Geoscience Reference
In-Depth Information
preserved in peat bogs scattered throughout Europe (Pearson et al.,
1986
). Beyond that
time, there have been progressive improvements in extending the calibrated time scale,
initially back to 30,000 years (Bard et al.,
1990
; Guilderson et al.,
2005
; Blackwell
et al.,
2006
) and later back to 50,000 years using pairs of pristine fossil corals dated
very precisely by both
14
Cand
230
Th/
234
U/
238
U (Fairbanks et al.,
2005
;Chiuetal.,
2005
), with the latter method being used to calibrate the former.
Three technical advances made this progress possible. First, the direct counting of
individual atoms of
14
C using cyclotrons as extremely sensitive mass spectrometers
has pushed the
potential
limits of radiocarbon dating back from approximately 40,000
years ago to approximately 100,000 years ago, with 1- to 100-mg samples (Muller,
1977
; Stephenson et al.,
1979
). Second, the use of Accelerator Mass Spectrometry
(AMS) and better preparation techniques have reduced the size of the sample needed
in radiocarbon dating by a factor of a thousand (Doucas et al.,
1978
; Muller,
1979
;
Hedges and Gowlett,
1984
). Third, the use of Thermal Ionization Mass Spectrometry
(TIMS) has enlarged the range of dating applications that were previously not possible
using the less sensitive alpha-counting technique (Chiu et al.,
2005
; Fairbanks et al.,
2005
). In establishing their calibration curve, Fairbanks et al. (
2005
) took great pains
to ensure that there had been minimal diagenetic alteration from aragonite to calcite
in their coral samples, rejecting any sample with more than 0.2 per cent calcite, in
contrast to other workers who used 1 per cent calcite detection limits and calcite
sample values of 1 per cent to 5 per cent. The reason for this precaution is that during
any chemical alteration from aragonite to calcite, there may have been a loss of
radiocarbon.
A major recent breakthrough in calibrating the terrestrial radiocarbon record from
11.2 ka back to 52.8 ka made excellent use of the annually laminated sediments in
Lake Suigetsu on the Sea of Japan coast in western Japan (Bronk Ramsey et al.,
2012
).
The age of 52.8 ka is the present limit of the radiocarbon method. The calibration was
based on 651 terrestrial radiocarbon dates. One outcome of this work was to show that
reservoir ages used to calibrate the Cariaco Basin and north-east Atlantic time scales
need some revision and have not been constant through late Quaternary time. It is
also worth noting that Lake Suigestu is an important site for paleoclimatic research in
that it provides an annual record of four climate-proxies, namely, winter and summer
monsoon intensity and the respective temperatures of the Siberian air mass and the
Pacific air mass (Nakagawa et al.,
2006
). This is because the lake is located north of
the monsoon front in winter and south of that front in summer, and so it is highly
sensitive to changes in Pacific air mass temperature in summer and Siberian air mass
temperature in winter.
Sample contamination by inert or by modern carbon may affect the reliability of the
radiocarbon dates (Polach and Golson,
1966
; Gillespie,
1982
).
Figure 6.3
shows the
effect of contamination by modern carbon, such as plant roots. The effect is substantial
and, unlike the effect of contamination by inert carbon shown in
Figure 6.4
, is not a
