Geoscience Reference
In-Depth Information
paintings was loosely based on the style of the images. This work offers the potential
to provide a much more rigorous basis for the development of a detailed chronology
of Upper Palaeolithic art across Europe, but some of the AMS results and their
interpretation have been contested (Pettitt and Bahn, 2003).
A key assumption of the method is that the ratio of radiocarbon (
14
C) to stable
carbon (
12
C) has remained constant in the Earth's atmosphere so that the measure-
ment of residual radiocarbon in a given sample provides a reliable indication of its
true age. However, it is now well established that radiocarbon production in the
upper atmosphere has fl uctuated markedly during the Quaternary (see Bard et al.,
1990; Mellars, 2006) and radiocarbon dates therefore have to be calibrated because
radiocarbon years are not directly equivalent to calendar years. In theory, radiocar-
bon dating can be used to date organic materials up to 50,000 years old, but in
practice many researchers do not place much faith in dates older than about 40,000
years because the ages can be distorted by sample contamination. Furthermore, the
development of calibration curves and algorithms for such old samples is still in its
infancy.
Since the radiocarbon method was pioneered by Libby in the 1940s, it has seen
a series of fundamental changes in the measurement and interpretation of results.
The key changes are largely responses to the problems associated with age calibra-
tion and sample contamination, and these factors are especially acute for radiocar-
bon determinations beyond six half-lives. However, in a stimulating and sanguine
review, Mellars (2006) argues that recent methodological advances have dramati-
cally reduced both of these sources of error. First, new pretreatments for the purifi -
cation of bone collagen have effectively removed the problem of contamination by
more recent carbon. Second, a new calibration model based on data from various
sites around the world now provides the best available means of calibrating radio-
carbon dates over the last 50,000 years (e.g., Hughen et al., 2004). This calibration
shows that a radiocarbon date of 35,000 years BP is equivalent to a calendar age
of approximately 40,500 years BP. It is therefore of crucial importance when report-
ing dating results to make a clear distinction between radiocarbon years and calen-
dar years. The systematic displacement of radiocarbon ages from true calendrical
ages has very clear implications for any comparison between radiocarbon-based
chronologies from archaeological sites and proxy climate records such as the Green-
land ice cores or other geological archives that have been dated by other methods
(Woodward, 2003; Mellars, 2006). If the purifi cation of bone samples and calibra-
tion back to 50,000 BP become routine over the next few years, this will present
exciting opportunities to test ideas about the nature of the Middle to Upper Palaeo-
lithic transition. Mellars (2006) has already begun to put forward a case for a much
more rapid transition and a more rapid dispersal of modern humans across
Europe.
Quaternary Geography: Transects Across Europe
Figure 13.4 shows transects across Europe from the Mediterranean Sea to the Arctic
Ocean under interglacial and glacial conditions. Each transect shows, in broad
terms, the major ecosystems present across the continent at the extremes of warm
and cold stages - the odd and even numbered stages, respectively, on fi gure 13.3.
The cold stage geography of Europe shows a large mid-latitude ice sheet fringed by
a belt of polar desert and steppe tundra. It shows a few trees on the southern slopes
