Environmental Engineering Reference
In-Depth Information
of known date, sequences of rings can be compared between progressively older samples
(Figure 1). The width of the rings is partly determined by the climatic conditions
prevailing each year, so they produce both an absolute dating system as well as climatic
information. In Europe oak chronologies have been developed extending back to 9971
BP, and in North America the bristlecone pine (
Pinus longaeva
) has been used to give a
continuous master chronology to 8681 BP for California. Lichens have also been used, on
the principle that lichen size is related to age. In areas experiencing glacier recession,
lichen growth is related to dated morainic surfaces to give a lichen growth curve.
Surfaces of unknown date within a similar area can then be dated by relating lichen
diameters at that site to the growth-rate curve and deriving a calendar age. It can be used
only for recent periods and where there is some other absolute dating method to produce
the dating curve.
Figure 1
Method of correlation in dendrochronology.
Of greatest use for absolute dating are the radiometric methods. These are based on the
radioactive properties of some unstable isotopes. The rate of radioactive decay is time
dependent, so if the rate of decay can be measured, the age of the sediment containing the
isotope can be established. Some elements decay in seconds, whilst others take millions
of years, allowing a range of sediment ages to be determined. Radiocarbon dating has
been of greatest value for the Late Quaternary period, as carbon is often contained within
sediment. It has a half-life (the period of time required to reduce a given quantity of
material to one half) of 5730 ± 40 years, so that material up to about 45,000 years old can
be dated. Methods have been developed to increase this slightly, but radiocarbon is still
restricted to the later phases of the Quaternary period.
For older material other isotopes such as potassium/argon and uranium/thorium can be
used to give an approximate absolute date of the material sampled. Other specialized
techniques such as luminescence and amino acid decay can also be used under particular
conditions. Taken together, there are a number of dating methods that are available for
giving an absolute date to sedimentation. Normally the most appropriate technique will
be used for any given material. No longer are we dependent on a relative dating, though
even absolute dates contain a margin of error and, more seriously, the radiocarbon clock
is not alway s directly related to the calendar. For example, radiocarbon is not produced
uniformly over time, as its production depends partly on solar activity. Radiocarbon dates
and calendar dates do not correspond precisely and have to be corrected if necessary.
