Fractionation and recycling of U and Th isotopes in a semi-arid endoreic depression of central Syria

0016-7037/90/%3.00t .0+-J

Geochrmica ef Cosmochunica Acla Vol. 54. pp. 1025-1035 Copyright 0 1990 Pergamon Press Printed in U.S.A.

plc.

Fractionation and recycling of U and Th isotopes in a semi-arid endoreic depression of central Syria BASSAM GHALEB,“~ CLAUDE HILLAIRE-MARCEL,~CHRISTIANECAUSSE,~ CLEMENT GARIEPY,~ and SYLVAIN VALLIERES~ ‘Atomic Energy Commission of Syria, P.O. Box 609 I. Damascus, Syria %EOTOP, Universit6 du Qu&ec 1 Mont&al, P.O. Box 8888, Sta. “A”, Montr~al~ Canada H3C 3P8 3Laboratoire de C%ologie du Quatemaire, C.N.R.S., case 907. 13288 Marseille-Luminy, CEDEX 9, France (Received April 25, 1989; accepted in revisedjbrm January 12, 1990)

Abstract-Mesozoic posits are exposed

and Cenozoic carbonate-rich in a structural

depression

formations including Late Cretaceous phosphate dewhich is known as the Palmyra aulacogen. The bottom of

this depression has been occasionally filled by water bodies during Quaterna~ humid episodes. The bottom of the depression now forms a sabkha as a consequence of modern semi-arid climatic conditions. Isotopic activity ratios and concentrations of U and Th were measured in bedrock formations, groundwaters, unsaturated soils, and in Quaternary lacustrine sediments cored in the sabkha in order to investigate the processes governing the fractionation and recycling of U and Th within the endoreic depression. The bedrock samples are close to secular equilibrium with the exception of phosphor&es which show a 234U deficit (234U/23*U= 0.83 + 0.04). Upper soil horizons have a strong excess of 230Th (230Th/234U = 1.7 f 0.2) as a consequence of U removal. The U loss occurs without fractionation (234U/2”8U= 1.03 + 0.0 1) as it is co-dissolved with the CaC03 matrix; this is also shown by an inverse relationship between CaC03 vs 232Thin soils Vertical profiles indicate transitional retention of U in the lower unsaturated soil horizons, where deficiency in 230Th (230Th/234Uas low as 0.37 + 0.02) and high U concentrations (up to ca. 100 fig. g-‘) characterize “secondary” co-precipitation of U and CaC03 which forms in response to downward decreasing pC0, during water percolation events. A rate of U removal of ca. 10m4to lo-’ a-’ from the upper soil horizons and a residence time of approximately 40 to 50 ka of soil particles in the U-ablation zone were calculated using a simplified mathematical model based on the assumption that z3aTh remains immobile in the soils. The residence time of U corresponds to a rate of erosion of ea. 1 cm for lo3 a. The activity ratios of the most recent lacustrine sediments of the sabkha (upper 2 m in cores) define a well correlated (r > 0.97) “dilution curve” of the form: [230Th/234U = ( 1.9 1 t 0.03) (234U/232Th)-1 + 0. I5 a 0.011. This indicates that the radioisotopes inherited with the detrital fraction were diluted by an authigeniclearly diagenetic phase of U fixation. A residence time for the diluting U (ca. 18,000 a) can be evaluated from the asymptote vaiue (0. I5 + 0.0 1). This age corresponds to the last humid episode recorded in central Syria (the equivalent of the last glacial maximum at higher latitudes) which was associated with lacustrine sedimentation in the basin. Detrital particles eroded from the surrounding soils were then characterized by 238U/232Thand 230Th/23sUratios lower than those observed in modem upper soil horizons: i.e., by a longer residence time and a higher U-leaching rate during their pedologic cycle.

BEYOND THEIR INTRINSICCHRONOLOGICALinterest (CHERDYNTSEV et al., 1955; cf. IVANOVICH and HARMON, 1982), radioactive disequilibria in the 238U and 232Th decay series

may be used as natural tracers of geochemical processes occurring in continental environments. For example. weathering and pedologic processes (e.g., ROSEIOLTet al., 1964, 1966; HANSEN and STOUT, 1968; SZABO and ROSHOLT, 1982;

LOWSONand SHORT, 1986) are responsible for disequilibria between U and/or Th isotopes in both the liquid and residual solid phases. These disequilibria have been used to monitor mixing and residence times of groundwate~ (CHALOVet al., 1964; OSMONDet al., 1968, 1974; KAUFMAN, 1969; OSMOND and COWART, 1976; SZABO, 1982), to assess erosion rates (MOREIRA-NORDEMANN, 1980; MICHEL, 1984), and potentially to date paleosoils or pedogenic precipitations (ROSHOLT, 1976, 1980; Ku et al., 1979; KNAUSS and Ku, 1980; BISHOFF et al., 1981: ROSHOLTet al., 1985). However, the recycling of U and Th isotopes in lacustrine sediments is still poorly documented, with the exception of authigenic carbonate sed-

iments (see examples in SCHWARCZ,1982). Similarly, little attention has been given to fractionation processes occurring in arid or semi-arid environments, especially in carbonate rich areas where groundwater mineralization processes, and therefore U and Th behaviour in unsaturated soils, are strongly dependent on carbonate equilibria between the gas, liquid, and solid phases (DEVER, 1985). Fu~he~ore, the role of the drastic late Quaternary climatic changes on the U and Th budgets, notably in soils, needs to be clarified. An evaluation of U and/or Th concentrations and isotopic compositions in bedrock, groundwaters, soils, and Pleistocene lacustrine sediments was undertaken in the Palmyra basin, an endoreic depression of central Syria (Fig. 1). This site offers several advantages with respect to investigations on Th and U isotope fractionation and recycling processes: excellent exposure of the carbonate bedrock; high potential sources of U in abundant phosphate deposits; unsaturated soils related to modern semi-arid conditions; thick late Quaternary sedimentary deposits at the bottom of the depression; no outlet for solid particles with the exception of wind erosion (which is not expected to induce fractionation between U and Th 1025

come from the deep groundwaters: nnxlng of’“shallo\\ il+utcr willcr.% with brines has been proposed at the margin oftiw whkha ((if;k.;i il.

i ‘)SSI. MATERIALS

LEGEND

__,:Wadls

lA& FI
AL SAWANA

MINE

.

