Geochemistry and radiometric dating of a Middle Pleistocene peat

Geochimicaet CosmochimicaActa, Vol. 61, No. 19, pp. 4201-4211, 1997 Copyright © 1997 ElsevierScienceLtd Printed in the USA. All rights reserved 0016-7037/97 $17.00 + .00

Pergamon

PII S0016-7037(97) 00213-5

Geochemistry and radiometric dating of a Middle Pleistocene peat PETER J. ROWE, 1 DAVID A. RICHARDS,2 TIMOTHY C. ATKINSON,J SIMON H. BOTTRELL,2 and ROBERT A. CLIFF2 ~School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK 2Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK (Received August 23 , 1996; accepted in revised form June 5, 1997) A b s t r a c t - - U r a n i u m , lead, and sulphur data for a Middle Pleistocene interglacial peat deposit from Norfolk, UK, suggest that uptake of these elements was synchronous and confined to a single early diagenetic episode, probably coeval with peat formation. Sulphur isotope data indicate that reducing conditions have been maintained within the deposit throughout its history. Both uranium and lead concentration profiles show a marked discontinuity near the middle of the bed, probably indicating an environmental change, possibly emergence. The lead isotope data are compatible with a single lead component below the discontinuity and two components above. Groundwater is thought to be the dominant source of lead with an additional airfall component present in the upper peat. The uranium and lead concentration profiles below the discontinuity and the sulfur isotope profile throughout the peat support the view that these elements were sequestered from upwelling groundwaters. The organic material is particularly suitable for 23°Th/238U dating because it contains a negligible allogenic mineral component and very low 232Th activity. A sequence of consistent ages through the peat profile (mean 317 +14 ka) over a wide range of uranium concentrations ( 7 - 6 5 mg g-~), strongly suggests that a discrete, short-lived, uranium-uptake event has been dated and that subsequent differential isotopic migration has not occurred. One sample, from immediately below the discontinuity, has an infinite apparent age, but there is strong evidence for sequestration of uranium from the peat into adjacent wood fragments found along the discontinuity. Calculated initial 234U/238Uvalues of 1.2--1.3 support a groundwater origin for the uranium, rather than a marine origin resulting from a subsequent rapid transgression. The very restricted range of U/Pb ratios in the lower part of the peat bed, and the heterogeneity of the initial lead isotopic composition in the upper part, preclude U-Pb isochron dating. 21°P0 measurements (as a proxy for 2~°Pb) also indicate possible post-depositional migration of 222Rn which, if active over a significant period, would bias any U-Pb age estimate. The 23°Th/238Uages are consistent with deposition during oxygen isotope Stage 9. Copyright © 1997 Elsevier Science Ltd 1. INTRODUCTION

geochemistry of U-series nuclides and other key species in the peat bed. Accurate ages can only be produced by the 23~l'h/z38U method if the deposit has remained closed to migration of uranium and thorium since its formation and, similarly, U-Pb ages are only valid if the system remains closed to migration of uranium, lead, and all intermediate nuclides. The chemical environment of accumulating peat is strongly reducing below a depth of only a few centimetres and porewaters are commonly sulphidic (Shotyk, 1988). Uranium solubility is strongly redox-sensitive: uranium is mobile under oxic conditions but immobile in reducing environments (Langmuir, 1978). Thus, uranium introduced into a peat (from atmospheric or groundwater inputs) will be trapped in the reducing zone. Lead too will be immobilized in the reducing zone, probably by incorporation into a metal sulphide phase since PbS is extremely insoluble (Shotyk, 1988). Direct complexation of uranyl species by carboxyl functional groups is also thought to play a major role in Ufixation within peat bodies (Read et al., 1993). Thorium is very insoluble in natural waters and generally considered immobile under most environmental conditions and would only be incorporated into the peat in detrital Th-bearing mineral matter. To establish that the peat has remained an isotopically closed-system since burial and that incursion of oxic groundwaters or any other desorbtive mechanism has not caused

Peats are common among Quaternary terrestrial deposits, and the pollen and macrofossils preserved within them can provide important paleoenvironmental and paleoclimatic information. It would, therefore, be extremely useful to derive radiometric ages for peats so that they can be correlated with other terrestrial deposits and the more continuous marine sequences on which the climate-related oxygen-isotope stratigraphy of the Quaternary is based (Imbrie et al., 1984). Previous studies have demonstrated that the 23°Th/238Udisequilibrium dating method can be applied to peats (Vogel and Kronfeld, 1980; van der Wijk et al., 1986; Heijnis and van der Plicht, 1992; Heijnis et al., 1993). In this study, we applied the 23°Th/238Udating technique to a Middle Pleistocene detrital wood peat from Tottenhill, Norfolk, UK, which is notable for its almost total lack of a 232Th-bearing allochthonous mineral component. In addition, we investigated the potential of measuring radiogenic 2°rpb ingrowth from decay of 238U as a technique for dating Quaternary peat deposits. This is of particular interest because it could be applied to deposits beyond the upper age limit of the 23°Th/ 238U techniques, effectively 350 ka for a-spectrometric methods and 500 ka for thermal-ionisation mass-spectrometric (TIMS) methods. Calculated ages from either dating method are only credible if certain geochemical criteria are rigorously met, and, to this end, we have investigated the 4201

P. J. Rowe et al.

4202

530N

)ximate of t n Ice

sas of

Tills sntioned

Fig. 1. Location of Tottenhill Quarry, Nar Valley, Norfolk, UK.

