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Environmental dose rates and radioactive disequilibrium from some Australian luminescence dating sites
~
Quaternary Science Reviews (Quaternary Geochronology), Vol. 14, pp. 439-448, 1995.
t Pergamon
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ENVIRONMENTAL DOSE RATES AND RADIOACTIVE DISEQUILIBRIUM FROM SOME AUSTRALIAN LUMINESCENCE DATING SITES J . R . P R E S C O T T a n d J.T. H U T T O N + Department o f Physics and Mathematical Physics, University o f Adelaide, SA 5005, Australia Abstract - - Determination of geological/archaeological ages by luminescence dating requires the measurement of the rate of delivery of radiation dose from the sample and its environment. For all age determinations it is necessary to assess whether the dose-rate has been constant during the time that the dose has been accumulating, and to make allowance for it if it was not. We report here results from a number of Australian sites to illustrate the information that can be found about element concentrations, about their distribution and about dose rates. Tests for radioactive disequilibrium form part of these measurements. Such tests take the form of comparison of independent measurements of dose-rates and/or individual elements. In general, there is little or no evidence for disequilibrium in aeolian dune systems, even when the age of the system exceeds 500 ka. However, disequilibrium has been found in several sites subject to wet conditions.
INTRODUCTION
QG
required. M o b i l i t y o f individual nuclides in the decay chain can result in parts of the chain being broken or augmented. An obvious example is the loss of 222Rn due to gaseous diffusion, which would reduce the lower part of t h e u r a n i u m s e r i e s r e l a t i v e to t h e p a r e n t 238U. Alternatively, transport in solution may carry parent or daughter nuclei into the environment. For a radioactive chain in equilibrium, each member of the chain decays to the next member at the same rate as it is p r o d u c e d from the i m m e d i a t e l y preceding one. Thus, all members of the chain show the same activity, i.e. the same n u m b e r o f disintegrations per unit time: e x p r e s s e d as b e c q u e r e l (Bq) or, m o r e usually, Bq/kg. Disequilibrium is indicated if there is a statistically significant difference between the activities of individual chain members. Minor disequilibrium does not usually have a significant effect on the dose rate. On the other hand we have found a few cases where disequilibrium is pronounced e n o u g h that it has to be a l l o w e d for ( B e l p e r i o et al., 1984; Smith and Prescott, 1987). The first o f these is from an underwater site in Spencer Gulf, while the second is from a dune site overlain with a lava flow at Mt Schank. Both are discussed in more detail below. Murray et al. (1992) give another example, from a fluvial environment, where an excess of 226Ra with respect to 23°Th was found. Krbetschek et al. (1994) have recently discussed the circumstances in which disequilibrium may be found in the context of luminescence dating. Readhead (1987) has published information that aids dose rate calculations when disequilibrium applies. The A d e l a i d e Physical Archaeometry laboratory has now been in operation for almost two decades and has environmental dose-rate data for a wide variety of locations. At many of these, measurements have been made
There is now a considerable body of data on luminescence dating in Australia. This includes thermoluminescence (TL), o p t i c a l l y s t i m u l a t e d l u m i n e s c e n c e ( O S L ) and, m o r e recently, infra-red s t i m u l a t e d l u m i n e s c e n c e (IRSL). Some recent representative examples are to be found in Roberts et al. (1990, 1994), Nanson et al. (1991) and Huntley et al. (1993a, b, 1994). All o f these reports include information about dose rates and many also give detailed data about concentrations of the individual elements, potassium, uranium and thorium, which are responsible for the dose. In addition there is a c o n t r i b u t i o n from c o s m i c rays, the r e l a t i v e i m p o r t a n c e o f which depends upon the location of the site (Prescott and Hutton, 1988, 1994). Ages found by luminescence dating are now c o m m o n in the interval from the present to 120 ka ago and the method is producing credible results for ages as large as 800 ka (Huntley et al., 1993a; Berger, 1994). For such long periods of time it is particularly necessary to include consideration of the change with time of the dose rate to find an average dose rate to go with the integrated dose. At a specific site, such information can often be deduced from measurements at points in a profile. Changes with depth of dose-rates and individual element concentrations may give information about the deposition history of the site. We illustrate this with some selected examples. T h e c o n t r i b u t i o n s f r o m U a n d Th c o m e f r o m the whole o f the radioactive decay chains that follow these parents and, for accurate evaluation o f dose rates, knowle d g e o f the e x t e n t o f r a d i o a c t i v e d i s e q u i l i b r i u m is
+Deceased. 439
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Quaternary Science Reviews (Quaternary Geochronology): Volume 14
on a complete excavation profile. It is the purpose of the present paper to report a variety of dose rates and elemental analyses from Australian sites. Most of this is previously unpublished but some previously published work is quoted for contrast and completeness. We have not attempted a comprehensive review of work from other laboratories. The sites discussed are shown on the map in Fig. 1. Seven different methods of element analysis are represented: delayed neutron analysis (DNA), neutron activation analysis (NAA), X-ray spectrometry (XRS), thick source (z-counting (TSAC), in situ y-ray scintillometry (scint), and high resolution c~ and y spectroscopy (c¢ and y, respectively). Their relative merits have been discussed in Hutton et al. (1985), Prescott and Hutton (1987, 1988) and Hutton and Prescott (1992). Briefly: DNA and NAA are reactor-based methods that give the parents, 238U and 232Th, respectively; XRS is used for K but it can also be used for U and Th at concentrations above about 5 gg/g, for other major elements and, if required, minor elements. In TSAC, all the o~ particles in both the U and Th decay chains are counted together; an estimate of Th is made by counting pairs of
o~ particles in the 22°Rn --) 216p0 --~ 212pb decay in the Th chain; the U-equivalent follows, assuming equilibrium in both cases. Although these estimates are individually of limited accuracy because of the low pairs count rate, the combined contribution of the U and Th series to the dose rate is almost independent of the relative concentrations of U and Th, for a given o~ count rate. The data in the tables should be read with this in mind. In scintillometry, y-ray spectra are measured in the field with a sodium iodide scintillometer (Prescott and Hutton, 1987). It measures individual y-rays for potassium (1.46 MeV), 214Bi (1.76 MeV) and 2°sPb (2.62 MeV). 214Bi is from a late daughter in the uranium series while 2°sPb is the last member of the thorium series. U- and Th-equivalents follow, again assuming equilibrium. The scintillometer is calibrated for the three nuclides individually and, independently, for the y-ray contribution to the dose rate. Comparisons among these methods allow an assessment of the existence and degree of disequilibrium. For example, if a comparison of parent 23sU by DNA and daughter 2HBi by scintillometry gives the same activity, then the chain is probably in equilibrium. A difference may indicate disequilibrium and this would be confirmed
• TC
°PJ oLA
MS
CA ,SK
SG
FIG. 1. Map showing the location of all the places named in the text. CA, Cooloola (CA, SK); EB, Roonka; ER Eyre Peninsula (HC, ML, WW); LA, Lake Amadeus; MS, Mound Springs; PJ, Puritjarra; SC, Mt Schank; SE, South East (MG, SQ, WKI ); SG, Spencer Gulf (SG, RED); TC, Tennant Creek; WR, Warrnambool.
J.R. Prescott and J.T. Hutton: Environmental Dose Rates
441
ENVIRONMENTAL DOSE RATES FOR SITES WITH LITTLE O R N O R A D I O A C T I V E DISEQUILIBRIUM
by an intermediate U-equivalent value found by TSAC. High resolution 7- or m-spectrometry, in which individual nuclides are identified by their 7 or ~ emissions, m a y then define the nature and details of the disequilibrium. A similar comparison can be made between 23ZTh by N A A and 2°8pb by scintillometry. In fact, we have never found any convincing evidence of disequilibrium in the thorium chain. This is not unexpected since the lifetimes of the thorium daughters are all short with respect to the time scales for their transport by geochemical processes. On the other hand, in the uranium chain, the 3.85 d halflife of =2Rn, for example, allows time for it to diffuse away from its production site and be lost; and the 1.6 ka half life of 226Ra is long enough for it to be transported. Since both o f these are in the m i d d l e o f the u r a n i u m series, any consequent disequilibrium has the potential to influence dose rates. Dose rates in this paper have been calculated from the data of Nambi and Aitken (1986) and/or Readhead (1987), with the e x c e p t i o n o f those in Table 4 (from Smith, 1983) which predate the former references and have not been recalculated.
Dune Systems Table 1 s h o w s e l e m e n t a n a l y s e s r e p r e s e n t a t i v e o f s o m e 200 sites at w h i c h the A d e l a i d e l a b o r a t o r y has obtained dates. Here, as elsewhere, sites are described by a site i d e n t i f i e r f o l l o w e d , after the "/", by the d e p t h below the present day surface in cm. They range from what is now the desert interior, to better-watered locations on the southern and eastern seaboards. They are all dune sites, although the context is variable. Our three lowest activity sites are included: SQ2/200 is in the bryozoal limestone near Mt Gambier, SA; CA9 is in the giant p o d s o l K a b a l i dune at C o o l o o l a , QLD; M L 2 is a recent dune on the western m a r g i n o f L a k e Malata, SA. It gave no reading above background with our scintillometer and is believed to be pure wind-sorted gypsum. Potassium data are included in the table, although they are not relevant to r a d i o a c t i v e disequilibrium. Rather,
TABLE 1. An illustrative selection of analyses from Australian sites [in part from Hutton and Prescott (1992)] %K Scintillometry
NAA
Th [.tg/g Scintillometry
XRS
Roonka, SA EB 1S/100
0.94
0.96
1.5
1.3
6.6
6.4
Cleland Hills, NT PJ1S/10 PJ 1S/75 PJ2S/100
0.23 0.75 0.10
0.25 0.75 0.12
0.8 1.9 0.35
1.0 1.7 0.52
4.0 8.9 2.0
4.1 10.0 2.0
Lake Amadeus, NT LA3S/200 LA4S/300
0.55 0.20
0.54 0.16
0.8 0.3
0.9 0.3
1.6 1.3
1.7 1.3
O.O9
4.1
4.0
1.6
1.6
Eyre Peninsula, SA HC5 ML2 WWO1
DNA
U ~tg/g Scintillometry
Locality
0.16
0.O8 1.1 1.1 No K, UorThdetectableabovebackground 0.18 1.7 1.5
South East, SA MG5/30 SQ2/200 WK1S/100
2.2 0.01 0.08
1.9 0.01 0.10
2.2 0.2 0.9
2.6 0.2 1.0
9.0 0.3 0.9
8.2 0.2 0.9
Warrnambool, Vic WR3 S
0.10
0.09
0.7
0.9
1.5
1.3
< 0.01 0.12 0.16 < 0.01
0.02 0.12 -0.01
0.92 2.7* 4.4 0.5 _+0.05?
