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thorium isotope systematics and open-system behaviour of peat layers
Chemical Geology (Isotope Geoscience Section), 94 ( 1992) 161-171 Elsevier Science Publishers B.V., Amsterdam
161
Uranium/thorium dating of Late Pleistocene peat deposits in NW Europe, uranium/thorium isotope systematics and open-system behaviour of peat layers H. Heijnis and J. van der Plicht Centrefor Isotope Research, UniversityofGroningen, Neijenborgb 4, NL-9747 AG Groningen, The Netherlands (Received July 16, 1991; revised and accepted October 31, 1991)
ABSTRACf Heijnis, H. and van der Plicht, J., 1992. Uranium/thorium dating of Late Pleistocene peat deposits in NW Europe, uranium/thorium isotope systematics and open-system behaviour of peat layers. Chern. Geol. (Iso!. Geosci. Sect.), 94: 161-171. The possibility of dating peat by the uranium-series disequilibrium method is discussed. In principle, this method can be used to date peat to -350 ka. The application of the U/Th disequilibrium method (UTD) on peat provides us with the probability of constructing a new chronology for the Late Pleistocene paleoclimatic record in NW Europe. The reliability of the obtained ages will be discussed as well as open-system behaviour and the contamination with detrital Th. By studying in detail interglacial peat profiles from the Tenagi Philippon site, Greece (a long terrestrial record), of an expected age of 125 ka and the Fenit site in Ireland of unknown age, we were able to explain the results in terms of the suspected open-system behaviour of top and bottom parts of these layers and how to avoid it by careful sampling. Peats contaminated with detrital Th were also analysed. Two peat layers, which were interpreted on basis of pollen analyses, stratigraphic position and TL dates to be early Last Glacial in age, were sampled. The first one is the Alit Odhar organic layer near Inverness, Scotland, and gave an age of 106 ka. The second is the key site to the British Last Glacial stratigraphy, the Chelford organic layer at Chelford, Cheshire, yielded an age of 86 ka which is in good agreement with the recently obtained TL dates.
1. Introduction
Dating peat by the U jTh disequilibrium (UTD) method provides the possibility of a detailed study in Late Pleistocene geology on a time scale beyond the range of 14c. The vegetational history preserved in peat (by means of pollen and macrofossils), combined with reliable UTD dates yields detailed information about paleoclimatic variations over the past 0.3 Ma. The dating of long terrestrial records makes correlation of the land chronology with the 18 0 deep-sea record possible. Earlier attempts to date peats have met with variable degrees of success (Vogel and Kronfeld, 1980; Kafri et aI., 1982; De Vernal et aI., 1986; van
der Wijk, 1987; van der Wijk et aI., 1988). The present study focuses on the behaviour ofU and Th isotopes in peat layers and various peat fractions during the chemical processes. The relationship between expected ages and observed ages was studied using sequences of peat. Open-system behaviour could be detected by analysing series ofsamples from these peat layers. Two sections were selected to study the open-system behaviour in detail. The first is an interglacial peat layer from the Tenagi Philippon borehole No. III (Wijmstra and Groenhart, 1983) in Macedonia, Greece, of an expected age of 125 ka (equivalent of 180 deepsea stage se; Martinson et aI., 1987) used as a standard of a known age. The second is an in-
0168-9622/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
162
H. HEIJNIS AND J. VAN DER PLICHT
terglacial peat layer from Fenit (County Kerry, Ireland) of an unknown age. The second study was focused on peat samples with a relatively low 230Thj232Th ratio, which are less suitable for dating by UTD. Nevertheless, attempts have been made to date two peat layers which were interpreted on the basis of pollen analyses, stratigraphic position and TL dates to be early Last Glacial interstadial organic deposits. The first one is the Alit Odhar organic layer (Walker, 1990) near Inverness, Scotland, and the second are the famous Chelford organic deposits (Simpson and West, 1958; Worsley, 1980) at Chelford, Cheshire. The 14C ages obtained for these sites were generally > 51 ka BP (Worsley, 1980; Harkness, 1990). 2. Experimental procedures
2.1. The dating UTD method
The natural isotope 238U decays - via other short-lived nuclides neglected here - to 234U, which in turn decays with a half-life of 248 ka (by emitting an alpha particle) into 230Th (Ivanovich and Harmon, 1982). The 230Th itself decays (also by emitting an alpha particle) with a half-life of 75 ka. This decay is the only source for 23°Th available in nature. Thus, as long as the production of 23°Th exceeds its radioactive decay, 230Th accumulates and the 230Thj234U activity ratio is a measure of age (Ivanovich and Harmon, 1982): 230Th 234U
l-e _230Al ( 1) 234Uj238U + 1- 234U j2 38U X
23°2 (1- _<230 A_234 A)I) 23°2_ 2342 e
( 1)
After -- 350 ka equilibrium is reached, where the decay of 230Th balances its production. The activity ratio e 30Thj234U) is a function of age, which can easily be determined if the initialratio e30Thj232Th)0=0. The (UjTh) activity ratios are measured by alpha-spectrom-
etry. Therefore, the U and Th have to be extracted from the material, and transferred into alpha-radioactive sources. The alphasources are obtained by electroplating U and Th on a stainless-steel disk. Consequently, alpha-spectra are analysed by means of an 8-fold alpha-spectrometer read out by a microcomputer (Ivanovich and Harmon, 1982; van der Wijk,1987). 2.2. The UTD method applied to peat
Peat can absorb large amounts of U from groundwater (Szalay, 1969). Especially the humic and fulvic acids absorb large amounts ofU and form large stable complexes. Since Th is highly insoluble in groundwater, the radiogenic isotope 230Th concentration should be zero at the time of peat formation. Thus the present-day 23crrh can be solely attributed to the decay of its radioactive precursor 234U. Violation of this assumption can be traced by the presence of the natural isotope 232Th (t l /2 = 1.39·\010 a). The reliability of the UTD dating for peat samples depends on whenever the system is closed. The compaction and burial by other sediments isolate the peat system from further U uptake and so, in principle, the method should be valid for dating this ubiquitous Quaternary material. 2.3. Chemical procedures
The peat samples are first dried overnight (60°C) and weighed to measure the water contents. Next the organic fraction of the peat is destroyed in a muffle furnace at a temperature of 550°C (24 hr.). The rather low temperature is to avoid the formation of insoluble Th-oxides (S.A. Short, pers. commun., 1990). Eventually the remaining organics are destroyed by adding a few drops of H 20 2 to the ashes during the leaching procedure. The remaining ashes are leached in 7 M HN0 3 and a spike solution, which is a solution of known
URANIUM/THORIUM DATING OF LATE PLEISTOCENE PEAT DEPOSITS
enU j2 28Th ) activity ratio, is added to yield tracing and is stored for 24 hr. to reach equilibrium between solution and spike. From the solution thus obtained, U and Th are isolated by coprecipitation with Fe(OHh U and Th are separated from each other by means of ion-exchange column chromatography [Biolcad'" AG 1X8 (100-200 mesh) ]. After the separation ofU and Th alpha-radioactive sources are obtained by electrodeposition on stainless-steel disks.
3. Open-system behaviour of peat layers and the consequences for dating
3.1. Experiments The research of Late Pleistocene peat layers has been traditionally focussed on its paleoclimatological implications, i.e , the story deduced from its pollen contents. However, this leads to an ambiguous approach in dating these layers. Paleoclimatologists/ecologists are in the first place interested in major climatological shifts which merely occur at the upper and lower boundaries of peat layers. These boundaries are not suitable for UTD dating because of the expected open-system behaviour of top and bottom parts of peat layers. To prove the open-system behaviour of these top and bottom parts, two systems were selected for a very detailed dating program. The first layer is a 4-m-thick peat layer from the Tenagi Philippon borehole III in Macedonia, Greece, which is the equivalent of 18 0 deep-sea stage se with an expected age of 125 ka. The pollen analysis of Tenagi Philippon is described in detail by Wijmstra and Groenhart (1983). From this interglacial peat layer 8 samples at 50-em intervals were taken for UTD analysis. The second site at Fenit, Co. Kerry, Ireland, consists of a complex of three peat layers with a maximum thickness of '" 40 em. This site was sampled at 2-cm intervals and is of unknown age. The pollen diagrams do not elucidate the
163
correlation with any of the 18 0 deep-sea stages. The age and correlation of this site will be published elsewhere (Heijnis, 1992; Heijnis et al., 1992). The Allt Odhar and Che\ford sites were studied in detail to investigate the contamination with detrital Th. These two sites have low 230Th/232Th ratios.
