Problems encountered with the 14C dating of peat

~

QuaternaryScienceReviews(QuaternaryGeochronology),Vol. 14, pp. 373-383, 1995.

Pergamon

Copyright © 1995 Elsevier Science Ltd. Printed in Great Britain. All rights reserved. 0277-3791/95 $29.00

0277-3791(95)00031-3

P R O B L E M S E N C O U N T E R E D WITH THE 14C DATING OF PEAT J.S. SHORE,* D.D. BARTLEY~"and D.D. HARKNESS:~ *Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 OQU, U.K. "~Department of Pure and Applied Biology, University of Leeds, Leeds LS2 9JT, U.K. ~NERC Radiocarbon Laboratory, East Kilbride, Glasgow G75 OQU, U.K.

Abstract- -

Pairs of neighbouring short columns of peat from two sites in Northern England were divided into contiguous 1 cm thick slices that were sub-sampled for pollen and plant macro fossil analysis. For each slice the remaining peat was then split into humic acid, humin and fulvic acid fractions and radiocarbon dated. This research demonstrates that significant variations can occur in the radiocarbon content of discrete chemically defined fractions of peat. A total of 127 radiocarbon dates was obtained. The results from the four columns of peat are highly variable and mutually inconsistent. The fulvic acid fraction, although normally younger than the corresponding humic acid and humin fractions, has very little effect on a combined date in these samples. However, it is removed from all samples prior to dating in order to remove any carbonates which could contaminate samples. The variations in the ages obtained for the humic acid and corresponding humin fractions range from the humin fraction being 630 years older than the humic acid (SRR-4029) to the humic acid being 1210 years older than the humin fraction (SRR-4007), even when a two sigma confidence is allowed. No correlation was apparent between pollen concentration, t3C enrichment, degree of humification and the conventional radiocarbon ages.

INTRODUCTION

QG

from blanket and reedswamp peats could be trusted for dating. Bartley and Chambers (1992) argued that the humin fraction of peat should be used, whereas Johnson et al. (1990) suggest that for some material the age obtained from the humic acid fraction is more realistic. Olsson (1986) found that charcoal from an archaeological site and peat from contemporaneous layers in a nearby Cladium peat bog yielded significantly different ages. The age of the peat was interpreted as being 500 years too old. In contrast Sheppard et al. (1979) have suggested from their own results that "the more mobile peat components (humic acids and lipids) do not on average have mean residence times which differ greatly from discrete radiocarbon ages of reference materials within the profile (charcoal, wood and humin)," although discrepancies as large as 225 years were observed. From a literature search, however, they recognize that large differences (100s-1000s years) are not uncommon. At the extreme, other researchers such as Mehringer and Warren (1976) have precluded the use of peat as a dating medium because of its unreliability. Lui Jinling et al. (1986) observed significant differences between the humin and humic material (1500-13,000 years) of a highly organic lake sediment. F o w l e r (1985) a t t e m p t e d to i d e n t i f y the v a r i o u s sources of carbon in peats and sediments by subjecting defined biogeochemical fractions such as lipids, humic acids and cellulose to AMS dating. Unfortunately, the precision obtainable by AMS dating on such small samples was at that time too poor to adequately resolve significant age differences. With the advent of r4C AMS

In the British Isles peat is one of the most important materials used in radiocarbon dating prehistoric events because of its widespread geographical occurrence and high carbon content. Peat affords an excellent palaeoecological record that details natural and anthropogenic environmental changes. Despite these attributes there has long been evidence that anomalous ages may be inferred from tac measurements made on peat when compared with other materials in secure association; while different physical and chemical fractions of the peat itself frequently produce different results (Dresser, 1971; Bartley and Chambers, 1992; Johnson et al., 1990; Mehringer and Warren, 1976). Unless such variations are identified and quantified, opportunities for misinterpretation of the timing of events recorded in the depositional record are considerable.