nuclide mobility, we have examined the internal consistency of U-series data, compared the uranium and lead distribution and isotopic data, and also used the geochemistry of S, another redox-sensitive species. In investigating the possible use of 238U to 2°6pb decay as a dating tool for peats, we also used measurements of 21°P0 activity (as a proxy for 21°pb) to test for possible loss of 222Rn, the most mobile nuclide in this decay series. 2. GEOLOGICAL SETTING AND SAMPLE DESCRIPTION The temperate freshwater and marine Pleistocene sediments of the lower Nar Valley have been exposed for many years in the sand and gravel quarry at Tottenhill, west Norfolk (Ordnance Survey Grid Reference TF638118; Fig. 1 ) and the site has been described by Gallois (1978) and Ventris ( 1985, 1986, 1996). The generalised Middle Pleistocene lithostratigraphy of the lower Nar Valley is shown in Fig. 2. At the base of the succession is the Woodlands Farm Till which is assigned to the Anglian Stage of the British Pleistocene sequence (Mitchell et al., 1973), and this is conformably overlain by the Setch Laminated Clays, a unit at least 13 m thick which is believed to represent the infilling of a large water body adjacent to the ice margin. The overlying 1 m thick fining-up sequence of sands to clays forms the basal unit of the Nar Valley Freshwater Beds (NVFB) and

represents the final stages of the infilling process. The clays are overlain in turn by a lignitic bed of black compressed wood peat (0.3-0.7 m), the lower contact of which, according to Ventris (1996), was very sharp and almost horizontal throughout the quarry. It is this peat which is the subject of the present paper. It derives from interglacial fen woodland which colonised the swamp areas around surviving isolated lakes and was conformably overlain, as rising water levels created more widespread lacustrine conditions, by a clay-marl (0.15 m) and then by the upper unit of the NVFB, a silty clay dominated by the mollusc Hydrobia (0.1 m). The succeeding Nar Valley Clay ( 1.0 m) was deposited as sea-level rise continued, and the area became first estuarine and then fully marine. According to Ventris (1986, 1996), the transgression eventually reached an altitude of 23 m above Ordnance Datum (present mean sea-level). Eventual regression and the subsequent emplacement of the Tottenhill Sands and Gravels during the succeeding (or some later) cold stage led to erosion and channelling of the marine and underlying freshwater deposits. Ventris (1986, 1996) follows Stevens (1960) and Gallois (1978) in correlating the basal Woodlands Farm Till Member with the Anglian Stage and the overlying temperate sediments, including the marine Nar Valley Clay, with the Hoxnian Stage (Mitchell et al., 1973), an important interglacial stage in the British Pleistocene sequence, the age of which is uncertain. The correlation with the stratotype at Hoxne

Isotopic dating of Pleistocene peat

• o~'. " ° • • o o. . . . . .

(65 cm). Nevertheless, pollen analyses of the lower part of the bed (below 16 cm), and of the 3 - 6 cm horizon, indicate that both peats represent approximately the same (early interglacial) period of time (F. Green pers. commun., 1997).

TOTTENHILL SANDS AND GRAVELS

erosional

3. ANALYTICAL PROCEDURES

contacl

I

NAR VALLEY CLAY (MARINE) SHELLY SILTY CLAY "~" "]

MARL

]WOODPI:AT CLAY SILT SAND - V

V

V

V

~' [ N A R V A L L E Y ~ | FRESHWATER 1 ] BEDS J

~ _J

V

~7

V

! /

SETCH LAMINATED CLAYS

_ _

V

4203

WOODLANDS FARM TILL

Fig. 2. Generalised lithostratigraphic succession of Middle Pleistocene sediments in the Nar Valley (modified from Ventris, 1986; not drawn to scale).

(West, 1956) is based not only on its stratigraphic position above a till, but also on palynological similarities, specifically (1) high late-glacial values for Hippophae, which is considered to be typical of the late Anglian, (2) the presence of palynomorph Type X throughout the sequence, and (3) the occurrence of significant amounts of Tilia and Taxus pollen in the thermophilous forest assemblage. Recent extension of the quarry revealed new sections which are described in detail by Gibbard et al. (1992). Although primarily concerned with the stratigraphically higher Tottenhill Sands and Gravels, they were able to establish that the underlying sequence of temperate sediments above till was the same as that observed by Ventris (1986, 1996). In 1991, samples were taken by two of the authors from a 5 m long block of wood peat and Nar Valley Clay, described by Gibbard et al. (1992) as occurring in a trench immediately north of their sites V and W and lying subhorizontally, having collapsed by rotation into a hollow scoured in the underlying deposits. There is no evidence that the block has been transported laterally. The peat seam was 32 cm thick at this location and after removing the outer 10 cm to avoid recently contaminated material, the freshly exposed face was sampled in 3--4 cm spits. The upper boundary of the peat was sharply defined, and a temporary datum was marked at this level. Samples are identified by their stratigraphic depth in relation to this datum. Several large fragments of wood, found parallel to the bedding between the 1 3 - 1 6 and 1 6 - 1 9 cm spits, were recovered. These consisted of branch pieces up to 20 cm long, originally about 5 cm in diameter but now compressed and flattened to only 1 cm or so thick. The sampling location is almost a kilometre from the Ventris site and, the peat bed here was only half the thickness that he recorded

Samples were analysed for uranium, thorium, lead, polonium, and sulfur according to the scheme illustrated in Fig. 3. After extraction/ separation of the relevant minor elements and species from the sediment, isotopic ratios and concentrations were measured by conventional techniques; alpha-spectrometry was used for uranium, thorium, and polonium, thermal-ionization mass-spectrometry for uranium and lead and gas-source mass-spectrometry for sulfur. Yields of uranium and thorium isotopes were determined using a 228Th/232U spike which was calibrated in 1981 and now has a transient equilibrium activity ratio of 1.027 (Ivanovich et al., 1984) with negligible error. A weighed aliquot was equilibrated with the nominated acid prior to sample dissolution. The specific activity of 21°Po (fin = 138.3 d) was determined by alpha-spectrometry using a 2°8po spike (tl/2 = 2.9 y). The activity of 2~°pb (tl/z = 22.6 y) was estimated by assuming secular equilibrium between 2~°po and 2t°pb. Determinations of uranium and lead concentrations and isotopic ratios by mass-spectrometry were performed on a VG Micromass 30 in single Faraday or Daly peak jumping modes. Isotope dilution analyses used a mixed 2°2pb-233U-236Uspike. Total procedural lead blank corrections of 400-600 pg were applied to all lead measurements. The blank composition is well known (2°Tpb/2°4Pb = 15.34, 2°rPb/2°4pb = 17.61, 2°sPb/2°4pb = 36.74; +- 2%, 2s) and in most cases represented < 3% of total lead. Larger corrections of - 10% blank contribution were needed only for the two residue fractions of peat where sample masses were extremely small (2-3 mg), Mean mass fractionation corrections, based on repeated analysis of NIST SRM 981, were 0.7%0 per a.m.u for Faraday collection, and 4.5%0 per a.m.u, for Daly collection (except 2°rpb/2°4pb - Daly, for which 2.4%0 per a.m.u, was used, based on a consistent bias in SRM 981 results). All mass-spectrometric determinations are quoted with 2a errors, while those by alpha-spectrometry are l cr. 4. RESULTS AND DISCUSSION