3.2. Results and discussion 3.2.1. The Tenagi Philippon site The results of a detailed study on the behaviour of different isotopes on the Tenaghi Philippon III peat layer are summarized in Tables I and II, and shown graphically in Fig. 1. 3.2.1.1. The organic contents. The organic contents differs with depth. The gyttja has a lower organic content and at the boundaries there seems to be partial mixing of the gyttja and peat. The centre of the peat layer has a very high organic content. There is hardly any inorganic matter which could be the source of detrital Th. 3.2.1.2. The different leachatefractions. An experiment to test the influence of the detritus on the activity ratios and age was performed. The resulting 3Drh/ 23 4 U ) activity ratios show no large differences between the 2 M HN0 3 and the HF-HCI0 4 fractions of samples Sand 8 (see Table I).
e
3.2.1.3. The open-system behaviour at sediment boundaries. At the upper and lower boundary of the peat layer one can notice a sudden increase in U (samples 4 to 3 and 7 to 8) while Th does not increase proportionally. The U/Th ratio and the e30Th/234U) activity ratios reflects this phenomenon very clearly. A proportional increase in U causes a change in age as one can see from the diagram in Fig. 1. Exchange of U between the gyttja (high in U content) and the peat (low in U content)
164
H. HEIJNISANDJ. VAN DER PLICHT
TABLE I Radiometric data obtained from the Tenagi Philippon III peat samples, Macedonia, Greece Sample
Depth (m)
% loss
671 672 673 674
27.10-27.20 28.20-28.30 29.00-29.10 29.90-30.00 30.70-30.80
88 82 76 88 88
675 676 677
31.60-31.70 32.50-32.60 33.40-33.50
83 66 62
670
UTDcode
234U/238U
23<>-rh/234U
23<>-rhF32Th
on ignition
Leachate age (ka) uncorrected"!
G89511 G89512 G89513 G89514 G89515 G89519"3 G89516 G89517 G89518 G89522"3
1.026±0.011 1.045±0.009 1.028±0.011 1.215± 0.030 1.659±0.147 1.110 ± 0.079 1.324 ± 0.044 1.135 ± 0.090 1.022 ± 0.0 14 1.028 ± 0.0 16
0.507 ± 0.023 0.523 ± 0.0 II 0.716±0.058 0.687±0.030 0.678±0.053 0.632 ± 0.063 0.650 ± 0.033 0.652 ± 0.051 0.591 ±0.015 0.574±0.018
19.75±2.95 15.75±0.83 6.667± 1.10 6.927±0.71 4.112±0.49 2.407±0.37 4.638±0.40 12.43± 2.44 88.33± 13.1 123.0±25.2
corrected'" (j= I)
76~~ 80~~ 135~~~
76~~ 124~~g
119~~o
109~~1
111~g
94~IJ
III ~l6 III~~
96~:A 106~:~
97~~
96~:
74~~
oIInitiaI 23<>-rhF 32Th=0. o2Initiai23<>-rh/232Th = I (j= I ). o3Sampies totally dissolved in HF-HCI04 _
TABLE II More radiometric data obtained from the Tenagi Philippon III peat samples, Macedonia, Greece Sample No.
670
671 672 673 674 674* 675 676 677 677*
UTD code
G89511 G89512 G89513 G89514 G89515 G89519 G89516 G89517 G89518 G89522
Ueone. (ppm)
18.16 13.74 13.22 2.102 0.750 0.831 1.324 1.135 16.67 6.21
Th cone. (ppm)
U/Th ratio
4.78 3.81 5.40 0.890 0.379 0.38 0.535 0.74 4.96 4.73
3.80 3.60 2.44 2.35 1.97 2.18 2.47 2.75 3.35 3.42
Content (mbq g:") 238U
234U
232Th
23<>-rh
222±2.5 168±1.5 162± 1.6 25.7±0.5 9.11 ±0.6 1O.2±0.5 16.22±0.5 25.13± 1.5 204±2.6 198±3.1
228±2.5 176± 1.5 166±1.6 31.3±0.6 15.12±0.8 11.29±0.6 17.75±0.6 28.5± 1.6 209±2.7 204±3.2
5.87±0.87 5.85±0.30 17.9±2.9 3.1±0.3 2.5±0.2 2.9±0.4 2.5±0.2 1.44±0.2 1.40±0.2 0.95±0.2
115.8±4.9 92.09± 1.6 119.5±9.6 21.5±0.8 1O.26±0.5 7.l4±0.6 11.55±0.4 18.6±0.97 123±2.5 117.3±3
* Samples totally dissolved in HF-HCI0 4 •
occurs only at the top and bottom of the peat layer. Samples 3 and 7 might be contaminated with U from the gyttja but the altered U/Th ratio could be caused by partially mixing of peat and other substances, like gyttja or clay (note that the organic contents are slightly higher in the middle part ofthe peat layer). The post-depositional U accumulation from seepage water at the sediment boundaries also leads to a disturbed U/Th ratio. From this
point of view sample 5 seems to be the most reliable one for straightforward dating purposes. 3.2.1.4. Age calculation using leaclzates alone. To obtain a reliable date from this data set the age was calculated using the leachate-alone method (Schwarcz and Latham, 1989). The principle of this method is that a datable substance consists of two components, a datable
165
URANIUM/THORIUM DATING OF LATE PLEISTOCENE PEAT DEPOSITS
Tenagi Philippon III
10
28
/
=', ~l~
...'-.
/
9
.
I'
I:" .,
',.",.
s: ....
~
;30
I';'
~
~
,;.; \~~ ~:
;31
:.:
---
~~~
~
32
~3-
I-
5
8 \
't
o '() 20 oUconc. ppm • Theone. wn
\
o5
.
too000
10
15
234U/232Th
20 130000
• v.clWijk, (1987)
BTIpeat
15
~gyttja
Tenagi Philippon III,Macedonia,Greece. Fig. I. Age-depth diagram and the relationships between sediment, age, U and Th isotopes, and organic contents of the Tenagi Philippon III peat samples, Macedonia, Greece.
fraction and a contaminant (detritus). As all samples have slightly different proportions of detritus one can expect that the activity ratios of the samples form a straight line in the isochron plot (Schwarcz and Latham, 1989), because of their more or less uniform age (the age differences are within the accuracy of the method). The activity ratios of samples 3-7 were plotted in isochron diagrams (Fig. 2) and the corrected ratios were calculated from the slopes of the least-squares fits of both diagrams. A corrected age of 122 ± l~ ka was calculated from the whole data set. This date agrees well with the age of the climatic maximum of the last interglacial ( 180 deep-sea stage 5e ) . 3.2.1.5. Conclusion. The top and bottom zones of this interglacial peat layer behave as an open system with regards to U due to the exchange
t 10
12
14
16
238U/232Th
Fig. 2. Plots of 230The32Th vs. 234U/ 232Th and 234U/ 232Th vs. 238U/232Th for leachates of the Tenagi Philippon samples. 230Th/ 234 U and 234 U /238U as determined from the slopes of the respective lines gives an apparent age of 122 ± ~ ka.
ofU from the gyttja and possibly also by postdepositional uptake of U from seepage water. As the right proportions of this open-system behaviour of top and bottom zones of peat layers are still not clear it was decided that the second site would be studied in more detail. 3.2.2. The Fenit site, Ireland The results are summarized in Tables III and IV and Fig. 3. The Fenit site (between Fenit
166
H. HEIJNISANDJ. VAN DER PLICHT
TABLE III Radiometric data obtained from the Fenit 89-1 peat samples, Co. Kerry, Ireland Sample
Depth (em)
% loss on ignition
UTD code
23·U/ 238U
230Th/234U
23°Th/232Th
Leachate age (ka) uncorrected>'
corrected'? (j= I)
692 696 699 700 701 702 703 706 708 709 7lJ
16-18 21-22 25-26 28-30 30-31 31-32 32-34 39-41 45-46 46-48 49-50
65 88 83 69 75 75 43 88 87 90 89
G89493 089494 089497 089495 089498 089496 089499 089489 089490 089506 089508
1.061± 0.022 1.054± 0.009 1.109 ± 0.024 1.156±0.007 1.206± 0.027 1.197±0.012 1.202± 0.022 1.043± 0.021 0.996±0.019 1.080± 0.028 0.973±0.009
0.699±0.O21 0.708±0.016 0.617 ± 0.090 0.499 ± 0.0 17 0.41O±0.014 0.594±0.013 0.731 ±O.O47 0.356±0.O13 0.652±0.019 0.657±0.O23 0.682 ± 0.025
8.725±0.524 9.054 ± 0.359 3.543 ± 0.678 9.690±0.411 9.330±0.495 10.890± 0.385 6.172±0.512 25.23 ± 3.322 12.43 ±0.693 20.5 I ± 2.25 I 27.87 ± 3.298
129~X
131~X 102~H 74~1 56::~
95::1 135:::~ 48::~ H5~X
120~X 123~~
82~~~
68::1 52::~
89::1 122:: :~ 46::~
109::X
114::~
llO::~
125::~o
123::~
>'lnitiaI23~h/mTh=0.
