Geoscience Reference
In-Depth Information
extend deep into the fundamental methodological foundations of all geographical sci-
ences concerned with matters related to the past. It relates predominantly to a change
in scale and complexity which, although hinted at earlier this century, could hardly
have been imagined at the opening of the post-World War II period.
As Bowen argued so forcefully, the oxygen isotope record from the marine archive
had major implications for all components of Quaternary geography - it made
geomorphologists, glaciologists and ecologists rethink the tempo of Quaternary
landscape and vegetation dynamics - it also led to new ideas in human evolution
and adaptation (Gamble, 1986). In short, this work forced a radical rethink about
the complexity and dynamics of Earth system change during the course of the Qua-
ternary Period and it signalled the end of the Alpine framework. The record was
soon replicated in all the major marine basins and a series of marine oxygen isotope
records were later used by Shackleton and co-workers to show that the rhythms of
the ice ages were controlled by astronomical parameters as predicted by Milanko-
vitch much earlier in the century (Hays et al., 1976; Imbric and Imbric, 1979).
Dating the Terrestrial Records: The Radiocarbon Method
Radiocarbon dating was developed by Willard Libby and his team at the University
of Chicago in the years immediately after the Second World War. Libby was awarded
The Nobel Prize for Chemistry in 1960 'for his method to use carbon-14 for age
determination in archaeology, geology, geophysics, and other branches of science'.
Radiocarbon (
14
C) is continually produced in the upper atmosphere and it enters all
living organisms via the carbon dioxide cycle. On the death of a plant or animal, the
uptake of radiocarbon ceases and the radiocarbon store in the organism continues to
decay, but without replenishment. So death sets the radiocarbon dating clock ticking
so that with a few assumptions, it is possible to establish the amount of residual
radioactivity per gram of carbon in a fossil sample and, using modern standards
and the measured half-life of radiocarbon (5,570
30 years), it becomes possible to
calculate a date for the death of the sample (Libby, 1955; Bowman, 1990).
The measurement of radiocarbon requires sensitive and specialist laboratory
equipment because for every one million million atoms of stable carbon (
12
C) in a
living organism, there is just a single atom of
14
C (Lowe and Walker, 1997). The sen-
sitivity of the method has been signifi cantly enhanced through the use of Accelerator
Mass Spectrometry (AMS) as this allows
14
C atoms to be detected and counted
directly in contrast to conventional dating which only detects those atoms that decay
during the time interval allotted for an analysis. AMS offers several advantages
because the measurement time is much quicker and only very small samples of carbon
(1 mg or less compared to 5 to 10 g for conventional dating) are needed for dating
(Gowlett et al., 1997; Bell and Walker, 2005). AMS represented a key breakthrough
for studies of the Middle and Upper Palaeolithic because it allowed small samples of
charcoal to be dated instead of bone samples - the latter are susceptible to contami-
nation by more recent carbon from percolating groundwater. This process can top up
the amount of residual radiocarbon in a bone sample to give a spuriously young age.
Another recent breakthrough has seen the application of the AMS approach to
obtain radiocarbon determinations directly from cave paintings by dating small
samples of the pigments and fragments of charcoal that form the images on the cave
walls (e.g., Valladas, 2003) (fi gure 13.2). Previously, the chronology of the cave
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