4.1. Sources of Components During Peat Diagenesis Concentrations of uranium and lead in the peat horizons are given in Table 1 and plotted as depth profiles in Fig. 4. Concentrations of lead range from 0.5 to 3.5 ( # g g-1. Concentrations of uranium are higher and range from 10 to 65 #g g-~. The depth distributions of uranium and lead are similar, and both exhibit different characteristics above and below the 16 cm level. Above the 16 cm level, concentrations of both elements are relatively constant at lower values, whereas, below the 16 cm layer, concentrations are higher, and there is a steep concentration gradient with values increasing from the base to a maximum in the 1 6 - 1 9 cm layer. Subsamples of the 1 3 - 1 6 cm horizon exhibit a wide range in lead values, most likely because of incomplete homogenization of the sample, indicating that high concentrations of lead may extend partly into this layer, and hence the profiles are not exactly coincident. A higher spatial resolution in sampling would be required to define the exact relationships. Concentrations of uranium and lead are plotted in Fig. 5, and the difference in geochemical characteristics above and below 16 cm are again apparent. Data from the lower part of the peat plot on a line which passes through the origin. The constant U / P b ratio for these lower samples suggests that uranium and lead entered this section of the peat by the

4204

P.J. Rowe et al.

Peat sampled in 3 cm thick layers I Peat dried at 105 ~Cto constsnt weight and ground to coarse powder

I Ash at 410 ~Cfor 15-20 hr I . . . . . . . . .IDles°lvein . .HCIO4/ . . . . .H . .NOa . . . . . . .and . . . . . i. . . . . . . . . . . . . . . . . Add (Newton . . . . .asidifled . . . . .e~ . . ~/, . . .CrCI2 .1995) ............

=w',

,

r . . . . . . . spike; . dlesolve;~'"'t ~Thr.~U i [~ ~eTh/~U dlesolve l d.i.s. .s.O '.iIspike; s. l.22eTh~U v. .e. .t . . tp. . . . . . . ]r~ ] i~. . . . . . . .k.~I. .2N . . .HNO~ . . .e. . . . . . .i . .il; . .~.iqaike; .p. b. .....U .diuolve'~ .. ~ e U . . . . .i ~

i

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Add 20epo. preoipitate on A~ d~cs (Hamilton and Smith, 1986)

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Residuum Totalsample dissolution

U and Rb asporation using HCI-I.,IBrHNOacolumn ¢hromleogmp~ (AG 1X8 anion exchange resin) and loaded on sing~ Re filaments with phoabhoric acid nnd silica gel.

1994)

I

I i =aOPo ,ISOTh

by combustion: of CuS and mass-spot. (Robinson & Kusnkabo, 1975) (Table 2 and Figure 4)

huron ~4 and mlms-i pec. 1~2) frable !and Figure ........

i

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I

I

I

I

U and Pb ¢oneentmtlens and Ilmlepi¢ rmlol measured by thermal ioni~tion rne~-spectrometry(VO MM30) using single D~y end Faraday colle~on in puk jumping mode. (Figures 4-7 end Table I)

U and Th concentrations and isotopic ratios measured by alpha-specb'ometry. (Figure 8 end Table 3 for isotope results)

~,nd concentration

astlvitloS

, .............................

,-

=¢zpb-~ U ~e U =¢ZPb-~U-=~mU s~ike" II spiM;ameo~w II

I' U ~ Th mpmlltlon by HCI-HNOsoolumn chmn'mtogrephy (AG 1X8 anion exchange resin) after co-precipitationwith Fe(OH)2;end electroplated onto stainlesssteel planchets.

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Conw

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Trtrate T~rat sulphide sulpl' yield yield

! Leachote

Residuum Le&chate Totalsample dissolution

!

i !~

i

................................. S .................................................. ..U.,....T.h.i i..................................

..P..q..

Fig. 3. Analytical scheme used to measure U, Th, Po, Pb, and S in the Nar Valley peat samples.

Table 1. Uranium and lead concentrations and lead isotopic ratios. Sample depth a

(cm) Peat 0-3 3-6" 6-10 10-13 13-16

16-19 19-22 22-26

Subsample a.b

A B* D(L) D(R) A* B*

a: b: c: d:

14.12 14.08 15.97 17.24 17.09 12.81 1.67 61.93 65.05

± ± ± ± ± ± ± ± ±

0.07 0.15 0.17 0.14 0.13 0.07 0.10 0.57 0.91

.

1.053 0.606 0.919 0.662 3.39 0.452 0.217 2.11 2.14

66.31 ± 1 . 2 0

A* C(L) A B

29-32 Wood 16

Pb concc'~ (#g g-~)

7.44 ± 0 . 1 5

C(R) 26-29

U concc (#g g-~)

45.64 39.06 5.04 34.25 34.03

± 0.81 _+ 0.75 ± 0.84 ± 0.12 ± 0.13

13.46 ± 0.29

25.93 ± 0.29

.

± ± ± ± ± ± ± ± ±

0.009 0.013 0.020 0.010 0.027 0.018 0.042 0.023 0.023

849 1456 1101 1651 317 1805 486 1877 1955

± 0.033 --. 0.040 ± 0.037 ± 0.007 ± 0.007

2050 1892 1345 2017 1944

.

1.424 1.316 0.239 1.095 1.116

238U/2°4pb

.

~ ± ± ± ± ± ± ± ±

17 30 22 24 1 71 40 15 19

46.2 79.6 59.6 89.1 17.4 97.1 26.8 100.1 103.7

± 31 ± 48 ± 181 ± 14 ± 15

109.2 103.7 72.4 106.6 104.1

.

.

0.444 ± 0.018

23SU/206pb

.

