Chlorine-36 production and distribution in Australia

Palaeogeography, Palaeoclimatology, Palaeoecology, 84 (1991): 299-307

299

Elsevier Science Publishers B.V., Amsterdam

Chlorine-36 production and distribution in Australia J . R . B i r d a, R . F . D a v i e a, A . R . C h i v a s b, L . K . F i f i e l d b a n d T . R . O p h e l b

"Australian Nuclear Science and Technology Organisation, Lucas Heights Research Laboratories, Menai, N.S. W. 2234, Australia 6Australian National University, G.P.O. Box 4, Canberra, A.C.T. 2601, Australia (Received April 12, 1989; revised and accepted January 11, 1990)

ABSTRACT Bird, J.R., Davie, R.F., Chivas, A.R., Fifield, L.K. and Ophel, T.R., 1991. Chlorine-36 production and distribution in Australia. Palaeogeogr., Palaeoclimatol., Palaeoecol., 84: 299-307. Chlorine-36 is an unstable isotope of chlorine produced by cosmic-ray bombardment of the atmosphere and the Earth's surface as well as by absorption of cosmic-ray or natural radioactivity-derived neutrons in stable chlorine. Nuclear weapons detonations, nuclear reactors and the use of neutron probes in environmental studies add to the inventory of 36C1. Radiochlorine is strongly associated with water in the atmosphere, environment, soils and groundwater. Chlorine-36 undergoes radioactive decay with a half-life of 301 ka. These properties make radiochlorine a valuable tracer in the study of aqueous and saline systems and a useful tool for dating closed systems (e.g. confined aquifers). Recent measurements of 36C1/C1,using the 14UD tandem accelerator at ANU, show that a variety of processes is important in groundwaters of the Murray-Darling Basin and in the Great Artesian Basin. The results also provide useful information on the levels of 3°C1 in soils, salt lakes and other parts of the Australian environment. The results confirm that estimates of 36C1 precipitation rates are reasonable for southeastern Australia but that more information is needed for other regions. Further modelling of chloride movement is also needed, particularly for arid Australia.

Production mechanisms I n f o r m a t i o n on the p r o d u c t i o n a n d m e a s u r e m e n t o f 36C1 in the e n v i r o n m e n t has been recently reviewed by Bentley et al. (1986a). T w o sources o f n a t u r a l r a d i a t i o n are responsible for 36C1 p r o d u c t i o n in the e n v i r o n m e n t : cosmic ray i n t e r a c t i o n s a n d n a t u r a l r a d i o a c t i v i t y . A third source is 36C1 p r o d u c e d in the last h a l f o f this c e n t u r y as a p a r t o f the d e v e l o p m e n t o f n u c l e a r science a n d technology.

Cosmic-ray production C o s m i c rays incident on the t o p o f the E a r t h ' s a t m o s p h e r e , t o g e t h e r with s e c o n d a r y r a d i a t i o n f r o m p r i m a r y i n t e r a c t i o n s in the a t m o s p h e r e p r o duce 36C1 t h r o u g h n u c l e a r r e a c t i o n s in elements 0031-0182/91/$03.50

o f higher a t o m i c mass. T h e only suitable element with a p p r e c i a b l e a b u n d a n c e in the a t m o s p h e r e is a r g o n a n d the m o s t i m p o r t a n t reactions are 4°Ar(x, x' ~)36C1 a n d 36Ar(n, p)36C1 (where x is a particle or p h o t o n with energy greater t h a n the n e u t r o n s e p a r a t i o n energy o f the o r d e r o f 10 MeV). T h e p r o d u c t i o n rate is m u c h lower than for lighter r a d i o n u c l i d e s such as 14C or 1°Be which can be p r o d u c e d with nuclear r e a c t i o n s in N a n d O. A t the surface o f the E a r t h , a n d for a few metres below, 36C1 is p r o d u c e d t h r o u g h s p a l l a t i o n reactions on 39K a n d 4°Ca. S p a l l a t i o n reactions in heavier elements require high-energy cosmic rays a n d c o n t r i b u t e less t h a n 5 % o f the total 36C1 p r o d u c t i o n . S e c o n d a r y c o s m i c - r a y n e u t r o n s also produce 36C1 t h r o u g h the r e a c t i o n 35C1(n, 7)36C1. In h i g h - c h l o r i d e e n v i r o n m e n t s , the reaction 37C1(x, x'n)36Cl will also be i m p o r t a n t but has n o t been investigated so far. T h e c o n c e n t r a t i o n s o f suitable

