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230Th234U and 14C dating of a late Pleistocene stalagmite in Lobatse II Cave, Botswana
Quaternary Geochronology (Quaternary Science Reviews), Vol. 13, pp. 111-119, 1994. Copyright © 1994 Elsevier Science Ltd. Printed in Great Britain. All rights reserved. 0277-3791/94 $26.00
Pergamon
0277-3791(94)E0019-7
23°Th/Z34UAND 14C DATING OF A LATE PLEISTOCENE STALAGMITE IN LOBATSE II CAVE, BOTSWANA Karin Holmgren,* Stein-Erik L a u r i t z e n t and G t r a n Possnert:~ *Department of Physical Geography, Stockholm University, 106 91 Stockholm, Sweden tDepartment of Geology, Section B, University of Bergen, All(gt, 41, 5007 Bergen, Norway "::The Svedberg Laboratory, Uppsala University, Box 533, 751 21 Uppsala, Sweden
A late Pleistocene stalagmite from a karst cave in southeastern Botswana has been used to compare the ~4C dating method on speleothem carbonates with the 23°ThfZ34Umethod. An age discrepancy between the two methods indicates that at least one of the dating series does not reflect the true growth ages. The deviation is smallest in the youngest part of the stalagmite, where 21,600 U-series years correspond to 17,800 ~4C BP. This is in accordance with the Jac and 23°Th/234U-dating results on Barbados corals (Bard et al., 1990). Subsequently, the deviation increases rapidly with age, resulting in a discrepancy of 20,000 years at the 23~l'h/234Uage of 50,000 years. Whilst the ~4C results exhibit a reversed order in the middle of the sequence, the 23°Th/234Uage estimates are stratigraphically ordered. We consider the U-series data reliable and conclude that these results do indeed reflect true calendar years. The ~4C age estimates are probably a result of the post~depositional introduction of ~4C, but may also reflect, to some extent, variations in the atmospheric ~4C content. This study indicates that ~4C dating of speleothem carbonate is problematic.
~TRODUCTION Increased attention has recently been paid to testing radiocarbon (14C) age estimates against other dating methods, in order to extend the calibration of tac beyond the Holocene tree-ring calibration (Becker and Kromer, 1986; Stuiver et al., 1986; Becker et al., 1991). Varve chronology from Swiss lakes covers the end of the Late Glacial period (Lotter, 1991). Comparative studies of the two methods, uranium-series (23°Th/234U)dating and t4C, have been carded out on evaporites (Peng et al., 1978), speleothems (Vogel, 1983; Geyh and Hennig, 1986; Shaw and Cooke, 1986; Goede and Vogel, 1991; Wildberger et al., 1991) and corals (Bard et al., 1990; Fairbanks, 1990; Edwards et al., 1993). The most extensive of these studies, on Barbados corals, has shown that the U-series age estimates for the last 10,000 years are well correlated with the tree ring ages (Bard et al., 1990). The U-series ages obtained are considered useful in calibrating the radiocarbon timescale back to 30,000 years. In this paper we have extended the comparisons of the two methods, 14C and 23°Th/234U,back to 50,000 years and have tested the validity of 14C measurements on speleothems, by analysing a stalagmite from Lobatse II Cave in southeastern Botswana (Fig. 1). The implications for the palaeoclimatic interpretation of the region, as inferred from the results from this study, will be discussed in a forthcoming paper. Speleothems (stalagmites, stalactites etc.) are cave sediments formed through the precipitation of calcium carbonate from saturated seepage water. The isotopic composition of a speleothem ideally reflects the isotopic composition of the regional annual precipitation at the time of deposition, thus making them excellent recorders of climate (e.g. Hendy, 1971; Yonge et al., 1985; Ford and
Williams, 1989; Talma and Vogel, 1992). Provided that it has remained a geochemically closed system, a speleothem can be dated by a variety of methods, with 23°Tla/234/Uand 14C dating amongst the most commonly used. Dating speleothems with 23°Th/234Uis generally assumed to be a reliable technique, with a potential range covering the past 350,000 years. For trustworthy results the uranium concentration in the sample must exceed 0.05 ppm and the sample should be free from detrital material, i.e. not contain any significant amounts of initial 23°Th (Schwarcz, 1989; Gascoyne, 1992; Ivanovich and Harmon, 1992). By applying the recently introduced mass spectrometry dating method (Li et al., 1989), it is possible to reduce the sample size to a few grams. Radiocarbon dating of speleothems has some critical constraints. The 14C content of speleothem carbonates has a more complex origin than that of corals for example, and is not directly coupled to the atmosphere. Because inactive carbon can enter the percolating seepage water from the surrounding carbonate bedrock (the 'dead carbon effect'), the t4C content in speleothems may vary, representing from 100% to only a few percent of the mean atmospheric level. Attempts to correct radiocarbon age estimates are based either on empirical studies of the 14C and 813C content in groundwater or on assumptions of an initial age. This results in corrected ages ranging from 90 to 30% of the apparent age estimates (Geyh, 1970; Cooke and Verhagen, 1977; Gascoyne and Nelson, 1983; Vogel, 1983; Bastin and Gewelt, 1986; Brook et al., 1990a; Goede and Vogel, 1991). Organic material imbedded in the carbonate may offer a better choice for radiocarbon dating, provided that the source of the organics can be identified. With the new accelerator technique only small amounts of carbon, in the order of 0.1-1 mg, are required (Possnert, 1990).
111
112
K. Holmgren et al.
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FIG. I. Map showing the location of the Lobatse Caves in southeastern Botswana, southern Africa.
SAMPLE DESCRIPTION AND S A M P L I N G TECHNIQUES Two caves in southeastern Botswana, Lobatse I and 11 (Fig. 1), which contain numerous fossil speleothem formations, are located in Proterozoic dolomite bedrock on hillsides well above the present groundwater level (Cooke, 1975). Except after heavy rains the caves are dry, and no active speleothem formation has been observed. One 330-ram-long stalagmite, labelled LII4, growing on top of an older stalagmite in Lobatse II Cave, was sawed parallel to its growth axis. The specimen was situated in a narrow area in connection with the main chamber, 2 m above the main passage and 30 m from the cave entrance, which consists of an 18-m-deep vertical shaft. The stalagmite was oval in cross section, with diameters of 60-80 mm near the base and 15-30 mm close to the top (Fig. 2). The exposed surface of stalagmite LII4 was soft and powdery, implying that it has suffered from postdepositional corrosion, probably owing to condensation moisture from the bat colonies that occupy the cave today. In section, the sample displays a series of more or less distinct growth layers. The colour of the layers vary from white to yellowish, greyish and light brown. There are few signs of recrystallisation, except for certain dark bands, presumed to represent longer growth hiatuses. Thirteen carbonate samples for -~3°Th/Z34U-dating were taken at intervals from the base to the top of the stalagmite (Fig. 2). A first series of seven carbonate samples for radiocarbon and stable isotope (~513C)analysis was drilled out close to the edge of a longitudinal slice cut from the stalagmite. A second series of six samples was later sawed out from the central part along the growth axis. Dark bands,
which may be contaminated, were avoided, but were sampled separately to check the organic content. Exact sampling positions are indicated in Fig. 2. Seepage water, to be used for ~4C-dating and stable isotope analysis, was collected from three sites in the cave in January 1993. Finally, one sediment sample was taken from the basal layers of the floor deposits, which are 2 m thick. These sediments consist of weathered bedrock and bat guano. Since the presence of bats requires an entrance, dating of the deepest (oldest) guano can give a minimum age for the opening of the cave to the surface environment.
RADIOCARBON DATING The carbonate samples, consisting of powder in the first series and small solid pieces in the second series, were immediately put into closed glass vessels filled with nitrogen, in order to avoid contamination by present day atmospheric CO 2. The amount sampled was in the range of 30-221 mg, so different tests of contamination of the material could be performed. The samples were leached stepwise by 0.5M HCI in order to extract carbon dioxide fractions reflecting different depths in the sample, thus revealing information about possible 14C gradients caused either during the storage in nature, sampling in the field or by the pretreatment work in the laboratory. The deepest (oldest) sample was divided into five fractions. All other samples were leached into two fractions in order to avoid material from the outer surface. The seepage water (50 ml from each sample) was similarly acidified to release the CO2 content. The wet sediment material was pretreated according to the standard acid-base-acid procedure. The soluble fraction obtained of 22 g was combusted by oxidation with CuO at
23°Th/234Uand ]4CDating
113
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L 114 1
2aOTh/ 234 U-sample • ~113C, 14C-sample LII4 1 sample identification
FIG. 2. Photo and sketch of the cross-section of a 330 mm high stalagmite, LiI4, Lobatse II Cave, Botswana. The exposed surface has suffered from biogenic corrosion. Sampling sites for 23(FFh/234U, I4Cand 8~3Canalyses are indicated on the sketch. The black dots indicate the first radiocarbon sampling-series and the black boxes the second series. The small drill-holes seen on the photo represent positions for 'sO-sampling.