± ± ± ± ± ± ± ± ±

0.3 1.6 1.1 1.2 0.1 3.6 2.1 0.8 1.0

0.8516 0.8480 0.8451 0.8413 0.8554 0.8400 0.8618 0.8327 0.8328

+ ± ± ± ±

1.5 2.4 9.3 0.7 0.7

0.8354 0.8388 0.8418 0.8335 0.8349

.

.

3603 ± 142

207pb/206pb

.

± 0.0015 ± 0.0013 ± 0.0016 ± 0.0014 ± 0.0003 ± 0.0021 --. 0.0040 ± 0.0004 ± 0.0012

.

.

201.5 ± 7.8

20spb/206pb

0.0012 0.0016 0.0070 0.0010 0.0012

.

0.8595 ± 0.0024

* denotes mean value of instrumental replicates. (L) and (R) denote leachate and residuum. Uranium and lead concentrations are reported relative to total mass of ashed subsample. Analyses in italics determined by a-spectrometry, otherwise thermal-ionisation mass-spectrometry.

.

206pbflO4pb

.

2.0794 2.0693 2.0721 2.0630 2.0905 2.0644 2.0997 2.0560 2.0629

± 0.0024 --. 0.0043 ± 0.0054 ± 0.0041 ± 0.0006 __+0.0049 ± 0.0116 ± 0.0009 ± 0.0044

2.0538 2.0567 2.0775 2.0533 2.0581

± 0.0042 -± 0.0057 ± 0.0166 ± 0.0050 ± 0.0054

.

± ± ± ± ±

207pb/204pb

15.64 15.57 15.60 15.58 15.88 15.94 15.64 15.91 15.85

± 0.04 -.+ 0.04 ± 0.05 ± 0.05 ± 0.02 ± 0.11 ± 0.09 ± 0.03 ± 0.04

18.37 18.36 18.46 18.53 18.21 18.60 18.15 18.74 18.85

± ± ± ± ± ± ± ± ±

0.04 0.04 0.05 0.05 0.02 0.14 0.07 0.04 0.04

15.67 15.66 15.74 15.77 15.60

-+ 0.05 ± 0.05 ± 0.16 ± 0.03 ± 0.05

18.77 18.67 18.57 18.91 18.69

_+ 0.05 ± 0.05 ± 0.18 ± 0.05 ± 0.06

.

.

2.0712 ± 0.0072

15.37 ± 0.07

17.88 ± 0.07

Isotopic dating of Pleistocene peat U concenntlon (v-g g-l) 20 i

0-3

40 i

i

60 i

-20

i

-15

-10

-5

i

3-6 6-10 10 - 13 Q. 13-16

.a

-].

II

Q. 16-19 Etll 19-22 22-26 26-29

tic-5

29-32

• Oqlanlc-5 I

I

1.0

I

I

2.0

I

I

I

3.0

Pb concenUation (~.g g-l)

Fig. 4. Depth profiles of U and Pb concentrations and sulfur isotope by species in the Nar Valley peat. The profile can be divided into upper and lower horizons based on differing U and Pb geochemical characteristics. A wood layer at 16 cm (shown by shading) separates these horizons. Relatively depleted 3~Sindicates that S was fixed in a reducing environment.

same mechanism and, presumably, at the same time. Most of the data from the upper peat samples plot away from the line and have lower U/Pb relative to those from the lower peat. The wood sample is enriched in uranium relative to the upper samples. On a plot of 2°Tpb/2°6pb vs. 23su/=°6pb (Fig. 6), the lower peat samples form a tight cluster, whereas the upper peat samples are relatively enriched in 2°Tpb and depleted in uranium. The wood sample has high 238u/E°6pb

4205

and 2°Tpb/2°rpb and is clearly distinguishable from the peat samples. It is clear from Figs. 5 and 6 that the upper samples have an additional fraction of lead in excess of that predicted by correlation of uranium and lead in the lower peat horizon. For ease of reference, the additional component in the upper horizon is called Pb, and the dominant component in the lower horizon, PbA. The amount of PbB present in any sample has been calculated by subtracting from the measured lead concentration the concentration predicted for that sample by the regression of uranium against lead in Fig. 5. A plot of the relative proportions of Pbg and PbB in the upper horizon against measured 2°7pb/2°6pb (Fig. 7) displays a linear trend, for which the least-squares linear regression line intersects 100% Pba at a :°Tpb/E°rpb value of 0.8375 + 0.0070 (95% confidence interval), within error of the mean for samples from the lower peat (0.8347). With increasing proportion of component PbB, 2°Tpb/2°rPbincreases to a predicted value of 0.8641 _+ 0.0085 at 100% PbB. We interpret this as good evidence that the relatively lower U/Pb in the upper part of the peat arises from a mixture of lead from the same source as lead (and uranium) in the lower peat (i.e., PbA) with an additional component of lead (2°Tpb/:°6pb = 0.8641 and no associated uranium) from another source (i.e., PbB). Further support for this interpretation comes from leaching experiments on two samples (Fig. 6, Table 1 ); a lower peat sample ( 2 2 - 2 6 cm), for which 2°7pb/2°6pbof the leachate and residuum agree within error, suggesting a single source for the lead, and an upper peat sample ( 1 3 - 1 6 cm), for which a marked difference in :°7pb/2°~Pb was found between leachate and residuum (0.8456 ___ 0.0060 and 0.8618 _+ 0.0040, respectively), indicating two lead components. The 2°Tpb/ 2°6pb ratio for the residuum of the 13-16 cm sample cannot be distinguished from the value predicted for the PbB component (0.8641 _ 0.0085).

70 16-19B 60

16-19A

50 O~ 4 0 O~ :=L "-," 3 0

22-26CIL)

I • 126-29B w°°d~

20

~ 1 3 - /16A ~

26-29A 13-16B

3-16D(L)=~~0"13 v/4~W6.10"'~r ,~22-26C(R)

10

¢~

0 0

1,3-16D(R) ,

0.5

1

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3-6

1.5

I

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2

2.5

3

3.5

Pb (~g g-l) Fig. 5. Uranium vs. lead concentrations of Nat Valley peat samples. Data are listed in Table 1 (see also Fig. 4). Lower peat samples (>16 cm, open squares) plot on the line Pb/U = 0.033. Upper peat samples (<16 cm, shaded diamonds) have higher Pb/U ratios and the wood (16 cm, shaded triangle) has a lower Pb/U ratio.