0.71 2.7 3.4 _+0.5t 0.48
Cooloola, Qld SK11-S CA2 CA5 CA9
1.3 8.5* 16.0 0.2 _ 0.17
1.05 7.5 16.9 +_5p 0.48
*By XRS; error +__3%. ~By thick source c~ counting. In TSAC the error in the thorium concentration depends on the (relatively poor) counting statistics for pairs. This affects the corresponding uranium concentration. The statistical precision of the total ¢x count is normally better than 3%. For DNA 1 SD counting error is 0.06 t.tg/g. For NAA 1 SD counting error is 0.5 [tg/g. For field scintillometry the relative counting error is 5-10%.
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they serve as an indicator of the usefulness of scintillometry at the l o w c o n c e n t r a t i o n l e v e l s shown, and as a reminder that in situ scintillometry is the best method available for potassium at low concentrations. In this table D N A (or N A A ) agree with scintillometry and, on the whole, it can be concluded from these data that disequilibrium is uncommon in Australian sites, particularly in interior aeolian dunes, and we ourselves have p r e v i o u s l y published data that support this contention (Hutton et al., 1985; Hutton and Prescott, 1989, 1992).
Profiles: Tennant Creek, Northern Territory A rare intra-plate earthquake at this site in 1988 ruptured the surface for about 30 kin. It was dated in order to place possible limits on the last previous m o v e m e n t of the fault (Crone et al., 1992; Hutton et al., 1994). Table 2 shows the dose rates for the hanging wall profiles from trenches cut across the K u n a y u n g k u (TC1) and Lake Surprise faults (TC2, TC3). Three almost independent measurements of dose rate are shown: in situ y scintillometry: D N A + N A A + X R S ; T S A C + X R S . C o s m i c ray dose rates have not yet been added. Field evidence suggests that depths below the present day land surface correspond at all three sites; there is little evidence of palaeosol development. A sedimentation rate o f 35 _+ 7 m m / k a for the r e g i o n was e s t a b l i s h e d (Hutton et al., 1994) so that the base of TCI is at about 35 ka and at TC2 it is beyond 70 ka. The table shows
only a small change with depth of the dose rates and that any changes with time have been steady. At this site the present day profile can be used to model its own past development and hence the time dependence of dose rate. In fact, within the uncertainties in the age determination itself, the age may be calculated on the basis that the dose rate has been the same as it is now at the level where the sample was collected.
Profiles: Puritjarra, Northern Territory The Puritjarra rock shelter in the Cleland Hills NT is the oldest human occupation site in the arid central interior (Smith, 1987). It has been comprehensively dated by ~4C and TL, although disagreement between ages found f r o m the two m e t h o d s r e m a i n s u n r e s o l v e d (Prescott, 1991). The dosimetry has been more thoroughly studied than at any other of our TL sites except, perhaps, Mt Schank referred to below. Dose rates, obtained by the same three independent methods as for Tennant Creek, are shown in Table 3. Samples denoted PJ1 are from within the shelter, sample PJ2 comes from an,open dune about 1 km away. The dose rates for PJ2 were the same at all levels down to 1.5 m and the figures shown are the site averages. The good agreement among the three essentially independent m e a s u r e m e n t s o f d o s e r a t e at T e n n a n t C r e e k a nd Puritjarra shows that radioactive disequilibrium is not important at these sites.
TABLE 2. Comparison of dose rates at Tennant Creek, Gy/ka, without cosmic ray contributions Site
Scintillometry
XRS + DNA + NAA
XRS + TSAC
TC l S/20 TCl S/45 TC1 S/80 TCIS/110
I. 12 _+0.035 1.27 +_0.035 1.34 _+0.035 1.28 _+0.035
1.34 + 0.043 1.30 _ 0.043 1.34 + 0.043 1.35 _+0.043
1.20 + 0.063 1.30 + 0.063 1.28 _+0.063 1.21 _+0.063
TC2S/65 TC2S/145 TC2S/185 TC2S/225 TC2S/250a TC2S/250b
1.26 _+0.037 1.34 + 0.037 1.41 _+0.037 1.46 _+0.037 1.60 _+0.08* 1.37 _+0.04+
1.25 _+0.043 1.34 _+0.044 1.33 _+0.044 1.42 + 0.044 1.46 _+0.044 1.33 + 0.044
1.30 + 0.11 1.30 _+0.06 1.29 + 0.08 1.40 _+0.08 1.40 + 0.1 [1.16 _+0.09]
TC3S/80 TC3S/170
1.31 _+0.038 1.54 _+0.041
1.24 _+0.05 1.32 _ 0.07
1.20 _+0.08 1.35 _+0.07
*This field scintillometer reading from the base of the profile is probably not measuring the same thing as the laboratory measurements. ?Sample TC2S/250b was taken on the foot wall side of the East Lake Surprise fault and corresponds stratigraphically to a depth of about 2 m. TABLE 3. Comparison of dose rates at Puritjarra, NT, Gy/ka, without cosmic ray contributions Sample
Scintillometry
DNA + NAA + XRS
TSAC + XRS
PJ 1S/10 PJ 1S/25 PJI S/40 PJ1 S/55 PJI S/75 PJI S/95
0.83 _+0.02 1.11 + 0.03 1.37 _+0.04 1.62 _+0.06 1.68 + 0.06 1.87 _+0.07
0.77 _+0.03 1.09 _+0.04 1.36 _ 0.04 1.77 _+0.06 1.82 _+0.06 1.86 + 0.09
0.85 _+0.05 1.28 _+0.06 1.36 _+0.07 1.76 _ 0.12 1.83 ___0.10 1.98 _ 0.11
PJ2S
0.41 _+0.02
0.36 _+0.02
0.40 +_0.03
J.R. Prescott and J.T. Hutton: Environmental Dose Rates Profiles: Roonka, South Australia
443
abrupt change in all properties to what appears to be an older surface on which the dune developed. This example illustrates the way in which complementary information can be obtained from an associated discipline (archaeology and soil science in the present case) to confirm the evidence from analyses and improve the accuracy of the TL ages. At this site discontinuities in deposition occurred at about 3 and 10 ka. In calculating ages the dose history was divided into four groups corresponding to the four different sections of the dune.
Roonka is one of the more significant prehistoric Australian aboriginal sites (Pretty, 1986). It is located on the banks of the River Murray, about 10 km north of Blanchetown in South Australia. Evidence for human occupation extends from 18 ka ago until the present day. Both TL and 14C dates have been reported for the East Bank site (Prescott, 1983; Prescott et al., 1983). Table 4 shows the dose rates (average of all methods) for this site as a function of depth, together with a selection of element analyses. They have been grouped by depth in accordance with the field evidence discussed below. The site is in an aeolian dune resting on indurated brown silt, which in turn overlies calcrete. The archaeological excavation showed that there was a veneer of recent deflation products about 10 cm deep, and former dune surfaces at depths of 45 cm and about 1.0 m. Archaeological feature F11 is a fireplace lit on the 45 cm surface and about 2.5 ka old (Prescott et al., 1983), while feature F13 is a burial probably associated with the 100 cm surface. Unpublished dates place this surface at about 10 ka ago and the base of the dune at about 20 ka. The general features are identifiable in discontinuities in the element analyses and dose records. The total potassium (about 1%) and A1203 (3.5%, not shown) are low and almost constant with depth, indicating a low clay and/or feldspar content and a nearly pure quartz sand dune. Ti, Zr and Th are about average for surface sediments except for the depth band 60-80 cm; the Ti/Zr and A1/Y ratios are also anomalous in this band. This suggests that the 60-80 cm samples are different from the rest of the dune and supports the field evidence for the two former dune surfaces, one at 45 cm and one at 100 cm with a zone of remobilization between. Y and Th commonly move together in the environment and the Y/Th ratio is almost constant down the profile. The Ti/Zr and A1/Y ratios, which are associated with minerals that do not weather, show the discontinuity. However, the Ca/Sr ratio progressively decreases from the surface to 180 cm, suggesting stability of long enough duration for the slightly soluble calcium carbonate minerals to come to an equilibrium with repeated solution and crystalization. During this process low strontium calcite forms at the surface and the solutions at lower depths contain progressively more strontium, leading to the lower Sr/Ca ratio (Hutton and Dixon, 1981). At 190 cm there is an
Dune Sequence, South-east South Australia The sequence of fossil dunes in the south-east of South Australia has now been comprehensively dated by both TL and IRSL. These dunes were built on a tectonically rising land surface by successive high stands of the sea as it advanced and retreated with the cycle of iceages. Because it is securely dated at the seaward end (Robe) by modern dune building, at the last interglacial (Woakwine Range 120-130 ka) by U-series and aminoacid r a c e m i z a t i o n and at N a r a c o o r t e where the Brnnhes-Matuyama geomagnetic field reversal (780 ka) lies between the East and West Naracoorte ranges, it provides a test sequence for luminescence dating for the past 800 ka (Huntley et al., 1993a, b, 1994). Over such a long period it is critical to understand the dose rate history. In this we were assisted by the high degree of uniformity of the materials of which the dunes are built, viz., quartz and shell-derived calcium carbonate. The local evidence from the more recent dunes is that induration of the surface begins shortly after formation and that this protects the interior of the dune from major additional weathering. Over longer periods, the older dunes e.g. Reedy Creek, Harper or Cave have gradually become indurated throughout. In this case it is safe to assume that the chemical composition and hence the dose rate has been essentially constant for most of the life of the dune. When we began the study (Huntley et al., 1985) we anticipated that a portion of the uranium would be incorporated partly in resistate minerals (Hutton and Prescott, 1989) and partly in the shell carbonate without its daughters; these would grow back in and at the same time uranium would be progressively leached out (Veeh and Burnett, 1982). Thus, uranium would be highest in the recent dunes and least in the older ones. Figure 2a and b collects together all the analyses for U
TABLE 4. Dose rates and chemical analyses of the Roonka, East Bank sand dunes Depth (cm) 10-30 60-80 100-120 170-180 190-230
No. of Dose rate s a m p l e s (Gy/ka) 4 2 3 2 2
1.91 1.39 1.83 2.15 2.30
K (%)
U
1.0 0.95 0.94 1.1 1.1
1.2 0.5 1.2 1.3 1.4
Th
Ti
Zr
(p.g/g) 7.0 3.0 6.3 7.6 10
1500 700 1100 1600 2800
Errors: dose rates, 4%; K, 3%; U, 0.1 p-g/g;Th, 0.5 p-g/g;Ti and Zr, 5%.