Soil

c)

Surface

samples

+

Dr,ll

A

Bedrock

.

water

samples

holes

05.

samples samples

,_u

**

I. Location map, structural units. and blow-up of the sampled

area

isotopes); and, last but not least, eficient logistical support provided by the Atomic Energy Commission of Syria. GEOLOGICAL

SETTING AND CLIMATIC OF THE PALMYRA AREA

CONDITIONS

The Palmyra basin. also known as the Al Mouh basin (Fig. 1). is a structural depression limited to the north and east by the relief of Paimyra auiacogen (PONIKAROV, 1966) of Miocene age. Sedimentary rocks largely ranging from the Triassic (in anticiinai cores) to the Paieogene are exposed. Cretaceous and Early Paleogene marine marls and limestones are particularly abundant in the lower part of the sequence and contain phosphorite deposits, some of which are mined. notably those of Campanian age at Al Sawana (Fig. I ). Detritai deposits dominate the upper part of the sequence and indicate that a transition from marine to freshwater conditions occurred during thv evolution of the Paimyra structure. The central part of the depression is presently occupied by a sabkha of some 160 km2 which is covered by a ca. 1 m thick evaporitic crust of gypsum and halite. Seismic surveys and drilling (open file reports in Russian language from the Syrian Hydrogeologicai Services) show that this crust overlies at least 200 m of unconsolidated deposits of Neogene to Pleistocene age. The Late Quaternar) portion of these sediments consists essentially of lacustrine silty-clays with abundant gypsum crystals (GHALEB, 1988). Dry alluvial channels and fans (wadis) with coarse to medium grained detrital deposits are located around the margin of the sabkha. Meteorological parameters have been recorded for I5 years at three stations located in the Palmyra area (KHOURI and TAIEB. 1983). Annual air temperature typically averages 20°C with a i-30”<’ winter/ summer shift. Rain essentially occurs between November and May and amounts to less than 140 mm per year. The relative air humidit! averages 50% at the Palmyra station. The area can be classified as semi-arid: the soils remain unsaturated almost all year long. Fresh groundwater resources are scarce. They are restricted to shallow aquifers in wadi channels. notably those recharged northwestwardly in the Palmyra Mountains. A few wells. some tens of meters deep, are exploited; some of them were dug by the Crusaders. A small number of springs located along major fault structures and including the well-known sulfur-water of Palmyra, tap deep “aged” groundwaters which originate to the west in the Anti-Lebanon coastal mountains (KHOURI and DROUBY, 198 I : GHALEB, 1988). The highly evaporated interstitial brines found in the sabkha sediments essentially

AND

RIE:~i’HODS

I he major objective of this study was to investigate the fractionation and recycling of U and Th isotopes from the catchment area to the bottom of the depression: sampling was therefore focused on soils and late Quaternary lacustrine sediments (Fig. I). Upper soil horizons were sampled at depths of 5 to 10 cm below the surfact! at 8 different sites around the sabkha, particularly in the area betwee:) the sabkha and the Al Sawana phosphate mine. Four soil profiles, most ofthem located close to the mine, were also studied: two were sub-sampled at ca. 25 cm intervals down to a depth of ca. I m; for ant’ of them, two sets of samples were analyzed. one corresponding to aiiquots of the total matrix and the second to the sub 120 brn fraction. (‘ontroi samples were collected in the lower soil horizons of the other two soil profiles (Fig. 1). The uppermost iacustrine sediments, ret&red to here as “surface sediments”, were cored in the sabkha to depths of I to 2 meters at places where the salt crust was thinner. and samples for analyses were extracted at IO or 20 cm intervals. Finail>. t!\o holes containing a continuous core of late Quaternary sediments down to 6 1 m and 35 m. respectively, wcrc drilled in the sabkha (Fig. I ). Both these cores were sub-sampled at IO m intervais: these samples will be referred to as “deep sediments”. Since the measurement of uranium concentration drrd isotope actlcny ratios in groundwaters requires relatively largr samples (ca 50 II. only four sites were found adequate (Fig. I ). rhcse were from a pumping station tapping a deep aquifer at the Al Sawana Mine 1#8: (a. 150 m). a deep hole SW of Palmyra (#50). the interstitial hrines of the sabkha (#F-2). and a brackish water well located at the transition between alluvial fan freshwaters and sabkha brines (tF64). It was not possible to filter the water samples nor to extract uranium i/l \iro on activated charcoal (e.g.. N~LIYE~. 1973). The samples were simply evaporated in large plastic containers thus the residllai soitds max. unfortunately contain traces of suspended matters. The bedrock was sampled at sites representative ofthe mobt abundant units. particularly of the Paleogenc marls and limestones (Fig. I ). while an uranium-rich phosphorite was collected at the Al Sauana mi nc.

Samples were treated using the routine laborator) procedures of GEOTOP (see CA~JSSE and HIL.L.AIRE-MARCEL. I989 for details). l‘hey were firstly dissolved in aquu rcgiti and then in a mixture of HCIO, and HF. to which a known mixture of a “XTh-‘34Th-“‘Ii spihe was added. After precipitation with ammonium hydroxide, II and Th were separated and purified using a combination of tonexchange resins and solvent-extraction techniques (BR(XCK~R. 1963: HII.LAIRE-MAKCEI. and CAUSSE, 1989). Finally. Th and Ii were electl-odeposited on separate steel disks and counted in fi- and/or cu-specuometers. The double Th-spike. which contains an (Y-and a e-emitter permits an evaluation of potential disequilibria in the ‘?‘Th decay series. due to “‘Ra migration. On a routine basis, the chemical yields for Th and U extractions respectively average 5O’“r and 35% in ciaqtype material (Fig. 2). Spectral resolution is usuali!, in the order of 05 KeV. RESULTS

The concentrations and activity ratios of U and Th determined for samples of bedrock. groundwaters. soil horizons. surface and deep sabkha sediments are listed in Tables i-5. with their respective I g errors, and graphically presented in Figs. 2 to 8. As shown on Fig. 2, significant departures from equilibrium conditions in the 232Th decay series are observed in most samples. particularly those from unconsolidated deposits (soils and sabkha sediments). Excesses of ““Th are

IO27

U-series isotopes in lacustrine carbonates

The Paleogene marls and limestones show a negative linear relationship between their CaC03 content and their 232Th or 23xU concentrations (Table 1b). The carbonate fraction is apparently acting as a dilutant of U and Th which are most likely associated with the detrital fraction. Uranium is slightly mobile in the carbonate-rich (U- and Th-poor) samples. as indicated by statistically significant excesses of 230Th over 234U in sample #C, and of 234U relative to 238U in sample #D (Table lb). These departures from secular equilibrium conditions may be accounted for by occasional water circulation in the highly fractured bedrock.

1

Groundwaters FIG. 2. Correlation of chemical yields and counting efficiencies with 228Thand 234Thspikes for Th isotope measurements.

systematically observed in Quaternary sediments cored in the sabkha at depth ranging from 5 to 50 m. In contrast, deficiencies in this isotope are seen in most surface samples of the sabkha and also in a few soil samples. These ***Th/ 232Th disequilibria are due to sub-actual ***Ra fluxes (see BRUNSKILL and WILKINSON, 1987). They will be discussed later on. Bedrock As expected (e.g., SLANSKY, 1980), the phosphate sample shows the highest U concentration (ca. 1400 pg/g; Table 1a) and a significant 234U/238U disequilibrium (0.83 ? 0.04) indicating relative enrichment of 23RUin the apatite lattice due to the preferential loss of 234U (KOLODNY and KAPLAN, 1970: BURNETT and VEEH, 1977). Similarly, the 230Th/234U activity ratio (0.86 f 0.10) may indicate small changes in the uranium fluxes at the outcrop surface (related to changes in the redox potential ?). However, the large uncertainty (1 u - 10%) of this ratio makes further comments irrelevant.

