Supplementary Information: Manuscript B09481
New ages for human occupation and climatic change at Lake
Mungo, Australia
James M. Bowler, Harvey Johnston, Jon M. Olley, John R. Prescott, Richard G. Roberts,
Wilfred Shawcross & Nigel A. Spooner
METHODS & TABLE 1
Optical dating
Optical dating provides an estimate of the time elapsed since luminescent minerals were
last exposed to sunlight1,2. Buried grains will accumulate the effects of the nuclear
radiation flux to which they are exposed, and the burial dose (equivalent dose, DE) can be
measured using the optically stimulated luminescence (OSL) signal. Optical ages were
calculated from the equivalent dose (DE), measured using the OSL signal, divided by the
dose rate due to ionizing radiation1,2. Dose rates were obtained from field gamma
spectrometry, delayed neutron activation, thick-source alpha counting and high-resolution
gamma spectrometry measurements, together with estimates of the cosmic-ray and
internal alpha dose rates. Not all methods were used for all samples; where they were, the
agreement among methods was excellent. Sample-specific details are given in the
footnotes to Table 1. Quartz grains of 90–125 and 180–212 µm diameter were extracted
from the sediment samples under dim red illumination using standard procedures,
including etching by HF acid to remove the outer alpha-dosed layer2. N.A.S. obtained DE
values by constructing combined multiple-aliquot, additive-dose and regenerative-dose
growth curves using the ‘Australian slide’ technique2 and a saturating exponential plus
linear function for curve fitting; experimental conditions and instrument specifications
are as reported elsewhere3. R.G.R. obtained DE values using the single-aliquot
regenerative-dose protocol, statistical models, and experimental apparatus as described
elsewhere4,5. For the latter, DE values were obtained using small (~10 grain) and large
(>100 grain) aliquots of each sample (to check for insufficient bleaching of the quartz
grains at deposition6), and the multiple-aliquot ‘slide’ technique and additive-dose
method with thermal transfer correction2,4 were also applied to two samples. The mean
DE values agreed in all cases, and the single-aliquot DE distributions showed no evidence
of partial bleaching6; the weighted mean DE values are listed in Table 1. Beta and gamma
dose rates due to 238U, 232Th (and their daughter products) and 40K were calculated from a
combination of high-resolution gamma spectrometry (J.M.O.) to check for disequilibria
in the 238U and 232Th decay series, and field gamma spectrometry (J.R.P. and N.A.S.) to
accommodate any in situ heterogeneity in the gamma radiation flux. Selected samples
were measured also by delayed neutron activation and thick-source alpha counting
(J.R.P.). A condition of secular equilibrium presently exists in the 238U and 232Th decay
chains for all samples, except those from the Mungo I Residual, which have 238U deficits
of 42–56% relative to 226Ra. Such deficits, however, have an insignificant effect on the
optical ages when, as here, the 238U chain accounts for a minor fraction (<20%) of the
total dose rate. The internal alpha dose rate for all samples was estimated from
instrumental neutron activation and inductively-coupled plasma mass spectrometry
measurements of U and Th concentrations in acid-etched quartz grains from 11 samples
(N.A.S.). Alpha-efficiency, beta-dose attenuation, and dose-rate conversion factors were
extracted from publications7–9 and the beta, gamma and cosmic-ray dose rates were
corrected2 for the estimated long-term water content of each sample. The cosmic-ray dose
rates were calculated from published data10, making allowance for site altitude,
geomagnetic latitude and time-averaged thickness of sediment overburden. The timeaveraged burial depth and long-term water content of each sample were estimated by
J.M.B. from reconstructions of the lunette profile and lake hydrology over the last 60 kyr,
with associated uncertainties sufficient to accommodate all likely possibilities. As an
inter-laboratory test of reproducibility, four duplicate samples were processed
independently by N.A.S. and R.G.R. using different protocols to determine the DE values,
laboratory beta-sources calibrated using different standards, and dose rates calculated
using largely independent data sets. The optical ages are in agreement, giving added
confidence to the OSL chronology for Lake Mungo.
Dust index
Desert dust contains a high percentage of 20–60 µm diameter quartz grains coated with
distinctive red clay skins (argillic cutans). Dust grains transported in suspension from red
desert soils are identical to the red silt-sized quartz identified as Sarahan-derived dust
(Wüstenquarz) in Atlantic marine sediments11. The red coatings and their silt size
distinguish dust grains from locally-derived lake and lake-shore sediments, which are
well sorted with modal peaks near 0.4 mm. Dust grains are almost always associated with
a reddish-brown clay matrix, the companion component of Australian desert dust. A ‘dust
index’ was established by counting silt-sized argillic quartz grains (over an area of 1 cm2
on a micrometer stage) in petrographic thin-sections of undisturbed sediment samples.
The resulting index is expressed as grains/cm2, a relative measure of the dust component.
Table 1 (to insert here)
References
1.
2.
3.
4.
Huntley, D.J., Godfrey-Smith, D.I. & Thewalt, M.L.W. Optical dating of sediments.
Nature 313, 105-107 (1985).
Aitken, M.J. An Introduction to Optical Dating (Oxford University Press, Oxford,
1998).
Spooner, N.A., Olley, J.M., Questiaux, D.G. & Chen, X.Y. Optical dating of an
aeolian deposit on the Murrumbidgee floodplain. Quat. Sci. Rev. 20, 835-840 (2001).
Galbraith, R.F., Roberts, R.G., Laslett, G.M., Yoshida, H. & Olley, J.M. Optical
dating of single and multiple grains of quartz from Jinmium rock shelter, northern
Australia: Part I, experimental design and statistical models. Archaeometry 41, 339364 (1999).
5. Yoshida, H., Roberts, R.G., Olley, J.M., Laslett, G.M. & Galbraith, R.F. Extending
the age range of optical dating using single ‘supergrains’ of quartz. Radiat. Meas. 32,
439-446 (2000).
6. Olley, J.M., Caitcheon, G.G. & Roberts, R.G. The origin of dose distributions in
fluvial sediments, and the prospect of dating single grains from fluvial deposits using
optically stimulated luminescence. Radiat. Meas. 30, 207-217 (1999).
7. Thorne, A. et al. Australia’s oldest human remains: age of the Lake Mungo 3
skeleton. J. Hum. Evol. 36, 591-692 (1999).
8. Mejdahl, V. Thermoluminescence dating: beta-dose attenuation in quartz grains.
Archaeometry 21, 61-72 (1979).
9. Adamiec, G. & Aitken, M. Dose-rate conversion factors: update. Ancient TL 16, 3750 (1998).
10. Prescott, J.R. & Hutton, J.T. Cosmic ray contributions to dose rates for luminescence
and ESR dating: large depths and long-term time variations. Radiat. Meas. 23, 497500 (1994).
11. Radczewski, O.E. in Recent Marine Sediments: A Symposium (ed P.D. Trask) 496502 (Special Publication 4, Society for Economic Paleontologists & Mineralogists,
Tulsa, Oklahoma, 1960).
12. Shawcross, W. Archaeological excavations at Mungo. Archaeol. Oceania 33, 183200 (1998).