BACKGROUND Dresser (1971) attempted to identify which physical or chemical fractions of various deposits were most suitable for radiocarbon dating. His study focused on some or all o f the empirically defined fractions for blanket bog, raised bog and reedswamp peats. Typically this involved whole sample, water soluble organics, humic acid, insoluble organic detritus larger than 250 la and insoluble fraction smaller than 250 ~t. Although relatively few measurements were obtained, Dresser (1971) advised that Sphagnum peats from raised bogs could provide a reliable chronology, but that only the fine insoluble fraction 373

374

Quaterna~ Science Reviews (Quaterna~ Geochronology): Volume 14

technology, several studies have focused on pollen extracted from peats and sediments (Brown et al. 1989, 1992; Regnell, 1992; Long et al. 1992). The implications of such work are discussed later. In the present study 127 ~4C analyses were undertaken on two pairs of peat columns. The prime objective was to determine the extent of variability contained within samples of the size routinely recovered for radiometric 14C dating and to establish whether consistent results could be obtained from bogs of different vegetational composition. Three scenarios were anticipated: (1) The traditional humin fraction (acid and alkali insoluble organic detritus) is consistently younger than the associated humic acid (alkali soluble, acid insoluble organic residues), implying that humin is dominated by intrusive rootlets. (2) The humic acid is consistently younger than the humin, implying that there is a net downward transport of the water soluble organics. (3) the humin and humic acid components are of the same age and therefore a bulk acid washed peat would give an acceptable mean age. THE SITES Lanshaw Moss and White Moss Each site investigated in this study had been used previously for palynological investigations (Jones, 1977; Bannister, 1985). That work had raised questions conceming the timing of palynological events which it was hoped could be answered by detailed 14C analyses. The peat at each site had accumulated relatively slowly in parts (from over 500 years/cm to less than 30 years/cm) with the suspicion of a hiatus during the mid-Holocene. The accumulation rates quoted above are based on the mean age for each 1 cm increment, however if larger increments or the ages of different fractions are used then different accumulation rates will be obtained. Detailed pollen diagrams for the monoliths at each site show the same major palynological trends. There are, of course, differences in detail though this is not uncommon when more than one core is studied at a site (Edwards, 1983). (a) Lanshaw Moss is a soligenous mire that lies in the NE of Rombalds Moor in West Yorkshire. It is centred around Grid Ref. S.E. 130 453 at a height of 350 m O.D. and extends approximately 100 m north to south and 250 m from east to west. At its deepest point, towards the centre, there are ca. 2.5 m of peat above a minerogenic substrate. The present day vegetation is dominated by

Eriophorum vaginatum, Eriophorum angustifolium, Sphagnum spp., Calluna vulgaris, Empetrum nigrum and Juncus effusus. (b) White Moss is part of a raised moss complex situated to the far west of the Craven district of West Yorkshire. It is centred around Grid Ref. S.D. 791 545 at a height of 190 m O.D. It extends approximately 800 m north to south and 630 m from east to west, at its widest point. At the deepest point sampled for palynological work (Jones, 1977), there is 7.4 m of peat above a

minerogenic substrate. In common with Lanshaw Moss, the present day vegetation is dominated by Eriophorum

vaginatum, Eriophorum angustifolium, Sphagnum spp., Calluna vulgaris, but in contrast Empetrum nigrum and Juncus effusus are rare or absent and a significant amount of Erica tetralix is present. METHODS Vertical columns of peat 115 cm apart horizontally and each measuring 38 x 15 x 15 cm were excavated from a freshly cleaned face at each site. After sub-sampling for pollen analyses each monolith was cut into 1 cm thick slices to provide adequately sized samples for ~4C dating. With Lanshaw Monolith ' R ' the depth to be analysed spanned two monolith cases and so the depth increment 87.5-88.5 cm was split into two 0.5 cm thick samples, 8 7 . 5 - 8 8 . 0 cm from the upper case and 88.0-88.5 cm from the lower case. These samples were not bulked, but kept as two discrete samples because a thin layer of peat from the end of each case has to be removed in order to avoid any possible contamination during sampling and storage. As a result, these two samples are not strictly contiguous. In order to investigate possible radiocarbon age differences within the organic content of the peat each sample was split into its component organic tractions. The subdivision was based on the definition of humification as proposed by Schnitzer and Levesque (1979) whereby the chemically complex bumic and fulvic acids are produced by the chemical and biological breakdown of organic materials. The precise definition of humic and fulvic acids varies according to the method of extraction (Fuchsman, 1980). Here they are defined as the acid and alkali soluble fulvic acid, the alkali soluble/acid insoluble humic acid and the alkali and acid insoluble humin fraction. The precise chemical definition is shown in Fig. 1. Each separation was carried out in a quantitative manner, with no material being discarded, i.e. the total budget of carbon in the raw peat was conserved within the component tractions recovered for radiometric analysis (Figs 2 - 5 ) . The HC m e a s u r e m e n t s made at the NERC Radiocarbon Laboratory were obtained by synthesis of sample carbon to benzene followed by liquid scintillation counting and are shown in Tables 1M-. ~3C analyses were performed on a Finegan MAT Delta stable isotope ratio mass spectrometer and are shown in Figs 6-9. Details of equipment and analytical procedures are as described by Harkness and Wilson (1972) and by Gupta and Polach (1985). C A L C U L A T I O N AND COMBINATION OF RADIOCARBON AGES The availability of ~4C enrichment values for all three component fractions at consecutive levels and their corresponding contribution to the total carbon content of the raw peat allowed the calculation of average ages for selected combinations of fractions (Tables 1-4). The