Table

la.

M~IIA e

E

I

WT 398

Concentrations

*=Th

23sU

and

actkity

=‘U ----238U

238U

2WU

23”Th

23’?h

22q,,

22q,,

22q,,

22,,,

164 f24

128 f20

111 f13

0.00 m.10

rDl9

MS9

27.43 f3.78

1395 flO6

0.83 f0.04

at the

1 (I level

Errors are quoted

ratios In the phosphorlte.

The U concentrations in deep groundwaters range from ca. 1 to 4 rig/g (Table 2). The maximum value is observed in sample #F-2 which was taken from the highly evaporated interstitial sabkha brines. This sample, together with #50 and #8, has a significant excess of 234U over 13*U (- 1.5: Table 2). Their enrichment in the lighter isotope is due to its preferential extraction from solid phases relative to that of ‘18U (FLEISCHER and RABBE, 1978). Because the deep groundwaters originate from distant recharge areas (the Anti-Lebanon Mountains to the west; KOURI and DROUBI, 198 I), the 234U excesses observed in the corresponding water samples cannot be related to geochemical processes occurring in the sabkha area. However, it is noteworthy that sample #64. taken from a well at the transition between an alluvial fan and the sabkha (Fig. I). has a 234U/238U activity ratio lower than that of the brines (Table 2). The “‘C and stable isotope characteristics of this sample are indicative of mixing between brines and freshwater from the alluvial fan aquifer (GHALEB, 1988). This points towards lower 234U excesses in waters from alluvial aquifers. Soils With the exception of sample #20. all upper soil horizons show an inverse relationship between their CaC03 content and 232Th concentration (Table 3a; Fig. 3a). Thorium is, like the rare earth elements, practically immobile during exogenous processes (MCLENNAN et al.. 1980: Fig. 3b). Its concentration in the bedrock indicates the relative dilution of the clay fraction by marine carbonates: conversely in soils, the relative enrichment in clay minerals may be attributed to CaC03 dissolution. As shown by a relatively uniform 230Th/ 234U activity ratio of ca. 1.7 -t 0.2 (Fig. 4a), the upper soil

of confidence. Table

Table

lb.

hkp

Leb

s

s

Concentrations

5 aclivlty

ratios

of U 8 Th

2. Concentrations

238U

2yU -----

23sU

2yU

23oTh

2=Th

oac03

M I

23qh

232Th

m I

23sU

“Y9

nw9

IWO

W’O

238U

232Th

2=Th

232Th

mIJ

Wr%

50

1.23 f0.12 1.00 f0.04 0.19 fO.O1 0.02 .x’.l.Ol

3.87 f0.08 1.78 f0.04 0.89 fO.01 0.47 fO.01

1.00 fO.O1 0.99 f0.02 1.08 f0.02 1.13 f0.03

9.55 *O.Q2 5.37 f0.23 14.57 il.17 91.04 f14.82

9.50 f0.92 5.34 f0.23 15.45 f1.24 102.8 f16.5

10.23 fO.60 5.14 f0.17 19.85 f1.52 102.0 it&l

1.08 jzO.09 0.96 fo.03 1.27 fo.04 0.99 MO.04

59.5

WT 580 WT 581 WT 582 WT 583

0.14 fO.01 0.08 Ko.01 0.05 4.01 0.04 .SO.Ol

1.22 f0.02 0.50 fO.O1 4.45 f0.00 1.74 f0.03

8 A 0 C D

UOT 703 w-f 704 UOT 708 WT 708

6 activity

ratios

of U (L Th

In groundwater.

In the bedrock.

F-2 77.0 84 78.4 94.8

l

Values

corrected

for

a deWHal

232Th, aasumlng a 238U1232Th from bedrock results.

23411 23sU

1.38 f0.02 1.37 f0.02 1.48 fO.01 1.29 f0.02 uranlum acllvity

23.5~.

234~.

n919

W

0.82 f0.02 0.33 fO.O1 4.31 f0.00 1.82 f0.03

1.58 f0.02 1.57 to.01 1.48 fO.O1 1.31 f0.03

content ratio

evaluated of 7

from

extrapolated

Table 3a. Concsntrailons and mL& "

x

20

UtX 479 WT 461 14 WT 465 106WT 466 16 WT 467 31 WT 515 39 UCiT 516 16

Table

*3qh

**U

role

w9

3.77 fO.Q6 6.64 *l.Ol 4.11 f0.74 2.43 i0.15 4.54 k0.34 4.50 i0.21 5.69 to.17

2.20 i0.05 2.66 iO.07 2.40 f0.06 1.71 iO.03 2.32 to.06 3.20 i0.06 3.26 iO.09

234~1

acllvllyratlos In upper soil horizons. *3qh -*32-rh 239Th

*?h

MXJ3

i3ou

*38U -*=Th

WU

WI%

1.06 i0.03 1.03 iO.03 1.03 iO.03 1.03 iO.02 1.00 f0.02 1.06 f0.02 1.03 i0.03

1.77 i0.46 1.27 i0.10 1.77 f0.32 2.14 f0.14 1.55 f0.12 2.16 fO.11 1.75 f0.07

1.66 i0.49 1.31 iO.20 1.62 f0.33 2.21 i0.14 1.55 i0.12 2.33 f0.12 1.61 iO.07

1.41 i0.37 1.91 kO.26 1.56 M.26 1.60 M.10 1.62 f0.13 1.73 HI.07 1.93 iO.07

60.5

3b. Concentrations and

*au

2.64 fO.10 2.50 iO.09 2.64 i0.13 3.52 i0.00 2.61 iO.ll 4.03 f0.17 3.50 iO.07

37.7 42.7 46.0 43.6 41.3 37.5

activity ratios in goi1profiles.