>2lnitial 23~h/mTh= I (f= I). TABLE IV More radiometric data obtained from the Fenit 89-1peat samples, Co. Kerry, Ireland Sample No.
692 696 699 700 701 702 703 706 708 709 7lJ
UTD code
U cone. (ppm)
089493 089494 089497 089495 089498 089496 089499 089489 089490 089506 089508
3.475 5.703 5.258 15.91 11.48 14.57 8.43 4.34 5.76 8.16 27.55
Th cone. (ppm)
1.374 2.253 2.150 4.630 2.78 5.10 3.85 0.794 1.98 2.83 9.27
U/Th ratio
2.529 2.53 2.445 3.43 4.13 2.85 2.19 5.5 2.91 2.88 2.97
and Spa, Co. Kerry, Ireland) was previously investigated by Mitchell (1970). As a result of his investigations he concluded that the organic deposits belonged to the penultimate interglacial period ( 18 0 deep-sea stage 7 or 9). However, Warren (1979, 1985) has subsequently put forward the view that the organic deposits are in fact Last Interglacial in age. Therefore, it was decided to study this contro-
Content (mBqg-l) 238U
234U
232Th
230Th
42.7±0.8 69.9±0.55 64.6± 1.5 195.6±2.1 140.7±3.5 179.4±2.2 103.5±2.3 53.11 ± 1.0 70.76± 1.3 99.78±2.1 336.5±3.6
45.3±0.8 73.7±0.6 71.6± 1.7 226.1 ±2.4 169.7±4.1 214.8±2.5 124.5±2.7 55.39± 1.0 70.45± 1.3 108±2.1 327.6±3.5
3.63±0.2 5.76±0.2 12.47±2.5 11.65±0.6 7.5±0.4 11.71±O.4 14.75± 1.4 0.78±0.1O 3.69±0.20 3.46±0.37 8.02±0.955
31.7±0.7 52.2±0.8 44.16±6.3 112.9±3.4 69.6± 1.6 127.6±2.0 91.1 ±5.4 19.72±0.58 45.94±0.92 71.08± 1.87 223.55±7.6
versial site in detail. Samples were taken in 1989 at the site were the peat deposit reached its maximum thickness. Each single layer was sampled every 1 or 2 em. The results are shown in Fig. 3. Although the scale is totally different from the Tenagi Philippon III site the general trend is more or less the same. The top and bottom parts show deviating U /Th ratios and therefore erroneous (much) younger ages. .
167
UR ANIUM/THORIUM DATING OF LATE PLEISTOCENE PEAT DEPOSITS
Fenit site89-1,(o.Kerry,lreland.
straight line of Fig. 4 was not the reason for their omission, but for the high U /Th ratio as shown in Fig. 3 and Table IV. The samples which are suspected to be in the zone of opensystem behaviour (deduced from the U /Th ratio) should therefore not be included in the corrections for the presence of detrital Th.
~c¢ $ ~"" {J" .~;:; q"'« ° l o,. on ignilion ~ V 0 so 100
" r"
.,. "
~ ,
"t,
-- -
~,
",~ 20
~
A
~~ t--- --
~-~--~------
--- - - ------- ------ -
"\,
40
-- --- - - - - - -- - -- - --
..?' ...-' ~ /~
I-~ ;~ ~
8
"
.. ---- - ------------ --
~
o-t--< o1 0 2 ~a
74 ~a
56k4-<
---
~,
,.E~ ,;
c
\-GJ •
.
I
--- ---- -- - - - - - - -- .----
;j ~ -t
o-t--<
o4Bka
...f-< o-f-i
--->20
~
3.2.2.3. Conclusions. From th e isochron plots a corrected age of 118 ± ~ ka was calculated . This is in good agreement with the paleo-ecological evidence which indicates a transition to a colder climate (Ruddock, 1990) and the position on top of a beach deposit which represents the period of high sea-level at the climatic ma ximum of the Last Interglacial (Heijnis et al., 1992). The zone of open sys-
.1
J; - --- . -- -- - - - 60
"
-- -
~ ni t89-1 - - ----
~:l
I: "" -;
+
lO
.
2
i
i
i
4
s
10
·U ppm ' Thppm .U/Thral io" IS:illcl a1 §
~" 15
100000
a B.P.
140000
+
20
+
+
peal
Fig. 3. Age- depth diagram and the relat ionships between sediment, age. and U and Th isotopes of the Fenit 89-1 peat sa mples. County Kerry. Ireland.
3.2.2.1. Open-system behaviour at the sediment boundary. The central parts of peat layers A and C indicate a more or less constant age between 120 and 110 kat Layer B does not show the same trend as it appeared to be an erosional block which was not found at site 2, which was only 2 m west from site 1. At the top ofthe peat layers the U /Th ratio decreases very rapidly with depth, from 5.5 at 40-cm depth to 2.1 at 45 em . 3.2.2.1. Age calculation using leachates alone. The isochron plots of the Fenit peat samples show a very high correlation coefficient. The two omitted points are two samples from the top ofthe erosional peat layer B (samples 5 and 6) . The fact that those samples are not on the
10
10
20
2lBU12l2Th 20
O l<::.----~----------~ 40 20 o 10
234U/232Th
Fig. 4. Plots of 230T h/232T h vs. 234 U /2J2T h and 234U/ 232T h vs. 238 U/232T h for leachates of the Fenit samples. 230Th /2 34 U and 234U/238U as determined from the slopes of th e respective lines gives an apparent age of 118± ~ kat
168
II. HEIJNIS AND J. VAN DER
tern is limited to the top few centimetres only, as the peat layers are very well protected by the clay layers which have a very low permeability. Because of its high U content complementary measurements of Pa/Th will be performed in the near future, in order to independently check the closed-system behaviour ofthe central parts of the peat layers. 4. Contamination with detrital Th and the consequences for dating
n.rcirr
scribed by Schwarcz and Latham (1989). The corrected ratios are shown in Fig. 5. Lines a and b in Fig. 5 represent the corrected ratios for two different samples (i.e. for samples AO JJ0 and AO190 the lines a and b, respectively) but as the lines are parallel the corrected ratio is identical. Sample A0090 which was taken near the top of the peat layer was excluded from the calculation of the corrected ratios as the data points are off the isochron plots and the 30Th/ 23 4 U ) ratio of 1.595 is erroneous (see Section 3.2.2.2). From the data set an age of 106 ± l~ ka was calculated for the Allt Odhar peat layer. This age is in good agreement with the age of the first Weichselian interstadial (Heijnis, 1992) and makes the interstadial equivalent to the 18 0 deep-sea stage 5 c (Martinson et aI., 1987).
e
4. J. The Allt Odhar site, near Inverness, Scotland
The Allt Odhar organics were interpreted on basis of pollen analyses to be interstadial in origin (Walker, 1990). Radiocarbon dates indicated an "infinite" age (> 51 ka BP) (Harkness, 1990). From the Allt Odhar layer a few samples were chosen from different parts ofthe peat layer, except from the very top and bottom to avoid open-system behaviour (which would lead to erroneous ages). The results are shown in Table V. Individual sample ages which could be calculated from the data shown in Table V are in the range of 150 to 120 ka. The presence of detrital Th made corrections necessary using the "leachate-alone" correction model as de-
4.2. The Chelford site, Cheshire, England
The Chelford organics were interpreted as early Last Glacial interstadial deposits. Previous radiocarbon dates on wood and peat samples indicated a minimum age of 63 ka BP (Worsley, 1980). From a large lump of peat several subsamples (with a different amount of detritus) were chosen for chemical analysis. The data are shown in Table VI and are corrected for the presence of environmental Th,
TABLE V
Radiometric data obtained from peat samples from Alit Odhar, near Inverness, Scotland Sample No.