;~ 1991 - - Elsevier Science Publishers B.V.

300

J.R. BIRD ETAL.

target elements, the intensities of each kind of incident radiation and the reaction cross-sections control the observed production rates in each environment.

Production from natural radioactivity Thermal neutron capture in 35C1 has a high cross-section (44 barns) and leads to the formation o f 36C1. Some neutrons are produced as a result of cosmic-ray interactions at the Earth's surface but radioactivity is the most important neutron source at depths of more than 1-2 m. Spontaneous fission o f 238U and alpha particle irradiation of light elements such as O, Na, Mg, A1, Si and K release neutrons which are moderated and captured in the surrounding material (Andrews et al., 1986). Neutron production is high in the neighbourhood of uranium ores and low but non-zero in rocks and soils with low levels of radioactivity. The concentrations of many elements are important for calculating 36C1 production rates because of their effect on neutron production, moderation and absorption in the environment. The local chloride concentration is also important as the target for neutron capture.

Anthropogenic production Several man-made sources of neutrons can have important consequences for a6C1 studies: -Nuclear weapons tests - - especially those carried out near the surface of the ocean; - - Nuclear reactor operation, presumably including nuclear-propelled ships and submarines especially in confined waters; - - The use of neutron sources in environmental studies for example those used for well logging and moisture probes and particularly in the presence of saline waters. Production

very low at high energies and increases rapidly towards low energies. Interactions with constituents of the atmosphere lead to the production of additional low-energy particles and the total flux peaks at an altitude of about 20 km. Below this altitude, the flux of lower energy cosmic-rays, which contribute most of the 36C1 production, decreases exponentially:

I = Ioexp ( - d/fl)

(1)

where I is the flux at depth d (g cm-2), io is the initial flux and fl is the attenuation coefficient. In the atmosphere fl= 160 g cm -2 (Yokoyama et al., 1977). Atmospheric production of 36C1 thus occurs mainly in the stratosphere although about 40% of the total production occurs in the troposphere. Cosmic-ray fluxes are affected by the Earth's magnetic field so that radionuclide production is dependent on geomagnetic latitude. However, mixing processes and transfer from the stratosphere to the troposphere modify this dependence. Lal and Peters (1967) have calculated the total 36C1 fallout as a function of latitude (Fig.l, curve I). An average rate of about 9 atoms m -2 s -1 is predicted at a latitude of 10°S (Cape York) rising to a peak of 28 atoms m - 2 S-1 at about 40°S (Tasmania) and falling again to 3-4 atoms m -2 s -1 in Antarctica. 36C1 in groundwaters in the ]

I

I

I

F

I /r

"I---

-1" --

--

/11I •

20

c~

10 8

rates

Atmospheric production C o s m i c - r a y s incident on the E a r t h ' s a t m o s p h e r e have an energy s p e c t r u m which extends to ultrahigh energies (e.g. 1018 eV). H o w e v e r , the flux is

[

I

I

I

]

I

I

I

10

20

30

~0

50

60

70

80

90

LATITUDE

Fig.1. Dependence of 36C1 precipitation as a function of geographic latitude (Curve I) and of surface production at sea level and longitude 110°E as a function of geomagnetic latitude (Curve//).

