Geoscience Reference
In-Depth Information
Figure 6.4. Effect of contamination by inert carbon on radiocarbon age. (Graph com-
piled from data provided in Polach and Golson,
1966
.)
dating. Owing to its ability to withstand chemical decay, charcoal may persist for a
long time in the landscape in temporary storage sites along the valley sides. Once
these stored sources of charcoal are remobilised, perhaps as a result of an extreme
rainfall event, they may become incorporated into much younger colluvial or alluvial
deposits. A perennial problem in dating river alluvium therefore concerns the possible
inheritance of charcoal or shell remains from an earlier depositional cycle. In an
elegant study of a small river in the Blue Mountains of eastern Australia, Blong and
Gillespie (
1978
) found that charcoal fragments of varying sizes deposited in channel
sand ripples during a single modern storm event ranged in age from 0 to more than
1,000 radiocarbon years, according to fragment size, indicating that previously stored
charcoal had been reworked by the flood.
Shells are less likely to survive multiple episodes of transport undamaged, and
ones that are broken can therefore be distinguished and not used for dating. A more
insidious problem with shells is the possibility that conventional methods of detecting
recrystallization from aragonite to calcite using X-ray diffraction and cathodolumines-
cence may fail to detect what Webb et al. (
2007
) termed 'cryptic meteoric diagenesis'
in freshwater snail shells.