UTD
U
code
(ppm)
A0190 A0190 A0190 A0190 A0190 A0190 AOO90 AOllOc AOllOd A0190a A0190c
G90445 G90446 G90500 G90501 G90502 G90503 G9lO40 G9lO43 G91044 G9lO45 G9lO46
2.490 2.075 3.945 3.828 3.740 4.119 0.199 0.542 0.635 1.549 1.564
234U/238U
230Th/ 234U
230Th/232Th
1.027±0.043 1.427 ± 0.084 1.141 ±0.048 1.120±0.046 1.165 ± 0.048 1.133±0.042 1.129±0.050 1.052± 0.049 1.239 ± 0.044 1.175 ± 0.025 1.189 ± 0.021
0.752±0.038 0.619 ± 0.040 0.745±0.037 0.764±0.038 0.861 ±0.042 1.279 ± 0.053 1.595±0.132 0.843±0.045 0.792±0.036 0.639 ± 0.034 0.606±0.037
1.166±0.059 1.140±0.073 1.399 ± 0.079 1.235 ± 0.064 1.452 ± 0.077 2.146±0.101 4.196 ± 0.654 1.688±0.102 1.911 ±0.102 1.244 ± 0.070 1.476±0.104
169
URANIUM/THORIUM DATING OF LATE PLEISTOCENE PEAT DEPOSITS
AUt Odhar
Chelford
l.,--..,.---,.----r---..-----.------r_----,_--.
+ ¥ /
++
#~
+ 1 2J . U/212Th
o
3 234U/232Th
f# .r: 0-
.., N
N
::>
;!,
'"
1
o
1
2
3
2l ! U/2l ZTh
Fig. 6. Plots of 2300[h/ 232Th vs, 234U j2 32T h and 234U / 232Th vs. 238U /232Th for leachates of the Chelford samples. 2300[h/234U and H4U/H8U as determined from the slopes of the respective lines gives an apparent age of 86±itka.
238U/232Th
Fig. 5. Plots of 2300[h/ 232Th vs. 234U/232Th and mU/ 232T h vs. 238Up32Th for leachates of the Alit Odhar samples. 2300[h/234U and 234U / 238U as determined from the slopes of the respective lines gives an apparent age of 106± IA ka.
using the "leachates alone" correction model as described by Schwarcz and Latham ( 1990). The corrected ratios are shown in Fig. 6. From the corrected ratios an age of86:t ~1 ka was calculated for the Chelford peat layer. This age is in good agreement with the age of the second Weichselian interstadial (Heijnis, 1992) and makes the interstadial equivalent to the 18 0
deep-sea stage 53 or 5 C (Martinson et al., 1987). Recently published TL dates (Rendell et al., 1991) for the overlying and underlying sediments (80 and 98 ka, respectively) are in good agreement with the obtained age for the organics and suggest a correlation with stage 53. 5. Discussion and conclusions We have shown that by careful sampling and good collaboration with field geologists it is possible to obtain reliable dates for peats using
170
H. HEIJNISANDJ. VAN DER
rucirr
TABLE VI Results of the Chelford peat samples Sample
UTDcode
U (ppm)
238U/234U
23OTh/234U
23oTh/232Th
783 783 783 783 783 783
G90453 G90504 G90505 G90506 G91003 G91004 G91049
1.600 2.285 2.406 2.254 2.070 2.100 1.866
1.49±0.04 1.50±0.06 l.52±0.05 1.54±0.04 1.41 ±0.05 1.43±0.04 1.36±0.04
0.70±0.04 0.74±0.04 0.68±0.03 0.62±0.02 0.69±0.03 0.66±0.03 0.55±0.03
1.61 ±0.12 1.81 ±0.1O 1.98±0.09 1.95±0.08 1.84±0.09 2.04±0.1O 1.79±0.11
800
the uranium-series disequilibrium method. Problems of undefined open-system behaviour of the peat to be dated could be avoided by careful sampling and by analysis of several series of samples from a peat profile. Depending on the sort of overlying and underlying sediments the minimum thickness of a peat suitable to be dated by the UTD method is in the order of 10-20 em, in order to fullfil the basic assumption of a closed system with regards to U. The corrections for detrital Th should be made by using the Ieachates-alone method as described by Schwarcz (1989). Dating long terrestrial records by UTD like the Tenagi Philippon and La Grande Pile (being processed at present) is extremely important to correlate the 18 0 deep-sea and terrestrial records over the last 125 ka. The attemps to date the important Quaternary marker sites is extremely important for regional correlations with deposits less suitable for dating. Acknowledgements
We would like to thank the Netherlands Foundation for Scientific Research (Z.W.O.), section Earth Sciences (grant No. 751.357.011) and the Quaternary Research Association for their financial support. We also like to thank M. Ivanovich and G.J. Hennig for reviewing the manuscript and their constructive remarks. Thanks are due to the following persons for providing us with the samples: C.
Auton, P. Worsley and T. Wijmstra. The collaboration and friendship with P. Coxon, J. Ruddock and R. Frank during the Fenit fieldwork is greatly appreciated. Mrs. H. Deenen is gratefully acknowledged for typing the manuscript. References De Vernal, A., Causse, C, Hillaire-Marcel, C., Mott, R.J. and Occhietti, S., 1986. Palynostratigraphy of Upper Pleistocene interglacial and interstadial deposits on Cape Breton Island, eastern Canada. Geology, 14: 554557. Harkness, D.O., 1990. Radiocarbon dating of the Odhar peat. In: CA. Auton, CR. Firth and J.W. Merritt (Editors), Beauly to Nairn. Quat. Res. Assoc., Coventry, Field Guide, pp. 69-70. Heijnis, H., 1992. Ph.D. Thesis, University of Groningen, Groningen. (In press.) Heijnis, H., Ruddock, J. and Coxon, P., in prep. Dating the interglacial peat deposits between Fenit and Spa, Co. Kerry, Rep. ofIreland. Ivanovich, M. and Harmon, R., 1982. Uranium Series Disequilibrium, Applications in Geochronology. Clarendon, Oxford, 571 pp. Kafri, U., Kaufman, A. and Magaritz, M., 1982. The rate of Pleistocene subsidence and sedimentation in the Hula Basin as compared with those with other time spans in other Israeli tectonic regions. Earth Planet. Sci. Lett., 65: 126-132. Martinson, D.G., Pisias, N.G., Hays, J.D., Imbrie, J., Moore, T.C and Shackleton, N.J., 1987. Age dating and the orbital theory of the ice ages: Development of a high resolution 0 to 300.000-year chronostratigraphy. Quat. Res., 27: 1-29. Mitchell, G.F., 1970. The Quaternary deposits between Fenit and Spa on the north shore of Tralee bay, Co.
URANIUM/THORIUM DATING OF LATE PLEISTOCENE PEAT DEPOSITS
Kerry. Proc. R. Irish Acad., 70b: 141-163. Rendell, H., Worsley, P., Green, F. and Parks, D., 1991. Thermoluminescence dating of the Chelford Interstadial. Earth Planet. Sci. Lett., 103: 182-189. Ruddock, J., 1990. An investigation ofan interglacial site between Spa and Fenit, Co. Kerry. M.Sc. Thesis, Trinity College, University of Dublin, Dublin (unpublished ). Schwarcz, H.P. and Latham, A.G., 1989. Dirty calcites, 1. Uranium series dating of contaminated calcite using leachates alone. Chern. Geol. (Isot. Geosci. Sect.), 80: 35-43. Simpson, l.M. and West, R.G., 1958. On the stratigraphy and palaeobotany of a late-Pleistocene organic deposit at Chelford, Cheshire. New Phytol., 57: 239-250. Szalay, A., 1969. Accumulation of uranium and other micrometals in coal and organic shales and the role of humic acids in these geochemical enrichments. Ark. Mineral. Geol., 5: 23-35. van der Wijk, A., 1987. Radiometric dating by alpha spectrometry on uranium series nuclides. Ph.D. Thesis, University of Groningen, Groningen (unpublished). van der Wijk, A., Ivanovich, M. and Mook, W.G., 1988. Correction for environmental 23lTfh in U /Th disequi-
171
librium dating of peat. Sci. Total Environ., 70: 19-40. Vogel, J. and Kronfeld, J., 1980. A new method for dating peat. S. Afr. J. Sci., 76: 557-558. Walker, MJ.C., 1990. The AlIt Odhar interstadial site, Moy, Inverness-shire: Results of pollen analyses. In: C.A. Auton, C.R. Firth and J.W. Merritt (Editors), Beauly to Nairn. Quat. Res. Assoc., Field Guide, pp. 70-72. Warren, W.P., 1979. The stratigraphic position and age of the Gortian Interglacial deposits. Geol. Surv. Ireland Bull., 2: 315-332. Warren, W.P., 1985. Stratigraphy. In: K.J. Edwards and W.P. Warren (Editors), The Quaternary History of Ireland. Academic Press, London, pp. 39-65. Wijrnstra, T.A. and Groenhart, M.C., 1983. Record of 700.000 years vegetational history in eastern Macedonia (Greece). Rev. Acad. Colomb. Cienc. Exactas, Fis. Nat., 15: 87-98. Worsley, P., 1980. Problems in radiocarbon dating the Chelford Interstadial ofEngland. In: R.A. Cullingford, D.A. Davidson and J. Lewin (Editors), Timescales in Geomorphology. Wiley, Chichester, pp. 289-304.