Ti/Zr 280 80 240 310 310
5.2 8.8 4.6 5.0 9.0
A1/Y 1.5 x 2.2 x 1.4 x 1.4 x 1.8 x
103 103 103 103 103
Y/Th
Ca/Sr
2.0 2.0 2.0 2.1 2.0
400 200 180 100 350
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Quaternary Science Reviews (Quaternary Geochronology): Volume 14
URANIUM 1.5
I
I
I
I
x
i
i n- situ
O
TSAC DNA XRS
X
+,
1.0
I
A
I
T-spectrometry
zk 0.5
RBTr W K I WD WA EA BA RBTrr WK[[ITrr RC RBI I I I I I 400 500 100 200 300 0 Age ( ko )
HA WB I 60~
ST
NW NE I 700
800
THORIUM
3.0 A
i in-situ
I
i
I
I
'
"7'- spectrometry
o TSAC • NAA x XRS
I
x
2.0 I
~L )¢
1.0
RBTT W K I WD RBI RB~ WK]~ITTT RC WA O-I I I 0 100 200 300
x EA BA I I 400 500
HA W B I 600
ST
NW NE I 700
800
Age ( ka ) FIG. 2. Concentrations in ~tg/g for most of the ranges in the south east of South Australia as found by field scintillometry, DNA, NAA, TSAC and XRS. (a) uranium; (b) thorium. Errors are not marked for XRS because the method is almost beyond its limits at these concentrations: U _+ 0.4, Th _ 0.5 ~tg/g. The notations for the ranges are: RB, Robe; WK, Woakwine; WD, West Dairy; RC, Reedy Creek; WA, West Avenue; EA, East Avenue; BA, Baker; HA, Harper; WB, Woolumbool; ST, Stewart/Cave; NW, West Naracoorte; NE, East Naracoorte. and Th. They are shown at the ages assigned to them by H u n t l e y et al. (1993a, b, 1994). The value plotted for each m e t h o d o f analysis is the average o f at least two independent observations. The uranium analyses shown in Fig. 2a appear at first sight to support the above expectation, in that they can be
represented by a continuously falling U concentration when going backwards in time. However, there is contrary evidence from the intercomparisons of these methods. i f radioactive disequilibrium is significant, then a comparison of D N A with scintillometry would be expected to reveal it, since D N A gives the concentration of the
J.R. Prescott and J.T. Hutton: Environmental Dose Rates parent 238U whereas scintillometry sees a daughter (214Bi) occurring late in the decay chain. It is clear from Fig. 2a that, for those sites at which both measurements are available, the analyses are in good agreement. A similar conclusion follows if the data from TSAC (which counts all the c~-emitting daughters) are included in the comparison. Other details of this argument can be found in Huntley et al. (1993a). The data in Fig. 2b suggest a Th concentration in equilibrium, generally decreasing with time, more scattered for the older dunes. An alternative interpretation of Fig. 2 is that, rather than showing a monotonic change with time, the thorium and uranium concentrations are constant (U: 0.5 ppm; Th: 1.5 ppm) for the ranges older than 350 ka, but that the more recent ranges had access to a source of reworked material higher in U content and perhaps lower in Th since about the time that the West Avenue (WA) Range formed 350 ka ago. The two graphs are then well represented by step functions with the step occurring at that time. The abnormally high values for all elements in WA are also consistent with this scenario.
SITES WITH RADIOACTIVE DISEQUILIBRIUM Steps should be taken to check on the presence (or absence) of radioactive disequilibrium as a matter of course by making measurements of dose rates and/or element concentrations by independent methods. As indicated above, disequilibrium is rare in Australian dune systems. However, we have found a number of examples of disequilibrium in circumstances where the environment is wet.
Spencer Gulf, South A u s t r a l i a Historically, one of our first examples of disequili-
445
brium was in northern Spencer Gulf where ages were being sought for marine sediments in connection with the environmental impact study for the proposed Redcliff petrochemical plant (Smith et al., 1982; Belperio et al., 1984). In brief, the floor of the Gulf consists of mixed terrigenous/estuarine sediments of Pleistocene age (last interglacial, Mambray Formation and older), overlain with r e w o r k e d H o l o c e n e seagrass bank deposits (Germein Bay Formation) dating from the time that the sea returned to the Gulf between 8 and 9 ka ago (Hails et al., 1984). They have therefore been underwater since that time, but would have had an arid to semi-arid environment for the preceding 100 ka. Table 5 shows U and Th analyses for cores SG182 and RED38. The Holocene/Pleistocene boundary lies at a depth of about 200 cm for SG182 and about 250 cm for RED38. DNA and NAA are compared with TSAC. The thorium concentrations are low, increase slightly with increasing depth and, within the counting errors, NAA and TSAC give the same results. This is not unexpected since the daughters in the Th chain all have short lifetimes with respect to likely geochemical separation processes. Uranium increases with depth in the Holocene sediments and changes abruptly at the boundary with the Pleistocene. It is clear that parent 238U exceeds the apparent concentration inferred from TSAC (the aggregate c~ count of U and its daughters) by a factor of the order of three in the Holocene sediments and much less than that in the Pleistocene sediments. The analyses are supported by spot checks of the parent concentrations by XRS, two of which are also shown in Table 5. Without further detailed studies of the individual elements in the decay chains it is not possible to construct a history of the movement of U and its daughters. For the purpose of the present exercise it is sufficient to note that
TABLE 5. Uranium and thorium analyses from Spencer Gulf, South Australia Locality
DNA
U (pg/g) TSAC
XRS
NAA
Th (~tg/g) TSAC
XRS
RED38/101 RED38/131 RED38/236
6.3 17.9 17.1
1.9 4.9 4.5
20.1
2.9 3.2 4.2
3.5 3.7 4.0
5.9
RED38/252 RED38/333 RED38/342
7.6 7.3 6.9
Approximate Pleistocene/Holocene boundary 2.8 4.0 4.0 3.3 5.2 4.9
3.7 4.5 4.8
SG 182/53 SG 182/69 SG182/128 SG182/146 SG182/167 SG 182/183
14.1 15.0 12.7 14.5 29.5 27.6
4.9 3.6 4.7 3.9 7.8 9.0
2.2 2.3 1.6 2.0 2.6 3.5
1.3 1.5 1.9 1.7 2.2 4.5
SG182/208 SG 182/228
6.6 11.9
Approximate Pleistocene/Holocene boundary 3.3 3.7 7.6 3.6
2.9 2.4
27.4
4.9
Errors for DNA and NAA are < 5% and for XRS are 3%. In TSAC the error in the thorium concentration depends on the counting statistics for pairs, and in this table is about 10%. This affects the corresponding uranium concentration. Although the statistical precision of the total alpha count, which is used in dose rate calculations, is everywhere better than 3% the error in U is about 5%.
446
Quaterna~ Science Reviews ( Quaternar3, Geochronology): Volume 14
coarse grain quartz TL dates for the Holocene differ by a factor of 1.45 depending on whether the dose rate is calculated using D N A for U, N A A for Th, and XRS for K (together with cosmic rays) or with U and Th found from TSAC. On the evidence of Table 5 the former would be too large and the latter w o u l d be too small. In fact, a model in which the dose rate changes continuously from beginning of the Holocene to the present day gives TL ages in g o o d agreement with 14C dates from the same depth in the core (Smith, 1983). Nevertheless the project illustrates the fact that, once disequilibrium has been identified, two alternatives face us: to calculate the TL age with error limits large enough to encompass the likely limits of dose rate (+ 25% in the present case) or to make detailed measurements of the daughters using high resolution c~- or y-spectroscopy. This is illustrated in the next example.
TABLE 6. Radio-element activities from Mt Schank, South Australia Element
LAVA
-~3sU e~4Th 234U
chemical 5' o~ ot O~
31 + l 30_+5 -25 _+7
2~°Th 22~'Ra 214Bi 2~°Pb
o~ ¥ 7 3'
2~2Th 2~Ra 22~Th 22*Ra 2°~Tl
chemical ? o~ 5' 5'
Activities (Bq/kg) SC3-6 SC3-7 24 _+2 40+6 22_+2 -22 _+2
25 _+ l 24+6 -26 _+9 --
35 _+5 31_+1 31 -!- 1 48-+24
110 _+5 121+2 160+ 10 130-+30
149 _+90 113_+2 -125+-25
34 _+ I 36+1 35 _+ 1 37 _+5 33 _+2
65 _+2 70+2 69 _+ 1
60 _+2 58_+2 58 + I 64 _+6 60 _+2
-
-
76 + 8
Mount Schank, South Australia c~and 5' indicate measurements by high resolution spectrometry. A site where very unusual conditions led to gross disequilibrium in the U chain is Mt Schank. Here TL dates w e r e o b t a i n e d f r o m d u n e q u a r t z o f the B r i d g e w a t e r Formation which had been overlain and intensely heated by a lava flow. The dune overlies bryozoal Mt Gambier limestone very close to the water table. Table 6, part of which is r e p r o d u c e d from Smith and Prescott (1987), shows detailed r a d i o e l e m e n t analyses for the lava and two s e p a r a t e d sites (SC3/5 and SC3/6) in the f o r m e r dune. To facilitate c o m p a r i s o n s for disequilibrium, the entries are expressed in Bq/kg. It is clear that, as above, the thorium series is in equilibrium, as is the uranium series in the lava. However, in the sand, there is a discontinuous change in activity between 2~4U and 23°Th. Before this break the concentrations are consistent with those found in other dunes of the Bridgewater formation elsewhere and due to U and Th in resistate minerals. After the break, the 23°Th and the subsequent daughters are enhanced by roughly a factor of five. Dose rate measurements with in situ TL dosimeters and scintillometry (not shown here) are in agreement with these analyses. The discontinuity in concentration is interpreted as due to :3°Th and its daughters being carried by water percolating through the still warm lava as it cooled, and being stopped on encountering the alkaline dune material. This interpretation is supported by measurements on trace elements (Smith and Prescott, 1987). On this site there can be no doubt that the dose rate is well-understood and that the TL dates are correct. Unravelling the radiochemical history, however, was very time-consuming and almost a decade elapsed between collecting the first samples and the appearance of the publication; but what is l0 years in 5000?