800°C for 5 min (27% carbon content). The CO2 of all the samples was finally converted to graphite by an iron catalytic reaction with hydrogen present, before making the accelerator measurements (Hut et al., 1986). Radiocarbon analyses were made on a total of 23 carbonate samples from stalagmite LII4, three recent seepage water samples and one sediment sample. The preparation background was controlled by a single crystal carbonate (an Icelandic Double spar), with no 14C activity remaining. An activity corresponding to an age of 50,000 _+ 1000 BP (years before present) was found and has
been corrected for in the analysis, although it is only of minor importance when ages < 30,000 BP are being measured. One aim o f the study was to apply the radiocarbon accelerator technique to small amounts of organic material, believed to have been deposited by bats living in the cave, and seen in the stalagmite as black-coloured layers. This would make it possible to avoid dating carbon of complex origin in the carbonate, and to obtain radiocarbon values that are closely related to the contemporary atmosphere at the time of formation of a specific layer. To test this possibility, a black deposit at about 50 mm from the top was scraped off
1 14
K. Holmgren et al.
(245 mg) and pretreated with HCI to remove the calcareous matrix. The insoluble remains (3.35 mg) were combusted but the CO2 yield of 0.08 mg carbon was too low to perform a radiocarbon measurement. Stable isotope analyses of the calcium carbonate samples and the modern seepage water were made with a conventional mass-spectrometer (Varian MAT) with a reproducibility of 0.2%~. 8~3C values were normalized and reported in standard notation relative to Peedee belemnite (PDB). The radiocarbon analyses were performed with the Uppsala tandem accelerator, which is an ultra-sensitive mass-spectrometer (Possnert, 1990). URANIUM SERIES DATING
Samples of 10 to 20 g were dissolved in 8M HNO3. A commercial 228Th/Z32U spike at transient equilibrium was used as an internal standard. The spike is regularly checked against uraninite and speleothem standards. The radionuclides were concentrated by scavenging with Fe(OH)3. U and Th fractions were separated by ion exchange chromatography (Dowex IX8 in 9M HCI and 8M HNO3, respectively) (Gascoyne, 19791. Because humic matter is known to affect the chemical yields in the dating procedure (Lauritzen et al., 1986), care was taken to oxidize the sample, prior to the scavenger step, by boiling it in HNOJH202 until all yellow humic stains were removed. The pure U and Th fractions were first electroplated onto stainless steel disks, and then counted to measure alpha particle activity on silicon surface barrier detectors in vacuo. Ages were calculated from isotope activity ratios using standard algorithms (lvanovich and Harmon, 1992). Most alpha particle spectra display a tailing towards lower energies for each peak. This
was corrected for by using exponential approximations to the tails (Lauritzen, 1991). RESULTS The Radiocarbon Analysis
A detailed presentation of data and results from the radiocarbon analysis is given in Tables la and b, and displayed in Fig. 3. The fractionation tests show only negligible effects of contamination by modem carbon. This is best illustrated by the small age-differences between the five fractions, LII4 1:1-1:5 (Table la). Furthermore, no systematic shifts in the results can be observed between those samples taken at the edge and those sampled at the centre of the stalagmite. Thus, no radiocarbon gradient is observed when moving along a growth layer. The modem seepage water contained a large amount of HCO? close to saturation and had a radiocarbon content close to the present atmospheric content (Table lb). Obtained 6'~C values, of-10.5 to -12%~, are more negative than the mean atmospheric 8~3Cvalue of-7.5%o, indicating the influence of soil CO2. Radiocarbon dating of the organic-rich sediment gave an age of 920 4- 80 BP, suggesting that the bat colonization has been extant at least this long. This may explain why only a very little organic matter was found in the stalagmite; the cave entrance did probably not exist at the time of speleothem formation. The Uranium Series Analysis
Details from the 23°Th/234U-analysis are given in Table 2. All U-series samples display U concentrations far above the detection limit (0.05 ppm) and were free from detrital 23°Th contamination, i.e. (23°Tl'l/232Th) > > 20. Moderate to high
TABLE I a. Radiocarbon data and derived ages for the carbonate samples from stalagmite LII4 Sample ld. Serie I LII4 6A LII4 5 LII4 5B LII4 4 LlI4 3 LlI4 2 LII4 t : I, LII4 1:2, LII4 1:3, LI14 1:4, LII4 1:5,
"outer' 'outer" 'outer" "outer" 'inner'
Serie 2 LII4 6B:l, "outer' LII4 6B:2 'inner' LII4 5A:l 'outer' LII4 5A:2. 'inner' LII4 4A: 1. 'outer' LII4 4A:2 'inner' LtI4 3A: 1. "outer' LII4 3A:2 'inner" LI14 2B:l 'outer' LII4 2B:2 'inner' LII4 IA:I 'outer' t,II4 IA:2 'inner"
Lab no.
Depth (mm)
Carbonate (mg)
Ua-2819 Ua-2823 Ua-2824 Ua-2820 Ua-2821 Ua-2822 Ua-2825 Ua-2826 Ua-2827 Ua-2828 Ua-2829
1(1 32 70 103 210 215 327 327 327 327 327
50 52 68 45 50 31 221
Ua-2877 Ua-2878 Ua-2875 Ua-2876 Ua-2873 Ua-2874 Ua-2871 Ua-2872 Ua-2869 Ua-2870 Ua-2867 Ua-2868
5 5 137 137 165 165 186 186 248 248 294 294
102 66 55 102 123 t04
Fraction* (%)
8~3C (%,~PDB)
65 68 68 51 64 61 12 9 14 9 19
~I).7 +0.2 - 3.2 3.5 4.8 -I.0 3.8
26 511 31) 61 34 62 21 56 25 32 24 72
I.I
3.8
-2.3 5.5 -6.5 - 1.6 6.5
~4C age (BP)
17,845 15,030 18,050 19,550 13,225 27,215 26,560 29,220 29,220 28,735 30,425
_+ 290 _+ 195 +.+_260 ___230 ~ 160 ~- 415 _+410 _ 520 _+ 500 _+ 655 _+ 635
17,165 17,305 18,995 18,855 14,180 13,240 12,660 12,330 28,625 29,280 34,870 35,510
_+ 140 _+ 175 _ 170 +- 185 _+ 170 ___175 -+ 160 +_ t55 -+ 435 +_ 350 -+ 935 +_760
*The measured fraction of the total amount carbonate after leaching of outer surface. The outermost surface has been removed in all samples, hence the total sum of sample is less than 100%.
115
23ff1~34U and ~4C Dating
1.94 mm/100 years and 3.00 mm/100 years are obtained for the two growth series.
TABLE lb. Radiocarbon data for modern seepage water samples, sampled in January 1993 Sample Id.
Lab. no.
Carbon (ktmol/ml)
613C
A]4C (%o)
Site 1 Site 2 Site 3
Ua-31M1 Ua-3042 Ua-3043
7.0 8.1 7.9
-12.0 -11.5 -10.5
164.47 + 11.15 170.48 _+ 10.34 173.82 + 10.66
DISCUSSION The age discrepancy observed between the two methods can be explained by (i) contamination of the material, (ii) methodological errors, (iii) Pleistocene 14C-variations in the atmosphere, or (iv) a combination of these factors. With the results gained so far, we are not likely to be able to determine conclusively the cause of the discrepancy, but we will discuss the plausibility of different explanations.
extraction yields and well resolved spectra provided analytically reliable results. Both dating series are plotted in Fig. 3. It can be observed that all data points of the U-series are in correct stratigraphic order, spanning a period from 50,000 to 21,600 years. When considering the error bars, the apparently inverse age (sample LII4 3) is statistically not different from the stratigraphically younger samples. A growth hiatus, located 212 mm from the top of the stalagmite, is bracketed between 44,300 and 34,950 years. Another hiatus, at 170 mm, lies between 36,000 and 27,000 years. The oldest and youngest 23°T13j234Uages correspond to the 14C values 29,400 and 17,800 BP, respectively (Fig. 3). The stratigraphic order of the ]4C age estimates is inconsistent, at places resulting in an inverse chronology. A remarkable dip in the curve occurs in association with the hiatuses and is located between 165 and 210 mm from the top of the stalagmite. Regression analysis of age versus height, carded out for samples taken from beneath and above the two hiatuses (Fig. 3), gives very high correlation coefficients for the 23°Thp34U series, of 0.95 and 0.98, respectively. The correlation coefficient for the ]4C series is fairly good for the younger part above the hiatuses; a value of 0.63 is obtained. Beneath the hiatuses the correlation is poorer, 0.48. Assuming that the U-series ages reflect true ages and that linear growth occurred beneath and above the hiatuses, growth rates of
The Uranium Series Systematics A common problem regarding U-series dating of speleothems is contamination by detrital 23°Th. In principle, uranium is mobilized through oxidation when bedrock is weathered, while the radiogenic daughter isotopes of Th and Pa remain insoluble and are absorbed onto clay minerals. (UO2)2÷is then incorporated into the carbonate lattice during precipitation as a zero-adjusted radioactive clock. In contaminated speleothems, clay-particles with attached 23°Th have been co-precipitated with the carbonate and disturb the radioactive clock. This can be checked for by measuring the 23°Th/232Th activity ratio. No significant inclusions of detrital 23°Th have been detected in stalagmite LII4. Another prerequisite for reliable 23°ThF34U-dating is that no post-depositional alteration of U and Th has occurred (Chen et al., 1991; Geyh, 1991; Wildberger et al., 1991; Ivanovich and Harmon, 1992). Compact, unrecrystallized speleothems are assumed to represent a closed system for U and Th exchange (Gascoyne, 1992). Stalagmite LII4 does show signs of porosity at certain places, but these places were avoided while collecting samples for 23°Th/234U-dating. Post-depositional addition of U would result in younger measured ages than the true ages and a correction would
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25000 20000 rn
15000 10000 0
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50
100
150
I I:
200
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250
300
350
Distance (mm) from top of stalagmite
FIG. 3. t4C and 23(yFh/234Uage estimates (Table 2) plotted against distance from the top of stalagmite LII4, Lobatse II Cave, Botswana. Shaded areas indicate periods of growth cessation. Regression lines are calculated for the 23°ThF34U and 14C values before and after the hiatuses. Correlation coefficient, r = 0.98 for 23°ThF34Uages and 0.63 for 14C ages after the hiatuses, r = 0.95 for 23Cq'hF34Uages and 0.48 for ~4C ages before the hiatuses.
116
K. Holmgren et al.
T A B L E 2. Uranium-series data, derived Th-ages and the corresponding uC ages Sample ld. LII4 LII4 LII4 LII4 Lll4 t,ll4 LII4 LI14 LII4 LIt4 LII4 LII4 LIt4 LII4 LII4
6B 6A 5 5B 4 5A 4A 3A 3 2 2A 2B 1B IA 1
Lab. no.
