living things, entering through food, water, and the air we breathe. It is
constantly being replaced in our tissue. But when we die, that process stops,
and the unstable radioactive isotopes begin to decay (Figure 2). In the case of
Carbon 14, because we know how long it takes to decay and at what rate that
happens, we can calculate how long it has been since the organism died–a
tree, an animal, a person.
The combination of stable and radioactive isotopes can also tell us
something about the diet of people in the past, where they were brought up as
children, and where they lived in their later adult years. Carbon isotopes can
indicate the climate in which a person grew up, because different plants take
up varying amounts of carbon depending on whether they are growing in
temperate or tropical environments; the amounts of carbon and nitrogen
isotopes can also indicate whether a person’s diet consisted of mostly
vegetable foods, or meat. Isotopes of the metal strontium are present in food
and water, and enter into our bones and teeth as we mature. The level of
strontium in different places varies, so by measuring strontium isotopes we
can establish where a person lived. Teeth are formed in the first decade of a
human life, so the strontium levels in them will reflect the place where a
person spent their childhood. Bones grow and change over time, so the levels
of strontium in them are a reflection of where they lived in the last ten or so
years of their life.
These analyses are useful in determining the identity of victims of disasters
such as plane crashes–working out where each person came from can help
put a name to their remains–and of course putting names to victims of crime.
They are very useful to archaeologists, not only because we can track how
people moved around in the ancient past (for example, finding out that a man
buried near Stonehenge in the early Bronze Age came originally from Central
ARCHAEOLOGY, HISTORY, CRIME AND PUNISHMENTFigure 2. The carbon14 curve (Source: HowardMorland/Wikimedia Commons)