301

CHLORINE-36 PRODUCTION AND DISTRIBUTION IN AUSTRALIA

USA and Europe give some support to the results in Fig.l but measurements on Greenland ice cores (latitude 65°N) show 36C1 contents from 3 to 5 times higher than expected (Elmore et al., 1987; Suter et al., 1987). It is clear therefore that atmospheric production and distribution are not yet fully understood but the predicted latitude effect would cause a variation by at least a factor of 3 from north to south across Australia. The production of cosmogenic isotopes in the atmosphere varies with solar activity and the measurements on Greenland ice cores show a rise in 36C1 levels during the Maunder minimum which is consistent with measurements of 1°Be and ~4C. However, other fluctuations are observed which indicate that 36C1 is subject to different processes which require further investigation. For example, the ratio l°Be/36Cl varies by more than a factor of two over the last 400 years (Elmore et al., 1987; Suter et al., 1987). Much further work is needed to establish the nature and extent of the production variations and mixing processes, especially in the Southern Hemisphere.

Bomb pulse Nuclear weapons tests which were carried out on Pacific atolls from 1954 to 1960 produced considerable quantities of 36C1 and the resulting fallout (known as the bomb pulse) has been observed in Greenland ice cores (Suter et al., 1987), soil profiles (Phillips et al., 1988) and groundwater (Bentley et al., 1986a). The observed fallout in Greenland ice is shown in Fig.2 as a function of time, peaking at about 500 times greater than the natural 36C1 deposition rate. A bomb pulse with isotope ratio of 6000 x 10 -15 is observed at a depth of the order of 1 m in soil. Weapons fallout varied considerably with latitude (Phillips et al., 1988) so that independent information is needed on the bomb pulse in the Southern Hemisphere. Some additional contribution may also be present in Australia from British weapons tests - - depending on the amount of salt present at the test sites.

Surface production In surface rocks and soils, spallation (removal of several nucleons) of 39K and ¢°Ca, together

T

T

1

10 ~

'E

r-

102

~o I £ _ _ _ _ L _ _ _ I 1950

L 1960

1970

1980

Fig.2. 36C1'bomb spike' in Greenland ice showing the change in estimated deposition rate from 1950 to 1985.

with the neutron-induced reactions 35C1(n, 7) and 39K(n, ~), are the most important sources of 36C1. Negative muon induced reactions such as 39K(t~-, p2n) and ¢°Ca(v-, e) make small contributions which may be important if the C1 concentration is low (e.g. in carbonates). Production of 36C1 by spallation of heavier elements such as Ti and Fe requires high-energy cosmic-rays - - being important in meteorites and the lunar surface (Nishiizumi et al., 1984) but insignificant at the Earth's surface. A possibly significant contribution from 3VCl(x, x'n)36Cl reactions in the salt component of the ocean, salt lakes, evaporites, etc. has not been investigated. Although this is likely to contribute -considerable 36C1 production, the high levels of stable chlorine atoms present will result in 36C1 ratios which are below the level of detection. The highest ratio will be observed in a matrix with low total chloride and high K and Ca concentrations. In high C1 and low K, Ca rocks, neutron capture is an important contributor and this is discussed in the following section. Some typical production rates for various rock types are listed in Table 1 for a latitude of 30°S (adapted from Fabryka-

302

J.R. BIRD ET AL,

TABLE 1 Typical rock composition and 36C1production at 30° latitude Type

Composition (~g/g)

36C1 P r o d u c t i o n

Surface CI

K ( X 103)

Ca ( X 103)

Th

Deep

U Prod. rate (atoms/m3 s)