Mound Springs, South Australia T h e w e s t e r n e d g e o f the G r e a t A r t e s i a n B a s i n is marked by a line of mound springs extending from the South Australia/Northern Territory border to Lake Frome (Boyd, 1990). T h e y exhibit a r e m a r k a b l e d i v e r s i t y o f
form. T h e y are built up from e v a p o r a t i o n of c a l c i u m bicarbonate solution carried up by the artesian water; in addition, aeolian debris is incorporated into the dune as it grows. Several periods of activity are evident on geological grounds, the earliest of which is probably older than 1 Ma; many springs are still active. In such a watery environment, radioactive disequilibrium is not unexpected. Table 7 shows a selection o f analyses, expressed in Bq/kg. KHI (Kewson Hill) and BS2 (Old Beresford) are representative of the older, inactive, springs with ages of the order of 1 Ma, ES1 (Elizabeth Springs) is old but still active, whereas BCI (Blanche Cup) and BB1 (Bubbler) are active and have been building their present mounds for times in excess of 20 ka. B S l a (New Beresford) is thought to be modern dune sand recently incorporated into the mound. So far as the thorium series is concerned, the data are c o n s i s t e n t with e q u i l i b r i u m a l t h o u g h , for K H I , the amounts present are so low that additional work may be desirable. In the uranium chain, with the exception of B S I a , disequilibrium is clearly established by the fact that parent uranium (as found with DNA) is insufficient to support its daughters as represented by either scintillometry or TSAC. This is confirmed by high resolution y and c~ spectroscopy. This disequilibrium is most conspicuous for the two young and active sites, BCI and BBI. With the possible exception of ESI the various 23~U and 234U analyses agree and there is some evidence that 23°Th may be a little in excess of equilibrium, as is 22~Th in the thorium series. For the younger springs, BC1 and BB1, the uranium chain at and b e y o n d 226Ra shows a spectacular increase in activity. This is not evident for the older springs, KH1 and E S I ; unfortunately 226Ra has not yet been measured for them. A provisional scenario on which to base dose rate calculations is that the :38U and 232Th observed (and their particular daughters) come from resistate minerals incorporated as wind-blown material, while the excess counts
447
J.R. Prescott and J.T. Hutton: Environmental Dose Rates TABLE 7. Analyses for Mound Springs, expressed as Bq/kg Uranium series High resolution
DNA U-238
"/ U-238
2.9 4.2 1.7 7.8 6.9 9.1
- 3 _+ 3
BSla BS2 ES1 KH1 BC1 BBI
~ U-238
e~ U-234
3.2
. --2 +_4 9 _+6
.
3.6 . 3.0 10.9 6.4 7.9
. 5.6 9.1 6.3 7.7
a Th-230
7 Ra-226
Scintillometry Bi-214
TSAC U equiv
7
4.5
2.1 12 9.1 11
--39.5 59.9
3.1 10.5 2.7 5.6 53 91
3.5 6.2 1.8 10.1 37.1 39.7
.
Thorium series High resolution
c~ Th-232 BS I a BS2 ES 1 KH1 BCI BB1
7 Ra-228
7
7.0
.
.
6.2 1.7 12 14
. --18.1 18.0
y Th-228
o~ Th-228
TSAC Po-216
Scintillometry T1-208
8.5
6
--17.3 18.4
5.8 3 15 14
4.9 None 4.5 0.7 11.1 12.5
6.2 0.65 4.9 0.8 13.0 16.5
.
The errors on the scintillation numbers are about 5%. The DNA error is about 7%; TSAC for uranium about 5%, for thorium about 15%. High-resolution y counts are better than 3% except for U-238. High resolution ~ counts: for uranium 10%, for thorium the absolute error is 1. in the lower part of the chain are due to radium, and perhaps a little thorium, carried up in solution by the water of the springs. The 1.6 ka half life of 226Ra would ensure that we would not expect to see this imbalance m a i n tained in the now rock-like material of the older springs. CONCLUSIONS It is common to calculate TL/OSL ages from a measured equivalent dose and a dose rate found from measurements with in situ dosemeters and/or individual element analyses. We suggest that sufficient measurements of dosimetry should be carried out that the dose rate history can be inferred, particularly when the anticipated age is large. At the very least, multiple samples should be collected, even if not all of them are analysed for equivalent dose. A set of analyses from half a dozen samples down a profile will give useful information even when the analyses are confined only to K, U and Th. If XRS is used for K analysis, then m a j o r e l e m e n t s are u s u a l l y available as well and this may help with the interpretation. Examples of this are to be found at Roonka above, in H u n t l e y et al. (1993a) and in Smith and Prescott (1987). We have long advocated the use of field scintillometry for measuring both gamma dose rate and individual K, U a n d Th c o n c e n t r a t i o n s ( P r e s c o t t a n d H u t t o n , 1987; Hutton and Prescott, 1992). It has the advantage of providing a completely self-contained dose rate; and this dose rate is immediately available without the need for further sample preparation. A separately calibrated gamma ray dose is obtained and this is averaged over the
same volume that provided the gamma dose to the sample. At very low K levels it is the preferred method for that element. Australian sites that have been built by aeolian activity show little if any evidence for radioactive disequilibrium. However, any site which is wet or has probably been wet for a significant time in its past history should be treated with caution, not to say suspicion. A test for radioactive disequilibrium should be carried out as a matter of course and reported in the published results. Such a test could comprise either at least one pair of independent measurements of dose rate (as in Table 2 above), or assay for individual radio elements by high resolution 7 spectrometry. E x a m p l e s of the latter are shown in Tables 6 and 7, although in both these cases the spectroscopy was ex post facto. High resolution ~ spectrometry is useful for clarifying details of decay chains once disequilibrium has been established, but is probably too time-consuming to be used routinely. Although the dose rate and element analyses discussed here were originally obtained for TL dating, they and the conclusions drawn would be just as valid for all aspects of trapped electron dating. ACKNOWLEDGEMENTS The late John Hutton was associated with all of the work described here and was involved with an early draft of the present paper. It is matter for regret that he did not see it completed. Our work has profited greatly from a long association with Gillian Robertson, Dave Huntley and Terry Wall. We are grateful to many colleagues who welcomed us to their field sites. We
448
Quaternary Science Reviews ( Quaternao' Geochronology): Volume 14
thank A. Murray and H. Heijnis for the high resolution 5' and o~ spectrometry of the mound springs. The work was supported by the Australian Institute of Nuclear Science and Engineering, the Australian Research Council, The National Science and Energy Research Council of Canada, the Mark Mitchell Foundation, the Australian Geological Survey Organization and the Research Fund of the University of Adelaide.
REFERENCES Belperio, A.E, Smith, B.W., Polach, H.A., Nittrouer, C.A., DeMaster, D.J., Prescott, J.R., Hails, J.R. and Gostin, V.A. (1984). Chronological studies of the Quaternary marine sediments of northern Spencer Gulf, Australia. Marine Geology, 61,256-296. Berger, G.W. (1994). Thermoluminescence dating of sediments older than about 100 ka. Quaternary Geochronology (Quaternary Science Reviews), 13, 445-456. Boyd, W.E. (1990). Mound Springs. In: Tyler, M.J., Twidale, C.R., Davies, M. and Wells, C.B. (eds), Natural Histor>' of the North East Deserts. Royal Society of South Australia. Crone, A.J., Machette, M.N. and Bowman, J.R. (1992). Geologic investigations of the 1988 Tennant Creek earthquakes - - implications for the paleoseismicity of stable continental regions. U.S. Geological Surve), Bulletin 2032A. Hails, J.R., Belperio, A.R, Gostin, V.A. and Sargent, G.E.G. (1984). The submarine stratigraphy of northern Spencer Gulf, South Australia. Marine Geology, 61, 345-372. Huntley, D.J., Hutton, J.T. and Prescott, J.R. (1985). South Australian sand dunes, a TL sediment test sequence: preliminary results. Nuclear Tracks, 10, 757-758. Huntley, D.J., Hutton, J.T. and Prescott, J.R. (1993a). The stranded b e a c h - d u n e sequence of south-east South Australia: a test of thermoluminescence dating, 0-800 ka. Quaternary Science Reviews, 12, 1-20. Huntley, D.J., Hutton, J.T. and Prescott, J.R. (1993b). Optical dating using inclusions within quartz grains. Geology, 21, 1087-1090. Huntley, D.J., Hutton, J.T. and Prescott, J.R. (1994). Further thermoluminescence dates from the dune sequence in the southeast of South Australia. Quaternao' Science Reviews, 13, 201-207. Hutton, J.T. and Dixon, J.C. (1981). The chemistry and mineralogy of some South Australian calcretes and associated soft carbonates and their dolomitisation. Journal o f the Geological SocieO, of Australia, 28, 71-79. Hutton, J.T. and Prescott, J.R. (1989). Low level U and Th in Australian dune systems. Proceedings, Sixth Australian Conference on Nuclear Techniques o f Analysis, pp. 184-186. AINSE, Sydney. Hutton, J.T. and Prescott, J.R. (1992). Field and laboratory measurements of low-level thorium, uranium and potassium. Nuclear Tracks and Radiation Measurements, 20, 367-370. Hutton, J.T., Wall, T.E. and Prescott, J.R. (1985). Comparison of methods of analysis for natural environmental radionuclides, especially Th and U, at the level of a few ~tg.g ~. Proceedings, Fourth Australian Con/erence on Nuclear Techniques of Analysis, pp. 58-63. AINSE, Sydney. Hutton, J.T., Prescott, J.R., Bowman, J.R., Dunham, M.N.E., Crone, A.J., Machette, M.N. and Twidale, C.R. (1994). Thermoluminescence dating of sediments as applied to the dating of palaeo-earthquakes in Australia. Quaternary Geochronology (Quaternary Science Reviews), 13, 143-147. Krbetschek, M.R., Rieser, U., Z611er, L. and Heinicke, J. (1994). About radioactive disequilibria in palaeodosimetric dating of sediments. Radiation Measurements, 23, 485-490.