4206

P.J. Rowe et al. 0.87

'

I

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I

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I

'

I

'

,\\\\~13-16D(R)

I

t

wood

0.86 •

J~

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0.85

N\

I

13-16B \ix 3-611\ \ ~

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6-10

13-16D(L) 10-13-/ ~~,~13-16A

a.

I',. O

\

0.84

I X ~ 22-28c(,)

16.19A ~v ..... 16-19B

0.83 ,

I

I

I

40

I

80

0.32 Ma--

I

,

I

120

,

I

160

I

200

I

240

238u/2O6pb Fig. 6. :°7Pb/2°rpb vs. 238U/2°rPb diagram for the peat samples and single wood sample. Samples are identified by depth (label) and horizon (upper: light shading; lower: no shading; wood: dark shading). The solid line represents a reference isochron of 0.32 Ma based on 23°Th/~3sU ages (Table 3) and constrained to pass through the weighted mean U-Pb results of the lower part of the peat where only one source of common Pb is assumed. The dashed line represents the mixing relationship between components Pbn and PbB, identified in Figs. 5 and 7.

These data and the relatively 345 depleted isotopic compositions for both organic- and pyritic-sulfur (Fig. 4, Table 2) indicate that sulfur was fixed in the peat from sulphide pro-

The sulfur speciation data in Table 2 show that sulfur is present predominantly in organic form ( 1 . 7 7 - 2 . 6 5 w t % ) with lesser concentrations of pyritic-S ( 0 . 0 8 - 0 . 8 7 w t % ) .

0.87

'

I

'

I

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I

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i

/ ~

.......

o.88

o~ooooo~sooo°. o°°~°°°°~.o~oo.,'°~°°°°[ J°°°°" 0.85

0.84

...."1

~

~

i PbA

s

mean Pb/ Pb of lower peat samples

,,~ 0.83

0

I

,

1:4

I

i 1:1

I

,

I 4:1

i PbB

Ratio of Pb components A and B Fig. 7.2°Tpb/E°rpb vs. relative proportion of components PbA and PbB in the Nar Valley peat. Pbg dominates the lower horizon of the peat deposit and has the same source as U, probably groundwater. PbB is an additional component that is found in the upper horizon. The lower peat has a mean 2°TPb/ZmsPbvalue of 0.8347 _ 0.0023 ( h r ) . With increasing proportion of PbB in the upper horizon, 2°Tpb/2°~pb increases to a predicted value of 0.8641 _+ 0.0085 at 100% PbB. Abscissa errors are derived by combining the U and Pb concentration errors and the slope error of the dashed regression line in Fig. 6.

Isotopic dating of Pleistocene peat Table 2. Sulphur speciation and isotopic data. Sample depth (cm)

Pyritic-S

Organic-S

Wt%

6345

Wt%

634S

Total S Wt%

0--3 3--6 6--10 10--13 13--16 16--19 19--22 22--26 26--29 29--32

0.87 0.33 0.32 0.11 0.12 0.10 0.08 0.19 0.47 0.25

-17.5 -12.2 -14.6 -13.7 -16.2 -12.9 -15.4 -16.2 -17.4 -15.3

1.77 2.11 2.50 2.65 2.51 2.24 2.18 2.25 2.21 2.01

-11.1 -8.0 -9.6 -12.2 -11.0 -11.2 -14.8 -12.4 -13.0 -16.3

2.64 2.44 2.82 2.76 2.63 2.34 2.26 2.44 2.68 2.26

duced by dissimilatory bacterial sulphate reduction in a reducing environment. This is usually the dominant mode of eariy diagenetic sulfur incorporation into peats and occurs during peat accumulation (Brown, 1985; Novak et al., 1994; Hannam et al., 1996; Bottrell and Novak, 1997). Importantly, there is no indication of later remobilization of sulfur by oxic groundwaters, which would rapidly remove pyritic and possibly organic sulfur. In cases where this has occurred, clear chemical and isotopic signals result from pyrite oxidation (e.g., silty peat of the West Runton Freshwater Bed, Norfolk, UK; Hannam et al. 1996; Bottrell et al., 1997). The organic-sulfur isotope data (Fig. 4) show a slight, but statistically significant trend to 34S enrichment upward through the peat ( r = 0.79, significant at the 95% confidence level). This might indicate a changing depositionai/diagenetic environment, with a different balance of sulfur sources as the peat accumulated (correlating with the changing balance of lead inputs described above). Alternatively, such a profile in organic-sulfur isotopic compositions might be the result of a minerotrophic (groundwater-fed) peat where the dominant sulfur source was SO4 in groundwater introduced at the base of the peat profile. In this case, S 2- and SO4 would become progressively 34S enriched upward through the peat profile as 34S-depleted SO4 is preferentially removed by bacterial reduction.

4207

a result of a slight temporary fall in the water table. The pollen in the lower part of our profile showed some evidence of corrosion, and it is possible that a retardation or pause in peat formation may have occurred at that time. Uranium uptake as a result of seawater intrusion during transgression can be ruled out on the basis of the (234U/Z38U)i,it values (Table 3, Fig. 8), which are consistently higher than the mean value of 1.147 for modem seawater (Chen et al., 1986), but within a range likely for groundwaters. The lower part of the peat thus represents a front where uranium and lead accumulated, possibly as a result of groundwater stasis when peat accumulation slowed. The process of deposition may have involved relatively oxic groundwater becoming reduced or much of the uranium and lead may have been directly complexed by organic macromolecules. Read et al. (1993) suggest that the latter process was important in the Brambster peat. The lower part of the peat contains only uranium and lead from this probable groundwater source. In the upper layer of peat, uranium and some lead is present from the groundwater source, but there is an additional lead component. The leaching experiment shows this to be associated with the residual fraction, and we believe it to be associated with wind-blown dust. This is less radiogenic and might represent either a local bulk-rock lead composition or be far-travelled and represent an exotic source. The reasons for the increase in the concentrations of this lead component in the upper peat are not clear. There may have been local environmental changes leading to higher dust production or the environment of peat deposition may have changed to one with more emergent vegetation or tree cover which intercepted more dust. The pollen data (Ventris, 1996) indicate the development of alder cart and thermophilous woodland at this time. What is most important is that all the data presented are consistent with uranium and lead accumulation in the Tottenhill peat as a result of chemically reducing conditions which prevailed during peat accumulation and early diagenesis. This was almost certainly accomplished in the time-span of one interglacial, since the overlying deposits of clay belong to the same interglacial period. There is no evidence for a later stage of oxic groundwater diagenesis which might have remobilized uranium or lead.