Equil. (atoms/g x

Ultramafic Basalt Granite Low Ca High Ca Sandstone Limestone Clay, shale

85 60 200 130 10 150 160

0.04 8.3

25 76

42 25.2 10.7 2.7 22.8

5.1 25.3 39.1 302 25.3

0.004 4 17 8.5 1.7 1.7 11

0.73 0.77

3.6 3.9

3 3 0.45 2.2 3.2

1.7 1.2 0.27 0.29 0.89

8.5 6.2 1.7 15 5.0

X

4.1 x

10 - 6

1 . 4 x 10 -3

1.9 x 10- 2 9.3 X 10 - 3 5.4 x 10-5 4.1 X 10 - 3 6.0 x 10 -3

10-11

"4,,d CALCULATED 36C1 PRODUCTION IN ROCKS a b c d • F g

UItramaFic Sandstone Bssalt Limestone Clsys, Shales 6rsnite (high Ca) Granite (}0. Ca)

10 12

Ez:

10-13

10-1}

b L_

\

e__ --

10-1~

\\ • •

\ I 1

Equil. (atoms/g 103)

0.02 7 98 47 0.32 21 34

( Y o k o y a m a et al., 1977) indicating an increase by a factor o f 3 between 10 ° a n d 40°S at the l o n g i t u d e o f eastern A u s t r a l i a (magnetic latitudes 21 a n d 41°S). I n t e g r a t i n g the surface p r o d u c t i o n t h r o u g h

M a r t i n , 1988). The calculated depth d e p e n d e n c e of the 36C1 ratio in each rock is s h o w n in Fig.3. The relative d e p e n d e n c e o f surface p r o d u c t i o n on geomagnetic latitude is s h o w n in C u r v e H in Fig. 1

~7---'~---...

106)

1

0.001

Prod. rate (atoms/m3 s)

\

\\k\ 10

100

DEPTH (m)

Fig.3. Calculated 36C1/C1ratios as a function of depth in various rock types (Fabryka-Martin, 1988).

1000

CHLORINE-36 PRODUCTIONAND DISTRIBUTIONIN AUSTRALIA

a 1 m layer gives values of the order of 0.3 atoms m - z s- 1 which are insignificant compared to atmospheric fallout. The latter is therefore expected to be dominant in surface water and groundwater recharge. Most in-situ production will remain locked within individual grains where the concentration of 36C1 builds up over times of many half-lives ( > 1 Ma) to secular equilibrium values which again depend on local geochemistry. Calculated equilibrium production rates in rock at sea level are included in Table 1 (adapted from Fabryka-Martin, 1988). The build-up of a6Cl within freshly exposed rock has been observed in a study of lavas of various ages (Leavy et al., 1987). Isotope ratios of the order of 10-12 or greater are reached in such samples. The release of high ratio CI to surface and ground water will depend on the speed of diffusion within a grain and of erosion of the surface. These are normally slow enough for there to be little effect on the chloride in groundwater but they may be significant in special circumstances such as the recent introduction of groundwater because of climate change or fresh exposure of the rock surface. In the case of production at the surface of the ocean, it is generally assumed that mixing of 36C1 into the vast quantity of stable chloride isotopes leads to Standard Mean Ocean Water (SMOW) having an isotope ratio well below the level of sensitivity of AM S ( < 10-15). In coastal and estuarine waters, in-situ production, the erosion of land surface and possible contributions from reactor cooling water may increase the local 36C1/C1 ratio. If the land surface is at an altitude higher than sea-level, the cosmic-ray flux and hence the 36C1 production rate are also higher. The relative variation in 36C1 production is shown in Curve I of Fig.4 (Yokoyama et al., 1977). A variation by a factor of 4 can be expected between surface production over most of the Australian continent and that at the highest point (2230 m) in the Great Dividing Range (southeast Australia). This factor will be reduced somewhat because snow cover for part of the year does not contain suitable target elements for 36C1 production and it shields the rocks to some extent.

303

1000

ALTITUDE (m} 2000

3000

WOO

10

g ~o -

\

\

"~. '~.

C3~

\

01

\

\

100] I DEPTH 2~(g. crn-2)[

\ 11 300 ~ \ J

05 10 DEPTH (m) (p=25)

~,1 1

Fig.4. Relative changes in low-energy cosmic-ray intensities

and hence of 36C1production rate as a function altitude in the atmosphere (Curve I) and of depth within rock of density 2.5 g cm -3 (Curve H) (inner section) and effect of water mixing (outer section).