Murray, A.S., Wohl, E. and East, J. (1992). Thermoluminescence and excess 226Ra decay dating of late Quaternary fluvial sands, East Alligator River, Australia. Quaternary Research, 37, 29-41. Nambi, K.S.V. and Aitken, M.J. (1986). Annual dose conversion factors for TL and ESR dating. Archaeometry, 28, 202-205. Nanson, G.C., Price, D.M., Short, S.A., Young, R.W. and Jones, B.G. (1991 ). Comparative uranium-thorium and thermoluminescence dating of weathered Quaternary alluvium in the tropics of northern Australia. Quaternary Research, 35, 347-366. Prescott, J.R. (1983). Thermoluminescence dating of sand dunes at Roonka, South Australia. PACT Journal, 9, 505-512. Prescott, J.R. (1991). A comparison of 14C and thermoluminescence dates from Puritjarra and new techniques for TL-dating of partially bleached sediments. In: Gillespie, R. (ed.), Quaternary Dating Workshop 1990, p. 34. Research School of Pacific Studies, Australian National University, Canberra. Prescott, J.R. and Hutton, J.T. (1987). Low level measurements of potassium, thorium and uranium by means of scintillometers in the field. Proceedings, Fifth Australian Conference on Nuclear Techniques of Analysis, pp. 76-78. AINSE, Sydney. Prescott, J.R. and Hutton, J.T. (1988). Cosmic ray and gamma ray dosimetry for TL and ESR. Nuclear Tracks, 14, 223 227. Prescott, J.R. and Hutton, J.T. (1994). Cosmic ray contributions to dose rates for luminescence and ESR dating: large depths and long-term time variations. Radiation Measurements, 23, 497 500. Prescott, J.R., Smith, B.W., Polach, H.A. and Pretty, G.L. (1983). Comparison of ~4C and thermoluminescence dates from Roonka, South Australia. PACT Journal, 8, 205-21 I. Pretty, G.L. (1986). Australian history at Roonka. Journal (~f the Historical Socie~_ of South Australia, 14, 107-122. Readhead, M.L. (1987). Thermoluminescence dose rate data and dating equations for the case of disequilibrium in the decay series. Nuclear Tracks and Radiation Measurements, 13, 197-207. Roberts, R.G., Jones, R. and Smith, M.A. (1990). Thermoluminescence dating of a 50,000-year-old human occupation site in northern Australia. Nature, 345, 153 156. Roberts, R.G., Jones, R., Spooner, N.A., Head, M.J., Murray, A.S. and Smith, M.A. (1994). The human colonisation of Australia: optical dates of 53,000 and 60,000 year bracket human arrival at Deaf Adder Gorge, Northern Territory. Quaternary Geochronology (Quaternary Science Reviews), 13, 575-583. Smith, B.W. (1983). New applications of thermoluminescence dating and comparisons with other methods. Ph.D. thesis, University of Adelaide (unpublished data). Smith, B.W. and Prescott, J.R. (1987). Thermoluminescence dating of the eruption of Mt. Schank, South Australia. Australian Journal of Earth Science, 34, 335-342. Smith, B.W., Prescott, J.R. and Polach, H.A. (1982). Thermoluminescent dating of marine sediments from Spencer Gulf. In: Ambrose, W. and Duerden, P. (eds), Archaeometry, An Australasian Perspective, pp. 282-289. Australian National University Press, Canberra. Smith, M.A. (1987). Pleistocene occupation in arid Central Australia. Nature, 328, 710 711. Veeh, H.H. and Burnett, W.C. (1982). Carbonate and phosphate sediments. In: lvanovic, M. and Harmon, R.S. (eds), Uranium Series Disequilibrium: Applications to Environmental Problems, pp. 459 480. Clarendon Press, Oxford.
Quaternary Science Reviews (Quaternary Geochronology), Vol. 14, pp. 439-448, 1995.
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ENVIRONMENTAL DOSE RATES AND RADIOACTIVE DISEQUILIBRIUM FROM SOME AUSTRALIAN LUMINESCENCE DATING SITES J . R . P R E S C O T T a n d J.T. H U T T O N + Department o f Physics and Mathematical Physics, University o f Adelaide, SA 5005, Australia Abstract - - Determination of geological/archaeological ages by luminescence dating requires the measurement of the rate of delivery of radiation dose from the sample and its environment. For all age determinations it is necessary to assess whether the dose-rate has been constant during the time that the dose has been accumulating, and to make allowance for it if it was not. We report here results from a number of Australian sites to illustrate the information that can be found about element concentrations, about their distribution and about dose rates. Tests for radioactive disequilibrium form part of these measurements. Such tests take the form of comparison of independent measurements of dose-rates and/or individual elements. In general, there is little or no evidence for disequilibrium in aeolian dune systems, even when the age of the system exceeds 500 ka. However, disequilibrium has been found in several sites subject to wet conditions.
INTRODUCTION
QG
required. M o b i l i t y o f individual nuclides in the decay chain can result in parts of the chain being broken or augmented. An obvious example is the loss of 222Rn due to gaseous diffusion, which would reduce the lower part of t h e u r a n i u m s e r i e s r e l a t i v e to t h e p a r e n t 238U. Alternatively, transport in solution may carry parent or daughter nuclei into the environment. For a radioactive chain in equilibrium, each member of the chain decays to the next member at the same rate as it is p r o d u c e d from the i m m e d i a t e l y preceding one. Thus, all members of the chain show the same activity, i.e. the same n u m b e r o f disintegrations per unit time: e x p r e s s e d as b e c q u e r e l (Bq) or, m o r e usually, Bq/kg. Disequilibrium is indicated if there is a statistically significant difference between the activities of individual chain members. Minor disequilibrium does not usually have a significant effect on the dose rate. On the other hand we have found a few cases where disequilibrium is pronounced e n o u g h that it has to be a l l o w e d for ( B e l p e r i o et al., 1984; Smith and Prescott, 1987). The first o f these is from an underwater site in Spencer Gulf, while the second is from a dune site overlain with a lava flow at Mt Schank. Both are discussed in more detail below. Murray et al. (1992) give another example, from a fluvial environment, where an excess of 226Ra with respect to 23°Th was found. Krbetschek et al. (1994) have recently discussed the circumstances in which disequilibrium may be found in the context of luminescence dating. Readhead (1987) has published information that aids dose rate calculations when disequilibrium applies. The A d e l a i d e Physical Archaeometry laboratory has now been in operation for almost two decades and has environmental dose-rate data for a wide variety of locations. At many of these, measurements have been made
There is now a considerable body of data on luminescence dating in Australia. This includes thermoluminescence (TL), o p t i c a l l y s t i m u l a t e d l u m i n e s c e n c e ( O S L ) and, m o r e recently, infra-red s t i m u l a t e d l u m i n e s c e n c e (IRSL). Some recent representative examples are to be found in Roberts et al. (1990, 1994), Nanson et al. (1991) and Huntley et al. (1993a, b, 1994). All o f these reports include information about dose rates and many also give detailed data about concentrations of the individual elements, potassium, uranium and thorium, which are responsible for the dose. In addition there is a c o n t r i b u t i o n from c o s m i c rays, the r e l a t i v e i m p o r t a n c e o f which depends upon the location of the site (Prescott and Hutton, 1988, 1994). Ages found by luminescence dating are now c o m m o n in the interval from the present to 120 ka ago and the method is producing credible results for ages as large as 800 ka (Huntley et al., 1993a; Berger, 1994). For such long periods of time it is particularly necessary to include consideration of the change with time of the dose rate to find an average dose rate to go with the integrated dose. At a specific site, such information can often be deduced from measurements at points in a profile. Changes with depth of dose-rates and individual element concentrations may give information about the deposition history of the site. We illustrate this with some selected examples. T h e c o n t r i b u t i o n s f r o m U a n d Th c o m e f r o m the whole o f the radioactive decay chains that follow these parents and, for accurate evaluation o f dose rates, knowle d g e o f the e x t e n t o f r a d i o a c t i v e d i s e q u i l i b r i u m is
+Deceased. 439
440
Quaternary Science Reviews (Quaternary Geochronology): Volume 14
on a complete excavation profile. It is the purpose of the present paper to report a variety of dose rates and elemental analyses from Australian sites. Most of this is previously unpublished but some previously published work is quoted for contrast and completeness. We have not attempted a comprehensive review of work from other laboratories. The sites discussed are shown on the map in Fig. 1. Seven different methods of element analysis are represented: delayed neutron analysis (DNA), neutron activation analysis (NAA), X-ray spectrometry (XRS), thick source (z-counting (TSAC), in situ y-ray scintillometry (scint), and high resolution c~ and y spectroscopy (c¢ and y, respectively). Their relative merits have been discussed in Hutton et al. (1985), Prescott and Hutton (1987, 1988) and Hutton and Prescott (1992). Briefly: DNA and NAA are reactor-based methods that give the parents, 238U and 232Th, respectively; XRS is used for K but it can also be used for U and Th at concentrations above about 5 gg/g, for other major elements and, if required, minor elements. In TSAC, all the o~ particles in both the U and Th decay chains are counted together; an estimate of Th is made by counting pairs of
o~ particles in the 22°Rn --) 216p0 --~ 212pb decay in the Th chain; the U-equivalent follows, assuming equilibrium in both cases. Although these estimates are individually of limited accuracy because of the low pairs count rate, the combined contribution of the U and Th series to the dose rate is almost independent of the relative concentrations of U and Th, for a given o~ count rate. The data in the tables should be read with this in mind. In scintillometry, y-ray spectra are measured in the field with a sodium iodide scintillometer (Prescott and Hutton, 1987). It measures individual y-rays for potassium (1.46 MeV), 214Bi (1.76 MeV) and 2°sPb (2.62 MeV). 214Bi is from a late daughter in the uranium series while 2°sPb is the last member of the thorium series. U- and Th-equivalents follow, again assuming equilibrium. The scintillometer is calibrated for the three nuclides individually and, independently, for the y-ray contribution to the dose rate. Comparisons among these methods allow an assessment of the existence and degree of disequilibrium. For example, if a comparison of parent 23sU by DNA and daughter 2HBi by scintillometry gives the same activity, then the chain is probably in equilibrium. A difference may indicate disequilibrium and this would be confirmed
• TC
°PJ oLA
MS
CA ,SK
SG
FIG. 1. Map showing the location of all the places named in the text. CA, Cooloola (CA, SK); EB, Roonka; ER Eyre Peninsula (HC, ML, WW); LA, Lake Amadeus; MS, Mound Springs; PJ, Puritjarra; SC, Mt Schank; SE, South East (MG, SQ, WKI ); SG, Spencer Gulf (SG, RED); TC, Tennant Creek; WR, Warrnambool.