J.S. Shore et al.: Problems Encountered With 14CDating of Peat

375

Raw sample i)Dry to constant weight and record ii)Digest in 0.5M KOH (20ml/g sample) at 80°C for 10 hrs iii)Centrifuge (20mins at 3000rpm) ,4,, Solution i)Adjust to pH3 by addition of 10% H)SOa

I

ii)Digest at 80oc to coagulate precipitate.

iii)Centrifuge (20mins at 3000rpm)

'4/ Solids i)Repeat KOH extraction as above ii)Continue extraction procedure with distilled water until no further colouration in solution. iii)Check residual solids are at neutral pH. iv)Acidify if necessary, wash, collect by filtration and dry in vacuum oven (50oc)

L

'4/ Solution Precipitate i)Evaporate to near dryness i)Wash with cold distilled on hotplate. water ii)Dry to constant weight in ii)Collect by filtration and vacuum oven (50°C) dry in vacuum oven (50°C)

1

1

Fulvie Acid

Humic Acid

Humm

(Record weight)

(Record weight)

(Record weight)

FIG. 1. Pretreatment of peat prior to 14C analysis.

weighted average enrichment values and their corresponding ages were determined as follows: D{~+b.... ~ = D , f b + D ~ + . . . where D = weighted mean estimate for a selected combination of component analyses, Dx = measured 14C enrichment for individual component fractions, f~ = mole fraction contribution of the individual component to the total carbon content o f the hypothetical composite sample. Then DT4C Age = - 8 0 3 3 1 n [1 +

] BP.

1000 DISCUSSION For Lanshaw and to a lesser extent White Moss, the

emphasis was on a quantitative study, not previously carried out in other research, of the carbon distribution within the fractions and on obtaining radiocarbon ages for all fractions at all levels. As in most samples, the humic acid contains most of the carbon and therefore has the greatest influence on a combined age. Therefore it is fundamentally important to establish whether or not it should be regarded as a contaminant in peat and therefore removed prior to dating. At Lanshaw P 14 of the 17 levels show the humic acid to be significantly older than the corresponding humin fraction. Other levels show no significant difference between the two fractions. In most cases the fulvic acid is the youngest fraction. Possible causes for the age differences of the three fractions include the upward movement of humic acid or the intrusion of younger rootlets from above. At this site the most plausible explanation is that younger rootlets (too fine to be removed by handpicking) penetrate down into older peat. As the plant

Quaternary Science Reviews (Quaternary Geochronology): Volume 14

376

% Carbon conlenl wrl Iolal dry peal 10

20

30

40

50

60

70

80

90

100

f

I

t

i

I

I

I

I

I

I

80 81 82

IIIIllllg

83 84 85 86 87 E

U J~

88

~o

89

~1~'~h~1~"as'~'~J

I

[] Humic Acid [] Humin

I

EIIIIIIII|

[] Fulvic Acid

l

0 !

90

]

91 7

92

1

93

LUlI.I

94

1111111

95 96 97

Itflllllllllllllltltlllltl"~

~mmmHmlu~mmmmmmtmlHmmtl

[illlllllllllllllllltitlllllllllll~

FIG. 2. Lanshaw monolith P. %Carbon

content

w~

tolaldrypeat

10

20

30

40

50

60

70

80

90

100

I

I

I

I

I

I

I

I

I

I

80 81 82 83 84

I 41111111qllqllB ! .,.,Imm..,,m.. 1

85 86 87

J ,i,..,.| I

87.75 E t.J J~

[] Humic Acid I []Humin

88.25

1

90 91

[] gulv c Ac d J

!

