There are different ways of dating old
sediments, such as peat and lake mud, but radiocarbon-dating
is probably the most commonly used technique. This
method uses the radioactive properties of the element
carbon (C in chemical notation). Carbon exists in
different forms in the atmosphere, because of
differences in the particles that make up each carbon
atom. Carbon-12 or 12C is the dominant
type while carbon-14 or 14C exists in much
smaller amounts. 14C is unstable so it
decays at a known rate to the stable element
nitrogen-14 or 14N. This is called radioactive
decay.
All living things take up carbon: plants
take in carbon dioxide (CO2) in
photosynthesis and animals (and people) then absorb
carbon by eating plants or animals that have eaten
plants. When an organism dies, the carbon is no
longer renewed and radioactive decay starts. By
measuring how much 14C is left, scientists
can work out how long ago an organism died.
There is always some uncertainty
associated with radiocarbon dates because of the
technical difficulties involved with these tiny
measurements and because of short-term fluctuations
in atmospheric carbon through time which adds extra
'wiggles' to the radioactive decay curve.
To date the vegetation history from
Greenhead Moss, we chose thin layers of peat where
important changes happen in the pollen diagram. These
10 peat samples were sent to a radiocarbon dating
laboratory. There they have the complicated
technology to extract pure carbon from the peat and
then measure how much of this is 14C to
tell us how old each bit of peat is.
We have used the 10 dates to draw a time-depth
curve. We use this to estimate the age of peat
anywhere in the peat sequence or stratigraphy and
this will tell us when and how quickly the vegetation
changed:
Time-depth curve
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