Undergroundproduction Below the land surface, the absorption of lowenergy cosmic-rays is given approximately by Eq. 1 with fl=190 g e m -2 (Nishiizumi et al., 1984). Spallation production therefore decreases as shown in the lower curve in Fig.4 and is concentrated in the first 1-2 m. Muons penetrate to greater depths and contribute significant 36C1 production in Carich rocks such as limestone between 10 and 100 m depth. At large depths, neutron capture is the only significant production mechanism (Bentley et al., 1986a; Fabryka-Martin, 1988). For example, the local neutron production rate is directly proportional to the U and Th content of Stripa granite (Andrews et al., 1986): = p(2cv + 0.7Crh) neutrons c m - 3 a - 1

(2)

where, p is the density (gcm -3) and c is the

304

J.R. BIRD ET AL.

concentration (lag/g). Neutron production is generally lower in other rocks and different numerical constants are required for Eq. 2 (Feige et al., 1968). The neutron flux is reduced in proportion to the concentration of elements with high absorption cross-sections and is thus very dependent on the total composition of the rock. It may be calculated using the concentration of 17 elements or it may be measured in situ. Fluxes up to 1-10 neutrons m -2 s-1 have been measured in high-U granite (Andrews et al., 1986). The 36C1/C1ratio at secular equilibrium is dependent only on the neutron flux (~b): R=4.55 x 10- l°~b

(3)

Calculated ratios range from 2 to 30 x 10 -15 (Fabryka-Martin, 1988 and Fig.3). Values up to 200 × 10-15 have been observed in high-U granite at Stripa, Sweden (Andrews et al., 1986) and in groundwaters close to U deposits in the Australian Northern Territory (Fabryka-Martin, 1987). Once again, the effect on groundwater depends on the release rate of chloride from within grains where 36C] is produced. The presence of saline water within a rock matrix increases the 36C1 production through capture of neutrons in this component which will move with the groundwater. The depth dependence of 36C1 production has been calculated for various rock types and is illustrated in Fig.3 (Fabryka-Martin, 1988).

Distribution

mechanisms

The distribution of 36C1 in the environment is affected by a number of processes (Fig.5), viz.: -Radioactive decay or in-situ production leading to changes in the 36C1 content of sample material; - - Addition of chloride from the local environment, ranging from dead chloride (no 36C1) or airborne sea salt with very low 36C1 content to high 36Cl material such as salt affected by anthropogenic production; - - Recycling of salt and dust from salt lakes, dry land, irrigation areas, etc.; - - Removal of water by evaporation or t r a n s p i -

ration (dilution by pure water is most unlikely to occur in the natural environment); - - The mixing of waters having different isotope ratios; a n d - - Ion exchange or filtration which may change isotope ratios. If one or two processes are dominant in a particular environment, these may be identified through their different effects on total chloride a n d the isotope ratio. Figure 5 shows the direction of movement between points for samples which a r e affected by the processes used on labels for the arrows. Any of the four types of plot is useful for identifying processes such as 36C1 decay or in-situ production. However, the plots in Fig.5b, d will involve non-linear sequences for the mixing of water bodies (shown at the upper left of each plot) a n d this is a situation which is frequently encountered. If more than one process is important then a vector addition of contributions from each one will describe the change between samples. For example, decay is always present but will only change the direction significantly over time-scales of the order of the 36C1 half-life (3.01 x 105 a). Other data (such as 14C, 180/O or 2D/H) are also very useful for identifying some of the processes which affect chloride content and ratio. In a closed system, radioactive decay should be the dominant process affecting 36C1 levels on long time-scales, but because of the long half-life, changes in the past affecting 36C1 concentrations (for example, variations in cosmic-ray flux, changes in climate and sea-level and variations in atmospheric mixing) could affect the accuracy of radiochlorine ages (Fabryka-Martin, 1987).