89

J,.........,.

92 93 94

!

!

95 96 97

FIG. 3. Lanshaw monolith R.

J.S. Shore et al.: Problems Encountered With 14C Dating of Peat

377

% Carbon conlent wrt tolal dry peat

0

10

20

30

40

50

60

70

8O

9O

100

I

I

I

I

I

I

1

I

I

I

75 76

~\\\\\\\~\\\\\\\\~1

77

~\~\\\\\\\\\\\\\\\\\~

I

78 79

~\\\\\\~\\\\\\\\\~|

8O

~\\\~\\\\~\\\\\\\\\~$

81 E

82

nHumic Acid1 [] Hum n

o

83 o

~\\\\\~\\\\\%.\\\%.\~1

84 85

~\\~\\\\\\\\\\\\\\X~I

86 87 88

I

~\\\\\\\\\\\%.\\\\\\~1

89 I

9O

FIG. 4. White Moss monolith A.

% Carbon conlenl wrl tolal dry peat 0

10

20

30

P

I

I

40 r

50 ~

60 I

70

8O

9O

tO0

I

I

I

F

75 76 77 78 79 80

~ \ \ \ \ \ \ \ ' %

.~\\\\\\\\',1

81

~

82

[] Humic Acid 1 [] I Ium n

~- 83 o

84 85 86 87

~,\\\\~XXX\\\\\\X'~

88 89 90

I

FIG. 5. White Moss monolith C.

378

Quaternary Science Reviews (Quaternary Geochronology): Volume 14 TABLE 1. Lanshaw Monolith P I

Laboratory no.

DL4C values (_+1~), ('f' values)

Depth (cm)

Humin

Humic acid

Fulvic acid

SRR 4000

81

SRR 4001

82

SRR 4002

83

SRR 4003

84

SRR 4004

85

SRR 4005

86

SRR 4006

87

SRR 4007

88

SRR 4008

89

SRR 4009

90

SRR 4010

91

SRR 4011

92

SRR4012

93

SRR 4013

94

SRR 4014

95

SRR 4015

96

SRR 4016

97

-442.73 _+5.96 (0.15) -488.74 _ 5.64 (0.14) -473.37 + 5.82 (0.13) -480.01 + 4.36 (0.35) -483.63 _+4.34 (0.26) -498.06 +_4.08 (0.30) -468.33 _+5.57 (0.15) -436.06 _+6.29 (0.20) -524.21 + 4.06 (0.25) -560.02 _+4.11 (0.31) -564.42 _+3.55 (0.22) -580.41 _+3.99 (0.27) -568.19 + 4.16 (0.23) -605.45 _ 3.05 (0.29) -592.44 + 4.26 (0.25) -595.03 + 4.19 (0.25) -578.58 + 4.84 (0.18)

-492.57 _+3.24 (0.75) -489.78 + 3.41 (0.78) -508.86 _+3.35 (0.74) -515.73 + 3.42 (0.58) -522.32 + 3.09 (0.68) -509.73 +_3.09 (0.65) -527.66 ± 3.09 (0.77) -533.93 _ 4.07 (0.73) -555.93 + 2.93 (0.69) -591.59 + 2.71 (0.63) -590.23 _+2.70 (0.71) -589.99 +_2.71 (0.67) 590.75 _+2.90 (0.72) -621.07 _+2.32 (0.66) -624.79 _+3.40 (0.55) -608.34 _+3.15 (0.40) -606.42 + 3.17 (0.55)

-477.82 + 5.83 (0.1 O) -437.52 + 7.06 (0,08) -473.03 + 7.39 (0,13) -467.40 + 8.45 (0.07) -423.65 _+9.43 (0.06) -427.93 _+7.63 (0.05) -482.10 _+8.42 (0.08) -427.04 +_8.77 (0.07) -490.72 _+9.43 (0.06) -486.53 _+ 13.58 (0.06) -547.20 _+6.11 (0.07) -496.32 _ 14.04 (0.06) 557.14 _+ 14.47 (0.05) -578.72 + 10.20 (0.05) 596.24 +_4.72 (0.20) -596.56 _+3.27 (0.35) -577.60 + 4.03 (0.27)

TABLE 2. Lanshaw Monolith R - - D ' 4 C values (±lc;), ('f' values) Laboratory no.