444 413 412 469 468 443 411 410 442 446 445 441 368
Depth (mini 5 10 32 7(7 103 137 165 186 210 215 238 248 290 294 327
U (ppm)
>aU/>sU
>I'ThF~4U
e~°Th/>-'Th
Th age (years)
1.60 1.60
2.24 -+ 0.04 2.45 -+ 0.03
0.184 -+ 0.006 0.180 -+ 0.011
> 1000 > 1000
21,600 -+ 750 21,600 _+ 1415
2.00 3.70 1.90 1.10 0.81 0.68 0.82 1.00 1.00 1.30 0.72
2.15 1.46 1.86 1.79 1.83 2.76 2.26 2.13 2.17 1.99 2.47
0.21 I 0.210 0.222 0,292 0.281 0.348 0.346 0.358 0.379 0.379 (7.385
-+ 0.03 _+0.02 -+ 0.02 -+ 0.04 + 0.05 -+ 0.04 -+ 0.05 -+ 0.05 _+ 0.04 -+ 0.03 -+ 0.06
increase the age discrepancy between the dating-series. A leakage of U would result in age estimates being too old. This would be documented in the radiometric data as a decrease in U concentration with time, which is not evident, either. Instead, the U concentration is relatively constant, except for in sample LII4 5A, which has much higher concentration. This higher concentration could be explained, for example, as a result of new uranium reservoirs from a local vein mineralization that has been exposed to the stalagmite, and has no implications for the dating. Since no indications of U and Th exchange in the material have been detected and the age estimates are in correct chronological sequence, it is most likely that the 23°Th/2-~4Udata obtained are reliable and reflect true growth ages. The >4UF38U and 23°Th/Z34U ratios are plotted in Fig. 4, with 1 ~ error ellipses. The sample points are connected by lines, in stratigraphic order. The stalagmite growth was interrupted by two hiatuses, as depicted in Fig. 3. 234Uexcess varies erratically with time, which is sometimes seen in speleothems (Latham and Schwarcz, 1992 and references therein). The changes in 234U/2~sU ratio are independent of the stratigraphically consistent ages, suggesting that these variations in the stalagmite reflect a similar 23~U/23~U variation in the seepage water composition. The other possible process that may affect the isotopic composition is post-depositional leaching of U, which would leave Th behind and cause an apparent increase in age. This effect may be ruled out, as we do not observe any significant disturbances in the stratigraphic order of ages across the zones where the corresponding change in the 234U/'--~sUratio is large. One explanation for the observed effect may be that a redox front developed above the cave and gradually reached it during each growth interval. This may be associated with soil development above the cave. A redox front of this type displays a core which is depleted with respect to ~34U, surrounded by a front that is enriched in 234U (Osmond and lvanovich, 1992). If such a front developed in the soil cover above the cave and gradually entered it, then the observed uranium disequilibria could be explained.
The Radiocarbon Series Systematics No correction for the 'dead carbon effect" has been made
-+ 0.005 _+ (7.004 + 0.1)03 -+ 0.008 + 0.035 -+ 0.019 -+ 0.011 _+0.012 + 0.010 -+ 0.009 _+0.014
185 -+ t 15 > 1000 > 1000 > 1000 > 1000 > 100(7 > 1000 > 1000 > 1000 > l(XI0 > 1000
25,220 25,270 26,760 36,500 34,950 44,300 44,200 46,200 49,300 49,600 50,060
HC age (100(7 BP) 17.3 + (7,2 17.8 + (7.3 15 _+0.2 18-+0.3 19.6 -+ 0.2 18.9_+0.2 13.2 -+ (7.2 12.3 _+ 0.2 13.2_+0.2 27.2 -+ 0.4
-+ 7(15 _+ 550 _+40(7 -+ 1225 _+ 5000 _+ 288(7 _+ 1610 _+ 1835 _+ 1570 -+ t 4 0 0 -+ 2240
29.3 + 0.4 35.5 + 0.8 30.4 _+0.6
to the J4C age estimates, therefore these ages represent maximum values, if incorporation of post-depositional modern carbon is believed to be negligible. HC analysis of recent seepage water from Lobatse II Cave showed very high concentrations of HC; 164 to 174%o (Table l b). Thus, only a minor amount of carbon is contributed from the surrounding dolomite bedrock today. The depleted ~3C values in the seepage water (-10.5 to - I 2%~), compared to atmospheric 8~3C (-7.5%~), reflect the presence of soil and vegetation above the cave. This has had no obvious effect on the water ~4C, indicating that the organic processes are fast and the soils quickly leached. It has also been observed that the present day seepage water flow into the cave is strongly correlated with precipitation, and has a short response time, in the order of a few days. The permeability and conductivity of the overlying bedrock today are such that most of the features of the meteoric water (humics, 8]so, trace and radionuclide element concentration etc.), are generally retained into the cave. The modern seepage water is therefore formed almost in equilibrium with the atmosphere outside !341 /238U
r
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FIG. 4. Uranium-series systematics m the Lobatse stalagmite LII4. The numbers refer to the laboratory numbers of the sample points. There is a tendency to increasing 234U/23~Uactivity ratios with decreasing ages (sample points represented by open ellipses), except for in the zones close to the hiatuses (sample points represented by filled ellipses).
117
23(Fl"~34Uand 14CDating
the cave. The circumstances at the end of the Pleistocene are more difficult to determine. The 14C content in a speleothem relative to that in the atmosphere depends on whether conditions typical of a closed or opened system have prevailed during the dissolution and reprecipitation of calcium carbonate (Hendy, 1970, 1971). In an open system, the seepage water is in contact with CO2 in the soil atmosphere during the dissolution of the bedrock. The 14C content in the reprecipitated calcium carbonate will be close to the atmospheric 14C. In a completely closed system, seepage water is isolated from the CO2 source before bedrock dissolution. The ~4C content in the speleothem will then be half of the ~4Cof the atmosphere. According to Hendy (1970), relatively positive values of 6~3C in a precipitate is indicative of open system conditions, but it also reflects the composition of the vegetation (Cerling et al., 1988; Brook et al., 1990b). High 613C values can also be a result of kinetic fractionation (Hendy, 1971). This latter possibility, as well as vegetation characteristics, will be discussed in detail in a forthcoming paper. 8~3C values obtained from the growth axis of stalagmite LII4 range from +0.15 to-6.54%o PDB (Table 1a). So far, these relatively enriched values, taken together with the previous discussion of the modem conditions, suggest that open system conditions dominated in the past. Hereof, no correction in this respect has been made to the apparent ~4C values. It should be noted that a correction of the ~4C values because of this 'dead carbon effect' always results in
younger ages than the apparent age estimates. Consequently, one would expect the apparent 14C age estimates to be older than the U-series ages. However, in the present study the situation is the opposite, and a correction will increase the discrepancy between the methods. Post-depositional introduction of younger atmospheric laC would result in '4C age estimates being too young. Biogenic corrosion at the surface of the stalagmite may facilitate an introduction of younger 14C into the stalagmite, with 14C values varying along a growth layer as a result. The 14C samples in this study have been extracted partly in the center and partly in the periphery of the speleothem, as indicated in Fig. 2. There are no major variations in the laCvalues that are related to the sample location along the growth layers, or to different fractions of these samples (Table la). Thus, if any post-depositional introduction of '4C has occurred, the younger 14C must have diffused into the stalagmite and been homogenized. If we assume incorporation from modem carbon to be the reason for the age difference between 14C and 23°ThF34U,21% of the carbon has to be modem, at the hiatuses at 165-210 mm depth. Circa 4.5% modern carbon will account for the age deviation above, and 2.7-1.8% is needed to explain the difference below the hiatuses. The calculations imply that all incorporation has occurred very recently. If, instead, the incorporation occurred several thousand years ago, or has occurred continuously since the stalagmite stopped growing,
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•
Barbados corals
FIG. 5.14C age estimates plotted against 23°Th/234Uage estimates. The solid line indicates the ideal relationship. The data represent the results from this study of stalagmite LII4, Lobatse II Cave, Botswana (apparent ~4Cvalues assumed to be close to corrected values), from a stalagmite from Cango Cave, South Africa (t4C values corrected with -1,500 year) (Vogel, 1983), from a stalagmite from Drotsky's Cave, Botswana (correction unknown) (Shaw and Cooke, 1986), from a stalagmite from Lynds Cave, Tasmania ('4C values corrected with -1,500 year) (Goede and Vogel, 1991) and from a study on Barbados corals (~4C values corrected with -400 year) (Bard et al., 1990; Fairbanks, 1990).
118
K. Holmgren et al.
much higher values for post-depositionally introduced carbon are needed to explain the difference. Beneath and above the hiatuses it appears that the lac values follow a more systematic deviation and the younger part is correlated with other data, as will be discussed in a forthcoming section. Thus, other explanations must also be considered. Atmospheric 14C Variations
The U-series ages are not directly comparable with the technical ~4C age estimates expressed in BP. If the true t4C half-life (5730 years) is used instead of the Libby half-life (5570 years), the radiocarbon age at 15,000 BP will become 300 years older and at 25,000 BP, 700 years older. This is not sufficient to explain the observed age deviation. A more critical parameter to discuss is the ancient production and concentration of atmospheric 14C. The short-term Jac variations in the atmosphere during the Holocene, observed as wiggles or as time intervals with constant /4C ages, have been linked to changes in cosmogenic J4C production owing to variations in solar activity (Oeschger et al., 1970; Suess, 1970; Wigley and Kelly, 1990). The long-term variations, observed as a progressive increase in discrepancy with time, indicate a larger ~4C production during the period from 30,000-10,000 BP than during the Holocene. This increased J4C production is commonly attributed to a weakening of the geomagnetic dipole (Vogel, 1983; Bard et al., 1990; Mazaud et al., 1991; Lao et al., 1992). Changes in solar radiation can not be excluded as another possible explanation for the variations. In Fig. 5 we have compiled our LII4 U-series t4C data with the few data available on speleothems from caves in southern Africa and Tasmania as well as data from the Barbados corals (Vogel, 1983; Shaw and Cooke, 1986; Bard et al., 1990: Fairbanks, 1990; Goede and Vogel, 1991). Considering the fact that the different authors have various ways of coping with the difficulties of ~4Ccorrection, we will compare the trends in the data rather than the details. The speleothem data from Drotsky's Cave, Botswana (Shaw and Cooke, 1986), and from Lynds Cave, Tasmania (Goede and Vogel, 1991 ), fit well with the coral data (Bard et al., 1990; Fairbanks, 1990). The age differences of the coral data have been related to increased atmospheric ~4C concentration. being due to a weakening of the geomagnetic field. The LII4 data between 25,000 and 21,000 years (U-series) are well correlated with both the coral and the speleothem data for that period of time. This is followed by a dip in the ~4C data from Lobatse II Cave, between about 35,000 and 26,000 years (U-series), resulting in a rapid increase in the age difference with time. There is no indication of a similar increase in age in the coral data. We believe that this immense deviation is related to the incorporation of modern carbon. Because of a change in the environment, stalagmite LII4 ceased to grow first at around 44,000 years ago and then again at 36,000 years ago, with each of the two periods lasting a few thousand years. The carbonate was eroded and redissolved, resulting in a porous structure in which it was possible for younger carbon to diffuse. Beneath the hiatuses the radiocarbon age estimates get systematically older, but a deviation of 15-20,000 years between the dating methods remains. There are no hiatuses or other indications in the
material to suggest an incorporation of modern carbon to explain this deviation. Changes in the atmospheric L4C concentration cannot be excluded as a possible explanation. The only other data available from this period of time are three poorly documented observations from Cango Cave, South Africa (Vogel, 1983). These age estimates in general deviate much less than the Lobatse II data, but exhibit a stagnation in the ~4C values at a U-series age of 35,000-30,000 years. Vogel (1983) has suggested that this ~4C plateau is related to the very high palaeomagnetic field strength around 30,000 BP that occurred locally because of the Lake Mungo polarity excursion in Australia (Barbetti and McElhinny, 1976). CONCLUSIONS Stalagmite LII4 from Lobatse I1 Cave in southeastern Botswana was deposited during the period spanning 50,000 to 21,600 U-series years. Gentle inclinations in the U-series curve, beneath and above two hiatuses, reflect periods of generally continuous deposition, with an average growth rate of roughly 2 mm per 100 years between 50,000 and 44,300 years and 3 mm per 100 years between 26,700 and 21,600 years. Dating carbonates with the t4C-method is problematic because of the complex dynamics involved between the source of ~4C in the atmosphere and the values obtained in the speleothem. Still, the age deviation observed between the radiocarbon and the U-series in studies of speleothems (Shaw and Cooke, 1986; Goede and Vogel, 1991) is in accordance with that of the Barbados corals (Bard et al., 1990). The age discrepancy seen in stalagmite LII4 correlates well with the above data fbr the period back to about 25,000 years (U-series). The deviation then increases rapidly. We propose that the ~4C-variations in the stalagmite are partly caused by a post-depositional introduction of younger ~4C and partly related to increased atmospheric ~4C concentrations. In order to understand the relative importance of these factors, stable isotope and mineral analyses will be carried out and additional age estimates will be made on this stalagmite, as well as on other material from the Lobatse I and II Cave.
