biologists have concentrated a tremendous amount of effort on
studying carbon and nitrogen isotopes to answer questions regarding subsistence and
paleodiet, and typically analyze these isotopes in tandem. Fortunately for the researcher,
both carbon and nitrogen can be analyzed simultaneously from a single sample of prepared
collagen. Today's mass spectrometers are capable of analyzing both isotopes concurrently;
therefore, two distinct datasets can be generated at once.
Carbon
The first stable isotope to receive attention in anthropological literature was carbon.
Initially, pioneering researchers realized that maize was often more difficult to date with
radiocarbon than other types of organic material like charcoal (
Katzenberg, 2008
). Second,
during this initial time researchers were continuing to differentiate the three photosynthetic
pathways that now form the basis of
d
13
C interpretations.
To sum, each of these photosynthetic pathways is represented by a distinct group of plants
that are differentiated by the way in which they convert atmospheric carbon dioxide (CO
2
)
into glucose. Ultimately, it is this process of CO
2
conversion to sugar (referred to as carbon
fixation) that results in distinct
d
13
C signatures, as the type of plant dictates the degree of
carbon fixation that occurs (
O'Leary, 1988; Brown and Brown, 2011
). The
C
3
,
or
Calvin photo-
synthetic pathway
, is found in plants common to temperate climatic regions and their
d
13
C
values average
e
26.5
(
Tykot, 2006
). Cultigens such as wheat, barley, and quinoa are exam-
ples of C
3
cultigens. The
Hatch-Slack photosynthetic pathway
, represented by
C
4
plants, fixes
carbon in an entirely different way. Maize, sorghum, millet, and sugarcane are examples of
C
4
plants and these plants are known to come frommore tropical climates
d
their
d
13
C values
average
e
12.5
&
(
Tykot, 2006
). Succulent species (e.g., cacti), plants that follow the
CAM
(
crassulacean acid metabolism
) pathway, yield
d
13
C values that fall between C
3
and C
4
plants.
Skeletal biologists interested in analyses of carbon isotopes should recognize that the
tissue type (e.g., biological apatite or collagen) under analysis plays an important part in
dietary reconstruction. This phenomenon was first realized after various controlled feeding
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