Tracing the Source of the Elephant And Hippopotamus Ivory from ...

study residence and mobility. Katzenberg and Harrison (1997) offer a comprehensive
review of the state of archaeological bone chemistry studies. The various isotopes of a
given element differ in the number of neutrons, while sharing the same number of
protons and electrons. Mass spectrometry is used to determine the amount of a given
isotope present in the sample. Stable isotopes do not change over time, and the isotope
ratios are used to characterize bone and ivory samples. The isotope ratios are expressed
using the delta (δ) notation in parts per thousand (per mil: ‰), and are derived from the
equation below, where R is the ratio of the heavier isotope to the lighter isotope:
Stable Carbon Isotope Ratios
δ(‰) = [(Rsample/Rstandard)-1] x 1000
The two stable carbon isotopes are 12 C and 13 C. 12 C accounts for 98.89% of
naturally occurring carbon, whereas 13 C accounts for 1.11%. The international standard
is derived from the Belemnitella americana marine fossil limestone from the Peedee
geological formation in South Carolina (PDB). Because limestone has more 13 C than
organic matter, the δ 13 C values from analyses of organic tissues are negative. The
variation in δ 13 C in plants reflects the different photosynthetic processes of C3, C4, and
CAM (crassulacean acid metabolism) plants, which all obtain their carbon from
atmospheric CO2. C3 plants generally grow in temperate climates and include trees,
shrubs, tubers, and temperate grasses, while C4 plants are subtropical grasses capable of
growing in hot and dry climates. Fractionation occurs in biological systems because 12 C
reacts first since it is lighter and leaves behind the heavier 13 C, so that plants end up with
far less 13 C than is present in the atmosphere. The δ 13 C values of C3 plants range from -
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