ACKNOWLEDGEMENTS This work was supported by the Swedish Agency for Research Cooperation with Developing Countries and by smaller grants from Stiftelsen L. Hiertas Minne, H. Ahlmanns Fond, Andrerfonden and C. Mannerfelts Fond. The radiocarbon analyses were supported by the Swedish National Science Research Council. W. Karl6n, Stockholm and P. Shaw, Luton, inspired and supported the work and made many valuable comments about the manuscript. Gratitude is extended to the manager of Lobatse Estates Ltd for permission to work in the cave and to G. Carlson and G. van Regenmortel for invaluable field assistance. Grateful acknowledgements are extended to E. Isaksson, G. Rosqvist, A. Stroeven, Stockholm, the referees R. G ~ n , Canberra and one anonymous referee for comments on the manuscript. J. Karlrn improved the English and H. Drake made the drawings. The research was authorized by the Office of the President. Botswana.
REFERENCES Barbetti, M.F. and McElhinny, M.W. (1976). The Lake Mungo Geomagnetic Excursion. Philosophical Transactions of the Royal Society of London, A 281,515-542.
23°Th/234Uand 14C Dating Bard, E., Hamelin, B., Fairbanks, R.G. and Zindler, A. (1990). Calibration of the '4C timescale over the past 30,000 years using mass spectrometric U-Th ages from Barbados corals. Nature, 345, 405-410. Bastin, B. and Gewelt, M. (1986). Analyse pollinique et datation ~4C de concr6tions stalagmitiques holoc6nes: apports compl6mentaires des deux m6thodes. Geographie Physique Quaternaire, 40, 185-196. Becket, B. and Kromer, B. (1986). Extension of the Holocene dendrochronology by the preboreal pine series, 8,800 to 10,100 BP. Radiocarbon, 28, 961-967. Becker, B., Kromer, B. and Trimborn, P. (1991). A stable-isotope tree-ring timescale of the Late Glacial/Holocene boundary. Nature, 353, 647--649. Brook, G.A., Burney, D.A. and Cowart, J.B. (1990a). Desert paleoenvironmental data from cave speleothems with examples from the Chihuahuan, Somali-Chalbi, and Kalahari deserts. Palaeogeography, Palaeoclimatology, Palaeoecology, 76, 311-329. Brook, G.A., Burney, D.A. and Cowart, J.B. (1990b). Paleoenvironmental data for Ituri, Zaire, from sediments in Matupi Cave, Mt. Hoyo. In: Boaz, N.T. (ed.), Evolution of Environments and Hominidae in the African Western Rift Valley, Virginia Museum National History Memoir 1, 49-70. Ceding, T.E., Bowman, J.R. and O'Neil, J.R. (1988). An isotopic study of fluvial-lacustrine sequence: the PlioPleistocene Koobi Fora sequence, East Africa. Palaeogeography, Palaeoclimatology, Palaeoeeology, 63, 335-356. Chen, J.H., Curran, H.A., White, B. and Wasserburg, G.J. (1991). Precise chronology of the last interglacial period: z34U-23°Thdata from fossil coral reefs in the Bahamas. Geological Society of Ameriea Bulletin, 103, 82-97. Cooke, H.J. (1975). The Lobatse Caves. Botswana Notes and Records, 7, 29-34. Cooke, H.J. and Verhagen, B.Th. (1977). The dating of cave development - - An example from Botswana. In: Proceedings of the 7th International Speleological Congress, pp. 122-136. Sheffield. Edwards, R.L., Beck, J.W., Burr, G.S., Donahue, D.J., Chappell, J.M.A., Bloom, A.L, Druffel, E.R.M. and Taylor, F.W. (1993). A large drop in atmospheric t4CpEC and reduced melting in the Younger Dryas, documented with 2azThages of corals. Science, 260, 962-968. Fairbanks, R.G. (1990). The age and origin of the 'Younger Dryas Climate Event' in Greenland ice cores. Paleooceanography, 5, 937-948. Ford, D.C. and Williams P. (1989). Karst Geomorphology and Hydrology. Unwin Hyman, London, 601 pp. Gascoyne, M. (1979). Pleistocene climates determined from stable isotope and geochronologic studies of speleothem. Unpublished Ph.D. Thesis, McMaster University, Hamilton, Ontario, 433 pp. Gascoyne, M. (1992). Palaeoclimate determination from cave calcite deposits. Quaternary Science Reviews, 11,609-632. Gascoyne, M. and Nelson, D.E. (1983). Growth mechanisms of recent speleothems from Castleguard Cave, Columbia Icefields, Alberta, Canada, inferred from a comparison of uranium-series and carbon-14 age data. Arctic andAlpine Research, 15, 537-542. Geyh, M.A. (1970). Isotopenphysikalische Untersuchungen an Kalksinter, ihre Bedeutung f'tir die 14C-Altersbestimmung von Grundwasser und die Erforschung des Pal~toklimas. Geologische Jahrbuch, 88, 149-158. Geyh, M.A. (1991). Zur Entvicklung der sinterchronologischen und isotopenphysikalischen Forschung in Deutschland. Karst und H6hle, 1989/90, 151-157. Geyh, M.A. and Hennig, G.J. (1986). Multiple dating of a long flowstone profile. Radiocarbon, 28, 503-509. Goede, A. and Vogel J.C. (1991). Trace element variations and dating of a Late Pleistocene Tasmanian speleothem. Palaeogeography, Palaeoclimatology, Palaeoecology, 88, 121-131. Hendy, C.H. (1970). The use of 14C in the study of cave processes. In: Olsson, I.U. (ed.), Twelfth Nobel Symposium, Radiocarbon Variations and Absolute Chronology, pp. 419-443. Uppsala 1969, Almqvist and Wiksell, Stockholm, and Wiley and Sons, New York. Hendy, C.H. (1971). The isotopic geochemistry of speleothems-l. The calculation of the effects of different modes of formation on the isotopic composition of speleothems and their applicability as palaeoclimatic indicators. Geochimica et Cosmochimica, 35, 801-824.
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Hut, G., Ostlund, H.G. and Van der Borg, K. (1986). Fast and complete CO 2 to graphite conversion for ~4C Accelerator Mass Spectrometry. Radiocarbon, 28, 186-190. Ivanovich, M. and Harmon, R.S. (eds) (1992). Uranium Series Disequilibrium. Applications to Environmental Problems. Clarendon, Oxford, 571 pp. Lao, Y., Anderson, R.F., Broecker, W.S., Trumbore, S.E., Hofmann, H.J. and Wolfli, W. (1992). Increased production of cosmogenic ~°Beduring the Last Glacial Maximum. Nature, 357, 576-578. Latham, A.G. and Schwarcz, H.P. (1992). Carbonate and sulphate precipitates. In: Ivanovich, M. and Harmon, R.S. (eds), Uranium-series
disequilibrium: Applications to Earth, Marine and Environmental Sciences, pp. 423-459. Clarendon, Oxford. Lauritzen, S.E. (1991)., "Age4U2U' Program for Reading ADCAM energy spectra, integration, peak-correction and calculation of 23°Thorium/ 234Uranium ages. Turbo Pascal Code, 5,000 lines. Lauritzen, S.E., Ford, D.C. and Schwarcz, H.P. (1986). Humic substances in speleothem matrix - - Paleoclimatic significance. IX International Congress of Speleology, pp. 77-79. Barcelona, Spain Proceedings. Li, W.-X., Lundberg, J., Dickin, A.P., Ford, D.C., Schwarcz, H.P., McNutt, R. and Williams, D. (1989). High-precision mass-spectrometric uranium-series dating of cave deposits and implications for palaeoclimate studies. Nature, 339, 534-536. Lotter, A.F. (1991 ). Absolute dating of the late glacial period in Switzerland using annually laminated sediments. Quaternary Research, 35, 321-330. Mazaud, A., Laj, C., Bard, E., Amold, M. and Tric, E. (1991). Geomagnetic field control of ~4C production over the last 80 ky: Implications for the radiocarbon time-scale. Geophysical Research Letters, 18, 1885-1888. Oeschger, H., Houtermans, J., Loosli, H. and Wahlen, M. (1970). The constancy of cosmic radiation from isotope studies in meteorites and on the earth. In: Olsson, I.U. (ed.), Twelfth Nobel Symposium, Radiocarbon Variations and Absolute Chronology, pp. 471-498. Uppsala 1969, Almqvist and Wiksell, Stockholm, and Wiley and Sons, New York. Osmond, J.K. and Ivanovich, M. (1992). Uranium-series disequilibrium mobilization and surface hydrology. In: Invanovich, M. and Harmon, R.S. (eds), Uranium-Series Disequilibrium: Applications to Earth, Marine and Environmental Sciences, pp. 259-289. Clarendon, Oxford. Peng, T.-H., Goddard, J.G. and Broecker, W.S. (1978). A direct comparison of 14Cand 23°Thages at Searles Lake, California. Quaternary Research, 9, 319-329. Possnert, G. (1990). Radiocarbon dating by the accelerator technique. Norweigian Archaeological Review, 23, 30-37. Schwarcz, H.P. (1989). Uranium series dating of quaternary deposits. Quaternary International, 1, 7-17. Shaw, P.A. and Cooke, H.J. (1986). Geomorphic evidence for the late Quaternary palaeoclimates of the middle Kalahari of northern Botswana. Catena, 13, 349-359. Stuiver, M., Kromer, B., Becker, B. and Ferguson, C.W. (1986). Radiocarbon age calibration of the german oak and us bristlecone pine chronologies. Radiocarbon, 28, 969-979. Suess, H.E. (1970). The three causes of the secular 14C fluctuations, their amplitudes and time constants. In: Olsson, I.U. (ed.), Twelfth Nobel Symposium, Radiocarbon Variations and Absolute Chronology, pp. 595-604. Uppsala 1969, Almqvist and Wiksell, Stockholm, and Wiley and Sons, New York. Talma, A.S. and Vogel, J.C. (1992). Late Quaternary paleotemperatures derived from a speleothem from Cango Caves, Cape Province, South Africa. Quaternary Research, 37, 203-213. Vogel, J.C. (1983). t4C variations during the upper Pleistocene. Radiocarbon, 25, 213-218. Wigley, T.M.L. and Kelly, P.M. (1990). Holocene climatic change, ~4C wiggles and variations in solar irradiance. Philosophical Transactions of the Royal Society of London, A 330, 547-560. Wildberger, A., Geyh, M.A. and Hennig, G. (1991 ). Datierungsversuche an Tropfsteinen aus dem H611och (Zentralschweiz)Akten des 9, nationalen Kongresses der SGH, 113-119. Yonge, C.J., Ford, D.C., Gray, J. and Schwarcz, H.P. (1985). Stable isotope studies of cave seepage water. Chemical Geology (Isotope Geoscience Section), 58, 97-106.