Depth (cm)

Humin

Humic acid

Fulvic acid

SRR 4017

83

SRR 4018

84

SRR 4019

85

SRR 4020

86

SRR 4021

87

SRR 4022

87.5-88

SRR 4023

88-88.5

SRR 4024

89

SRR 4025

90

SRR 4026

91

SRR 4027

92

SRR 4028

93

SRR 4029

94

SRR 4030

95

-450.72 + 4.58 (0.29) -461.84 _+4.92 (0.27) -436.29 + 4.74 (0.23) -446.18 + 5.61 (0.14) -500.06 + 5.25 (0.18) -480.67 -2_5.49 (0.23) -551.71 _+4.80 (0.24) -531.04 _+4.96 (0.12) -539.53 _+5.45 (0.13) -545.44 + 4.77 (0.21) -526.93 -+ 4.86 (0.25) -532.86 -+ 3.30 (0.26) -582.95 +_3.66 (0.16) -593.16 _+4.14 (0.24)

-418.88 _+4.31 (0.59) 4 1 6 . 7 9 _+4.51 (0.54) 4 5 1 . 6 1 +_4.12 (0.64) M50.24 _+3.43 (0.71) -477.67 + 4.50 (0.74) -486.54 _+4.14 (0.67) -515.43 + 3.98 (0.56) -516.60 _+3.20 (0.74) -525.58 _ 3.97 (0.74) 533.60 _+3.92 (0.67) -507.77 _+2.63 (0.67) -517.36 _+2.70 (0.67) -532.97 _+2.59 (0.70) -547.83 _+2.65 (0.65)

-460.77 _+ 12.7 I (0.12) -431.55 _+6.27 (0.19) -444.63 _+5.87 (0.13) -418.78 + 5.06 (0.15) -483.96 _+9.40 (0.08) N/A (0.10) N/A (0.20) --491.63 -+ 4.24 (0.14) -524.98 + 4.47 (0.13) -471.20 + 5.39 (0.12) -491.55 + 5.19 (0.08) -516.88 + 6.79 (0.07) -543.12 _+4.34 (0.14) -517.69 _+5.40 (0.11 )

J.S. Shore et al.: Problems Encountered With 14C Dating of Peat

379

TABLE 3. White Moss Monolith A - - D l a C values (±1~), ('f' values) Laboratory no.

Depth (cm)

Humin

Humic acid

SRR 4031

76-76.5

SRR 4032

77

SRR 4033

79

SRR 4034

80

SRR 4035

81

SRR 4036

82

SRR 4037

83

SRR 4038

84

SRR 4039

85

SRR 4040

86

SRR 4041

88

SRR 4042

90

-502.52 _+4.40 (0.33) -487.74 ± 4.33 (0.34) -510.52 _+4.17 (0.37) -516.74 _+4.00 (0.30) -535.23 + 3.89 (0.37) -543.34 + 3.81 (0.37) -551.38 _+3.63 (0.34) -566.85 ± 3.42 (0.34) -570.45 ± 3.82 (0.34) -574.48 ± 3.54 (0.32) -580.02 ± 3.51 (0.37) ~514.83 ± 3.44 (0.19)

-492.56 ± 3.46 (0.67) -453.49 ± 3.79 (0.66) -516.02 ± 4.01 (0.63) -551.59 ± 2.46 (0.70) , -566.19 + 2.60 (0.63) -560.98 ± 2.70 (0.63) -573.03 ± 2.68 (0.66) -588.38 ± 2.70 (0.66) -594.61 ± 2.65 (0.66) -592.49 _ 3.12 (0.68) -608.96 ± 2.81 (0.63) -633.88 ± 2.56 (0.81)

TABLE 4. White Moss Monolith C - - D~4C values (_+la), ( ~ values) Laboratory no.