J.R. Prescott and J.T. Hutton: Environmental Dose Rates
441
ENVIRONMENTAL DOSE RATES FOR SITES WITH LITTLE O R N O R A D I O A C T I V E DISEQUILIBRIUM
by an intermediate U-equivalent value found by TSAC. High resolution 7- or m-spectrometry, in which individual nuclides are identified by their 7 or ~ emissions, m a y then define the nature and details of the disequilibrium. A similar comparison can be made between 23ZTh by N A A and 2°8pb by scintillometry. In fact, we have never found any convincing evidence of disequilibrium in the thorium chain. This is not unexpected since the lifetimes of the thorium daughters are all short with respect to the time scales for their transport by geochemical processes. On the other hand, in the uranium chain, the 3.85 d halflife of =2Rn, for example, allows time for it to diffuse away from its production site and be lost; and the 1.6 ka half life of 226Ra is long enough for it to be transported. Since both o f these are in the m i d d l e o f the u r a n i u m series, any consequent disequilibrium has the potential to influence dose rates. Dose rates in this paper have been calculated from the data of Nambi and Aitken (1986) and/or Readhead (1987), with the e x c e p t i o n o f those in Table 4 (from Smith, 1983) which predate the former references and have not been recalculated.
Dune Systems Table 1 s h o w s e l e m e n t a n a l y s e s r e p r e s e n t a t i v e o f s o m e 200 sites at w h i c h the A d e l a i d e l a b o r a t o r y has obtained dates. Here, as elsewhere, sites are described by a site i d e n t i f i e r f o l l o w e d , after the "/", by the d e p t h below the present day surface in cm. They range from what is now the desert interior, to better-watered locations on the southern and eastern seaboards. They are all dune sites, although the context is variable. Our three lowest activity sites are included: SQ2/200 is in the bryozoal limestone near Mt Gambier, SA; CA9 is in the giant p o d s o l K a b a l i dune at C o o l o o l a , QLD; M L 2 is a recent dune on the western m a r g i n o f L a k e Malata, SA. It gave no reading above background with our scintillometer and is believed to be pure wind-sorted gypsum. Potassium data are included in the table, although they are not relevant to r a d i o a c t i v e disequilibrium. Rather,
TABLE 1. An illustrative selection of analyses from Australian sites [in part from Hutton and Prescott (1992)] %K Scintillometry
NAA
Th [.tg/g Scintillometry
XRS
Roonka, SA EB 1S/100
0.94
0.96
1.5
1.3
6.6
6.4
Cleland Hills, NT PJ1S/10 PJ 1S/75 PJ2S/100
0.23 0.75 0.10
0.25 0.75 0.12
0.8 1.9 0.35
1.0 1.7 0.52
4.0 8.9 2.0
4.1 10.0 2.0
Lake Amadeus, NT LA3S/200 LA4S/300
0.55 0.20
0.54 0.16
0.8 0.3
0.9 0.3
1.6 1.3
1.7 1.3
O.O9
4.1
4.0
1.6
1.6
Eyre Peninsula, SA HC5 ML2 WWO1
DNA
U ~tg/g Scintillometry
Locality
0.16
0.O8 1.1 1.1 No K, UorThdetectableabovebackground 0.18 1.7 1.5
South East, SA MG5/30 SQ2/200 WK1S/100
2.2 0.01 0.08
1.9 0.01 0.10
2.2 0.2 0.9
2.6 0.2 1.0
9.0 0.3 0.9
8.2 0.2 0.9
Warrnambool, Vic WR3 S
0.10
0.09
0.7
0.9
1.5
1.3
< 0.01 0.12 0.16 < 0.01
0.02 0.12 -0.01
0.92 2.7* 4.4 0.5 _+0.05?
0.71 2.7 3.4 _+0.5t 0.48
Cooloola, Qld SK11-S CA2 CA5 CA9
1.3 8.5* 16.0 0.2 _ 0.17
1.05 7.5 16.9 +_5p 0.48
*By XRS; error +__3%. ~By thick source c~ counting. In TSAC the error in the thorium concentration depends on the (relatively poor) counting statistics for pairs. This affects the corresponding uranium concentration. The statistical precision of the total ¢x count is normally better than 3%. For DNA 1 SD counting error is 0.06 t.tg/g. For NAA 1 SD counting error is 0.5 [tg/g. For field scintillometry the relative counting error is 5-10%.
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Quaternary, Science Reviews ( Quaterna~ Geochronology): Volume 14
they serve as an indicator of the usefulness of scintillometry at the l o w c o n c e n t r a t i o n l e v e l s shown, and as a reminder that in situ scintillometry is the best method available for potassium at low concentrations. In this table D N A (or N A A ) agree with scintillometry and, on the whole, it can be concluded from these data that disequilibrium is uncommon in Australian sites, particularly in interior aeolian dunes, and we ourselves have p r e v i o u s l y published data that support this contention (Hutton et al., 1985; Hutton and Prescott, 1989, 1992).
Profiles: Tennant Creek, Northern Territory A rare intra-plate earthquake at this site in 1988 ruptured the surface for about 30 kin. It was dated in order to place possible limits on the last previous m o v e m e n t of the fault (Crone et al., 1992; Hutton et al., 1994). Table 2 shows the dose rates for the hanging wall profiles from trenches cut across the K u n a y u n g k u (TC1) and Lake Surprise faults (TC2, TC3). Three almost independent measurements of dose rate are shown: in situ y scintillometry: D N A + N A A + X R S ; T S A C + X R S . C o s m i c ray dose rates have not yet been added. Field evidence suggests that depths below the present day land surface correspond at all three sites; there is little evidence of palaeosol development. A sedimentation rate o f 35 _+ 7 m m / k a for the r e g i o n was e s t a b l i s h e d (Hutton et al., 1994) so that the base of TCI is at about 35 ka and at TC2 it is beyond 70 ka. The table shows
only a small change with depth of the dose rates and that any changes with time have been steady. At this site the present day profile can be used to model its own past development and hence the time dependence of dose rate. In fact, within the uncertainties in the age determination itself, the age may be calculated on the basis that the dose rate has been the same as it is now at the level where the sample was collected.
Profiles: Puritjarra, Northern Territory The Puritjarra rock shelter in the Cleland Hills NT is the oldest human occupation site in the arid central interior (Smith, 1987). It has been comprehensively dated by ~4C and TL, although disagreement between ages found f r o m the two m e t h o d s r e m a i n s u n r e s o l v e d (Prescott, 1991). The dosimetry has been more thoroughly studied than at any other of our TL sites except, perhaps, Mt Schank referred to below. Dose rates, obtained by the same three independent methods as for Tennant Creek, are shown in Table 3. Samples denoted PJ1 are from within the shelter, sample PJ2 comes from an,open dune about 1 km away. The dose rates for PJ2 were the same at all levels down to 1.5 m and the figures shown are the site averages. The good agreement among the three essentially independent m e a s u r e m e n t s o f d o s e r a t e at T e n n a n t C r e e k a nd Puritjarra shows that radioactive disequilibrium is not important at these sites.
TABLE 2. Comparison of dose rates at Tennant Creek, Gy/ka, without cosmic ray contributions Site
Scintillometry
XRS + DNA + NAA
XRS + TSAC
TC l S/20 TCl S/45 TC1 S/80 TCIS/110
I. 12 _+0.035 1.27 +_0.035 1.34 _+0.035 1.28 _+0.035
1.34 + 0.043 1.30 _ 0.043 1.34 + 0.043 1.35 _+0.043
1.20 + 0.063 1.30 + 0.063 1.28 _+0.063 1.21 _+0.063
TC2S/65 TC2S/145 TC2S/185 TC2S/225 TC2S/250a TC2S/250b
1.26 _+0.037 1.34 + 0.037 1.41 _+0.037 1.46 _+0.037 1.60 _+0.08* 1.37 _+0.04+
1.25 _+0.043 1.34 _+0.044 1.33 _+0.044 1.42 + 0.044 1.46 _+0.044 1.33 + 0.044
1.30 + 0.11 1.30 _+0.06 1.29 + 0.08 1.40 _+0.08 1.40 + 0.1 [1.16 _+0.09]
TC3S/80 TC3S/170
1.31 _+0.038 1.54 _+0.041
1.24 _+0.05 1.32 _ 0.07
1.20 _+0.08 1.35 _+0.07
*This field scintillometer reading from the base of the profile is probably not measuring the same thing as the laboratory measurements. ?Sample TC2S/250b was taken on the foot wall side of the East Lake Surprise fault and corresponds stratigraphically to a depth of about 2 m. TABLE 3. Comparison of dose rates at Puritjarra, NT, Gy/ka, without cosmic ray contributions Sample
Scintillometry
DNA + NAA + XRS
TSAC + XRS
PJ 1S/10 PJ 1S/25 PJI S/40 PJ1 S/55 PJI S/75 PJI S/95
0.83 _+0.02 1.11 + 0.03 1.37 _+0.04 1.62 _+0.06 1.68 + 0.06 1.87 _+0.07
0.77 _+0.03 1.09 _+0.04 1.36 _ 0.04 1.77 _+0.06 1.82 _+0.06 1.86 + 0.09
0.85 _+0.05 1.28 _+0.06 1.36 _+0.07 1.76 _ 0.12 1.83 ___0.10 1.98 _ 0.11
PJ2S
0.41 _+0.02
0.36 _+0.02
0.40 +_0.03
J.R. Prescott and J.T. Hutton: Environmental Dose Rates Profiles: Roonka, South Australia
443
abrupt change in all properties to what appears to be an older surface on which the dune developed. This example illustrates the way in which complementary information can be obtained from an associated discipline (archaeology and soil science in the present case) to confirm the evidence from analyses and improve the accuracy of the TL ages. At this site discontinuities in deposition occurred at about 3 and 10 ka. In calculating ages the dose history was divided into four groups corresponding to the four different sections of the dune.