Pergamon
0277-3791(94)E0019-7
23°Th/Z34UAND 14C DATING OF A LATE PLEISTOCENE STALAGMITE IN LOBATSE II CAVE, BOTSWANA Karin Holmgren,* Stein-Erik L a u r i t z e n t and G t r a n Possnert:~ *Department of Physical Geography, Stockholm University, 106 91 Stockholm, Sweden tDepartment of Geology, Section B, University of Bergen, All(gt, 41, 5007 Bergen, Norway "::The Svedberg Laboratory, Uppsala University, Box 533, 751 21 Uppsala, Sweden
A late Pleistocene stalagmite from a karst cave in southeastern Botswana has been used to compare the ~4C dating method on speleothem carbonates with the 23°ThfZ34Umethod. An age discrepancy between the two methods indicates that at least one of the dating series does not reflect the true growth ages. The deviation is smallest in the youngest part of the stalagmite, where 21,600 U-series years correspond to 17,800 ~4C BP. This is in accordance with the Jac and 23°Th/234U-dating results on Barbados corals (Bard et al., 1990). Subsequently, the deviation increases rapidly with age, resulting in a discrepancy of 20,000 years at the 23~l'h/234Uage of 50,000 years. Whilst the ~4C results exhibit a reversed order in the middle of the sequence, the 23°Th/234Uage estimates are stratigraphically ordered. We consider the U-series data reliable and conclude that these results do indeed reflect true calendar years. The ~4C age estimates are probably a result of the post~depositional introduction of ~4C, but may also reflect, to some extent, variations in the atmospheric ~4C content. This study indicates that ~4C dating of speleothem carbonate is problematic.
~TRODUCTION Increased attention has recently been paid to testing radiocarbon (14C) age estimates against other dating methods, in order to extend the calibration of tac beyond the Holocene tree-ring calibration (Becker and Kromer, 1986; Stuiver et al., 1986; Becker et al., 1991). Varve chronology from Swiss lakes covers the end of the Late Glacial period (Lotter, 1991). Comparative studies of the two methods, uranium-series (23°Th/234U)dating and t4C, have been carded out on evaporites (Peng et al., 1978), speleothems (Vogel, 1983; Geyh and Hennig, 1986; Shaw and Cooke, 1986; Goede and Vogel, 1991; Wildberger et al., 1991) and corals (Bard et al., 1990; Fairbanks, 1990; Edwards et al., 1993). The most extensive of these studies, on Barbados corals, has shown that the U-series age estimates for the last 10,000 years are well correlated with the tree ring ages (Bard et al., 1990). The U-series ages obtained are considered useful in calibrating the radiocarbon timescale back to 30,000 years. In this paper we have extended the comparisons of the two methods, 14C and 23°Th/234U,back to 50,000 years and have tested the validity of 14C measurements on speleothems, by analysing a stalagmite from Lobatse II Cave in southeastern Botswana (Fig. 1). The implications for the palaeoclimatic interpretation of the region, as inferred from the results from this study, will be discussed in a forthcoming paper. Speleothems (stalagmites, stalactites etc.) are cave sediments formed through the precipitation of calcium carbonate from saturated seepage water. The isotopic composition of a speleothem ideally reflects the isotopic composition of the regional annual precipitation at the time of deposition, thus making them excellent recorders of climate (e.g. Hendy, 1971; Yonge et al., 1985; Ford and
Williams, 1989; Talma and Vogel, 1992). Provided that it has remained a geochemically closed system, a speleothem can be dated by a variety of methods, with 23°Tla/234/Uand 14C dating amongst the most commonly used. Dating speleothems with 23°Th/234Uis generally assumed to be a reliable technique, with a potential range covering the past 350,000 years. For trustworthy results the uranium concentration in the sample must exceed 0.05 ppm and the sample should be free from detrital material, i.e. not contain any significant amounts of initial 23°Th (Schwarcz, 1989; Gascoyne, 1992; Ivanovich and Harmon, 1992). By applying the recently introduced mass spectrometry dating method (Li et al., 1989), it is possible to reduce the sample size to a few grams. Radiocarbon dating of speleothems has some critical constraints. The 14C content of speleothem carbonates has a more complex origin than that of corals for example, and is not directly coupled to the atmosphere. Because inactive carbon can enter the percolating seepage water from the surrounding carbonate bedrock (the 'dead carbon effect'), the t4C content in speleothems may vary, representing from 100% to only a few percent of the mean atmospheric level. Attempts to correct radiocarbon age estimates are based either on empirical studies of the 14C and 813C content in groundwater or on assumptions of an initial age. This results in corrected ages ranging from 90 to 30% of the apparent age estimates (Geyh, 1970; Cooke and Verhagen, 1977; Gascoyne and Nelson, 1983; Vogel, 1983; Bastin and Gewelt, 1986; Brook et al., 1990a; Goede and Vogel, 1991). Organic material imbedded in the carbonate may offer a better choice for radiocarbon dating, provided that the source of the organics can be identified. With the new accelerator technique only small amounts of carbon, in the order of 0.1-1 mg, are required (Possnert, 1990).
111
112
K. Holmgren et al.
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FIG. I. Map showing the location of the Lobatse Caves in southeastern Botswana, southern Africa.
SAMPLE DESCRIPTION AND S A M P L I N G TECHNIQUES Two caves in southeastern Botswana, Lobatse I and 11 (Fig. 1), which contain numerous fossil speleothem formations, are located in Proterozoic dolomite bedrock on hillsides well above the present groundwater level (Cooke, 1975). Except after heavy rains the caves are dry, and no active speleothem formation has been observed. One 330-ram-long stalagmite, labelled LII4, growing on top of an older stalagmite in Lobatse II Cave, was sawed parallel to its growth axis. The specimen was situated in a narrow area in connection with the main chamber, 2 m above the main passage and 30 m from the cave entrance, which consists of an 18-m-deep vertical shaft. The stalagmite was oval in cross section, with diameters of 60-80 mm near the base and 15-30 mm close to the top (Fig. 2). The exposed surface of stalagmite LII4 was soft and powdery, implying that it has suffered from postdepositional corrosion, probably owing to condensation moisture from the bat colonies that occupy the cave today. In section, the sample displays a series of more or less distinct growth layers. The colour of the layers vary from white to yellowish, greyish and light brown. There are few signs of recrystallisation, except for certain dark bands, presumed to represent longer growth hiatuses. Thirteen carbonate samples for -~3°Th/Z34U-dating were taken at intervals from the base to the top of the stalagmite (Fig. 2). A first series of seven carbonate samples for radiocarbon and stable isotope (~513C)analysis was drilled out close to the edge of a longitudinal slice cut from the stalagmite. A second series of six samples was later sawed out from the central part along the growth axis. Dark bands,
which may be contaminated, were avoided, but were sampled separately to check the organic content. Exact sampling positions are indicated in Fig. 2. Seepage water, to be used for ~4C-dating and stable isotope analysis, was collected from three sites in the cave in January 1993. Finally, one sediment sample was taken from the basal layers of the floor deposits, which are 2 m thick. These sediments consist of weathered bedrock and bat guano. Since the presence of bats requires an entrance, dating of the deepest (oldest) guano can give a minimum age for the opening of the cave to the surface environment.
RADIOCARBON DATING The carbonate samples, consisting of powder in the first series and small solid pieces in the second series, were immediately put into closed glass vessels filled with nitrogen, in order to avoid contamination by present day atmospheric CO 2. The amount sampled was in the range of 30-221 mg, so different tests of contamination of the material could be performed. The samples were leached stepwise by 0.5M HCI in order to extract carbon dioxide fractions reflecting different depths in the sample, thus revealing information about possible 14C gradients caused either during the storage in nature, sampling in the field or by the pretreatment work in the laboratory. The deepest (oldest) sample was divided into five fractions. All other samples were leached into two fractions in order to avoid material from the outer surface. The seepage water (50 ml from each sample) was similarly acidified to release the CO2 content. The wet sediment material was pretreated according to the standard acid-base-acid procedure. The soluble fraction obtained of 22 g was combusted by oxidation with CuO at
23°Th/234Uand ]4CDating
113
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114 2
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FIG. 2. Photo and sketch of the cross-section of a 330 mm high stalagmite, LiI4, Lobatse II Cave, Botswana. The exposed surface has suffered from biogenic corrosion. Sampling sites for 23(FFh/234U, I4Cand 8~3Canalyses are indicated on the sketch. The black dots indicate the first radiocarbon sampling-series and the black boxes the second series. The small drill-holes seen on the photo represent positions for 'sO-sampling.