Depth (cm)

Humin

Humic acid

SRR 4043

80

SRR 4044

84

SRR 4045

85

SRR 4046

86

SRR 4047

87

SRR 4048

90

-356.03 _ 4.47 (0.36) -425.13 _+4.81 (0.33) -495.63 ± 4.53 (0.29) -527.42 _+4.30 (0.36) -588.88 __3.79 (0.26) -594.21 e 4.20 (0.18)

-338.64 + 4.61 (0.64) -419.82 _+4.05 (0.73) -501.63 e 3.66 (0.71) -528.29 _+3.36 (0.64) -608.17 ± 2.83 (0.74) -623.59 ± 3.10 (0.82)

litter d e c o m p o s e s to form humic acid the younger less decomposed rootlets form the majority of the humin fraction at any given level. Fulvic acid, soluble in both acid and alkali, is likely to be very mobile within peat, possibly being leached downwards to give ages which in most cases are y o u n g e r than both other c o r r e s p o n d i n g fractions. It could therefore be concluded that the age closest to the ' t r u e ' age o f d e p o s i t i o n w o u l d be given b y the h u m i c a c i d fraction, a l t h o u g h even this c o u l d be too young as the younger intrusive rootlets deacy and contribute younger carbon to the humic acid. However, when this theory was tested at Lanshaw R the opposite pattern was found. O f the 14 levels dated, eight humins were found to be older than the corresponding humic acids, the others showing no significant differences. Five levels also show the fulvic acid to be older than one or both o f the other fractions. This could be due

to a rise in the water table or lateral drainage possibly along root paths. The macro fossil content o f Lanshaw R indicates that the site was probably always a little drier than Lanshaw E It may be possible that the humic acid has been washed downwards by water draining through the peat in a way which would not happen in the saturated peat of Lanshaw E However, it is difficult to explain the downward movement of humic acid while the fulvic acid was moving upwards or laterally. Another possible e x p l a n a t i o n o f the age d i f f e r e n c e s c o u l d be that the h u m i n f r a c t i o n at L a n s h a w R is c o m p r i s e d o f o l d e r woody material around which the finer peat has accumulated. This h y p o t h e s i s is supported by the presence o f wood and bark in the peat. However, the amount of this material found is less than the amount of other organics present except at levels 8 3 - 8 6 cm and this does not coincide with any pattern in ages.

Quaternary Science Reviews (Quaternary Geochronology): Volume 14

380

weighted mean age (years BP ¢1o) 5000 5500 6000 6500 7000 7500 8000

82

Age Difference (fraction mean ages) Humic Acid - Humin 1000 2000

8°3C Enrichment

Age Dillerence (traction m e a n ages) H u m i c Acid - Fufvic A c i d 1000 2000

8~3CpoeJ0.1%o 28 26

30

I

u\

I~

A'."

24

,-'",k •

i

84

laN 86

<._ ~ 88

£

IOl

Alnus rise

lel 92

tel WN

94

NM

96

HH 95

FIG. 6. Lanshaw monolith P.

weighted mean age

[years BP _+1o) 4000 4500 5000 5500 6000 6500 7000

MH

Age Difference (fraction mean ages) H u m i c Acid - H u m i n -1000 -500 500

\

4.

84

Age Difference (Iraction m e a n ages) H u m i c Acid - F u l v i c Acid - t 000 1000 2000

8'~C Enrichment 8 t~Cpdb~o. 1%o 29 28

30

31

A

27

!

7 86

I~

4

HH HH 88

[3

I~H

90

\

, \

I~H 92

94

I~

]

AI~Ius rise

.

L \

i .

,'

&.~"l'

M

I

.-

;

Hurnin

I

- • - Humic Acid 96

]- - ~ -- Fulvic Acid I

FIG. 7, Lanshaw monolith R.

At White Moss, Sphagnum is present in large quantities in the upper half of both profiles. If the conclusions of Dresser (1971) could be applied to this site then it would be expected that the ages from this Sphagnum peat would not show any significant differences in the ages of the two fractions. However, the presence of Sphagnum in both profiles does not coincide with any change in the pattern of ages and significant differences occur between the h u m i c a c i d and h u m i n f r a c t i o n s e v e n w h e r e Sphagnum is dominant.

It would be expected that two neighbouring profiles should give almost identical ages for the same pollen feature. Therefore, by comparing humin age pairs and humic acid age pairs for the same feature it should be possible to infer which fraction is the more consistent and thereby most likely to be nearer the 'true' age of the feature. The level at which Alnus becomes significant is shown on the age/depth profiles (Figs 6-9) and this feature is used in order to test this theory (Shore, 1988). At Lanshaw the humin ages for the rise in AInus are closer to each other

J.S. S h o r e et al.: P r o b l e m s E n c o u n t e r e d W i t h 14C D a t i n g o f P e a t

weighted mean age (years BP ±1o1 5000 5500 6000 6500 7000 7500 8000 76 I +-- + + i i

Age Ollterence (fraction mean ages) Humic Acid - Humin -1000 -500 0 500 1000 .... I I +

381

,~'c Enrichment ~'3cpue+0.1~, -29 -28 I I

-30 I-

?'