ProMe x 1 t,+q.~Lab *=Th I 6

x u(x 402 UOT 401 WT 400 WT 399

5 4 3

*=U

rvo

w/9

5.64 i0.24 1.20 iLi.r? 2.47 iO.lQ 1.10 i0.12

4.73 f0.12 5.65 i0.12 10.6 f0.29 102 i4.90

238~ -*32Th

*3"Th

cac03

14C

iseu

*%U

Wt%

Act%

1.00 to.03 1.06 i0.03 1.07 f0.03 1.01 i0.02

2.55 i0.13 14.2 il.56 13.3 il.07 261 i33.Q

1.45 f0.06 0.99 f0.07 0.92 f0.04 0.37 f0.02

234U

Prollle 12
a

*32Th

*3L)U

Iwo

w/9

WT 4.84 lOlli0.34 WT 0.62 1012f0.03 UOT 2.66 1013i0.10 WT 0.94 1014*0.09

4.43 i0.11 5.52 iO.16 15.93 i0.62 13.26 f0.37

*34U -238~ 1.02 iO.01 1.06 iO.O1 1.04 iO.01 1.04 iO.01

Depth cm

53.3

nd

10

72.2

r-d

35

44.4

nd

65

96.6

1.7 iO.2

95

p

*36U

*3@Th

csDo314C

*3qh

*34u

WI%

Act%

2.72 i0.20 26.7 il.7 16.0 iO.l 42.5 i4.3

1.20 fO.06 0.64 i0.03 0.46 i0.02 0.63 i0.05

nd

nd

10

nd

1.4 iO.1 1.2 ko.3 3.5 f0.5

36

nd nd

D+h cm

have a near secular equilibrium acttvity ratlo 01’ 1.03 :I cl.0I (Fig. 4b). Because a loss in U is demonstrated bv the presence of unsupported ‘30Th, it is concluded that the dissolution process did not allow preferential departure oi’ ‘lJ(J vs. “‘1 :. The very oxidizing conditions in the unsaturated soils and the co-dissolution of U and CaCC& may both account for identical loss rates of 234U and “‘II. Additional constraints on the mechanism of C dissolutton in these soils are provided by the study of sori profiles (Fig. 5a and b). When compared to the upper horizons, the lower soil horizons present much higher contents in I;, up to 100 Fg/g near the Al Sawana mine, and an inverse 230Th/2341~ disequilibrium (with an activity ratio as low as 0.37 i 0.02. at the same site). The high II concentrations and the dehciencies in 230Th (vs 234U) in lower soil horizons match the U depletions and 23”Th excesses of the upper horizons (Fig. 5a). This suggests that a vertical transfer of li takes place from the upper to the lower soil horizons. All control samples collected at depths greater than 50 cm below the surface in soil profiles (Table 3b) similarly show 230Th/L1JU activity ratios lower than unity and relatively high U contents: i.e., evidences of U uptake. The 2?J/23RU activity ratios remain close to secular equilibrium when proceeding downwards along soil profiles, (Fig. 5a and b), indicating that the transfer process did not fractionate the U isotopes dunng uptake and reprecipitation. As seen on Figure 5b, U fluxes from the upper to the lower soil horizon essentially concern the fine fraction of the soil matrix. Moderate U losses and U uptakes are observed in the coarse fraction. which shows ‘30Th/23411 activity ratios

75 100

65

Profile # 2 total matrix h+@ Lab

*32Th

238~~

NV9

lwl

zqj

*36U -239Th

1.21

2.52

1.06

6.30

1.04

fO.06 2.66

iO.03 0.99

i0.40 20.73

1026f0.03 Ux 1.64 1027iO.09 UM 0.75

iO.07 6.33 fO.10 7.79

i0.03 1.05 fO.O1 1.03

il.6 11.74 i0.71 31.5

1026f0.10

i0.12

i0.02

t4.3

to.04 0.97 f0.05 0.66 f0.04 1.10 f0.1

I

*

33

UYr

34

1025i0.07 WT 0.42

35 36

*w

Depth

*3qh

cac0314C

*%lJ

Wt%

Act%

nd

nd

10

nd

1.4 iO.l 1.2 io.3 3.5 Kl.5

35

nd nd

cm

75 100

40. I/ 35.

U-enrichedsoilhorizonsfrom pmfiles3.3 4 Mp x 40 25 26

L;t, 232Th x

w9

u(x 2.16 517 f0.12 WT 3.06 1009i0.1 WT 3.45 lOlOf0.13

nd-not

238U W/B 4.06 f0.12 4.44 f0.32 6.02 iO.16

234U --236~ 1.05 f0.03 1.05 f0.04 1.06 iO.01

238u

23oTh

(ra33

14C

*32Th

234~

wt%

k3-s

5.72 i0.06 4.37 f0.37 5.26 i0.27

0.75 iO.04 0.60 f0.05 0.67 iO.03

46.4

5.6 iO.2 2.9 f0.3 r!d

nd nd

Depth cm 50 50 75

determlned.

horizons are characterized by significant excesses of 23@Th, which result from U loss. In contrast to what is usually observed in soil samples (e.g., ROSHOLT, 1982), the Palmyra soils do not show any deficit in 234U relative to 238U: they

FIG.3. (a) 2’2Th concentration vs CaCO, content in soil samples and (b) 232Thand La concentrations in al4 samples.

U-series isotopes in lacustrine carbonates

1029

to the 230Th/234U time scale, most samples are close to secular equilibrium or show a slight depletion in uranium (Table 5). However, the samples located between depths of ca. 30 and 50 m show significant U losses on the 274U/2’RU time scale, as indicated by activity ratios lower than the secular equilibrium value (Fig. 8). All samples from the “deep sediments” of the sabkha show unsupported ‘**Th (22sTh/232Th - 1.15: r = 0.98; Fig. 2) indicating flows of interstitial waters recently enriched in 228Ra. DISCUSSION

Soil prcicessw

FIG. 4. Frequency histograms of z3”Th/2’JlJand 23aU/*‘XUactivity ratios: each activity ratio is plotted as a gauss curve with its 30 error bar: (a) and (b) upper soil horizons;(c) surface sediments ofthe sabkha.

Latham and Schwartz ( 1987) developed a model for determining rates of U leaching in weathered crystalline rocks. This model can be simplified here, to account for U dissolution without preferential departure of ‘?J (vs 23xU). The homogeneity of the 2’0Th-excesses (over 234U) in the upper soil horizons (Fig. 4a) points towards a relatively uniform negative flux of U within the basin. This flux can be monitored using steady-state conditions. i.e., a U-dissolution process of constant rate “F” (flux = F X L’). Erosion of solid particles at the soil surface will also be considered negligible. The concentrations of U and Th isotopes will then respond

not significantly different from 1: however. the fine fraction shows large fluctuations in both its U content and 230Th/Z”41J ratio. This difference is simply due to the larger water/matrix reactive surface of the fine particles.

The clayey sediments of the sabkha, sampled down to 2 m below the salt crust, are characterized by a large range of U and Th concentrations and activity ratios (Table 4). A few samples are depleted in U (230Th/2’4U > 1) whereas secondary U uptake is indicated in others by higher 23sU/‘“2Th ratios. 234U/2”RU > 1 and 230Th/234U < I. Secondary U was trapped in these sediments, in addition to the radioisotopes inherited with the detrital particles eroded from the surrounding soils. That this likely occurred in response to low Eh conditions is shown by the high sulfide content of these horizons (GH-ILEB, 1988). This ~‘authigenic” or early-diagenetic (?) U was characterized by a slight excess in 234U over 23RU, as shown by the departure of a few samples from secular equilibrium (Table 4: Fig. 4~). Most of the surface sediment samples are characterized by a disequilibrium between 221(Thand 232Th (activity ratio lower than the unity; Fig. 2) due to modern “‘Ra losses (cf. BRUNWLL and WILKINSON, 1987). This does not imply sub-actual fluxes of U which should more likely remain immobile in view of the redox conditions.