800°C for 5 min (27% carbon content). The CO2 of all the samples was finally converted to graphite by an iron catalytic reaction with hydrogen present, before making the accelerator measurements (Hut et al., 1986). Radiocarbon analyses were made on a total of 23 carbonate samples from stalagmite LII4, three recent seepage water samples and one sediment sample. The preparation background was controlled by a single crystal carbonate (an Icelandic Double spar), with no 14C activity remaining. An activity corresponding to an age of 50,000 _+ 1000 BP (years before present) was found and has
been corrected for in the analysis, although it is only of minor importance when ages < 30,000 BP are being measured. One aim o f the study was to apply the radiocarbon accelerator technique to small amounts of organic material, believed to have been deposited by bats living in the cave, and seen in the stalagmite as black-coloured layers. This would make it possible to avoid dating carbon of complex origin in the carbonate, and to obtain radiocarbon values that are closely related to the contemporary atmosphere at the time of formation of a specific layer. To test this possibility, a black deposit at about 50 mm from the top was scraped off
1 14
K. Holmgren et al.
(245 mg) and pretreated with HCI to remove the calcareous matrix. The insoluble remains (3.35 mg) were combusted but the CO2 yield of 0.08 mg carbon was too low to perform a radiocarbon measurement. Stable isotope analyses of the calcium carbonate samples and the modern seepage water were made with a conventional mass-spectrometer (Varian MAT) with a reproducibility of 0.2%~. 8~3C values were normalized and reported in standard notation relative to Peedee belemnite (PDB). The radiocarbon analyses were performed with the Uppsala tandem accelerator, which is an ultra-sensitive mass-spectrometer (Possnert, 1990). URANIUM SERIES DATING
Samples of 10 to 20 g were dissolved in 8M HNO3. A commercial 228Th/Z32U spike at transient equilibrium was used as an internal standard. The spike is regularly checked against uraninite and speleothem standards. The radionuclides were concentrated by scavenging with Fe(OH)3. U and Th fractions were separated by ion exchange chromatography (Dowex IX8 in 9M HCI and 8M HNO3, respectively) (Gascoyne, 19791. Because humic matter is known to affect the chemical yields in the dating procedure (Lauritzen et al., 1986), care was taken to oxidize the sample, prior to the scavenger step, by boiling it in HNOJH202 until all yellow humic stains were removed. The pure U and Th fractions were first electroplated onto stainless steel disks, and then counted to measure alpha particle activity on silicon surface barrier detectors in vacuo. Ages were calculated from isotope activity ratios using standard algorithms (lvanovich and Harmon, 1992). Most alpha particle spectra display a tailing towards lower energies for each peak. This
was corrected for by using exponential approximations to the tails (Lauritzen, 1991). RESULTS The Radiocarbon Analysis
A detailed presentation of data and results from the radiocarbon analysis is given in Tables la and b, and displayed in Fig. 3. The fractionation tests show only negligible effects of contamination by modem carbon. This is best illustrated by the small age-differences between the five fractions, LII4 1:1-1:5 (Table la). Furthermore, no systematic shifts in the results can be observed between those samples taken at the edge and those sampled at the centre of the stalagmite. Thus, no radiocarbon gradient is observed when moving along a growth layer. The modem seepage water contained a large amount of HCO? close to saturation and had a radiocarbon content close to the present atmospheric content (Table lb). Obtained 6'~C values, of-10.5 to -12%~, are more negative than the mean atmospheric 8~3Cvalue of-7.5%o, indicating the influence of soil CO2. Radiocarbon dating of the organic-rich sediment gave an age of 920 4- 80 BP, suggesting that the bat colonization has been extant at least this long. This may explain why only a very little organic matter was found in the stalagmite; the cave entrance did probably not exist at the time of speleothem formation. The Uranium Series Analysis
Details from the 23°Th/234U-analysis are given in Table 2. All U-series samples display U concentrations far above the detection limit (0.05 ppm) and were free from detrital 23°Th contamination, i.e. (23°Tl'l/232Th) > > 20. Moderate to high
TABLE I a. Radiocarbon data and derived ages for the carbonate samples from stalagmite LII4 Sample ld. Serie I LII4 6A LII4 5 LII4 5B LII4 4 LlI4 3 LlI4 2 LII4 t : I, LII4 1:2, LII4 1:3, LI14 1:4, LII4 1:5,
"outer' 'outer" 'outer" "outer" 'inner'
Serie 2 LII4 6B:l, "outer' LII4 6B:2 'inner' LII4 5A:l 'outer' LII4 5A:2. 'inner' LII4 4A: 1. 'outer' LII4 4A:2 'inner' LtI4 3A: 1. "outer' LII4 3A:2 'inner" LI14 2B:l 'outer' LII4 2B:2 'inner' LII4 IA:I 'outer' t,II4 IA:2 'inner"
Lab no.
Depth (mm)
Carbonate (mg)
Ua-2819 Ua-2823 Ua-2824 Ua-2820 Ua-2821 Ua-2822 Ua-2825 Ua-2826 Ua-2827 Ua-2828 Ua-2829
1(1 32 70 103 210 215 327 327 327 327 327
50 52 68 45 50 31 221
Ua-2877 Ua-2878 Ua-2875 Ua-2876 Ua-2873 Ua-2874 Ua-2871 Ua-2872 Ua-2869 Ua-2870 Ua-2867 Ua-2868
5 5 137 137 165 165 186 186 248 248 294 294
102 66 55 102 123 t04
Fraction* (%)
8~3C (%,~PDB)
65 68 68 51 64 61 12 9 14 9 19
~I).7 +0.2 - 3.2 3.5 4.8 -I.0 3.8
26 511 31) 61 34 62 21 56 25 32 24 72
I.I
3.8
-2.3 5.5 -6.5 - 1.6 6.5
~4C age (BP)
17,845 15,030 18,050 19,550 13,225 27,215 26,560 29,220 29,220 28,735 30,425
_+ 290 _+ 195 +.+_260 ___230 ~ 160 ~- 415 _+410 _ 520 _+ 500 _+ 655 _+ 635
17,165 17,305 18,995 18,855 14,180 13,240 12,660 12,330 28,625 29,280 34,870 35,510
_+ 140 _+ 175 _ 170 +- 185 _+ 170 ___175 -+ 160 +_ t55 -+ 435 +_ 350 -+ 935 +_760
*The measured fraction of the total amount carbonate after leaching of outer surface. The outermost surface has been removed in all samples, hence the total sum of sample is less than 100%.
115
23ff1~34U and ~4C Dating
1.94 mm/100 years and 3.00 mm/100 years are obtained for the two growth series.
TABLE lb. Radiocarbon data for modern seepage water samples, sampled in January 1993 Sample Id.
Lab. no.
Carbon (ktmol/ml)
613C
A]4C (%o)
Site 1 Site 2 Site 3
Ua-31M1 Ua-3042 Ua-3043
7.0 8.1 7.9
-12.0 -11.5 -10.5
164.47 + 11.15 170.48 _+ 10.34 173.82 + 10.66
DISCUSSION The age discrepancy observed between the two methods can be explained by (i) contamination of the material, (ii) methodological errors, (iii) Pleistocene 14C-variations in the atmosphere, or (iv) a combination of these factors. With the results gained so far, we are not likely to be able to determine conclusively the cause of the discrepancy, but we will discuss the plausibility of different explanations.
extraction yields and well resolved spectra provided analytically reliable results. Both dating series are plotted in Fig. 3. It can be observed that all data points of the U-series are in correct stratigraphic order, spanning a period from 50,000 to 21,600 years. When considering the error bars, the apparently inverse age (sample LII4 3) is statistically not different from the stratigraphically younger samples. A growth hiatus, located 212 mm from the top of the stalagmite, is bracketed between 44,300 and 34,950 years. Another hiatus, at 170 mm, lies between 36,000 and 27,000 years. The oldest and youngest 23°T13j234Uages correspond to the 14C values 29,400 and 17,800 BP, respectively (Fig. 3). The stratigraphic order of the ]4C age estimates is inconsistent, at places resulting in an inverse chronology. A remarkable dip in the curve occurs in association with the hiatuses and is located between 165 and 210 mm from the top of the stalagmite. Regression analysis of age versus height, carded out for samples taken from beneath and above the two hiatuses (Fig. 3), gives very high correlation coefficients for the 23°Thp34U series, of 0.95 and 0.98, respectively. The correlation coefficient for the ]4C series is fairly good for the younger part above the hiatuses; a value of 0.63 is obtained. Beneath the hiatuses the correlation is poorer, 0.48. Assuming that the U-series ages reflect true ages and that linear growth occurred beneath and above the hiatuses, growth rates of
The Uranium Series Systematics A common problem regarding U-series dating of speleothems is contamination by detrital 23°Th. In principle, uranium is mobilized through oxidation when bedrock is weathered, while the radiogenic daughter isotopes of Th and Pa remain insoluble and are absorbed onto clay minerals. (UO2)2÷is then incorporated into the carbonate lattice during precipitation as a zero-adjusted radioactive clock. In contaminated speleothems, clay-particles with attached 23°Th have been co-precipitated with the carbonate and disturb the radioactive clock. This can be checked for by measuring the 23°Th/232Th activity ratio. No significant inclusions of detrital 23°Th have been detected in stalagmite LII4. Another prerequisite for reliable 23°ThF34U-dating is that no post-depositional alteration of U and Th has occurred (Chen et al., 1991; Geyh, 1991; Wildberger et al., 1991; Ivanovich and Harmon, 1992). Compact, unrecrystallized speleothems are assumed to represent a closed system for U and Th exchange (Gascoyne, 1992). Stalagmite LII4 does show signs of porosity at certain places, but these places were avoided while collecting samples for 23°Th/234U-dating. Post-depositional addition of U would result in younger measured ages than the true ages and a correction would
55000
m
w
50000
=!~
7=
45000 -- 4 0 0 0 0 L
= =
~0
35000 3000O
• U/l'h-ages a
m
C14-ages
25000 20000 rn
15000 10000 0
~
I
I
I
50
100
150
I I:
200
t
I
I
250
300
350
Distance (mm) from top of stalagmite
FIG. 3. t4C and 23(yFh/234Uage estimates (Table 2) plotted against distance from the top of stalagmite LII4, Lobatse II Cave, Botswana. Shaded areas indicate periods of growth cessation. Regression lines are calculated for the 23°ThF34U and 14C values before and after the hiatuses. Correlation coefficient, r = 0.98 for 23°ThF34Uages and 0.63 for 14C ages after the hiatuses, r = 0.95 for 23Cq'hF34Uages and 0.48 for ~4C ages before the hiatuses.