Mix/ng In the simplest situation, such as may apply to recharge groundwater or surface soil, the local 36C1 concentration will be dominated by atmospheric precipitation. At the other extreme, a Grand Uniform Mixing (GUM) model may be postulated which assumes that all the 36C1 from atmospheric, surface and anthropogenic production (plus any underground 36C1 which is released by groundwater discharge, volcanic action, etc.) is mixed with the free chloride content of a

CHLORINE-36

PRODUCTION

AND DISTRIBUTION

305

IN A U S T R A L I A

36ci PRODUCTION

"-

~' SORPT,ON

F~ ~J

/*x~J, ~ ,

DILUTIONIPure water

36CI DECAY

F,tTRAT,ON ION

3601 DECAY

CI (mg/L}

CI (mg/L)

i

Ic.

36CI PRODUCTION

,...,oe".,.~_ • ~././~Q~,O

.9 ~5 FILTRATION~' [

P e~

EVAP

O"

SORPTION DILUTION(Pure water)

~CI DECAY I/CI (mg/L)

)EVAPO".,ON 3eCI DECAY 3601 (atoms/L)

Fig.5. The effect of various processes on 36C1 and CI levels can be illustrated by plotting different parameters: a. 36C1versus C1; b. 36C1/C1 versus C1; c. 36C1/C1versus I/CI; d. 36C1/C1 versus 36C1.

large area such as a complete drainage basin, to a depth accessible to recycling by pluvial and aeolian processes. Loss by deeper seepage, river flow and long distance dust transport must also be taken into account but will not affect the 36C1/ CI ratios. The observed ratio will be dependent on the amount of marine chloride introduced, the level ofin-situ production of 36C1 and the residence time of chloride near the surface. There are many parameters involved in such a calculation most of which are amenable to separate experimental determination. As a guide to the development of such a model, it is worthwhile to assemble all information already available o n 36C1 levels in the Australian environment (see the following section). A sampling program has also been initiated with the aim of quantifying atmospheric precipitation rates, the variation of 36C1/C1 ratios as a function of distance from the coast and the significance of other production processes.

The Australian hemisphere A considerable number of 36C1/C1 measurements have now been made for samples from various parts of the Australian environment. These show major differences in the processes that are dominant in specific studies.

Meteoric deposition Davie et al. (1989) have combined the predicted latitude dependence of atmospheric 36C1 production with literature values of total chloride precipitation.to obtain estimates of 36C1/C1 ratios expected in southeastern Australia. These agree well with measurements for recharge water in the Mallee area of the Murray-Darling Basin (about 20 × 10 15) __ reflecting the proximity to the South Australian coast as a source of marine chloride.

306

Recharge water in the upper reaches of the Lachlan River region shows ratios from 140 to 170 x 10 -15 (Bird et al., 1989) which are somewhat higher than those calculated (80-90 x I0-is, Davie et al., 1989). Likewise, ratios of 70-120 x 10 -is, observed in the recharge area of the great Artesian Basin (Bentley et al., 1986b), are higher than expected on the basis of the one calculated value (50 x 10 -15) available for this region. More information is clearly needed on chloride and 36C1 precipitation rates as a function of latitude and distance from the coast in order to be able to satisfactorily interpret groundwater measurements. Great Artesian Bas& Groundwater enters the Great Artesian Basin along the coastal ranges in northeast Australia and emerges near Lake Eyre in central Australia. Observed 36C1/C1 ratios provide the best example of dating in a confined aquifer allowing estimation of groundwater ages of > 1 Ma in agreement with calculations from hydraulic models of water flow (Bentley et al., 1986b). However, after four halflives 36C1 levels are approaching the present sensitivity of the AMS technique and a level at which natural fluctuations may be very important. There are also departures from the systematic decay curve which indicate that the addition of chloride is important in some parts of the aquifer. Murray Basin-Mallee region The C1 isotope ratios for groundwaters in the Mallee region of the Murray-Darling Basin (southeast Australia) are relatively low (20-40 x 10-15). This is associated with an increase along the flow line in chloride content by a factor of 8 or more suggesting the percolation of rainwater downwards at several places within the region (Davie et al., 1989). Additional variations in isotope ratio are associated with discharge zones. These results illustrate the use of 36C1 as a tracer to identify chlorides of different origins. Lachlan Fan A small number of measurements on groundwaters from the Lachlan river region show falling