Deplh

238”

(cm)

(Pg/g?ir)

23qJ/238U (*IT1

230~~,23”(J (r1.T)

+

:.

Q

+

i

The Late Quaternary sequenc’c cored in the sabkha The sediments sub-sampied at depth ranging from 5 to 60 m in the two cores show “‘U and ‘?‘Th concentrations not unlike those of the surface sediments (Table 5). With respect

FIG. 5. ThjU changes in two soil profiles from the western part of the Palmyra depression.

Table 4: Comxntratlons 8 actlvlty ratlos In the surface sediments.

___.-.

(1 activities ,which

,“41J changes 72a WT

4.54

12.93 1.13

306 72b IKYT 451 72~ LJDT 459 72d WT 403 96a UOT 461 96b WT 457 06~ WT 453 7Qb WT 397 79c WT 417 1OOaUQT

f0.21 4.16 kO.16 2.60 fO.ll 7.01 f0.27

f0.02 1.07 f0.03 1.05 to.03 1.07 f0.03 1.15 f0.03 1.05 to.03 1.09 i0.04 1.12 f0.03 1.21 i0.05 1.02

6.64 i0.44 3.07 fO.16 4.46 f0.21 2.72 f0.12 7.47 i0.36 4.01 to.22 1.64 iO.ll 1.05 f0.07 4.01 f0.26 1.77

0.76 i0.40 3.29

f0.16 3.55 f0.17 7.56 io.37 3.62 i0.13 1.09 iO.06 5.71

iO.28 4.23 f0.12 4.25 io.ll 6.26 to.14 11.01 i0.36 4.69 io.12 4.60 i0.14 2.33 to.05 1.44 iO.04 3.34

465 lOC4UOT

to.23 6.01

iO.06 3.62

iO.03 1.02

to.06 1.37

420 f0.26 6Oa Ull 4.01

to.06

f0.03

4.33 i0.13 4.51 iO.10 2.70 i0.07 2.60 iO.10 2.64 fO.06 3.17 iO.ll 3.35 fO.ll 2.69 i0.07 3.52 iO.11 3.66 fO.10

1.06 f0.04 1.12 iO.03 1.10 f0.03 1.09 iO.05 1.09 f0.04 0.94 iO.04 1.23 f0.05 1.21 f0.03 1.03 f0.04 1.03 io.03

4.47

466 i0.20 60b lKlT 5.09 421 f0.21 56a UJT 5.02 454 i0.20 66b WT 6.63 452 f0.37 69a KIT 6.33 456 f0.24 69b WT 4.13 462 f0.20 76a WT 4.66 416 f0.21 76b WT 3.06 416 f0.16 71a NIT 6.21 416 f0.31 77a UQT 6.55 419 i0.21

3.16 f0.12 2.40 iO.06 2.54 fO.06 2.31 f0.06 3.23

0.32 ka.01 0.73 iO.03 0.54 i0.02 0.79 No.03 0.36

iO.00 2.46 ztO.06 2.04 i0.05 2.26

iO.02 0.59 Ho.03 1.02 iO.06 1.10

i0.05 3.03 f0.14 2.14

fo.04 0.62 M.03 1.16

26.1

fO.06 1.39

fO.06 1.67

i0.05 1.20

22.7

f0.05

0.05

to.04

fl.05

3.27 f0.10 2.26 iO.00 1.60 iO.06 1.24 fO.06 1.36 iO.06 2.33 i0.14 2.09 f0.12 2.21 fO.10 1.30 fO.06 1.60 f0.07

3.53 iO.20 2.55 fO.10 1.65 fO.00 1.36 iO.00 1.40 iO.07 2.16 f0.13 2.57 io.14 2.66 i0.12 1.34 i0.07 1.64 fO.06

2.44 iO.06 2.20 iO.06 2.00 f0.05 1.71 i0.06 1.02 f0.07 1.93 f0.06 2.22 f0.07 2.33 fO.06 1.65 f0.04 1.66 f0.05

0.60 iO.04 0.66 i0.03 1.06 iQ.05 1.26 ~3.06 1.29 f0.05 0.66 M.05 0.67 10.04 0.67 to.04 1.23

to.17 4.67 f0.22 2.92 f0.13 6.56 io.44 4.21

f0.23 2.00

to.12 2.07 fO.06 4.66 f0.31 1.61

iO.06 1.01 io.04

time according

14.4 15.5 17.7

The daughter equation:

5.5 __

dTh/dt

34.5

product

230Th varies according

(8) to the following

= -(A” 4 P’“)Th + X’i”

Since the solubility

-(A"

t

F”)7h

of Th is considerably

i XC’. (9)

lower than that

10.0 Table 5: Concentrations and activityratios In the deep

17.6 41.0

Hole'

46.5

.

Ld, s

=Th

23sU

w3

!Jglg

cares.

w _----

23sU

234U

23cTh

2=ThCX03

23sU

23qh

232Th

z3qh

234U

Wt%

____-

13.6

Fl-02

6.6 f

24.2

+

30.0

Fl-10

rela-

(2)

dTh/dt

(3)

f f Fl-40 f

where: f

CL C” and 7% = the concentrations of 23xU, ?J. and ““Th. respectively, F’. F’, F” = their respective dissolution rates (in a ‘), h. x’. X” = their respective radioactive decay constants (in a-‘). With respect to dfJ, the radioactive decay of 23xU can be neglected: the loss should essentially be due to dissolution (t 9 h). Then: (4)

then:

C’ x L:,c !‘I,

, ?I

12.5

u’[:‘f& = -(p” + h’)[:’

constant,

lo ix.

t A’II ’ ! Ai Jtli

--

(I)

As F was assumed

may then shwn

# I :i I ” 2 L’;,c-’ r ‘: (X/h )I ,l(’

5.5

t X)1‘.

dl’ = -FL’dt.

l il.

trt

As there is no significant departure from secular equilibrium between ?J and *?I in all analyzed soil samples (XL’ -_ XL”). the loss due to 234U decay is constanti) matched b> a gain due to “*U decay, hence

12.5

to the following

= -(I.‘” + x”)Th

to ~hlw

1.~

of time

I/C” = [-(F’

f

dL’/dt = -(F

concentrattonsj

“immobik’

as a function

10.6

Fl-30

through

absolute

5.0

Fl-20

lo l-1 ablation tions:

(or

is considered

f Fl-50 f

Because U concentrations (e.g., in dpm *g. ’ or in fig. g-.‘) may vary without any loss in U but as a consequence of CaC03 dissolution in the soils, it seems preferable to report

6.01

4.46

404

i0.41

iO.11 *0.03

1.09

WT

4.69

2.61

592

XI.14

i0.03 kO.02

1.15 1.16

1.70

1.65

1.04

1.05

to.10

kO.10

kO.07

f0.05

1.69

1.04

2.02

1.04

f0.05

fO.06

to.03

i0.03

WT

5.27

3.16

1.62

2.09

2.03

0.07

594

MO.16

f0.04 to.01

kO.06

i0.07

to.03

to.03

UYT

5.92

5.27

2.70

2.70

2.93

1.09

406

a.29

f0.12 f0.03

fO.15

f0.15

i0.13

io.05

1.00

LJX

4.03

3.66

2.76

2.76

3.06

1.12

406

x).20

fO.06 iO.03

1.00

f0.15

f0.15

f0.13

iO.05

WT

5.60

6.47

3.50

3.26

3.72

1.13

407

iO.30

f0.14 io.02

0.94

kO.20

iO.10

i0.17

M.06

WT

5.60

4.43

2.40

2.29

2.62

1.14

601

i0.14

~0.05 iO.01

0.95

to.07

f0.06

i0.04

i0.03

WT

6.07

4.32

2.16

2.15

2.50

1.16

602

i0.19

to.05 fO.O1

1.00

iO.07

f0.07

f0.04

i0.04

WT

5.79

4.62

2.42

2.29

2.65

1.16

603

fO.16

to.06 iO.01

0.95

20.07

kO.07

f0.05

so.03

WT

6.05

4.57

2.29

2.16

2.56

1.16

406

iO.34

fO.ll f0.03

to.14

f0.13

fO.ll

iO.06

4.62

4.07

4.55

1.12

to.15

f0.12

i0.07

i0.03

0.94

LICIT3.75

5.96

506

kO.10

i0.06 iO.01

0.65

WT

3.41

5.62

4.09

4.47

4.42

0.90

599

iOo.ll *0.06f0.01

fO.16

fO.16

f0.10

to.03

NIT

4.34

5.32

3.71

3.41

3.62

1.06

600

f0.19

f0.07 iO.O1

i0.17

fO.15

i0.07

M.04

0.69 0.92

lKJT 5.50

3.95

2.17

2.20

2.53

1.14

40s

f0.25

fO.10 io.03

1.01

iO.ll

io.11

fO.10

io.05

UJT

4.33

3.59

2.62

2.42

2.53

1.04

505

M.14

f0.04 fO.O1

fO.00

f0.06

f0.04

M.03

f

UJT

4.10

3.37

2.49

2.46

2.76

1.13

f

596 kg.20 tXIT 4.09

f0.04 iO.O1 3.42 0.99

i0.12 2.53

f0.12 2.50

fO.06 2.71

f0.05 1.09

597

iO.16

f0.04f0.01

f0.10

fO.10

f0.04

iO.04

UX

6.36

6.17

2.94

2.64

3.19

1.13

411

i0.76

fO.16 f0.03

f0.36

f0.35

i0.12

~3.13

WT

3.50

3.11

2.60

2.69

2.49

0.66

412

Xl.56

f0.36 i0.16

i0.54

f0.57

iO.ll

i0.17

F2-05 FZ-15 F2-25

(5)

WT

F2-35

0.06 0.00

0.96 1.07

WT

4.32

3.64

2.56

2.47

2.96

1.20

413

itI.

fO.06 i0.03

0.97

i0.16

i0.15

f0.13

to.06

UX

2.30

3.56

4.67

4.76

5.09

1.06

414

kg.15

iO.00 f0.03

f0.33

i0.33

f0.26

iO.06

1.02

* Sample depth (m) Is Indicated by the last two dlglts. f Replicate analyses from the same

level.

16.1

36.1 13.4 44.3

51.0

53.4

52.2 50.4 66.7 63.3

1031

U-series isotopes in lacustrine carbonates

of U, we assume that F” < A”: the loss in 230Th is essentially due to its decay. Then: dTh/dt

x

-X”Th + AU.

Converting

- F)-‘(emF’ - em”“).

(1 I)

234U and

+ @).

(15)

For 230Th, assuming that its solubility is negligible (F” < A”), the differential equation is then: dTh/dt

into activity ratios: By integration,

X”Th/XU x XrTho/XUoe(F-h”)’

between

U N Ub + c#Jtx (X/x’)(U,

(10)

By integrating: Th sz Thee-“” + AU&”

No fractionation should be expected 238U during precipitation. Then:

x

-x”Th

+ X’U’.

(16)

we obtain:

Th Y ThoemX”’+ (XU,/A”)( 1 - em’“‘) - [A”/(F - xII)][l - e@-“‘)‘],

(12) + (Xl$/h”)[(P”‘/X”)

I.e.. R = Roex’ - (Y/x)(1

- @I”),

(13)

where R and R. stand for 230Th/238U activity ratios, respectively, at time, t, and at the origin, and x for the critical factor (F - A”), i.e., the difference between the rate of U removal (assumed constant) and the decay constant of 230Th. Equation ( 13) contains three unknowns. In most geological settings, it is safe to assume that unweathered bedrock will have an age large enough to attain secular equilibrium in the 238U decay series, and therefore R. = 1. Two unknowns thus remain: F and t. This point will be addressed later. An inverse situation (U uptake) is observed proceeding down the soil profile (Fig. 5a and b). This is shown by the strong deficit in 230Th vs its parent isotope 234U (e.g., samples UQT-399, -517, -1012, -1013, -1014, -1009; Table 3). The vertical transfer of U from the upper to the lower horizons can be linked to the peculiar geochemical behaviour, at various latitudes, of unsaturated zones in carbonate-rich soils (DEVER, 1985). During a given rainy event, water percolates through the profile and dissolution of the carbonate matrix occurs (with co-dissolution of unfractionated U as shown above) in response to high partial pressures of CO2 in the A horizon. The pC02 decreases with depth and calcium (+Mg) carbonate can precipitate once again in lower horizons (transition B-/C-horizon, in the present case). Evidence for the occurrence of this mechanism can be found in the 14C activities measured in the carbonate matrix of the lower soil horizons developed here on a bedrock of Cretaceous age (that should normally be free of any measurable 14C content). Activities ranging from 1.2 -t 0.3 to 5.6 rt 0.2% of “modern carbon”, Table 3b) are observed. The secondary carbonate precipitation can induce U uptake as shown by the present example. Since this process is controlled by the hydric conditions and organic activity responsible for pC02 changes in the soil, the positive U fluxes in lower soil horizons will strongly depend on climatic conditions. In spite of this complication, these positive U fluxes characterizing lower soil horizons can also be quantified. The above Eqn. (13) cannot be used. The positive flux of U in lower soil horizons is independent of the quantity of U which is already present, but it depends on what is left in the upper soil horizons. It may also respond to other distinct parameters (e.g., K02 profile, Eh conditions, etc.). Setting 4 as the absolute flux (e.g., in pg.g-’ .a-‘) and neglecting the loss of 238U due to radioactive decay (i.e., assuming that I$ % XU), then, u N u, + c$t.