116
K. Holmgren et al.
T A B L E 2. Uranium-series data, derived Th-ages and the corresponding uC ages Sample ld. LII4 LII4 LII4 LII4 Lll4 t,ll4 LII4 LI14 LII4 LIt4 LII4 LII4 LIt4 LII4 LII4
6B 6A 5 5B 4 5A 4A 3A 3 2 2A 2B 1B IA 1
Lab. no.
444 413 412 469 468 443 411 410 442 446 445 441 368
Depth (mini 5 10 32 7(7 103 137 165 186 210 215 238 248 290 294 327
U (ppm)
>aU/>sU
>I'ThF~4U
e~°Th/>-'Th
Th age (years)
1.60 1.60
2.24 -+ 0.04 2.45 -+ 0.03
0.184 -+ 0.006 0.180 -+ 0.011
> 1000 > 1000
21,600 -+ 750 21,600 _+ 1415
2.00 3.70 1.90 1.10 0.81 0.68 0.82 1.00 1.00 1.30 0.72
2.15 1.46 1.86 1.79 1.83 2.76 2.26 2.13 2.17 1.99 2.47
0.21 I 0.210 0.222 0,292 0.281 0.348 0.346 0.358 0.379 0.379 (7.385
-+ 0.03 _+0.02 -+ 0.02 -+ 0.04 + 0.05 -+ 0.04 -+ 0.05 -+ 0.05 _+ 0.04 -+ 0.03 -+ 0.06
increase the age discrepancy between the dating-series. A leakage of U would result in age estimates being too old. This would be documented in the radiometric data as a decrease in U concentration with time, which is not evident, either. Instead, the U concentration is relatively constant, except for in sample LII4 5A, which has much higher concentration. This higher concentration could be explained, for example, as a result of new uranium reservoirs from a local vein mineralization that has been exposed to the stalagmite, and has no implications for the dating. Since no indications of U and Th exchange in the material have been detected and the age estimates are in correct chronological sequence, it is most likely that the 23°Th/2-~4Udata obtained are reliable and reflect true growth ages. The >4UF38U and 23°Th/Z34U ratios are plotted in Fig. 4, with 1 ~ error ellipses. The sample points are connected by lines, in stratigraphic order. The stalagmite growth was interrupted by two hiatuses, as depicted in Fig. 3. 234Uexcess varies erratically with time, which is sometimes seen in speleothems (Latham and Schwarcz, 1992 and references therein). The changes in 234U/2~sU ratio are independent of the stratigraphically consistent ages, suggesting that these variations in the stalagmite reflect a similar 23~U/23~U variation in the seepage water composition. The other possible process that may affect the isotopic composition is post-depositional leaching of U, which would leave Th behind and cause an apparent increase in age. This effect may be ruled out, as we do not observe any significant disturbances in the stratigraphic order of ages across the zones where the corresponding change in the 234U/'--~sUratio is large. One explanation for the observed effect may be that a redox front developed above the cave and gradually reached it during each growth interval. This may be associated with soil development above the cave. A redox front of this type displays a core which is depleted with respect to ~34U, surrounded by a front that is enriched in 234U (Osmond and lvanovich, 1992). If such a front developed in the soil cover above the cave and gradually entered it, then the observed uranium disequilibria could be explained.
The Radiocarbon Series Systematics No correction for the 'dead carbon effect" has been made
-+ 0.005 _+ (7.004 + 0.1)03 -+ 0.008 + 0.035 -+ 0.019 -+ 0.011 _+0.012 + 0.010 -+ 0.009 _+0.014
185 -+ t 15 > 1000 > 1000 > 1000 > 1000 > 100(7 > 1000 > 1000 > 1000 > l(XI0 > 1000
25,220 25,270 26,760 36,500 34,950 44,300 44,200 46,200 49,300 49,600 50,060
HC age (100(7 BP) 17.3 + (7,2 17.8 + (7.3 15 _+0.2 18-+0.3 19.6 -+ 0.2 18.9_+0.2 13.2 -+ (7.2 12.3 _+ 0.2 13.2_+0.2 27.2 -+ 0.4
-+ 7(15 _+ 550 _+40(7 -+ 1225 _+ 5000 _+ 288(7 _+ 1610 _+ 1835 _+ 1570 -+ t 4 0 0 -+ 2240
29.3 + 0.4 35.5 + 0.8 30.4 _+0.6
to the J4C age estimates, therefore these ages represent maximum values, if incorporation of post-depositional modern carbon is believed to be negligible. HC analysis of recent seepage water from Lobatse II Cave showed very high concentrations of HC; 164 to 174%o (Table l b). Thus, only a minor amount of carbon is contributed from the surrounding dolomite bedrock today. The depleted ~3C values in the seepage water (-10.5 to - I 2%~), compared to atmospheric 8~3C (-7.5%~), reflect the presence of soil and vegetation above the cave. This has had no obvious effect on the water ~4C, indicating that the organic processes are fast and the soils quickly leached. It has also been observed that the present day seepage water flow into the cave is strongly correlated with precipitation, and has a short response time, in the order of a few days. The permeability and conductivity of the overlying bedrock today are such that most of the features of the meteoric water (humics, 8]so, trace and radionuclide element concentration etc.), are generally retained into the cave. The modern seepage water is therefore formed almost in equilibrium with the atmosphere outside !341 /238U
r
t
3.0.
=
i
I .,
...............
10
0"20 20ky
]
0:~0
30ky
40ky
................. ;
/
.
.
.
.
.
.
.
.
.
0.'4C
50ky
FIG. 4. Uranium-series systematics m the Lobatse stalagmite LII4. The numbers refer to the laboratory numbers of the sample points. There is a tendency to increasing 234U/23~Uactivity ratios with decreasing ages (sample points represented by open ellipses), except for in the zones close to the hiatuses (sample points represented by filled ellipses).
117
23(Fl"~34Uand 14CDating
the cave. The circumstances at the end of the Pleistocene are more difficult to determine. The 14C content in a speleothem relative to that in the atmosphere depends on whether conditions typical of a closed or opened system have prevailed during the dissolution and reprecipitation of calcium carbonate (Hendy, 1970, 1971). In an open system, the seepage water is in contact with CO2 in the soil atmosphere during the dissolution of the bedrock. The 14C content in the reprecipitated calcium carbonate will be close to the atmospheric 14C. In a completely closed system, seepage water is isolated from the CO2 source before bedrock dissolution. The ~4C content in the speleothem will then be half of the ~4Cof the atmosphere. According to Hendy (1970), relatively positive values of 6~3C in a precipitate is indicative of open system conditions, but it also reflects the composition of the vegetation (Cerling et al., 1988; Brook et al., 1990b). High 613C values can also be a result of kinetic fractionation (Hendy, 1971). This latter possibility, as well as vegetation characteristics, will be discussed in detail in a forthcoming paper. 8~3C values obtained from the growth axis of stalagmite LII4 range from +0.15 to-6.54%o PDB (Table 1a). So far, these relatively enriched values, taken together with the previous discussion of the modem conditions, suggest that open system conditions dominated in the past. Hereof, no correction in this respect has been made to the apparent ~4C values. It should be noted that a correction of the ~4C values because of this 'dead carbon effect' always results in
younger ages than the apparent age estimates. Consequently, one would expect the apparent 14C age estimates to be older than the U-series ages. However, in the present study the situation is the opposite, and a correction will increase the discrepancy between the methods. Post-depositional introduction of younger atmospheric laC would result in '4C age estimates being too young. Biogenic corrosion at the surface of the stalagmite may facilitate an introduction of younger 14C into the stalagmite, with 14C values varying along a growth layer as a result. The 14C samples in this study have been extracted partly in the center and partly in the periphery of the speleothem, as indicated in Fig. 2. There are no major variations in the laCvalues that are related to the sample location along the growth layers, or to different fractions of these samples (Table la). Thus, if any post-depositional introduction of '4C has occurred, the younger 14C must have diffused into the stalagmite and been homogenized. If we assume incorporation from modem carbon to be the reason for the age difference between 14C and 23°ThF34U,21% of the carbon has to be modem, at the hiatuses at 165-210 mm depth. Circa 4.5% modern carbon will account for the age deviation above, and 2.7-1.8% is needed to explain the difference below the hiatuses. The calculations imply that all incorporation has occurred very recently. If, instead, the incorporation occurred several thousand years ago, or has occurred continuously since the stalagmite stopped growing,
5OOOO
L~DO00 35OOO 3OOOO
5
I1•,~
2OO0O
I0000
0
~
&50gO ,~000 4,.~00 40000 3,.'.'.'.'.'.'.'.~30000 25000 20000 15000 10000 ,5000
0
U/'lt~ y~Kn •
LobaJse C o v e
D
DroJsk~s C a v e
• C a n g o Cave
o Lynds C a v e
•
Barbados corals
FIG. 5.14C age estimates plotted against 23°Th/234Uage estimates. The solid line indicates the ideal relationship. The data represent the results from this study of stalagmite LII4, Lobatse II Cave, Botswana (apparent ~4Cvalues assumed to be close to corrected values), from a stalagmite from Cango Cave, South Africa (t4C values corrected with -1,500 year) (Vogel, 1983), from a stalagmite from Drotsky's Cave, Botswana (correction unknown) (Shaw and Cooke, 1986), from a stalagmite from Lynds Cave, Tasmania ('4C values corrected with -1,500 year) (Goede and Vogel, 1991) and from a study on Barbados corals (~4C values corrected with -400 year) (Bard et al., 1990; Fairbanks, 1990).