4.2. A Conceptual Model for Diagenesis of the Peat The lower section of the peat profile has higher concentrations of uranium relative to the upper section. We interpret this as the result of diagenetic uranium enrichment of the peat by inflowing groundwater rising from below. A similar profile was produced by rising groundwater depositing dissol~ed uranium in a Recent peat at Brambster, Caithness, with uranium concentrations of up to 1000 ppm being observed (Read et al., 1993), whilst Yliruokanen (1980), in a study of Finnish peat bogs, noted that vertical uranium distribution was often very erratic, but with a marked tendency towards higher concentrations at the base. The uranium and lead distribution in the Tottenhill peat can probably be explained by changing contemporary environmental conditions. Ventris (1996) noted that pollen preservation was extremely poor in a 25 cm wide band just below the middle of the peat bed and attributed this to oxidation, probably as

4.3. Geochronology of the Peat 4.3.1.

U/Pb

Geochronology

The amount of radiogenic lead produced in a sample from uranium decay over timescales applicable to the Quaternary is extremely small relative to the amount of initial lead present, the isotopic composition of which is very poorly known. To calculate ages of such young deposits, therefore, it is necessary to use isochron techniques. Data are plotted in Fig. 6 in 2°Tpb/2°rpb - 238U/2°6pb space and, for cases where radioactive equilibrium has been maintained for the entire history of the mineral, isochron relationships are preserved in this diagram with slope (M) defined by: M

1 137.88(e%35r

-

1) -

1 (e x23,r -

1)

4208

P.J. Rowe et al.

1.16

~L

g

1.08

o,I

1.00 0.96

1.04

1.12

1.20

23o-rh/238U Fig. 8. Plot of activity ratios 234U/238Uvs. 23°Th/238Ufor samples from the Nat Valley peat for total sample dissolution analyses using aqua regis. Ellipses are lcr precisions based on counting statistics. Isochrons and closedsystem trajectories from initial 234U/23sUvalues are labelled. Shaded regions correspond to interglacial oxygen-isotope stages based on Imbrie et al. (1984). The analyses cluster about a mean age of 317 ka, with only one sample, from just below the wood layer, showing evidence of U migration.

where I is the initial ratio of 2°Tpb/2°6pb (Tera and Wasserburg, 1972). However, it is well known that uranium is deposited in peats from groundwaters with an initial state of U-series disequilibrium. In these circumstances, modification of the terms within brackets of the above equation are required (see Wendt and Carl, 1985; Eqns. 2 - 6 ) . Preconditions for successful age derivation from isochron plots are (1) the isotopic composition of the initial lead should be homogenous throughout the deposit, (2) there must be a sufficient range in 238U/2°6pb values to obtain precise values of M and I, and (3) the relevant deposit must have remained a closed-system with respect to all the isotopes in the decay chains since uranium was deposited. However, it is clear that these conditions have not been met. Initial lead was probably derived from two sources in the upper part of the peat. In the lower part of the peat, where a single source of uranium and lead appears to dominates the isotopic signal, there is a very limited range of 23sU/ 2°6pb (Fig. 6). Data on 2~°Pb activities in two subsamples of the peat profile indicate that 2~°Pb is not in secular equilibrium with 23°Th in either case. Activity ratios (calculated for the time of collection in 1991 ) of 1.173 _ 0.044 for the 6 10 cm layer, and 0.455 _ 0.013 for the 2 6 - 2 9 cm were obtained. These data demonstrate that, at the scale of subsampling of the peat, there must have been post-depositional mobility of nuclides in the decay series intermediate between thorium and lead. The most probable candidate is 222Rn which may have migrated by gaseous diffusion from the lower layer and to a lesser extent accumulated in the upper layer. It is impossible to construct a mass balance for Rn

migration within or from the peat or determine the timing of open-system behaviour on the basis of the data available, but these data do serve to demonstrate that the peat did not remain a closed-system for nuclides in the decay chain between Z3°Th and z°6pb. This particular peat deposit, therefore, cannot be successfully dated by U-Pb methods. A linear trend does exist in the data in 2°Tpb/2°6pb-z38U/2°6pb space (Fig. 6), but this results from the mixing relationships of lead sources identified in Figs. 5 and 7, and the slope and intercept are too high for the age of the peat, which is known on stratigraphic grounds to be Middle Pleistocene. A reference isochron is illustrated in Fig. 6 with an age of 320 ka (based on U-Th analyses for this peat deposit; Table 3) and constrained to pass through the U-Pb results for the lower section.

4.3.2. U/Th Geochronology The uranium and thorium isotope data determined by alpha-spectrometry are shown in Table 3. Of particular note are the high 23°Zh/232Th, indicating negligible activities of 232Th in all but the basal sample ( 2 9 - 3 2 cm) where slight detrital contamination occurs. Initially, ashed samples were leached in 2 N HNO3 to determine uranium content and the degree of detrital thorium contamination. Subsequent total dissolution of one of the small insoluble residues (identified as haematite by XRD analysis) revealed that U-Th fractionation had occurred during leaching and that the residue was enriched in thorium relative to the leachate. Therefore, isotopic data derived from the leachates must be considered unre-

Isotopic dating of Pleistocene peat

4209

Table 3. Uranium and thorium isotope data and calculated ages for different peat sample dissolutions. Sample depth (cm) 2 N HNO3 I)-3 3-6 15-10 10-13 13-16 16-19 19-22 22-26 7 N HNO3 10-13 13-16 16-19 19-22 26-29 29-32 Total sample dissolution 0-3 3-6 6-10 10-13 13-16 16-19 19-22 2?-26 26-29 29-32

23°Th/238U

1.071 1.081 1.038 1.087 1.090 1.084 1.134 1.070

+ 0.029 ___0.022 ± 0.025 _+ 0.036 ± 0.036 ± 0.043 ± 0.037 ± 0.026

.

