Luminescence chronologies for coastal and marine sediments

BOREAS Luminescence chronologies for coastal and marine sediments 527
Olley et al. 2004a, b; Goodwin et al. 2006; Ballarini
et al. 2007; Bostock et al. 2007; Carr et al. 2007; Brooke
et al. 2008a, c) in aeolian, marine and estuarine sediments.
No single-grain D e distribution data were presented
or discussed for some of the studies (e.g.
Goodwin et al. 2006; Bostock et al. 2007; Brooke et al.
2008c). Bostock et al. (2007), however, mentioned that
some samples required the minimum age model to estimate
the burial dose; this suggests that these D e distributions
probably showed some evidence of partial
bleaching. Good agreement was obtained between
the quartz OSL and 14 C results, suggesting that singlegrain
analysis was advantageous in this estuarine
environment.
Zazo et al. (2005) reported results for one sample
analysed using 4200 small aliquots (each composed of
o10 grains) and 24 larger aliquots (50 grains). Even
though the large aliquots are small compared to those
measured at most laboratories, the difference in results
between their small and large aliquots is striking. The
D e values for the small aliquots ranged from 55 to
240 Gy, compared to 130 to 400 Gy for the larger
aliquots. Such a range cannot be explained by beta microdosimetry,
but partial bleaching or post-depositional
mixing cannot be excluded. In this situation,
neither small nor large aliquots will necessarily yield the
correct depositional age, and single grains ought to be
measured. The calculated ages should, therefore, be
viewed with caution.
Olley et al. (2004b) measured single grains from a
marine sediment sample collected from a 3500-year-old
storm deposited beach ridge in Queensland, Australia.
The D e values, when plotted as a radial plot, indicated
that the sample had been affected significantly by partial
bleaching, but these authors were able to obtain
excellent agreement with a 14 C age when the minimum
age model was applied. They also constructed 96 synthetic
aliquots, of which 48 contained 100 grains while
the other 48 contained 10 grains. When they applied the
minimum age model to these two data sets, they obtained
ages that were too old by 32% for the 10-grain
and by 57% for the 100-grain aliquots. This illustrates
clearly that caution should be exercised when using
multi-grain aliquots, even small aliquots, for sediments
that may have been poorly bleached; this may explain
the anomalous results obtained by Zazo et al. (2005).
Partial bleaching was also observed, using single grains,
for two samples from a deep-sea ocean core off the
coast of northwestern Australia (Olley et al. 2004a). In
addition to analysing the D e distributions for these two
samples, they also estimated the D e values associated
with the fast-dominated signal from LM-OSL measurements
(e.g. Yoshida et al. 2003); this resulted in a
similar D e distribution. Applying the minimum age
model to these two samples resulted in age estimates
consistent with 14 C ages. Marine sediments have,
therefore, been shown not to be immune to partial or
heterogeneous bleaching. Accordingly, caution should
be taken in general for coastal and marine sediments,
and especially when dating Holocene samples, where
the effects of incomplete bleaching at burial will be exacerbated
by the comparatively small dose absorbed
after deposition.
Single-grain analysis was also applied to aeolian deposits
by Ballarini et al. (2007), who measured single
grains for two modern samples from Texel Island in
The Netherlands. One sample was known to be o10
years old and the other had been deposited 1 year before
collection. Single-aliquot data suggested that the
former had been well bleached, whereas the D e for the
latter sample was significantly overestimated and resulted
in an age of 7424 years. Using a modified SAR
approach specifically designed for young samples, Ballarini
et al. (2007) were able to obtain ages similar to
those obtained from the single aliquots when a mean D e
value was used. The single-grain D e distributions of
these samples, however, provided valuable information
that clearly indicated the presence of some grains for
which not even the fast component had been sufficiently
bleached. The presence of such grains in a multigrain
aliquot will result in age overestimation, just as it
will when using the mean of the single-grain D e values.
Partial bleaching, therefore, can also afflict to a measurable
extent samples with an aeolian origin, and the
extent of this bias will be exacerbated in age determination
for young samples. At present, application of
the minimum age model to such young samples is not
straightforward, but use of the mean for such distributions
is not appropriate either; a revision of the minimum
age model that does not log transform the D e
values has, therefore, been developed (R. F. Galbraith,
pers. comm. 2008).
In South Africa, Jacobs et al. (2003a) calculated D e
values for two aeolianite samples at sea level, and for an
uncemented dune sand deposited inside a coastal cave,
using large, single aliquots (1000 grains), single grains
and synthetic aliquots (each of 100 grains). They obtained
consistent results for all three aeolian units,
when applying the central age model of Galbraith et al.
(1999). The single-grain D e distribution, however, was
wider than expected (‘overdispersed’), but similar in
spread to laboratory-controlled (dose recovery) measurements
of quartz grains given a known dose. Based
on the appearance of the data on a radial plot, it can be
suggested that such scatter in apparent D e results from
natural variability in quartz OSL that is not fully accounted
for in the SAR procedure. Microdosimetry
variations in the beta dose to individual grains is a
possibility, but is considered unlikely as similar results
were obtained from both cemented and uncemented
samples. Also along the southern Cape coast of South
Africa, Bateman et al. (2004) and Carr et al. (2007)
measured single grains for a subset of their samples.
For one of the older samples, about 5 m below the