J.R. BIRD ET AL.

isotope ratios (170-80 x 10 -15) associated with a major increase in CI content down flow. Most of the samples have the higher ratios and lie on a straight line through the origin on a plot of 36C1 versus C1 values. This is characteristic of evapotranspiration or addition of constant-ratio chloride (Bird et al., 1989). Inland Australia Relatively low ratios (30-60 x 10 -15) are observed in chloride from salt lakes and playas almost independent of location in central and western Australia (Fifield et al., 1987). Because of the absence of major mountain ranges to concentrate precipitation, equal 36C1/C1 ratio contours might be expected to be more widely spaced than in the western USA. Low ratios can be expected in tropical Australia because of the very high rainfall carrying marine chloride. However, for large areas of inland Australia, rainfall is very low and chloride fallout is often dominated by recirculated material. One report of chloride precipitation near Alice Springs gives a predicted ratio of 110 x 10-15 but much of the chloride is recycled solids from the surroundings (Hutton, 1983) which would make the ratio for meteoric fallout higher and certainly not as low as the results for salt lakes. Several factors may operate to affect the isotope ratio of surface chloride: (a) much of the long-term average rainfall comes from intense tropical storms which sweep across the inland from the north and northwest (or occasionally from other directions) bringing a predominantly low-ratio chloride to inland precipitation; (b) large-scale dust storms redistribute chloride from surface layers, including salt lakes and make it impossible to distinguish dry precipitation of cosmogenic 36C1from recycled chloride; most chloride travels relatively short distances but occasional events move massive quantities of solids over large distances; (c) massive floods, though they occur infrequently, will wash surface chloride of whatever origin back to the salt lakes and playas. This is an ideal scenario for application of the GUM model to explain the relatively uniform

CHLORINE-36 PRODUCTION AND DISTRIBUTION IN AUSTRALIA

ratios observed. The low values indicate that either more marine chloride or dead chloride from inland rock formations is input than indicated from the one available measurement or that the residence time of surface chloride is greater than 0.3-0.6 Ma. The latter possibility could have involved geological and climatic changes which influenced the C1 and 36C1/C1 values.

Uranium Province Ratios up to ~500 x 10 -15 are observed in the neighbourhood of Northern Territory uranium deposits showing the expected increase caused by localised neutron fluxes (Fabryka-Martin, 1987).

Bomb spike Some evidence has been obtained for bomb spike 36C1, in soil profiles with a 36C1/C1 ratio peaking at 350 x 10-15 at a depth of 1-2 m. At greater depths, ratios drop to 40 x 10-15 or less (Fifield et al., 1987).

Anthropogenic production 36C1 produced by neutron absorption is also observed with isotope ratios up to 10 -11 in the vicinity of the HIFAR reactor at Lucas Heights but not as an environmental hazard. High ratios are observed for samples from Maralinga nuclear weapons sites in South Australia where very localised values up to 10 -11 also occur.

Acknowledgements We wish to acknowledge many valuable discussions with G. Allison, G. Calf, R. Evans, R. Habermehl and J. Kellett during the development of the ANU/ANSTO 36CI measurement program and J. Fabryka-Martin for access to the information used for Table 1 and Fig.3.

References Andrews, J. N., Fontes, J.-C., Michelot, J.-L. and Elmore, D., 1986. In-situ neutron flux. 36C1 production and groundwater evolution in crystalline rocks at Stripa, Sweden. Earth Planet. Sci. Lett., 77: 49-58.