1.039 1.060 1.136 1.321 1.027 1.001

± + . ± ± ± ±

0.021 0.023 0.021 0.023 0.024 0.024

1.065 1.057 1.070 1.082 1.104 1.124 1.097 1.066 1.073 1.079

± 0.022 ± 0.021 + . 0.024 ± 0.022 ___0.018 ± 0.021 .+ 0.021 ± 0.021 ± 0.023 ± 0.032

234U/238ua

.

.

.

.

23°Th/234U

1.124 1.112 1.092 1.130 1.110 1.096 1.094 1.126

÷ 0.020 . ± 0.020 ± 0.021 ± 0.025 + 0.020 ± 0.014 ± 0.014 _+_0.020

0.953 . 0,972 0.951 0.962 0.982 0.989 1.037 0.950

+ ± ± ± ±

0.026 0.020 0.023 0.032 0.032 0.039 0.034 0.022

1.082 1.108 1.103 1.092 1.100 1.104

± 0.016 + 0.016 . ± 0.016 ___0.017 ± 0.018 ± 0.024

0.960 0.957 . 1.030 1.021 0.933 0.907

± ÷ _ ± ± ±

0.019 0.021 0.019 0.021 0.023 0.022

1.089 1.092 1.102 1.114 1.123 1.093 1.118 1.098 1.099 1.099

+ + + ± ± ± ÷ ± ± ±

0.978 0.968 0.971. 0.971 0.983 1.028 0.981. 0.971 0.976 0.982

± 0.020 ± 0.019 + 0.022 ± 0.020 --- 0.016 ± 0.019 + 0.019 ± 0.019 ± 0.021 ___0.029

0.019 0.017 0.016 . 0.018 0.015 0.014 0.015 . 0.015 0.015 0.028

23(~1/232Th

.

.

.

.

120 200 130 230 400 230 350 140

÷ ± ± ± ± ± ±

230 150 240 310 145 18 190 250 190 190 350 190 430 320 180 12

30. 40 20 40 80 30 40 20

(234U/238U)init a

1.270 1.266 1.205 1.292 1.276 1.256

+ ± ± ± ± ± -1.272 ±

+ ± ± + ± ±

0.065

278 309 285 287 327 350 >350 274

± 40 + 20. ± 32 _+ 50 ± 22 ± 1

1.192 ± 0.054 1.242 ÷ 0.056 --1.209 _ 0.053 1.200 ± 0.057

301 287 >350 >350 261 234

± ~7 ° + 33 4~

-+ 30 + 40 + 30. ± 30 ± 70 -+ 20 + 90. ± 60 ± 20 ± 0.5

1.215 1.221 1.243 1.269 1.305

331 311 311 306 323 >350 322 314 321 333

± ± ÷ ± ±

78 ,~ ~7 41 ~g ~0

÷ ± ± ±

~5 ~7 46~ t49°

± ± + ± ± -1.292 ÷ 1.236 ± 1.244 ± 1.253 ±

0.087 0.074 0.067 0.095 0.088 0.086

Age b (ka)

0.065 0.061 0.065 0.068 0.064 0.068 0.059 0.067 0.103

49 36 60 35~ Iv° J~0 ~v96 °

± ~]

± 4~ ± 28

40

a: 234U/238U is the measured value. (234U/238U)init is the initial value and is given by [(234U/238U)init - 1] = [(234U/238U) - 1]" e~23~r) where k234 is a decay constant and T is the age in years. b: Ages are quoted in ka and are calculated using 23°Th/238U - 1 = -e-~23or + [(234U/238U) - 1]" (~230/~230 -- ~k234)" (1 -- e-~X230 k234)T), where T is the age in years. Errors are 1cr and may be overestimated since no allowance has been made for error correlations. Such correlations have been incorporated into the error ellipses of Fig. 8.

liable. A stronger leaching agent, 7 N HNO3, was used in order to bring the samples completely into solution but proved to be unsuccessful. Consequently, in all subsequent analyses, samples were subjected to boiling aqua regia under reflux conditions for several hours until negligible insoluble residue remained (total sample dissolution or T S D ) . These analyses are considered to have produced the most reliable age estimates since all u r a n i u m and thorium isotopes were brought into solution. The nitric acid leaches produced a wide scatter o f ages ( T a b l e 3 ) , probably resulting from differential U - T h fractionation between the leachates and insoluble residues. The ages derived from the T S D data, in contrast, tend to cluster, with nine out of ten determinations lying between 306 ka and 333 ka and overlapping at one standard deviation, derived from counting statistics. This is illustrated in Fig. 8 where the nine concordant ages can be seen to fall within the time period of oxygen isotope Stage 9 ( 3 0 3 - 3 3 9 ka, Imbrie et al., 1984). The only discordant age in the TSD dataset is from the sample at 1 6 - 1 9 c m depth. T w o nitric acid leach analyses and a TSD analysis produced m e a n ages -- 350 ka ( T a b l e 3), ~md there can be little doubt as to the anomalous nature of this subsample. The most likely explanation for the older apparent age is a loss of uranium into the large fragments of compressed wood w h i c h were present immediately above this horizon. M o d e r n wood contains almost no uranium, but a sample from one of the wood fragments at this level con-

tained 25.6 /~g g - l U in the ashed residue (Table 1 ). The m e c h a n i s m was probably transport within the peat in the form of U-fulvate or U - h u m a t e complexes. Permeation of wood by humic and fulvic acids has been noted by Worsley ( 1 9 8 0 ) , and van der Wijk et al. ( 1 9 8 6 ) and Read et al. ( 1 9 9 3 ) have s h o w n that uranium can b e readily complexed by these acids. That uranium may have been lost from the 1 6 - 1 9 c m horizon is also suggested by the relationships of the measured 23°Th/238U and 2 3 4 U / 2 3 8 U values at this level to the stratigraphic distributions of these ratios (Fig. 9). Both distributions show a pattern in w h i c h values rise towards the centre of the deposit, and whereas 23°Th/238U at 1 6 - 1 9 cm conforms with this trend, 2 3 4 U / / 3 8 U lies well below the value expected from interpolating the data. W e interpret this as indicating preferential 234U loss relative to 2 3 8 U ( a n d to the very i m m o b i l e Z3°Th). This is an expected consequence of uranium migration since alpha recoil effects tend to favour mobility of 23nU o v e r 238U (Kigoshi, 1971; Fleischer, 1982). This explanation suggests that the inconsistent age data from the 1 6 - 1 9 c m horizon can be discounted. Trial calculations show that uranium migration from layer 1 6 - 1 9 c m to other parts of the peat is highly unlikely as it leads to discordance between all the dates from the different sub-samples, with no systematic pattern discernable. The ashed wood sample contains 445 ng g-1 of lead with z°Tpb/Z°6pb of 0.8595 _+ 0.0024, close to the value of the additional c o m p o n e n t PbB present in the upper peat. The lead in the 1 6 - 1 9 c m