Roonka is one of the more significant prehistoric Australian aboriginal sites (Pretty, 1986). It is located on the banks of the River Murray, about 10 km north of Blanchetown in South Australia. Evidence for human occupation extends from 18 ka ago until the present day. Both TL and 14C dates have been reported for the East Bank site (Prescott, 1983; Prescott et al., 1983). Table 4 shows the dose rates (average of all methods) for this site as a function of depth, together with a selection of element analyses. They have been grouped by depth in accordance with the field evidence discussed below. The site is in an aeolian dune resting on indurated brown silt, which in turn overlies calcrete. The archaeological excavation showed that there was a veneer of recent deflation products about 10 cm deep, and former dune surfaces at depths of 45 cm and about 1.0 m. Archaeological feature F11 is a fireplace lit on the 45 cm surface and about 2.5 ka old (Prescott et al., 1983), while feature F13 is a burial probably associated with the 100 cm surface. Unpublished dates place this surface at about 10 ka ago and the base of the dune at about 20 ka. The general features are identifiable in discontinuities in the element analyses and dose records. The total potassium (about 1%) and A1203 (3.5%, not shown) are low and almost constant with depth, indicating a low clay and/or feldspar content and a nearly pure quartz sand dune. Ti, Zr and Th are about average for surface sediments except for the depth band 60-80 cm; the Ti/Zr and A1/Y ratios are also anomalous in this band. This suggests that the 60-80 cm samples are different from the rest of the dune and supports the field evidence for the two former dune surfaces, one at 45 cm and one at 100 cm with a zone of remobilization between. Y and Th commonly move together in the environment and the Y/Th ratio is almost constant down the profile. The Ti/Zr and A1/Y ratios, which are associated with minerals that do not weather, show the discontinuity. However, the Ca/Sr ratio progressively decreases from the surface to 180 cm, suggesting stability of long enough duration for the slightly soluble calcium carbonate minerals to come to an equilibrium with repeated solution and crystalization. During this process low strontium calcite forms at the surface and the solutions at lower depths contain progressively more strontium, leading to the lower Sr/Ca ratio (Hutton and Dixon, 1981). At 190 cm there is an
Dune Sequence, South-east South Australia The sequence of fossil dunes in the south-east of South Australia has now been comprehensively dated by both TL and IRSL. These dunes were built on a tectonically rising land surface by successive high stands of the sea as it advanced and retreated with the cycle of iceages. Because it is securely dated at the seaward end (Robe) by modern dune building, at the last interglacial (Woakwine Range 120-130 ka) by U-series and aminoacid r a c e m i z a t i o n and at N a r a c o o r t e where the Brnnhes-Matuyama geomagnetic field reversal (780 ka) lies between the East and West Naracoorte ranges, it provides a test sequence for luminescence dating for the past 800 ka (Huntley et al., 1993a, b, 1994). Over such a long period it is critical to understand the dose rate history. In this we were assisted by the high degree of uniformity of the materials of which the dunes are built, viz., quartz and shell-derived calcium carbonate. The local evidence from the more recent dunes is that induration of the surface begins shortly after formation and that this protects the interior of the dune from major additional weathering. Over longer periods, the older dunes e.g. Reedy Creek, Harper or Cave have gradually become indurated throughout. In this case it is safe to assume that the chemical composition and hence the dose rate has been essentially constant for most of the life of the dune. When we began the study (Huntley et al., 1985) we anticipated that a portion of the uranium would be incorporated partly in resistate minerals (Hutton and Prescott, 1989) and partly in the shell carbonate without its daughters; these would grow back in and at the same time uranium would be progressively leached out (Veeh and Burnett, 1982). Thus, uranium would be highest in the recent dunes and least in the older ones. Figure 2a and b collects together all the analyses for U
TABLE 4. Dose rates and chemical analyses of the Roonka, East Bank sand dunes Depth (cm) 10-30 60-80 100-120 170-180 190-230
No. of Dose rate s a m p l e s (Gy/ka) 4 2 3 2 2
1.91 1.39 1.83 2.15 2.30
K (%)
U
1.0 0.95 0.94 1.1 1.1
1.2 0.5 1.2 1.3 1.4
Th
Ti
Zr
(p.g/g) 7.0 3.0 6.3 7.6 10
1500 700 1100 1600 2800
Errors: dose rates, 4%; K, 3%; U, 0.1 p-g/g;Th, 0.5 p-g/g;Ti and Zr, 5%.
Ti/Zr 280 80 240 310 310
5.2 8.8 4.6 5.0 9.0
A1/Y 1.5 x 2.2 x 1.4 x 1.4 x 1.8 x
103 103 103 103 103
Y/Th
Ca/Sr
2.0 2.0 2.0 2.1 2.0
400 200 180 100 350
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Quaternary Science Reviews (Quaternary Geochronology): Volume 14
URANIUM 1.5
I
I
I
I
x
i
i n- situ
O
TSAC DNA XRS
X
+,
1.0
I
A
I
T-spectrometry
zk 0.5
RBTr W K I WD WA EA BA RBTrr WK[[ITrr RC RBI I I I I I 400 500 100 200 300 0 Age ( ko )
HA WB I 60~
ST
NW NE I 700
800
THORIUM
3.0 A
i in-situ
I
i
I
I
'
"7'- spectrometry
o TSAC • NAA x XRS
I
x
2.0 I
~L )¢
1.0
RBTT W K I WD RBI RB~ WK]~ITTT RC WA O-I I I 0 100 200 300
x EA BA I I 400 500
HA W B I 600
ST
NW NE I 700
800
Age ( ka ) FIG. 2. Concentrations in ~tg/g for most of the ranges in the south east of South Australia as found by field scintillometry, DNA, NAA, TSAC and XRS. (a) uranium; (b) thorium. Errors are not marked for XRS because the method is almost beyond its limits at these concentrations: U _+ 0.4, Th _ 0.5 ~tg/g. The notations for the ranges are: RB, Robe; WK, Woakwine; WD, West Dairy; RC, Reedy Creek; WA, West Avenue; EA, East Avenue; BA, Baker; HA, Harper; WB, Woolumbool; ST, Stewart/Cave; NW, West Naracoorte; NE, East Naracoorte. and Th. They are shown at the ages assigned to them by H u n t l e y et al. (1993a, b, 1994). The value plotted for each m e t h o d o f analysis is the average o f at least two independent observations. The uranium analyses shown in Fig. 2a appear at first sight to support the above expectation, in that they can be
represented by a continuously falling U concentration when going backwards in time. However, there is contrary evidence from the intercomparisons of these methods. i f radioactive disequilibrium is significant, then a comparison of D N A with scintillometry would be expected to reveal it, since D N A gives the concentration of the
J.R. Prescott and J.T. Hutton: Environmental Dose Rates parent 238U whereas scintillometry sees a daughter (214Bi) occurring late in the decay chain. It is clear from Fig. 2a that, for those sites at which both measurements are available, the analyses are in good agreement. A similar conclusion follows if the data from TSAC (which counts all the c~-emitting daughters) are included in the comparison. Other details of this argument can be found in Huntley et al. (1993a). The data in Fig. 2b suggest a Th concentration in equilibrium, generally decreasing with time, more scattered for the older dunes. An alternative interpretation of Fig. 2 is that, rather than showing a monotonic change with time, the thorium and uranium concentrations are constant (U: 0.5 ppm; Th: 1.5 ppm) for the ranges older than 350 ka, but that the more recent ranges had access to a source of reworked material higher in U content and perhaps lower in Th since about the time that the West Avenue (WA) Range formed 350 ka ago. The two graphs are then well represented by step functions with the step occurring at that time. The abnormally high values for all elements in WA are also consistent with this scenario.
SITES WITH RADIOACTIVE DISEQUILIBRIUM Steps should be taken to check on the presence (or absence) of radioactive disequilibrium as a matter of course by making measurements of dose rates and/or element concentrations by independent methods. As indicated above, disequilibrium is rare in Australian dune systems. However, we have found a number of examples of disequilibrium in circumstances where the environment is wet.
Spencer Gulf, South A u s t r a l i a Historically, one of our first examples of disequili-
445
brium was in northern Spencer Gulf where ages were being sought for marine sediments in connection with the environmental impact study for the proposed Redcliff petrochemical plant (Smith et al., 1982; Belperio et al., 1984). In brief, the floor of the Gulf consists of mixed terrigenous/estuarine sediments of Pleistocene age (last interglacial, Mambray Formation and older), overlain with r e w o r k e d H o l o c e n e seagrass bank deposits (Germein Bay Formation) dating from the time that the sea returned to the Gulf between 8 and 9 ka ago (Hails et al., 1984). They have therefore been underwater since that time, but would have had an arid to semi-arid environment for the preceding 100 ka. Table 5 shows U and Th analyses for cores SG182 and RED38. The Holocene/Pleistocene boundary lies at a depth of about 200 cm for SG182 and about 250 cm for RED38. DNA and NAA are compared with TSAC. The thorium concentrations are low, increase slightly with increasing depth and, within the counting errors, NAA and TSAC give the same results. This is not unexpected since the daughters in the Th chain all have short lifetimes with respect to likely geochemical separation processes. Uranium increases with depth in the Holocene sediments and changes abruptly at the boundary with the Pleistocene. It is clear that parent 238U exceeds the apparent concentration inferred from TSAC (the aggregate c~ count of U and its daughters) by a factor of the order of three in the Holocene sediments and much less than that in the Pleistocene sediments. The analyses are supported by spot checks of the parent concentrations by XRS, two of which are also shown in Table 5. Without further detailed studies of the individual elements in the decay chains it is not possible to construct a history of the movement of U and its daughters. For the purpose of the present exercise it is sufficient to note that
TABLE 5. Uranium and thorium analyses from Spencer Gulf, South Australia Locality
DNA
U (pg/g) TSAC
XRS
NAA
Th (~tg/g) TSAC
XRS
RED38/101 RED38/131 RED38/236
6.3 17.9 17.1
1.9 4.9 4.5
20.1
2.9 3.2 4.2
3.5 3.7 4.0
5.9
RED38/252 RED38/333 RED38/342
7.6 7.3 6.9
Approximate Pleistocene/Holocene boundary 2.8 4.0 4.0 3.3 5.2 4.9
3.7 4.5 4.8
SG 182/53 SG 182/69 SG182/128 SG182/146 SG182/167 SG 182/183
14.1 15.0 12.7 14.5 29.5 27.6
4.9 3.6 4.7 3.9 7.8 9.0
2.2 2.3 1.6 2.0 2.6 3.5
1.3 1.5 1.9 1.7 2.2 4.5
SG182/208 SG 182/228
6.6 11.9
Approximate Pleistocene/Holocene boundary 3.3 3.7 7.6 3.6
2.9 2.4
27.4
4.9
Errors for DNA and NAA are < 5% and for XRS are 3%. In TSAC the error in the thorium concentration depends on the counting statistics for pairs, and in this table is about 10%. This affects the corresponding uranium concentration. Although the statistical precision of the total alpha count, which is used in dose rate calculations, is everywhere better than 3% the error in U is about 5%.