78

+27

/

/

/

I 80

82

Alnus rise

/

4 /

Nil 84

1414 1411 •

86

/ m

I I

t

88

90

I

¢

Humin

]

-- • - Humic Acid

/ iI I

F I G . 8. W h i t e M o s s m o n o l i t h A.

3000 80

m

weighted mean age (years B P ± l a ) 4000 5000 6000 7000 ~

,

i

,

i

,

8000

i

Age Dilference (fraction mean ages) Humic Acid - Humin - 500 0 500 1000

~':~C Enrichment

t:~Cpos.O. 1%° -28.5 -28

-29

-27.5 f

+

\ % \

81

\

82

\ %%

83

84

m

E u Q.

85

m

Q

////

86

n Alnus rise

87

m

] / / /

88

/ / / /

89

/ ! /

90

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Quaternary, Science Reviews (Quaternary Geochronology): Volume 14

than the corresponding ages for the humic acid. From this it could be implied that at this site the humin fraction gives an age nearest to the 'true' age. However, when the same test was performed on the White Moss data the humin ages are not so tightly matched and the argument becomes less convincing. A l t h o u g h all the ages in this study fall within the range which can now be calibrated, it was decided that there was little to be g a i n e d f r o m such a study. The process of calibration acts to increase the range of possible error within each age while converting the measure of 14C in a sample to an approximate calendrical date. The calendrical date of the samples within this study is not relevant. The salient fact remains that different fractions of the same bulk sample can contain significantly different levels of ~4C and not that the same level of t4C occurred in the atmosphere over different periods of time. Any further calculations performed on these initial measurements only adds to the uncertainty of the resulting age and by doing so can act to blur real differences. Other approaches which have been carried out on the dating of sediments, including peats, have tended recently to concentrate on dating the pollen within the deposit. This has been attempted in different ways: (1) Use of fine fraction because it contains pollen - - it may be argued that as pollen may be expected to give the 'true' age of a feature (often a palynological change) then by dating only the fine fraction of the peat, as Dresser advised, a date closest to the 'true' age will be obtained. However, as the fine fraction is that which is most likely to contain wind borne particles from elsewhere, and these particles may well contain older reworked erosional sediments, the possibility of obtaining an erroneous age is high. These reworked particles will of course be present in the peat regardless of pre-treatment, however their effect on the final age will be much diluted if they are incorporated with samples derived from in situ organics. For this reason no sieving was performed on the humin fraction of this study. (2) AMS dating of pollen - - to take the above argument to its logical conclusion some have attempted to separate pollen from sediments and peat for AMS dating. This method may well be extremely useful in areas where hard water errors may cause problems with the dating of bulk sediments (Regnell, 1992). However, the technique for separation is laborious and if not completed until only a pure pollen sample remains, the problem of contamination by older reworked matter is even greater as it will form a larger proportion of the sample. The effect of any reworked pollen within the sample on the final age will also increase. (3) A M S dating of small samples - - A M S is not intrinsically any better than radiometric techniques for dating peat. Indeed, without extreme care in sample selection AMS could give misleading results. It is still necessary to know which fraction, chemical or physical, is contemporary with the feature to be dated. It should always be borne in mind that the smaller the sample dated the greater the effect of any contaminant present.

CONCLUSIONS Approximately 120 age measurements on peat are perlbrmed by the NERC laboratory each year. The majority of these have in the past been carried out on samples which have only been subjected to digestion in mineral acid followed by washing to neutral pH. The majority of published ages for peat over the last 30 years have been determined from such 'acid insoluble' residues. These data form the basis of our present knowledge concerning the timing of events throughout the Holocene and in many cases it is against these ages that new techniques are compared (Regnell, 1992; Brown et al., 1992). It is i m p o r t a n t to know how m a n y of these ages can be regarded as accurate indicators of the age of the samples and which should be reassessed in the light of recent work in the field of radiocarbon dating. At p r e s e n t the v a r i o u s r a d i o c a r b o n l a b o r a t o r i e s throughout the world use various pre-treatment techniques. Although these methods are noted when reporting date lists often the precise details are absent. Our work calls into question the conclusions reached by previous research and the validity of preferred pre-treatments. Despite the limitations of the data presented here, significant differences have been shown to exist which are not consistent, either within or between sites. No generally applicable advice on pre-treatment can be given other than to advise that all available carbon within a sample is dated and the contribution of each fraction to the total carbon budget measured. This would allow the direct comparison of published data, which at present is not possible.