118
K. Holmgren et al.
much higher values for post-depositionally introduced carbon are needed to explain the difference. Beneath and above the hiatuses it appears that the lac values follow a more systematic deviation and the younger part is correlated with other data, as will be discussed in a forthcoming section. Thus, other explanations must also be considered. Atmospheric 14C Variations
The U-series ages are not directly comparable with the technical ~4C age estimates expressed in BP. If the true t4C half-life (5730 years) is used instead of the Libby half-life (5570 years), the radiocarbon age at 15,000 BP will become 300 years older and at 25,000 BP, 700 years older. This is not sufficient to explain the observed age deviation. A more critical parameter to discuss is the ancient production and concentration of atmospheric 14C. The short-term Jac variations in the atmosphere during the Holocene, observed as wiggles or as time intervals with constant /4C ages, have been linked to changes in cosmogenic J4C production owing to variations in solar activity (Oeschger et al., 1970; Suess, 1970; Wigley and Kelly, 1990). The long-term variations, observed as a progressive increase in discrepancy with time, indicate a larger ~4C production during the period from 30,000-10,000 BP than during the Holocene. This increased J4C production is commonly attributed to a weakening of the geomagnetic dipole (Vogel, 1983; Bard et al., 1990; Mazaud et al., 1991; Lao et al., 1992). Changes in solar radiation can not be excluded as another possible explanation for the variations. In Fig. 5 we have compiled our LII4 U-series t4C data with the few data available on speleothems from caves in southern Africa and Tasmania as well as data from the Barbados corals (Vogel, 1983; Shaw and Cooke, 1986; Bard et al., 1990: Fairbanks, 1990; Goede and Vogel, 1991). Considering the fact that the different authors have various ways of coping with the difficulties of ~4Ccorrection, we will compare the trends in the data rather than the details. The speleothem data from Drotsky's Cave, Botswana (Shaw and Cooke, 1986), and from Lynds Cave, Tasmania (Goede and Vogel, 1991 ), fit well with the coral data (Bard et al., 1990; Fairbanks, 1990). The age differences of the coral data have been related to increased atmospheric ~4C concentration. being due to a weakening of the geomagnetic field. The LII4 data between 25,000 and 21,000 years (U-series) are well correlated with both the coral and the speleothem data for that period of time. This is followed by a dip in the ~4C data from Lobatse II Cave, between about 35,000 and 26,000 years (U-series), resulting in a rapid increase in the age difference with time. There is no indication of a similar increase in age in the coral data. We believe that this immense deviation is related to the incorporation of modern carbon. Because of a change in the environment, stalagmite LII4 ceased to grow first at around 44,000 years ago and then again at 36,000 years ago, with each of the two periods lasting a few thousand years. The carbonate was eroded and redissolved, resulting in a porous structure in which it was possible for younger carbon to diffuse. Beneath the hiatuses the radiocarbon age estimates get systematically older, but a deviation of 15-20,000 years between the dating methods remains. There are no hiatuses or other indications in the
material to suggest an incorporation of modern carbon to explain this deviation. Changes in the atmospheric L4C concentration cannot be excluded as a possible explanation. The only other data available from this period of time are three poorly documented observations from Cango Cave, South Africa (Vogel, 1983). These age estimates in general deviate much less than the Lobatse II data, but exhibit a stagnation in the ~4C values at a U-series age of 35,000-30,000 years. Vogel (1983) has suggested that this ~4C plateau is related to the very high palaeomagnetic field strength around 30,000 BP that occurred locally because of the Lake Mungo polarity excursion in Australia (Barbetti and McElhinny, 1976). CONCLUSIONS Stalagmite LII4 from Lobatse I1 Cave in southeastern Botswana was deposited during the period spanning 50,000 to 21,600 U-series years. Gentle inclinations in the U-series curve, beneath and above two hiatuses, reflect periods of generally continuous deposition, with an average growth rate of roughly 2 mm per 100 years between 50,000 and 44,300 years and 3 mm per 100 years between 26,700 and 21,600 years. Dating carbonates with the t4C-method is problematic because of the complex dynamics involved between the source of ~4C in the atmosphere and the values obtained in the speleothem. Still, the age deviation observed between the radiocarbon and the U-series in studies of speleothems (Shaw and Cooke, 1986; Goede and Vogel, 1991) is in accordance with that of the Barbados corals (Bard et al., 1990). The age discrepancy seen in stalagmite LII4 correlates well with the above data fbr the period back to about 25,000 years (U-series). The deviation then increases rapidly. We propose that the ~4C-variations in the stalagmite are partly caused by a post-depositional introduction of younger ~4C and partly related to increased atmospheric ~4C concentrations. In order to understand the relative importance of these factors, stable isotope and mineral analyses will be carried out and additional age estimates will be made on this stalagmite, as well as on other material from the Lobatse I and II Cave.
ACKNOWLEDGEMENTS This work was supported by the Swedish Agency for Research Cooperation with Developing Countries and by smaller grants from Stiftelsen L. Hiertas Minne, H. Ahlmanns Fond, Andrerfonden and C. Mannerfelts Fond. The radiocarbon analyses were supported by the Swedish National Science Research Council. W. Karl6n, Stockholm and P. Shaw, Luton, inspired and supported the work and made many valuable comments about the manuscript. Gratitude is extended to the manager of Lobatse Estates Ltd for permission to work in the cave and to G. Carlson and G. van Regenmortel for invaluable field assistance. Grateful acknowledgements are extended to E. Isaksson, G. Rosqvist, A. Stroeven, Stockholm, the referees R. G ~ n , Canberra and one anonymous referee for comments on the manuscript. J. Karlrn improved the English and H. Drake made the drawings. The research was authorized by the Office of the President. Botswana.
REFERENCES Barbetti, M.F. and McElhinny, M.W. (1976). The Lake Mungo Geomagnetic Excursion. Philosophical Transactions of the Royal Society of London, A 281,515-542.
23°Th/234Uand 14C Dating Bard, E., Hamelin, B., Fairbanks, R.G. and Zindler, A. (1990). Calibration of the '4C timescale over the past 30,000 years using mass spectrometric U-Th ages from Barbados corals. Nature, 345, 405-410. Bastin, B. and Gewelt, M. (1986). Analyse pollinique et datation ~4C de concr6tions stalagmitiques holoc6nes: apports compl6mentaires des deux m6thodes. Geographie Physique Quaternaire, 40, 185-196. Becket, B. and Kromer, B. (1986). Extension of the Holocene dendrochronology by the preboreal pine series, 8,800 to 10,100 BP. Radiocarbon, 28, 961-967. Becker, B., Kromer, B. and Trimborn, P. (1991). A stable-isotope tree-ring timescale of the Late Glacial/Holocene boundary. Nature, 353, 647--649. Brook, G.A., Burney, D.A. and Cowart, J.B. (1990a). Desert paleoenvironmental data from cave speleothems with examples from the Chihuahuan, Somali-Chalbi, and Kalahari deserts. Palaeogeography, Palaeoclimatology, Palaeoecology, 76, 311-329. Brook, G.A., Burney, D.A. and Cowart, J.B. (1990b). Paleoenvironmental data for Ituri, Zaire, from sediments in Matupi Cave, Mt. Hoyo. In: Boaz, N.T. (ed.), Evolution of Environments and Hominidae in the African Western Rift Valley, Virginia Museum National History Memoir 1, 49-70. Ceding, T.E., Bowman, J.R. and O'Neil, J.R. (1988). An isotopic study of fluvial-lacustrine sequence: the PlioPleistocene Koobi Fora sequence, East Africa. Palaeogeography, Palaeoclimatology, Palaeoeeology, 63, 335-356. Chen, J.H., Curran, H.A., White, B. and Wasserburg, G.J. (1991). Precise chronology of the last interglacial period: z34U-23°Thdata from fossil coral reefs in the Bahamas. Geological Society of Ameriea Bulletin, 103, 82-97. Cooke, H.J. (1975). The Lobatse Caves. Botswana Notes and Records, 7, 29-34. Cooke, H.J. and Verhagen, B.Th. (1977). The dating of cave development - - An example from Botswana. In: Proceedings of the 7th International Speleological Congress, pp. 122-136. Sheffield. Edwards, R.L., Beck, J.W., Burr, G.S., Donahue, D.J., Chappell, J.M.A., Bloom, A.L, Druffel, E.R.M. and Taylor, F.W. (1993). A large drop in atmospheric t4CpEC and reduced melting in the Younger Dryas, documented with 2azThages of corals. Science, 260, 962-968. Fairbanks, R.G. (1990). The age and origin of the 'Younger Dryas Climate Event' in Greenland ice cores. Paleooceanography, 5, 937-948. Ford, D.C. and Williams P. (1989). Karst Geomorphology and Hydrology. Unwin Hyman, London, 601 pp. Gascoyne, M. (1979). Pleistocene climates determined from stable isotope and geochronologic studies of speleothem. Unpublished Ph.D. Thesis, McMaster University, Hamilton, Ontario, 433 pp. Gascoyne, M. (1992). Palaeoclimate determination from cave calcite deposits. Quaternary Science Reviews, 11,609-632. Gascoyne, M. and Nelson, D.E. (1983). Growth mechanisms of recent speleothems from Castleguard Cave, Columbia Icefields, Alberta, Canada, inferred from a comparison of uranium-series and carbon-14 age data. Arctic andAlpine Research, 15, 537-542. Geyh, M.A. (1970). Isotopenphysikalische Untersuchungen an Kalksinter, ihre Bedeutung f'tir die 14C-Altersbestimmung von Grundwasser und die Erforschung des Pal~toklimas. Geologische Jahrbuch, 88, 149-158. Geyh, M.A. (1991). Zur Entvicklung der sinterchronologischen und isotopenphysikalischen Forschung in Deutschland. Karst und H6hle, 1989/90, 151-157. Geyh, M.A. and Hennig, G.J. (1986). Multiple dating of a long flowstone profile. Radiocarbon, 28, 503-509. Goede, A. and Vogel J.C. (1991). Trace element variations and dating of a Late Pleistocene Tasmanian speleothem. Palaeogeography, Palaeoclimatology, Palaeoecology, 88, 121-131. Hendy, C.H. (1970). The use of 14C in the study of cave processes. In: Olsson, I.U. (ed.), Twelfth Nobel Symposium, Radiocarbon Variations and Absolute Chronology, pp. 419-443. Uppsala 1969, Almqvist and Wiksell, Stockholm, and Wiley and Sons, New York. Hendy, C.H. (1971). The isotopic geochemistry of speleothems-l. The calculation of the effects of different modes of formation on the isotopic composition of speleothems and their applicability as palaeoclimatic indicators. Geochimica et Cosmochimica, 35, 801-824.
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