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Quaterna~ Science Reviews ( Quaternar3, Geochronology): Volume 14
coarse grain quartz TL dates for the Holocene differ by a factor of 1.45 depending on whether the dose rate is calculated using D N A for U, N A A for Th, and XRS for K (together with cosmic rays) or with U and Th found from TSAC. On the evidence of Table 5 the former would be too large and the latter w o u l d be too small. In fact, a model in which the dose rate changes continuously from beginning of the Holocene to the present day gives TL ages in g o o d agreement with 14C dates from the same depth in the core (Smith, 1983). Nevertheless the project illustrates the fact that, once disequilibrium has been identified, two alternatives face us: to calculate the TL age with error limits large enough to encompass the likely limits of dose rate (+ 25% in the present case) or to make detailed measurements of the daughters using high resolution c~- or y-spectroscopy. This is illustrated in the next example.
TABLE 6. Radio-element activities from Mt Schank, South Australia Element
LAVA
-~3sU e~4Th 234U
chemical 5' o~ ot O~
31 + l 30_+5 -25 _+7
2~°Th 22~'Ra 214Bi 2~°Pb
o~ ¥ 7 3'
2~2Th 2~Ra 22~Th 22*Ra 2°~Tl
chemical ? o~ 5' 5'
Activities (Bq/kg) SC3-6 SC3-7 24 _+2 40+6 22_+2 -22 _+2
25 _+ l 24+6 -26 _+9 --
35 _+5 31_+1 31 -!- 1 48-+24
110 _+5 121+2 160+ 10 130-+30
149 _+90 113_+2 -125+-25
34 _+ I 36+1 35 _+ 1 37 _+5 33 _+2
65 _+2 70+2 69 _+ 1
60 _+2 58_+2 58 + I 64 _+6 60 _+2
-
-
76 + 8
Mount Schank, South Australia c~and 5' indicate measurements by high resolution spectrometry. A site where very unusual conditions led to gross disequilibrium in the U chain is Mt Schank. Here TL dates w e r e o b t a i n e d f r o m d u n e q u a r t z o f the B r i d g e w a t e r Formation which had been overlain and intensely heated by a lava flow. The dune overlies bryozoal Mt Gambier limestone very close to the water table. Table 6, part of which is r e p r o d u c e d from Smith and Prescott (1987), shows detailed r a d i o e l e m e n t analyses for the lava and two s e p a r a t e d sites (SC3/5 and SC3/6) in the f o r m e r dune. To facilitate c o m p a r i s o n s for disequilibrium, the entries are expressed in Bq/kg. It is clear that, as above, the thorium series is in equilibrium, as is the uranium series in the lava. However, in the sand, there is a discontinuous change in activity between 2~4U and 23°Th. Before this break the concentrations are consistent with those found in other dunes of the Bridgewater formation elsewhere and due to U and Th in resistate minerals. After the break, the 23°Th and the subsequent daughters are enhanced by roughly a factor of five. Dose rate measurements with in situ TL dosimeters and scintillometry (not shown here) are in agreement with these analyses. The discontinuity in concentration is interpreted as due to :3°Th and its daughters being carried by water percolating through the still warm lava as it cooled, and being stopped on encountering the alkaline dune material. This interpretation is supported by measurements on trace elements (Smith and Prescott, 1987). On this site there can be no doubt that the dose rate is well-understood and that the TL dates are correct. Unravelling the radiochemical history, however, was very time-consuming and almost a decade elapsed between collecting the first samples and the appearance of the publication; but what is l0 years in 5000?
Mound Springs, South Australia T h e w e s t e r n e d g e o f the G r e a t A r t e s i a n B a s i n is marked by a line of mound springs extending from the South Australia/Northern Territory border to Lake Frome (Boyd, 1990). T h e y exhibit a r e m a r k a b l e d i v e r s i t y o f
form. T h e y are built up from e v a p o r a t i o n of c a l c i u m bicarbonate solution carried up by the artesian water; in addition, aeolian debris is incorporated into the dune as it grows. Several periods of activity are evident on geological grounds, the earliest of which is probably older than 1 Ma; many springs are still active. In such a watery environment, radioactive disequilibrium is not unexpected. Table 7 shows a selection o f analyses, expressed in Bq/kg. KHI (Kewson Hill) and BS2 (Old Beresford) are representative of the older, inactive, springs with ages of the order of 1 Ma, ES1 (Elizabeth Springs) is old but still active, whereas BCI (Blanche Cup) and BB1 (Bubbler) are active and have been building their present mounds for times in excess of 20 ka. B S l a (New Beresford) is thought to be modern dune sand recently incorporated into the mound. So far as the thorium series is concerned, the data are c o n s i s t e n t with e q u i l i b r i u m a l t h o u g h , for K H I , the amounts present are so low that additional work may be desirable. In the uranium chain, with the exception of B S I a , disequilibrium is clearly established by the fact that parent uranium (as found with DNA) is insufficient to support its daughters as represented by either scintillometry or TSAC. This is confirmed by high resolution y and c~ spectroscopy. This disequilibrium is most conspicuous for the two young and active sites, BCI and BBI. With the possible exception of ESI the various 23~U and 234U analyses agree and there is some evidence that 23°Th may be a little in excess of equilibrium, as is 22~Th in the thorium series. For the younger springs, BC1 and BB1, the uranium chain at and b e y o n d 226Ra shows a spectacular increase in activity. This is not evident for the older springs, KH1 and E S I ; unfortunately 226Ra has not yet been measured for them. A provisional scenario on which to base dose rate calculations is that the :38U and 232Th observed (and their particular daughters) come from resistate minerals incorporated as wind-blown material, while the excess counts
447
J.R. Prescott and J.T. Hutton: Environmental Dose Rates TABLE 7. Analyses for Mound Springs, expressed as Bq/kg Uranium series High resolution
DNA U-238
"/ U-238
2.9 4.2 1.7 7.8 6.9 9.1
- 3 _+ 3
BSla BS2 ES1 KH1 BC1 BBI
~ U-238
e~ U-234
3.2
. --2 +_4 9 _+6
.
3.6 . 3.0 10.9 6.4 7.9
. 5.6 9.1 6.3 7.7
a Th-230
7 Ra-226
Scintillometry Bi-214
TSAC U equiv
7
4.5
2.1 12 9.1 11
--39.5 59.9
3.1 10.5 2.7 5.6 53 91
3.5 6.2 1.8 10.1 37.1 39.7
.
Thorium series High resolution
c~ Th-232 BS I a BS2 ES 1 KH1 BCI BB1
7 Ra-228
7
7.0
.
.
6.2 1.7 12 14
. --18.1 18.0
y Th-228
o~ Th-228
TSAC Po-216
Scintillometry T1-208
8.5
6
--17.3 18.4
5.8 3 15 14
4.9 None 4.5 0.7 11.1 12.5
6.2 0.65 4.9 0.8 13.0 16.5
.
The errors on the scintillation numbers are about 5%. The DNA error is about 7%; TSAC for uranium about 5%, for thorium about 15%. High-resolution y counts are better than 3% except for U-238. High resolution ~ counts: for uranium 10%, for thorium the absolute error is 1. in the lower part of the chain are due to radium, and perhaps a little thorium, carried up in solution by the water of the springs. The 1.6 ka half life of 226Ra would ensure that we would not expect to see this imbalance m a i n tained in the now rock-like material of the older springs. CONCLUSIONS It is common to calculate TL/OSL ages from a measured equivalent dose and a dose rate found from measurements with in situ dosemeters and/or individual element analyses. We suggest that sufficient measurements of dosimetry should be carried out that the dose rate history can be inferred, particularly when the anticipated age is large. At the very least, multiple samples should be collected, even if not all of them are analysed for equivalent dose. A set of analyses from half a dozen samples down a profile will give useful information even when the analyses are confined only to K, U and Th. If XRS is used for K analysis, then m a j o r e l e m e n t s are u s u a l l y available as well and this may help with the interpretation. Examples of this are to be found at Roonka above, in H u n t l e y et al. (1993a) and in Smith and Prescott (1987). We have long advocated the use of field scintillometry for measuring both gamma dose rate and individual K, U a n d Th c o n c e n t r a t i o n s ( P r e s c o t t a n d H u t t o n , 1987; Hutton and Prescott, 1992). It has the advantage of providing a completely self-contained dose rate; and this dose rate is immediately available without the need for further sample preparation. A separately calibrated gamma ray dose is obtained and this is averaged over the
same volume that provided the gamma dose to the sample. At very low K levels it is the preferred method for that element. Australian sites that have been built by aeolian activity show little if any evidence for radioactive disequilibrium. However, any site which is wet or has probably been wet for a significant time in its past history should be treated with caution, not to say suspicion. A test for radioactive disequilibrium should be carried out as a matter of course and reported in the published results. Such a test could comprise either at least one pair of independent measurements of dose rate (as in Table 2 above), or assay for individual radio elements by high resolution 7 spectrometry. E x a m p l e s of the latter are shown in Tables 6 and 7, although in both these cases the spectroscopy was ex post facto. High resolution ~ spectrometry is useful for clarifying details of decay chains once disequilibrium has been established, but is probably too time-consuming to be used routinely. Although the dose rate and element analyses discussed here were originally obtained for TL dating, they and the conclusions drawn would be just as valid for all aspects of trapped electron dating. ACKNOWLEDGEMENTS The late John Hutton was associated with all of the work described here and was involved with an early draft of the present paper. It is matter for regret that he did not see it completed. Our work has profited greatly from a long association with Gillian Robertson, Dave Huntley and Terry Wall. We are grateful to many colleagues who welcomed us to their field sites. We
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Quaternary Science Reviews ( Quaternao' Geochronology): Volume 14
thank A. Murray and H. Heijnis for the high resolution 5' and o~ spectrometry of the mound springs. The work was supported by the Australian Institute of Nuclear Science and Engineering, the Australian Research Council, The National Science and Energy Research Council of Canada, the Mark Mitchell Foundation, the Australian Geological Survey Organization and the Research Fund of the University of Adelaide.
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