4210

P.J. Rowe et al. 23OTh /23a U 1.00 0 4

1.05 i

1.10 i

1.20

1.15 i

a)

m m m

8

m m

g12

mm "0

m 20

m

24

m

28

m m ml m

32

i

2a,~U/2aeu 1.04 0 "'

1.06

1.08

1.10

1.12

1.14 r

1.18

o) 4

8

g

1,~

"u

20 24 28 32

t

Ii

i

Fig. 9. Profiles of (a) 23°Th/238U and (b) 234U/238U through the Nar Valley peat. The bars represent the _+l a errors (counting statistics only) of the measured values (tick marks).

horizon has a m e a n 2 ° 7 p b / 2 ° 6 p b of 0.8367, and thus the lead in the wood appears to be dominated by the PbB component and not to have migrated with the uranium. The coherence of the 23°Wh/238Uages over a wide range of uranium concentrations is powerful evidence that uranium uptake was a rapid, unique event in the diagenetic history of the peat. This coherence, together with the observed relative constancy of the U/Pb ratios in the lower part of the peat, also suggests that there has been little or no post-depositional remobilisation of uranium and that closed-system conditions have persisted since burial. The pattern of the uranium, lead, and sulfur profiles in the peat-bed suggests that these elements were introduced into the deposit by groundwater from below. The sharp breaks in the uranium and lead profiles in the middle of the peat, coincident with a layer rich in wood fragments, are likely to be attributable to a stasis or slight fall and subsequent recovery of the water table, probably

involving temporary emergence. The secondary lead component, found only in the upper past of the peat, above the woody layer, postdates this phase and probably had an aeolian origin. The cumulative weight of these observations suggests that uranium and lead uptake occurred contemporaneously with peat formation. That being so, the U-series dates should reflect the actual depositional age of the peat. The close clustering of the finite dates around their mean of 319 ka indicates that the peat bed probably can be regarded as a single geochemically homogenous unit with respect to the U-series isotope ratios, and, therefore, that it is valid to consider the isotopic data collectively in order to derive a best-estimate of the true age of peat deposition. This is confirmed by inspection of the ratio data. With the exception of the sample which is believed to have experienced uranium migration, all the 234U/ 238U and all but one of the 23°Th/23sU ratios overlap at 1 (Fig. 9). The mean square of weighted deviates (MSWD) values for these ratios are 0.524 and 0.596, respectively, for the nine finite-aged samples, both of which are >[1 - (2/f)1/2], where f = degrees of freedom, i.e., within lcr of the expected value of unity (Wendt and Carl, 1991 ). This is a strong indication that analytical error is the sole cause of data scatter, that there is no additional geological error due to inhomogenous samples or isotopic mobility and, therefore, that it is appropriate to pool the ratio data in order to derive a best-estimate of true age. The error-weighted means of the 23°Th/238U and 2 3 4 U / 2 3 8 U ratios produce a calculated age of 317 ___ 14, using a representative error correlation value of 0.35. The 95% confidence interval for peat deposition is, therefore, 289-345 ka. This corresponds closely to the astronomically-tuned age estimate for oxygen isotope Stage 9, which covers the period 303-339 ka (Imbrie et al., 1984). This is known to have been a global warm stage, and, therefore, the dating results are compatible with pollen analyses (Ventris, 1996), which demonstrated deposition during the early stages of an interglacial phase. 5. CONCLUSION This work has demonstrated that closed-system conditions with respect to uranium, thorium, lead, and sulfur have been maintained in a 32 cm thick Pleistocene peat-bed for over 300 ka. However, there is evidence of ZZZRnmigration, and, at one horizon, of minor sequestration of uranium from the peat into wood fragments. Previous suggestions that some peats might be suitable deposits for 2 3 ° T h / 2 3 8 U dating are confirmed (although the effects of wood on uranium distribution require further study), but it appears that U/Pb dating is likely to be less successful. The available evidence suggests that lead, uranium, and sulfur were introduced into the peat by groundwaters from below, with a secondary lead component probably entering the upper layers from above. The 23°Th/23sU age concordance shows that only a single uptake event was involved. It is highly likely that the entry of these elements into the peat bed was early in its diagenetic history and coeval with peat formation. The calculated U-series ages, therefore, are likely to represent the true age of peat deposition. The 95% confidence interval derived from weighted averaging of the isotopic data

Isotopic dating of Pleistocene peat is 2 8 9 - 3 4 5 ka. This is very close to the limits of alpha spectrometric U-series dating but gives only a 2.5% chance that the true age is > 3 4 5 ka. When it is taken in conjunction with the interglacial character of the pollen within the peat, then correlation of peat deposition with oxygen isotope Stage 9 is very strongly suggested. The till underlying the dated deposits, therefore, cannot be younger than oxygen isotope Stage 10. Acknowledgments This study was supported by NERC grants GR3 / 8315 and GR3/9358. We thank Dr G. Shimmield, Dunstaffnage Marine Laboratory, Oban, U.K. for supplying 2°SPo spike solution. The helpful comments of J. Bischoff, M. Gascoyne, and an anonymous referee improved the manuscript. We are grateful to K. Ludwig for providing the Isoplot program and for valuable discussion. Philip Judge drew Figs. 1, 2, and 9. Ed#orial handling: K. R. Ludwig REFERENCES

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