(14)

Converting Th* e

+ t - (l/x”)].

(17)

into activities:

Th; e@l + U$ ( 1 _ +“‘) + ($*/X”)(X”t + eP

- l),

(18)

where Th*, U*, and 4* stand for activities (in dpm *g-l). In most cases, secular equilibrium can be assumed between The and U. (i.e., in unweathered bedrock). The equation can then be simplified as below: Th* -

uo* + (c$*/x”)(x”t + P

- I ).

(19)

We are still left with three unknowns (U,*, 4*, and t). Using Eqn. (13) we are left with two: U,* and t. With the limited number of measurements made on bedrock samples, it is impossible to set a “regional” value for U$ . However, a reasonable approximation can be made when U $ U,, i.e., by assuming that all the U present originates from 4; such is the case of sample UQT-399 (Table 2) collected at a depth of ca. 95 cm, in the soil profile (Fig. 5a), which shows more than 100 pg *g-’ of U, i.e., very likely two orders of magnitude more than in the bedrock (see Table 1). In this case: - uo*j/t -

4* ^I (u* Th* -

w/t

Th* - c:;

(20) (21)

Then: Th* -

(u*/x”t)(x”t

R = (Th*/U*)

+ P -

- 1)

(I/A”t)(A“t + e+“’ - 1).

(22) (23)

Thus when large amounts of U have been accumulated in such soil horizons, t depends essentially on the 230Th/234U disequilibrium. Then: R-

%X”t

for low values oft

(e.g., t 5 l/x”).

(24)

In the present example, R - 0.37, and t - 80,000 a. For sample UQT-1013 (second soil profile; Fig. 5b), a similar approximation yields a value of ca. 70,000 a for t. These ages give an order of magnitude for the residence time of soil particles in the U precipitation horizon. The lower soil horizons appear therefore to be a transitory reservoir for the U dissolved above which is far from being negligible. As a whole, the soils constitute U-accumulation zones: U concentrations in soils (including the “U-depleted” upper horizons) are higher than those measured in the unaltered bedrock. This transitory accumulation is simply due to the lowering of soils in the topography (essentially through CaC03 dissolution

1032

B. Ghaleb et al.

processes): lower overall rates of co-dissolution of U result in a relative enrichment of soils in this element. When attempting to quantify U fluxes, a few approximations were made and steady state conditions assumed. It seems unlikely that the removal rate of U will remain constant through time, especially in an area which experienced significant climatic changes during the Late Quaternary (KAISER et al., 1973). IfFand 4 are not constant, the above equations are no longer valid. Moreover, the soil particles of this basin have been subjected to a complete geochemical cycle (U fixation/U dissolution) through time, by the means of topographic lowering of the soil profile which induces mass loss by (1) dissolution and (2) mechanical erosion at the surface. The key problem concerns the duration of this cycle: i.e., the residence time of the particles in the weathered profile. The difficulty of setting time constraints lies in the observation that the original term (the unweathered mother rock) cannot be directly related to the U-depleted particles at the soil surface, since an intermediate phase of fixation occurred. This allows U fluxes to vary, for given climatic conditions, from -0 (bedrock) to a maximum positive value (4 max./ lower soil horizons), then to a maximum negative value (corresponding to F max./upper soil horizons). Considering climatic changes and the subsequent grain size variation in soils, the flux is likely to vary through time for any given depth in the soil profile. Nevertheless, orders of magnitude of U-dissolution rates in upper soil horizons and of the residence time of soils can be set from Eqns. (6) and (13) by calculating the U-negative flux “F” and the time “t” needed for any given U-enriched sample in a lower soil horizon to be progressively U depleted until it reaches the U content (and the inverse 230Th/234U disequilibrium state) ofthe corresponding upper soil horizon sample. In the example of the two soil profiles of Fig. 5, calculations of “F” and “t” values, respectively, between samples UQT-399 and -402 and between samples UQT- 10 13 and -1011, yield a similar “t” of ca. 48,000 a and “F” values of -0.98 - 10m4and -0.4 - 10m4a-‘, respectively. It should be emphasized here that these “F” values are minimum (and therefore that the “t” values are maxima). The 230Th/234U disequilibria measured in upper soil samples around the sabkha are generally higher (mean: 1.7 + 0.2) than those observed in samples UQT-402 and -1011 (Fig. 5). The “F” and “c” values that can be calculated to bring the few U-enriched samples of lower soil horizons that were available for this study (Table 3) to the mean U and Th content and isotopic composition of the upper soil horizons, all point towards “F” values ranging from - 1O-4to - lo-’ a-’ and “t” values varying from 40,000 to 50,000 a. In view of the depth of the U-accumulation zone in soil profiles (ca. 50 cm to 1 m?), erosion rates 2 1 cm for lo3 a can be derived.

of a typical dilution curve (Y = UP-’ + b), with 230Th/234U activity ratio (Y) decreasing as a function of incoming U (X = 234U/232Th)(Fig. 6); the high correlation coefficient of this curve (r > 0.97) indicates that the diluting U was deposited during a single phase of precipitation (GOETZ et al., 1989). The “b-asymptote” of the dilution curve corresponds to the 23@Th/234Uratio of the pure diluting phase. Because a large uncertainty exists in the calculation of its value, the residence time of the diluting U can be better determined through usual “isochrone” constructions (ROSHOLT, 1976; see Fig. 7). However, as seen in Fig. 6, all samples with 230Th/234U> 1 fit on the steepest end of the dilution curve as they incorporated very small amounts of “secondary” U; the variability of their 230Th/234Uactivity ratios may be primarily “inherited”, i.e., due to the original spread of 230Th/234Uvalues in the detrital particles eroded from the surrounding soils. Therefore, this group of samples should not be used for the calculation of the 230Th/234Uratio of the diluting U. All other samples beyond the threshold value of 234U/232Th- 1.8 can then be used to plot the *30Th/232Thvs 234U/232Thisochrone (Fig. 7b) yielding precisely the 230Th/234Uratio of the diluting U (the slope of the isochrone), i.e., the “b-asymptote” of the dilution curve Y = (1.9 1 t 0.03)X-’ + (0.15 f 0.0 1). This value combined with the 234U/23*Uratio of the diluting U derived from the 234U/232Thvs 238U/232Th“isochrone” slope (Fig. 7a), finally yields the residence time of the diluting U in the sediments, with the usual equation: (230Th/238U)= (1 - eex”‘) + ({234U/238U}- 1) X (A”/{h” - x’))( 1 - e-l’“-“)‘).