161
Uranium/thorium dating of Late Pleistocene peat deposits in NW Europe, uranium/thorium isotope systematics and open-system behaviour of peat layers H. Heijnis and J. van der Plicht Centrefor Isotope Research, UniversityofGroningen, Neijenborgb 4, NL-9747 AG Groningen, The Netherlands (Received July 16, 1991; revised and accepted October 31, 1991)
ABSTRACf Heijnis, H. and van der Plicht, J., 1992. Uranium/thorium dating of Late Pleistocene peat deposits in NW Europe, uranium/thorium isotope systematics and open-system behaviour of peat layers. Chern. Geol. (Iso!. Geosci. Sect.), 94: 161-171. The possibility of dating peat by the uranium-series disequilibrium method is discussed. In principle, this method can be used to date peat to -350 ka. The application of the U/Th disequilibrium method (UTD) on peat provides us with the probability of constructing a new chronology for the Late Pleistocene paleoclimatic record in NW Europe. The reliability of the obtained ages will be discussed as well as open-system behaviour and the contamination with detrital Th. By studying in detail interglacial peat profiles from the Tenagi Philippon site, Greece (a long terrestrial record), of an expected age of 125 ka and the Fenit site in Ireland of unknown age, we were able to explain the results in terms of the suspected open-system behaviour of top and bottom parts of these layers and how to avoid it by careful sampling. Peats contaminated with detrital Th were also analysed. Two peat layers, which were interpreted on basis of pollen analyses, stratigraphic position and TL dates to be early Last Glacial in age, were sampled. The first one is the Alit Odhar organic layer near Inverness, Scotland, and gave an age of 106 ka. The second is the key site to the British Last Glacial stratigraphy, the Chelford organic layer at Chelford, Cheshire, yielded an age of 86 ka which is in good agreement with the recently obtained TL dates.
1. Introduction
Dating peat by the U jTh disequilibrium (UTD) method provides the possibility of a detailed study in Late Pleistocene geology on a time scale beyond the range of 14c. The vegetational history preserved in peat (by means of pollen and macrofossils), combined with reliable UTD dates yields detailed information about paleoclimatic variations over the past 0.3 Ma. The dating of long terrestrial records makes correlation of the land chronology with the 18 0 deep-sea record possible. Earlier attempts to date peats have met with variable degrees of success (Vogel and Kronfeld, 1980; Kafri et aI., 1982; De Vernal et aI., 1986; van
der Wijk, 1987; van der Wijk et aI., 1988). The present study focuses on the behaviour ofU and Th isotopes in peat layers and various peat fractions during the chemical processes. The relationship between expected ages and observed ages was studied using sequences of peat. Open-system behaviour could be detected by analysing series ofsamples from these peat layers. Two sections were selected to study the open-system behaviour in detail. The first is an interglacial peat layer from the Tenagi Philippon borehole No. III (Wijmstra and Groenhart, 1983) in Macedonia, Greece, of an expected age of 125 ka (equivalent of 180 deepsea stage se; Martinson et aI., 1987) used as a standard of a known age. The second is an in-
0168-9622/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
162
H. HEIJNIS AND J. VAN DER PLICHT
terglacial peat layer from Fenit (County Kerry, Ireland) of an unknown age. The second study was focused on peat samples with a relatively low 230Thj232Th ratio, which are less suitable for dating by UTD. Nevertheless, attempts have been made to date two peat layers which were interpreted on the basis of pollen analyses, stratigraphic position and TL dates to be early Last Glacial interstadial organic deposits. The first one is the Alit Odhar organic layer (Walker, 1990) near Inverness, Scotland, and the second are the famous Chelford organic deposits (Simpson and West, 1958; Worsley, 1980) at Chelford, Cheshire. The 14C ages obtained for these sites were generally > 51 ka BP (Worsley, 1980; Harkness, 1990). 2. Experimental procedures
2.1. The dating UTD method
The natural isotope 238U decays - via other short-lived nuclides neglected here - to 234U, which in turn decays with a half-life of 248 ka (by emitting an alpha particle) into 230Th (Ivanovich and Harmon, 1982). The 230Th itself decays (also by emitting an alpha particle) with a half-life of 75 ka. This decay is the only source for 23°Th available in nature. Thus, as long as the production of 23°Th exceeds its radioactive decay, 230Th accumulates and the 230Thj234U activity ratio is a measure of age (Ivanovich and Harmon, 1982): 230Th 234U
l-e _230Al ( 1) 234Uj238U + 1- 234U j2 38U X
23°2 (1- _<230 A_234 A)I) 23°2_ 2342 e
( 1)
After -- 350 ka equilibrium is reached, where the decay of 230Th balances its production. The activity ratio e 30Thj234U) is a function of age, which can easily be determined if the initialratio e30Thj232Th)0=0. The (UjTh) activity ratios are measured by alpha-spectrom-
etry. Therefore, the U and Th have to be extracted from the material, and transferred into alpha-radioactive sources. The alphasources are obtained by electroplating U and Th on a stainless-steel disk. Consequently, alpha-spectra are analysed by means of an 8-fold alpha-spectrometer read out by a microcomputer (Ivanovich and Harmon, 1982; van der Wijk,1987). 2.2. The UTD method applied to peat
Peat can absorb large amounts of U from groundwater (Szalay, 1969). Especially the humic and fulvic acids absorb large amounts ofU and form large stable complexes. Since Th is highly insoluble in groundwater, the radiogenic isotope 230Th concentration should be zero at the time of peat formation. Thus the present-day 23crrh can be solely attributed to the decay of its radioactive precursor 234U. Violation of this assumption can be traced by the presence of the natural isotope 232Th (t l /2 = 1.39·\010 a). The reliability of the UTD dating for peat samples depends on whenever the system is closed. The compaction and burial by other sediments isolate the peat system from further U uptake and so, in principle, the method should be valid for dating this ubiquitous Quaternary material. 2.3. Chemical procedures
The peat samples are first dried overnight (60°C) and weighed to measure the water contents. Next the organic fraction of the peat is destroyed in a muffle furnace at a temperature of 550°C (24 hr.). The rather low temperature is to avoid the formation of insoluble Th-oxides (S.A. Short, pers. commun., 1990). Eventually the remaining organics are destroyed by adding a few drops of H 20 2 to the ashes during the leaching procedure. The remaining ashes are leached in 7 M HN0 3 and a spike solution, which is a solution of known
URANIUM/THORIUM DATING OF LATE PLEISTOCENE PEAT DEPOSITS
enU j2 28Th ) activity ratio, is added to yield tracing and is stored for 24 hr. to reach equilibrium between solution and spike. From the solution thus obtained, U and Th are isolated by coprecipitation with Fe(OHh U and Th are separated from each other by means of ion-exchange column chromatography [Biolcad'" AG 1X8 (100-200 mesh) ]. After the separation ofU and Th alpha-radioactive sources are obtained by electrodeposition on stainless-steel disks.
3. Open-system behaviour of peat layers and the consequences for dating
3.1. Experiments The research of Late Pleistocene peat layers has been traditionally focussed on its paleoclimatological implications, i.e , the story deduced from its pollen contents. However, this leads to an ambiguous approach in dating these layers. Paleoclimatologists/ecologists are in the first place interested in major climatological shifts which merely occur at the upper and lower boundaries of peat layers. These boundaries are not suitable for UTD dating because of the expected open-system behaviour of top and bottom parts of peat layers. To prove the open-system behaviour of these top and bottom parts, two systems were selected for a very detailed dating program. The first layer is a 4-m-thick peat layer from the Tenagi Philippon borehole III in Macedonia, Greece, which is the equivalent of 18 0 deep-sea stage se with an expected age of 125 ka. The pollen analysis of Tenagi Philippon is described in detail by Wijmstra and Groenhart (1983). From this interglacial peat layer 8 samples at 50-em intervals were taken for UTD analysis. The second site at Fenit, Co. Kerry, Ireland, consists of a complex of three peat layers with a maximum thickness of '" 40 em. This site was sampled at 2-cm intervals and is of unknown age. The pollen diagrams do not elucidate the
163
correlation with any of the 18 0 deep-sea stages. The age and correlation of this site will be published elsewhere (Heijnis, 1992; Heijnis et al., 1992). The Allt Odhar and Che\ford sites were studied in detail to investigate the contamination with detrital Th. These two sites have low 230Th/232Th ratios.