ACKNOWLEDGEMENTS JSS gratefully acknowledges the U.K. Natural Environment Research Council for the C.A.S.E. studentship that supported this work. Current support by the Leverhulme Trust (grant F 704) and the Carnegie Trust for the Universities of Scotland is also gratefully acknowledged.

REFERENCES Bannister, J. (1985). The vegetational and archaeological history of Rombalds Moor, West Yorkshire. Unpublished Ph.D. thesis, University of Leeds. Bartley, D.D. and Chambers, C. (1992). A pollen diagram, radiocarbon ages and evidence of agriculture of Extwistle Moor, Lancashire. New Phytologist, 121, 311-320. Brown, T.A, Nelson, D.E., Mathewes, R.W., Vogel, J.S. and Southon, J.R. (1989). Radiocarbon dating of pollen by accelerator mass spectrometry. Quaterna~ Research, 32, 205-212. Brown, T.A., Farwell, G.W., Grootes, RM. and Schmidt, F.H. (1992). Radiocarbon AMS dating of pollen extracted from peat samples. Radiocarbon, 34, 550-556. Dresser, EQ. (1971). A study of sampling and pretreatment for radiocarbon dating. Unpublished Ph.D. thesis, Queens University, Belfast. Edwards, K.J. (1983). Quaternary palynology: multiple profile studies and pollen variability. Progress in Physical Geography, 7, 587-609. Fowler, A.J. (1985). Radiocarbon dating of the lake sediments and peats by accelerator mass spectrometry. Unpublished Ph.D. thesis, University of Oxford.

J.S. Shore et al.: Problems Encountered With ~4C Dating of Peat Fuchsman, C.H. (1980). Peat, Industrial Chemistry and Technology. Academic Press, London. Gupta, S.K. and Polach, H.A. (1985). Radiocarbon Dating Practices at A.N.U. Handbook. A.N.U. Printing Services, Australia. Harkness, D.D. and Wilson, H.W. (1972). Some applications in radiocarbon measurement at the Scottish Research and Reactor Centre. In: Rafter, T.A. and Grant Taylor, T. (eds), Proceedings o f the 8th International Conference on Radiocarbon Dating, pp. 210-223. Wellington, Royal Society of New Zealand. Johnson, R.H., Tallis, J.H. and Wilson, E (1990). The seal edge combes, North Derbyshire - - a study of their erosional and depositional history. Journal of Quaternary Science, 5, 83-94. Jones, I.R (1977). Studies in the Flandrian vegetational history of the Craven district of Yorkshire. Unpublished Ph.D. thesis, University of Leeds. Lui Jinling, Tang Lingyu, Qiao Yulou, Head, M.J. and Walker, D. (1986). Late Quaternary vegetation history at Menghai, Yunnan Province, Southwest China. Journal o f Biogeography, 13, 399-418.

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Mehringer, RJ. and Warren (1976). Marsh dune and archaeological chronology, Ash Meadows, Amargosa Desert, Nevada. Nevada Archaeological Research Paper No. 6, pp. 120-150, University of Nevada. Olsson, I.U. (1986). A study of errors in ~4C dates of peats and sediment. Radiocarbon, 28, 429-435. Regnell, J. (1992). Preparing pollen concentrates for AMS dating - - a methodological study from a hard-water lake in southern Sweden. Boreas, 21,373-377. Schnitzer, M. and Levesque, M. (1979). Electron spin resonance as a guide to the degree of humification of peats. Soil Science, 127, 140-145. Sheppard, J.C., Ali, S.Y. and Mehringer, P.J., Jr (1979). Radiocarbon dating of organic components of sediments and peats. In: Berger, R. and Suess, H.E. (eds), Radiocarbon Dating: Proceedings 9th International Conference, 1976, Los Angeles and La Jolla. University of California Press, Berkeley, CA. Shore, J.S. (1988). The radiocarbon dating of peat in relation to pollen analysis. Unpublished Ph.D. thesis, University of Leeds.