(25)

Since (234U/238U)is not much different from I, t - A’-’ In (1 - {230Th/238U}).

(26)

A residence time of ca. 18,000 a is indicated here for the diluting U. As mentioned previously, this age corresponds to the last humid episode in the area and is equivalent to the last glacial maximum at higher latitudes or altitudes. This

The fwo-component system in the surface sediments of the sabkha

AS seen on Fig. 4C, the U diluting the radioisotopes inherited with the detrital fraction in the surface sediments of the sabkha shows a slight departure from secular equilibrium. The dilution process can be fingerprinted by the calculation

FIG. 6. Dilution curve (z3?h/234U vs 234U/232Th)defined surface sediments of the sabkha.

by the

1033

U-series isotopes in lacustrine carbonates

Uranium behaviour in the deep sediments of the sabkha

_.

0

i

4

6

io

i

i2

236”/232~h

2%

accYnl”I.1m”

,ron %-decay . “-“pIaLe

0

2

4

6

6

10

FIG. 7. Activity ratios (230Th/234Uand 23“U/238U)of the dilution phase in the surface sediments of the sabkha.

“humid” episode allowed infilling of the sabkha by a relatively permanent water body (KAISER et al., 1973) where continuous sedimentation prevailed. This episode also allowed U fixation at the water/sediment interface. Since then, arid conditions prevailed and mass budgets in the depression are currently negative with wind erosion resulting in losses of fine sediments. With respect to the recycling of U in such a basin, it seems obvious that it is strongly climate-dependent. Significant changes in U fluxes through time can be expected. Evidences for such changes can be found in the U content of the surface sediments of the sabhka showing minimum dilution by secondary U (e.g., samples UQT4 18, -456, -452, and -420, Table 4). Their 238U activities reported relative to those of the relevant Z32Th(to allow comparison with other types of samples), yield ratios of ca. 1.25 which are considerably lower than those of modern upper soil horizons (238U/232Th - 1.8). This shows that the particles deposited some 18,000 a ago suffered significantly larger uranium losses during their pedological cycle. Accordingly, the latter also show smaller excesses of 230Th over 238U: their present 230Th/238U of - 1.25 would have corresponded to - 1.47 some 18,000 a ago, a value that is lower than that of - I .7 which characterizes modern upper soil horizons (Fat 18,000 BP < Fat Present). This indicates that during humid episodes, the detrital particles in upper soil horizons had a longer residence time in the leaching horizon, possibly due to lower erosional rates in relation to a more abundant vegetation cover.

A few meters below the surface, the sediments do not show any significant disequilibrium between 230Th and 234U. Several complementary causes may account for this: (1) radioactive decay through time, (2) smaller initial disequilibria in detrital particles, or (3) secondary fixation of U during sedimentation, supporting the inherited 230Th-excess. Consequently, Th and U isotopes cannot be used to “date” sedimentation events in such depressions, although this may be possible in deepsea sediments (Ku, 1966). A significant departure from equilibrium between 234U and 238U is however observed in the sediments collected at depths ranging from ca. 30 to 50 m (Fig. 8). The relative loss in 234U can be explained by groundwater dissolution of U, without dissolution of the carbonate fraction as shown by increasing concentrations in CaC03 downcore (GHALEB, 1988). The “deep” groundwaters pumped at a depth of ca. 35 m in the sabkha do show the highest 234U/238U activity ratio (ca. 1.48). Strong hydraulic gradients from SW to NE, towards the Euphrate River, are probably responsible for “deep” negative U fluxes. At a smaller time scale, i.e., that of 228Th/232Th disequilibria through “*Ra fluxes, the excess of 228Th over 232Th is interpreted as a consequence of adsorption by the sediments of some 228Th produced by the decay of 2’*Ra transported by groundwater. This “‘Ra had to be dissolved during the last few decades at the most; however, with the limited number of data available on groundwaters, its origin is difficult to hypothesize. CONCLUSIONS The recycling history of U and Th isotopes in the basin is summarized in Fig. 9 which illustrates the sequential dis234

u/238,~

0.9

0,6

OJ”“““““““”

.

5-

lo-

15 -

20 -

25l

Surface

A

“deep”

sediments in the

M

30-

sediments

Sabkha

t

35-

40-

A

A

AA

FIG. 8. 234U/2’sUactivity ratios down core in the sabkha.

Beyond their chronoIogica1 interest ( ROSHOI I. ! 985). i iir)c! 111isotope measurements in palctrG1~ mu! iiiCrcl;x~ \ ;tG btsrful paieoclimatic information: I(,Anr~lc’/~~~~crr/r--Discussions\clth Jean-Claude Marrschal. Jam?, Bourne (UQAM), and Christian GCWZ (CNRS. France) helped to develop the model for the U tluxcs. (‘omments iiom I+. P. Schwarl-/ ( McMaster1Jniv.) and 7‘. L. Ku (Um. ofS. California) substantwel) improved our manuscript; thanks are due to all of them. This work w;1s financial]> supported by the Narural Science and Engineering Council of Canada (grants to CHM and C’G) and thr .,\romic Energy C‘ommission of Syria.

REFERENCES BISCHOFFJ. L.. SHELMONR. J., Kv T.-L.. SIMPSONK. D.. ROSI:N-

FIG;. 9. Radioactive disequilibria (““Th/“%) and mass budgets (*34U/“ZTh) induced by the recycling, chemical fractionation. and radioactive decay of U and Th isotopes in the Palmyta area.

equilibria

created

velopment

between

on a relatively

‘jOTh U-rich

and

its parent

bedrock

causes

U. Soil deexcesses

of

Detrital particles are transported from soils during “humid” episodes and deposited at the bottom of the depression. During drier periods. wind erosion prevails. Secondary U uptake at the water/sediment interface may create an inverse disequilibrium. Secular equilibrium will be restored through time. if the system remains closed with respect to U. A few conclusions can be drawn from the study of this sabkha. Firstly, lithology and hydric conditions strongly control the behaviour of U in soils and may produce “unsupported” or “excesses” in 23”Th. Secondly, the absence ofan\ significant *34U/z”xU fractionation can only be explained by its co-dissolution with carbonate phases, under the very oxydizing conditions of these unsaturated soils. Thirdly, the transitory retention of U essentially occurs in lower unsaturated and carbonate-rich soil horizons. Finally. a two-component system characterizes the surface sediments ofthe sabkha, whereby radioisotopes from the detrital fraction are diluted by an “authigenic” U component. The measured activity ratios define a typical dilution curve which can be used. in combination with isochron plots, to compute the residence time of IJ. In this study, the residence time appears ““Th

(over

to be 18,000

‘34U) in upper soil horizons.

a.

in such an area are strongly dependent on climatic changes. With reference to modern conditions, the past “humid” climatic episode was characterized by a longer residence time of particles in soils and higher U-leaching rates. IJranium

fluxes

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