3.2. Results and discussion 3.2.1. The Tenagi Philippon site The results of a detailed study on the behaviour of different isotopes on the Tenaghi Philippon III peat layer are summarized in Tables I and II, and shown graphically in Fig. 1. 3.2.1.1. The organic contents. The organic contents differs with depth. The gyttja has a lower organic content and at the boundaries there seems to be partial mixing of the gyttja and peat. The centre of the peat layer has a very high organic content. There is hardly any inorganic matter which could be the source of detrital Th. 3.2.1.2. The different leachatefractions. An experiment to test the influence of the detritus on the activity ratios and age was performed. The resulting 3Drh/ 23 4 U ) activity ratios show no large differences between the 2 M HN0 3 and the HF-HCI0 4 fractions of samples Sand 8 (see Table I).
e
3.2.1.3. The open-system behaviour at sediment boundaries. At the upper and lower boundary of the peat layer one can notice a sudden increase in U (samples 4 to 3 and 7 to 8) while Th does not increase proportionally. The U/Th ratio and the e30Th/234U) activity ratios reflects this phenomenon very clearly. A proportional increase in U causes a change in age as one can see from the diagram in Fig. 1. Exchange of U between the gyttja (high in U content) and the peat (low in U content)
164
H. HEIJNISANDJ. VAN DER PLICHT
TABLE I Radiometric data obtained from the Tenagi Philippon III peat samples, Macedonia, Greece Sample
Depth (m)
% loss
671 672 673 674
27.10-27.20 28.20-28.30 29.00-29.10 29.90-30.00 30.70-30.80
88 82 76 88 88
675 676 677
31.60-31.70 32.50-32.60 33.40-33.50
83 66 62
670
UTDcode
234U/238U
23<>-rh/234U
23<>-rhF32Th
on ignition
Leachate age (ka) uncorrected"!
G89511 G89512 G89513 G89514 G89515 G89519"3 G89516 G89517 G89518 G89522"3
1.026±0.011 1.045±0.009 1.028±0.011 1.215± 0.030 1.659±0.147 1.110 ± 0.079 1.324 ± 0.044 1.135 ± 0.090 1.022 ± 0.0 14 1.028 ± 0.0 16
0.507 ± 0.023 0.523 ± 0.0 II 0.716±0.058 0.687±0.030 0.678±0.053 0.632 ± 0.063 0.650 ± 0.033 0.652 ± 0.051 0.591 ±0.015 0.574±0.018
19.75±2.95 15.75±0.83 6.667± 1.10 6.927±0.71 4.112±0.49 2.407±0.37 4.638±0.40 12.43± 2.44 88.33± 13.1 123.0±25.2
corrected'" (j= I)
76~~ 80~~ 135~~~
76~~ 124~~g
119~~o
109~~1
111~g
94~IJ
III ~l6 III~~
96~:A 106~:~
97~~
96~:
74~~
oIInitiaI 23<>-rhF 32Th=0. o2Initiai23<>-rh/232Th = I (j= I ). o3Sampies totally dissolved in HF-HCI04 _
TABLE II More radiometric data obtained from the Tenagi Philippon III peat samples, Macedonia, Greece Sample No.
670
671 672 673 674 674* 675 676 677 677*
UTD code
G89511 G89512 G89513 G89514 G89515 G89519 G89516 G89517 G89518 G89522
Ueone. (ppm)
18.16 13.74 13.22 2.102 0.750 0.831 1.324 1.135 16.67 6.21
Th cone. (ppm)
U/Th ratio
4.78 3.81 5.40 0.890 0.379 0.38 0.535 0.74 4.96 4.73
3.80 3.60 2.44 2.35 1.97 2.18 2.47 2.75 3.35 3.42
Content (mbq g:") 238U
234U
232Th
23<>-rh
222±2.5 168±1.5 162± 1.6 25.7±0.5 9.11 ±0.6 1O.2±0.5 16.22±0.5 25.13± 1.5 204±2.6 198±3.1
228±2.5 176± 1.5 166±1.6 31.3±0.6 15.12±0.8 11.29±0.6 17.75±0.6 28.5± 1.6 209±2.7 204±3.2
5.87±0.87 5.85±0.30 17.9±2.9 3.1±0.3 2.5±0.2 2.9±0.4 2.5±0.2 1.44±0.2 1.40±0.2 0.95±0.2
115.8±4.9 92.09± 1.6 119.5±9.6 21.5±0.8 1O.26±0.5 7.l4±0.6 11.55±0.4 18.6±0.97 123±2.5 117.3±3
* Samples totally dissolved in HF-HCI0 4 •
occurs only at the top and bottom of the peat layer. Samples 3 and 7 might be contaminated with U from the gyttja but the altered U/Th ratio could be caused by partially mixing of peat and other substances, like gyttja or clay (note that the organic contents are slightly higher in the middle part ofthe peat layer). The post-depositional U accumulation from seepage water at the sediment boundaries also leads to a disturbed U/Th ratio. From this
point of view sample 5 seems to be the most reliable one for straightforward dating purposes. 3.2.1.4. Age calculation using leaclzates alone. To obtain a reliable date from this data set the age was calculated using the leachate-alone method (Schwarcz and Latham, 1989). The principle of this method is that a datable substance consists of two components, a datable
165
URANIUM/THORIUM DATING OF LATE PLEISTOCENE PEAT DEPOSITS
Tenagi Philippon III
10
28
/
=', ~l~
...'-.
/
9
.
I'
I:" .,
',.",.
s: ....
~
;30
I';'
~
~
,;.; \~~ ~:
;31
:.:
---
~~~
~
32
~3-
I-
5
8 \
't
o '() 20 oUconc. ppm • Theone. wn
\
o5
.
too000
10
15
234U/232Th
20 130000
• v.clWijk, (1987)
BTIpeat
15
~gyttja
Tenagi Philippon III,Macedonia,Greece. Fig. I. Age-depth diagram and the relationships between sediment, age, U and Th isotopes, and organic contents of the Tenagi Philippon III peat samples, Macedonia, Greece.
fraction and a contaminant (detritus). As all samples have slightly different proportions of detritus one can expect that the activity ratios of the samples form a straight line in the isochron plot (Schwarcz and Latham, 1989), because of their more or less uniform age (the age differences are within the accuracy of the method). The activity ratios of samples 3-7 were plotted in isochron diagrams (Fig. 2) and the corrected ratios were calculated from the slopes of the least-squares fits of both diagrams. A corrected age of 122 ± l~ ka was calculated from the whole data set. This date agrees well with the age of the climatic maximum of the last interglacial ( 180 deep-sea stage 5e ) . 3.2.1.5. Conclusion. The top and bottom zones of this interglacial peat layer behave as an open system with regards to U due to the exchange
t 10
12
14
16
238U/232Th
Fig. 2. Plots of 230The32Th vs. 234U/ 232Th and 234U/ 232Th vs. 238U/232Th for leachates of the Tenagi Philippon samples. 230Th/ 234 U and 234 U /238U as determined from the slopes of the respective lines gives an apparent age of 122 ± ~ ka.
ofU from the gyttja and possibly also by postdepositional uptake of U from seepage water. As the right proportions of this open-system behaviour of top and bottom zones of peat layers are still not clear it was decided that the second site would be studied in more detail. 3.2.2. The Fenit site, Ireland The results are summarized in Tables III and IV and Fig. 3. The Fenit site (between Fenit
166
H. HEIJNISANDJ. VAN DER PLICHT
TABLE III Radiometric data obtained from the Fenit 89-1 peat samples, Co. Kerry, Ireland Sample
Depth (em)
% loss on ignition
UTD code
23·U/ 238U
230Th/234U
23°Th/232Th
Leachate age (ka) uncorrected>'
corrected'? (j= I)
692 696 699 700 701 702 703 706 708 709 7lJ
16-18 21-22 25-26 28-30 30-31 31-32 32-34 39-41 45-46 46-48 49-50
65 88 83 69 75 75 43 88 87 90 89
G89493 089494 089497 089495 089498 089496 089499 089489 089490 089506 089508
1.061± 0.022 1.054± 0.009 1.109 ± 0.024 1.156±0.007 1.206± 0.027 1.197±0.012 1.202± 0.022 1.043± 0.021 0.996±0.019 1.080± 0.028 0.973±0.009
0.699±0.O21 0.708±0.016 0.617 ± 0.090 0.499 ± 0.0 17 0.41O±0.014 0.594±0.013 0.731 ±O.O47 0.356±0.O13 0.652±0.019 0.657±0.O23 0.682 ± 0.025
8.725±0.524 9.054 ± 0.359 3.543 ± 0.678 9.690±0.411 9.330±0.495 10.890± 0.385 6.172±0.512 25.23 ± 3.322 12.43 ±0.693 20.5 I ± 2.25 I 27.87 ± 3.298
129~X
131~X 102~H 74~1 56::~
95::1 135:::~ 48::~ H5~X
120~X 123~~
82~~~
68::1 52::~
89::1 122:: :~ 46::~
109::X
114::~
llO::~
125::~o
123::~
>'lnitiaI23~h/mTh=0.