307 Bentley, H. W., Phillips, F. M. and Davis, S. N., 1986a. Chlorine-36 in the terrestrial environment. In: P. Fritz and J. C. Fontes(Editors), Handbook of Environmental Isotope Geochemistry. Elsevier, Amsterdam, 2, pp. 427-480. Bentley, H.W., Phillips, F.M., Davis, S.N., Habermehl, M.A., Airey, P. L., Calf, G.E., Elmore, D., Gove, H.E. and Torgersen, T., 1986b. Chlorine-36 dating of very old groundwater 1. The Great Artesian Basin, Australia. Water Resour. Res., 22: 1991-2001. Bird, J. R., Calf, G.E., Davie, R.F., Fifield, L.K., Ophel, T. R., Evans, W. R., Kellett, J.R. and Habermehl, M.A., 1989. The role of 36C1 and 14C measurements in Australian groundwater studies. Radiocarbon, 31:877-883. Davie, R. F., Kellett, J. R., Fifield, L. K., Evans, W. R., Calf, G.E., Bird, J.R., Topham, S. and Ophel, T.R., 1989. *Chlorine-36 measurements in the Murray Basin: preliminary results from the Victorian and South Australian Mallee region. BMR J. Aust. Geol. Geophys., 11: 261-272. Elmore, D., Conard, N. J., Kubik, P. W., Gove, H. E., Wahlen, M., Beer, J. and Suter, M., 1987. 36C1 and 1°Be profiles in Greenland ice: dating and production rate variations. Nucl.Instrum. Methods, B29: 207-210. Fabryka-Martin, J., 1988. Production of radionuclides in the Earth and their hydrogeologic significance with emphasis on chlorine-36 and iodine-129. Thesis. Univ. Arizona. Fabryka-Martin, J., Davis, S. N. and Elmore, D., 1987. Applications of 129I and 36C1 in hydrology. Nucl. Instrum. Methods, B29:361 371. Feige, Y., Oltman, B.G. and Kastner, J., 1968. Production rates of neutrons soils due to natural radioactivity. J, Geophys. Res., 73: 3135-3142. Fifield, L.K., Ophel, T. R., Bird, J. R., Calf, G. E., Allison, G. B. and Chivas, A. R., 1987. The chlorine-36 measurement program at the Australian National University. Nucl. Instrum. Methods, B29:114-119. Hutton, J.T., 1983. Soluble ions in rainwater collected near Alice Springs N.T., and their relation to locally derived atmospheric dust. R. Soc. South Aust., 107: 138. Lal, D. and Peters, B., 1967. Cosmic ray produced radioactivity on the Earth.In: Handbuch der Physik, 46: 551-612. Leavy, B.D., Phillips, F. M., Elmore, D., Kubik, P.W. and Gladney, E., 1987. Measurement of cosmogenic 36CI/Cl in young volcanic rocks: an application of accelerator mass spectrometry in geochronology. Nucl. Instrum. Methods, B29: 246-250. Nishiizumi, K., Elmore, D., Ma, X. Z. and Arnold, J. R., 1984. 1°Be and 36C1 depth profiles in an Apollo 15 drill core. Earth Planet. Sci. Lett., 70: 157-163. Phillips, F.M., Mattick, J. L., Duval, T. A., Elmore, D. and Kubik, P.W., 1988.Chlorine 36 and tritium from nuclear weapons fallout as tracers for long-term liquid and vapor movement in desert soils. Water Resour. Res., 24: 18771891. Suter, M., Beer, J., Bonani, G., Hofmann, H.J., Michel, D., Oeschger, H., Synal, H. A. and Wolfli, W., 1987.36C1 studies at the ETH/SIN-AMS Facility. Nucl. Instrum. Methods, B29:211-215. Yokoyama, Y., Reyss, J.-L. and Guichard, F., 1977. Production of radionuclides by cosmic-rays at mountain altitudes. Earth Planet. Sci. Lett., 36: 44-50.