Biology Reference
In-Depth Information
While archaeological data are important tools for the reconstruction of prehistoric social
hierarchies, stable isotope analyses also represent a unique lens in which to view social rela-
tionships. In an elegant example, Ambrose and colleagues (2003) utilized stable isotope data
of both carbon and nitrogen isotopes to investigate status differences between a subsample
of 272 individuals interred in Mound 72 at Cahokia (just outside present-day St. Louis,
Missouri). Of these individuals, archaeological evidence suggested at least two distinct social
groupswere interred at the site. These distinctions were based upon the presence of extra-local
artifacts that had been buriedwith some individuals and not others. Moreover, bioarchaeolog-
ical analyses of the two groups indicated differences in the frequency of paleopathological
lesions, with lower status individuals described as having higher frequencies of
nonspecific
indicators of stress
1
(see Smith [Chapter 7], this volume, for a review of stress indicators).
Isotopic data revealed several interesting characteristics about the mortuary sample from
Mound 72. Data from bone collagen (the organic component of bone) yielded nitrogen signa-
tures that suggested the diet of high-status individuals was more enriched from animal
protein sources. Further, analysis of carbon from biological apatite (the mineral part of
bone) indicated an even more marked disparity between the high-status and low-status
groups. Given that carbon isotopes derived from biological apatite provide an indicator of
whole or bulk diet, differences in maize consumption between groups were even more
apparent. Overall, these isotopic data suggest that the high-status individuals had access
to additional food sources while the lower status individuals did not (
Ambrose et al., 2003
).
WHAT ARE ISOTOPES?
As many skeletal biologists may have limited experience in chemistry, a rudimentary
understanding of the concepts involved is necessary before proceeding further.
Isotopes
are variations of chemical elements that differ in the number of neutrons but have the
same number of protons. As a result, isotopes of the same element will differ in their
atomic
mass,
or the sum of the number of protons and neutrons. For example, strontium-86 (written
86
Sr) is one isotope of the element strontiumwith a mass number of 86. The
atomic number
of
strontium is 38, indicating that every strontium atom has 38 protons, so that the
neutron
number
of this isotope is 48 (38 protons
þ
¼
86 mass number). A list of isotopes
and their masses is presented in
Table 15.1
. The isotopes presented in this table are all defined
as
stable isotopes
, (as opposed to unstable or
radioactive isotopes,
i.e., those that decay over
time), and are commonly analyzed by skeletal biologists.
While each of the isotopes of an element have had the same atomic number, their different
atomic masses influence the ways in which they behave during physical and chemical
processes (
Brown and Brown, 2011
). For example,
13
C is 8.3% heavier than
12
C, which means
that it reacts more slowly in biochemical reactions such as photosynthesis (the process plants
use to convert atmospheric carbon dioxide to glucose for energy). During photosynthesis, the
lighter isotope becomes enriched more rapidly than the heavier isotope, resulting in
isotope
fractionation
, or a change in isotope ratios due to chemical processes (i.e., the ratio of
48 neutrons
13
C
12
C). The degree of isotope fractionation is plant-specific and ultimately
compared to
1
All bolded terms are defined in the glossary at the end of this volume.