>2lnitial 23~h/mTh= I (f= I). TABLE IV More radiometric data obtained from the Fenit 89-1peat samples, Co. Kerry, Ireland Sample No.
692 696 699 700 701 702 703 706 708 709 7lJ
UTD code
U cone. (ppm)
089493 089494 089497 089495 089498 089496 089499 089489 089490 089506 089508
3.475 5.703 5.258 15.91 11.48 14.57 8.43 4.34 5.76 8.16 27.55
Th cone. (ppm)
1.374 2.253 2.150 4.630 2.78 5.10 3.85 0.794 1.98 2.83 9.27
U/Th ratio
2.529 2.53 2.445 3.43 4.13 2.85 2.19 5.5 2.91 2.88 2.97
and Spa, Co. Kerry, Ireland) was previously investigated by Mitchell (1970). As a result of his investigations he concluded that the organic deposits belonged to the penultimate interglacial period ( 18 0 deep-sea stage 7 or 9). However, Warren (1979, 1985) has subsequently put forward the view that the organic deposits are in fact Last Interglacial in age. Therefore, it was decided to study this contro-
Content (mBqg-l) 238U
234U
232Th
230Th
42.7±0.8 69.9±0.55 64.6± 1.5 195.6±2.1 140.7±3.5 179.4±2.2 103.5±2.3 53.11 ± 1.0 70.76± 1.3 99.78±2.1 336.5±3.6
45.3±0.8 73.7±0.6 71.6± 1.7 226.1 ±2.4 169.7±4.1 214.8±2.5 124.5±2.7 55.39± 1.0 70.45± 1.3 108±2.1 327.6±3.5
3.63±0.2 5.76±0.2 12.47±2.5 11.65±0.6 7.5±0.4 11.71±O.4 14.75± 1.4 0.78±0.1O 3.69±0.20 3.46±0.37 8.02±0.955
31.7±0.7 52.2±0.8 44.16±6.3 112.9±3.4 69.6± 1.6 127.6±2.0 91.1 ±5.4 19.72±0.58 45.94±0.92 71.08± 1.87 223.55±7.6
versial site in detail. Samples were taken in 1989 at the site were the peat deposit reached its maximum thickness. Each single layer was sampled every 1 or 2 em. The results are shown in Fig. 3. Although the scale is totally different from the Tenagi Philippon III site the general trend is more or less the same. The top and bottom parts show deviating U /Th ratios and therefore erroneous (much) younger ages. .
167
UR ANIUM/THORIUM DATING OF LATE PLEISTOCENE PEAT DEPOSITS
Fenit site89-1,(o.Kerry,lreland.
straight line of Fig. 4 was not the reason for their omission, but for the high U /Th ratio as shown in Fig. 3 and Table IV. The samples which are suspected to be in the zone of opensystem behaviour (deduced from the U /Th ratio) should therefore not be included in the corrections for the presence of detrital Th.
~c¢ $ ~"" {J" .~;:; q"'« ° l o,. on ignilion ~ V 0 so 100
" r"
.,. "
~ ,
"t,
-- -
~,
",~ 20
~
A
~~ t--- --
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--- - - ------- ------ -
"\,
40
-- --- - - - - - -- - -- - --
..?' ...-' ~ /~
I-~ ;~ ~
8
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~
o-t--< o1 0 2 ~a
74 ~a
56k4-<
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;j ~ -t
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...f-< o-f-i
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~
3.2.2.3. Conclusions. From th e isochron plots a corrected age of 118 ± ~ ka was calculated . This is in good agreement with the paleo-ecological evidence which indicates a transition to a colder climate (Ruddock, 1990) and the position on top of a beach deposit which represents the period of high sea-level at the climatic ma ximum of the Last Interglacial (Heijnis et al., 1992). The zone of open sys-
.1
J; - --- . -- -- - - - 60
"
-- -
~ ni t89-1 - - ----
~:l
I: "" -;
+
lO
.
2
i
i
i
4
s
10
·U ppm ' Thppm .U/Thral io" IS:illcl a1 §
~" 15
100000
a B.P.
140000
+
20
+
+
peal
Fig. 3. Age- depth diagram and the relat ionships between sediment, age. and U and Th isotopes of the Fenit 89-1 peat sa mples. County Kerry. Ireland.
3.2.2.1. Open-system behaviour at the sediment boundary. The central parts of peat layers A and C indicate a more or less constant age between 120 and 110 kat Layer B does not show the same trend as it appeared to be an erosional block which was not found at site 2, which was only 2 m west from site 1. At the top ofthe peat layers the U /Th ratio decreases very rapidly with depth, from 5.5 at 40-cm depth to 2.1 at 45 em . 3.2.2.1. Age calculation using leachates alone. The isochron plots of the Fenit peat samples show a very high correlation coefficient. The two omitted points are two samples from the top ofthe erosional peat layer B (samples 5 and 6) . The fact that those samples are not on the
10
10
20
2lBU12l2Th 20
O l<::.----~----------~ 40 20 o 10
234U/232Th
Fig. 4. Plots of 230T h/232T h vs. 234 U /2J2T h and 234U/ 232T h vs. 238 U/232T h for leachates of the Fenit samples. 230Th /2 34 U and 234U/238U as determined from the slopes of th e respective lines gives an apparent age of 118± ~ kat
168
II. HEIJNIS AND J. VAN DER
tern is limited to the top few centimetres only, as the peat layers are very well protected by the clay layers which have a very low permeability. Because of its high U content complementary measurements of Pa/Th will be performed in the near future, in order to independently check the closed-system behaviour ofthe central parts of the peat layers. 4. Contamination with detrital Th and the consequences for dating
n.rcirr
scribed by Schwarcz and Latham (1989). The corrected ratios are shown in Fig. 5. Lines a and b in Fig. 5 represent the corrected ratios for two different samples (i.e. for samples AO JJ0 and AO190 the lines a and b, respectively) but as the lines are parallel the corrected ratio is identical. Sample A0090 which was taken near the top of the peat layer was excluded from the calculation of the corrected ratios as the data points are off the isochron plots and the 30Th/ 23 4 U ) ratio of 1.595 is erroneous (see Section 3.2.2.2). From the data set an age of 106 ± l~ ka was calculated for the Allt Odhar peat layer. This age is in good agreement with the age of the first Weichselian interstadial (Heijnis, 1992) and makes the interstadial equivalent to the 18 0 deep-sea stage 5 c (Martinson et aI., 1987).
e
4. J. The Allt Odhar site, near Inverness, Scotland
The Allt Odhar organics were interpreted on basis of pollen analyses to be interstadial in origin (Walker, 1990). Radiocarbon dates indicated an "infinite" age (> 51 ka BP) (Harkness, 1990). From the Allt Odhar layer a few samples were chosen from different parts ofthe peat layer, except from the very top and bottom to avoid open-system behaviour (which would lead to erroneous ages). The results are shown in Table V. Individual sample ages which could be calculated from the data shown in Table V are in the range of 150 to 120 ka. The presence of detrital Th made corrections necessary using the "leachate-alone" correction model as de-
4.2. The Chelford site, Cheshire, England
The Chelford organics were interpreted as early Last Glacial interstadial deposits. Previous radiocarbon dates on wood and peat samples indicated a minimum age of 63 ka BP (Worsley, 1980). From a large lump of peat several subsamples (with a different amount of detritus) were chosen for chemical analysis. The data are shown in Table VI and are corrected for the presence of environmental Th,
TABLE V
Radiometric data obtained from peat samples from Alit Odhar, near Inverness, Scotland Sample No.
UTD
U
code
(ppm)
A0190 A0190 A0190 A0190 A0190 A0190 AOO90 AOllOc AOllOd A0190a A0190c
G90445 G90446 G90500 G90501 G90502 G90503 G9lO40 G9lO43 G91044 G9lO45 G9lO46
2.490 2.075 3.945 3.828 3.740 4.119 0.199 0.542 0.635 1.549 1.564
234U/238U
230Th/ 234U
230Th/232Th
1.027±0.043 1.427 ± 0.084 1.141 ±0.048 1.120±0.046 1.165 ± 0.048 1.133±0.042 1.129±0.050 1.052± 0.049 1.239 ± 0.044 1.175 ± 0.025 1.189 ± 0.021
0.752±0.038 0.619 ± 0.040 0.745±0.037 0.764±0.038 0.861 ±0.042 1.279 ± 0.053 1.595±0.132 0.843±0.045 0.792±0.036 0.639 ± 0.034 0.606±0.037
1.166±0.059 1.140±0.073 1.399 ± 0.079 1.235 ± 0.064 1.452 ± 0.077 2.146±0.101 4.196 ± 0.654 1.688±0.102 1.911 ±0.102 1.244 ± 0.070 1.476±0.104
169
URANIUM/THORIUM DATING OF LATE PLEISTOCENE PEAT DEPOSITS
AUt Odhar
Chelford
l.,--..,.---,.----r---..-----.------r_----,_--.
+ ¥ /
++
#~
+ 1 2J . U/212Th
o
3 234U/232Th
f# .r: 0-
.., N
N
::>
;!,
'"
1
o
1
2
3
2l ! U/2l ZTh
Fig. 6. Plots of 2300[h/ 232Th vs, 234U j2 32T h and 234U / 232Th vs. 238U /232Th for leachates of the Chelford samples. 2300[h/234U and H4U/H8U as determined from the slopes of the respective lines gives an apparent age of 86±itka.
238U/232Th
Fig. 5. Plots of 2300[h/ 232Th vs. 234U/232Th and mU/ 232T h vs. 238Up32Th for leachates of the Alit Odhar samples. 2300[h/234U and 234U / 238U as determined from the slopes of the respective lines gives an apparent age of 106± IA ka.
using the "leachates alone" correction model as described by Schwarcz and Latham ( 1990). The corrected ratios are shown in Fig. 6. From the corrected ratios an age of86:t ~1 ka was calculated for the Chelford peat layer. This age is in good agreement with the age of the second Weichselian interstadial (Heijnis, 1992) and makes the interstadial equivalent to the 18 0
deep-sea stage 53 or 5 C (Martinson et al., 1987). Recently published TL dates (Rendell et al., 1991) for the overlying and underlying sediments (80 and 98 ka, respectively) are in good agreement with the obtained age for the organics and suggest a correlation with stage 53. 5. Discussion and conclusions We have shown that by careful sampling and good collaboration with field geologists it is possible to obtain reliable dates for peats using
170
H. HEIJNISANDJ. VAN DER
rucirr
TABLE VI Results of the Chelford peat samples Sample
UTDcode
U (ppm)
238U/234U
23OTh/234U
23oTh/232Th
783 783 783 783 783 783
G90453 G90504 G90505 G90506 G91003 G91004 G91049
1.600 2.285 2.406 2.254 2.070 2.100 1.866
1.49±0.04 1.50±0.06 l.52±0.05 1.54±0.04 1.41 ±0.05 1.43±0.04 1.36±0.04
0.70±0.04 0.74±0.04 0.68±0.03 0.62±0.02 0.69±0.03 0.66±0.03 0.55±0.03
1.61 ±0.12 1.81 ±0.1O 1.98±0.09 1.95±0.08 1.84±0.09 2.04±0.1O 1.79±0.11
800
the uranium-series disequilibrium method. Problems of undefined open-system behaviour of the peat to be dated could be avoided by careful sampling and by analysis of several series of samples from a peat profile. Depending on the sort of overlying and underlying sediments the minimum thickness of a peat suitable to be dated by the UTD method is in the order of 10-20 em, in order to fullfil the basic assumption of a closed system with regards to U. The corrections for detrital Th should be made by using the Ieachates-alone method as described by Schwarcz (1989). Dating long terrestrial records by UTD like the Tenagi Philippon and La Grande Pile (being processed at present) is extremely important to correlate the 18 0 deep-sea and terrestrial records over the last 125 ka. The attemps to date the important Quaternary marker sites is extremely important for regional correlations with deposits less suitable for dating. Acknowledgements
We would like to thank the Netherlands Foundation for Scientific Research (Z.W.O.), section Earth Sciences (grant No. 751.357.011) and the Quaternary Research Association for their financial support. We also like to thank M. Ivanovich and G.J. Hennig for reviewing the manuscript and their constructive remarks. Thanks are due to the following persons for providing us with the samples: C.
Auton, P. Worsley and T. Wijmstra. The collaboration and friendship with P. Coxon, J. Ruddock and R. Frank during the Fenit fieldwork is greatly appreciated. Mrs. H. Deenen is gratefully acknowledged for typing the manuscript. References De Vernal, A., Causse, C, Hillaire-Marcel, C., Mott, R.J. and Occhietti, S., 1986. Palynostratigraphy of Upper Pleistocene interglacial and interstadial deposits on Cape Breton Island, eastern Canada. Geology, 14: 554557. Harkness, D.O., 1990. Radiocarbon dating of the Odhar peat. In: CA. Auton, CR. Firth and J.W. Merritt (Editors), Beauly to Nairn. Quat. Res. Assoc., Coventry, Field Guide, pp. 69-70. Heijnis, H., 1992. Ph.D. Thesis, University of Groningen, Groningen. (In press.) Heijnis, H., Ruddock, J. and Coxon, P., in prep. Dating the interglacial peat deposits between Fenit and Spa, Co. Kerry, Rep. ofIreland. Ivanovich, M. and Harmon, R., 1982. Uranium Series Disequilibrium, Applications in Geochronology. Clarendon, Oxford, 571 pp. Kafri, U., Kaufman, A. and Magaritz, M., 1982. The rate of Pleistocene subsidence and sedimentation in the Hula Basin as compared with those with other time spans in other Israeli tectonic regions. Earth Planet. Sci. Lett., 65: 126-132. Martinson, D.G., Pisias, N.G., Hays, J.D., Imbrie, J., Moore, T.C and Shackleton, N.J., 1987. Age dating and the orbital theory of the ice ages: Development of a high resolution 0 to 300.000-year chronostratigraphy. Quat. Res., 27: 1-29. Mitchell, G.F., 1970. The Quaternary deposits between Fenit and Spa on the north shore of Tralee bay, Co.
URANIUM/THORIUM DATING OF LATE PLEISTOCENE PEAT DEPOSITS
Kerry. Proc. R. Irish Acad., 70b: 141-163. Rendell, H., Worsley, P., Green, F. and Parks, D., 1991. Thermoluminescence dating of the Chelford Interstadial. Earth Planet. Sci. Lett., 103: 182-189. Ruddock, J., 1990. An investigation ofan interglacial site between Spa and Fenit, Co. Kerry. M.Sc. Thesis, Trinity College, University of Dublin, Dublin (unpublished ). Schwarcz, H.P. and Latham, A.G., 1989. Dirty calcites, 1. Uranium series dating of contaminated calcite using leachates alone. Chern. Geol. (Isot. Geosci. Sect.), 80: 35-43. Simpson, l.M. and West, R.G., 1958. On the stratigraphy and palaeobotany of a late-Pleistocene organic deposit at Chelford, Cheshire. New Phytol., 57: 239-250. Szalay, A., 1969. Accumulation of uranium and other micrometals in coal and organic shales and the role of humic acids in these geochemical enrichments. Ark. Mineral. Geol., 5: 23-35. van der Wijk, A., 1987. Radiometric dating by alpha spectrometry on uranium series nuclides. Ph.D. Thesis, University of Groningen, Groningen (unpublished). van der Wijk, A., Ivanovich, M. and Mook, W.G., 1988. Correction for environmental 23lTfh in U /Th disequi-
171
librium dating of peat. Sci. Total Environ., 70: 19-40. Vogel, J. and Kronfeld, J., 1980. A new method for dating peat. S. Afr. J. Sci., 76: 557-558. Walker, MJ.C., 1990. The AlIt Odhar interstadial site, Moy, Inverness-shire: Results of pollen analyses. In: C.A. Auton, C.R. Firth and J.W. Merritt (Editors), Beauly to Nairn. Quat. Res. Assoc., Field Guide, pp. 70-72. Warren, W.P., 1979. The stratigraphic position and age of the Gortian Interglacial deposits. Geol. Surv. Ireland Bull., 2: 315-332. Warren, W.P., 1985. Stratigraphy. In: K.J. Edwards and W.P. Warren (Editors), The Quaternary History of Ireland. Academic Press, London, pp. 39-65. Wijrnstra, T.A. and Groenhart, M.C., 1983. Record of 700.000 years vegetational history in eastern Macedonia (Greece). Rev. Acad. Colomb. Cienc. Exactas, Fis. Nat., 15: 87-98. Worsley, P., 1980. Problems in radiocarbon dating the Chelford Interstadial ofEngland. In: R.A. Cullingford, D.A. Davidson and J. Lewin (Editors), Timescales in Geomorphology. Wiley, Chichester, pp. 289-304.