Speleothem records older than 500 ka from Southern Siberia

S. Breitenbach1*, D. Fernandez2, J. Adkins2, B. Mingram1, H. Oberhänsli1, G. Haug1

1 GeoForschungsZentrum Potsdam, Section 3.3, Telegrafenberg, 14473 Potsdam, Germany
2 California Institute of Technology, Division of Geological and Planetary Science, 91125 Pasadena, CA, 
   USA
* Corresponding author. Tel.: 0049 331 288 1347; Fax: 0049 331 288 1302 
   E-mail address: troll@gfz-potsdam.de (S. Breitenbach)

Abstract
Speleothems have proven to be generally well-preserved and widely distributed terrestrial tools for tracing palaeoclimatic history (Vaks et al. 2003, Hou et al. 2003, Burns et al. 2001, Gascoyne 1992). Furthermore, some speleothems (especially stalagmites) have proven to be datable with high precision, using U/Th decay schemes (Richards & Dorale 2003, Neff et al. 2001, Wang et al. 2001). Few speleothems have been dated with U/Pb series so far (Richards et al. 1998).
Here we present the U/Th and stable isotope results obtained from stalagmites retrieved from Southern Siberian caves documenting the climate variability of the interior of Central Eurasia. Furthermore we carried out ICP-MS and XRD measurements in order to screen selected elements and identify the mineralogy of the speleothems. 

We report data from two caves (i) Botovskaya cave located in Ordivician limestone 670 km NE of Irkutsk and (ii) Polytechnika cave located in Rhipaeic limestones NW of the vicinity of Lake Baikal. All stalagmites (with one exception) analyzed show 230Th/238U >1 and hence are in secular equilibrium and older than ~500ka (Edwards et al. 1986/87). Stalagmite Boti 881 (Botovskaya cave) consists of both, calcite and aragonite minerals. We have found high uranium concentrations of up to 73.6 ppm. Therefore this stalagmite seems suitable for U/Pb dating (Richards et al. 1998).
Stable oxygen isotope ratios range from -11.2 ‰ to -14.1 ‰ (VPDB standard). δ13C values vary from +1.8 ‰ to -1.6 ‰. Stalagmite Poly 1 (Polytechnika cave) consists of pure calcite. Its uranium concentration reaches 0.95 ppm. Stable oxygen isotope ratios range from -11.7 ‰ to -14.2 ‰ (VPDB standard). δ13C values vary between -3.9 ‰ and -7 ‰. These changes in the isotopic composition in the stalagmite’s calcite are most probably climate-related.
Future work will be focused on U/Pb dating. This method can give speleothem ages well beyond the established U/Th dating method and therefore these potential data may be extremely interesting for Neogene palaeoclimate reconstruction.
Comparison of our data with results from the Baikal Drilling Project will help us to validate independently both records and to elucidate climatic history of this still under-represented continental region.

Introduction
Speleothems (secondary cave deposits) are widely distributed powerful archives of palaeoclimatic proxies (McDermott 2004). Advances in mass spectrometry enable us to analyse trace elements and stable isotopes, as well as radioactive elements with very high precision. Since stalagmites are very stable over long periods of time, and can be dated precisely with U/Th and U/Pb methods (Richards et al. 1998), they are most useful for paleoclimate research.
Here we present first stalagmite records from Southeast Siberia (see fig. 1). Two cave locations have been sampled and three stalagmites analysed. Botovskaya cave is located near river Lena, about 670 km NE of Irkutsk. The plateau, in which the cave is developed, consists of Ordovician sediments (sandstone, siltstone and limestone). The cave is a horizontal maze labyrinth and is longer than 60 km (Göbel & Breitenbach 2003).
Polytechnika cave is a vertical cave located in the Pribaikalsk Mountain ridge, about 70 km E of Irkutsk. The host rock is Riphaeic limestone and the cave is a multi-level system, bound to tectonic faults. Lake Baikal is ca. 7 km to the SE (Breitenbach et al. 2004).
Both caves are situated in a region with strong continentality, and thus high annual temperature amplitude, though lake Baikal has a mitigating effect (Endlicher 2000, Breitenbach 2004).

Fig. 1 Map of lake Baikal and surrounding region. Localities of Botovskaya and Polytechnika caves are shown as stars.

Methods
Samples were prepared for petrologic and stratigraphic observations by cutting and polishing stalagmites from both caves. Thin slides were prepared for CL (cathodoluminescence) microscopy. We carried out X-Ray diffraction (XRD) measurements for mineralogy and ICP-MS analysis for elemental screening. To distinguish between calcite and aragonite, we used both, cathodoluminescence microscopic techniques and XRD (Breitenbach 2004). We used a ThermoFinnigan DELTAplusXL mass spectrometer with coupled Gasbench II for stable isotope measurements (d13C and d18O). To determine ages, U and Th isotopic ratios have been measured on a Finnigan Neptune multi collector inductively coupled mass spectrometer (MC ICP-MS). Although this method allows high precision age determination up to ~500,000 years, we found that the isotopic ratios in the analysed stalagmites are in secular equilibrium, showing that they have been precipitated beyond the limit of the U/Th method (Edwards et al. 1986/87). Therefore we concentrate on U/Pb techniques for dating. This method is well established for rock older than half a million years (Tera & Wasserburg 1972) and only recently for comparatively young carbonates (Richards et al. 1998). 
Stable isotope samples (2 × 250±50μg each) have been taken along the growth axis with a cleaned in 10% HCl stainless steel drill bit (Ø 0.5 mm) and along individual growth layers for Hendy tests (Fig. 2 and 3). After closure of the samples in glass vials under helium inert atmosphere at 72.0±0.1°C, phosphoric acid (concentrated H3PO4) was added and the CO2 generated transferred through a GC column into the mass spectrometer (see Spötl & Vennemann 2003 for methodical details). The 2σ error for δ13C and δ18O is <0.1 ‰.
ICP-MS samples have been taken with a 1 mm drill bit for U concentration measurements. Samples for U/Th measurements for age calculations were taken using a 0.5 mm drill bit. Sample size ranges from 0.14 to 0.19 g (Botovskaya cave) and 0.1 to 1.04 g (Polytechnika cave). Analytical sample treatment is described in Edwards et al. 1986/87.

Results
Petrography
Stalagmite Boti 881 from Botovskaya cave is a 162 mm long cone with a maximum diameter of 75 mm. The samples show a dense palisade crystal structure, with laminae in sub millimetre scale. The crystal growth is radial. The top of this stalagmite shows a broken remnant of a stalactite. Sample Boti 881 consists of both, aragonite and calcite (three XRD samples). Layered Mn and Mg enrichment are vaguely

Tab. 1 U and Th samples taken from Boti 881 and Poly 11
Sample	238U (ng/g)	232Th (pg/g)	δ234Um2	(230Th/238U)act3	(230Th/232Th)act3 ×103	
Poly 1-1a	711.1 ± 0.1	15,262 ± 16	4.9 ± 0.2	1.023 ± 0.004	145.6 ± 0.5	
Poly 1-1b	710.5 ± 0.2	19,084 ± 42	5.5 ± 0.3	1.017 ± 0.007	115.7 ± 0.8	
Poly 1-1c	704.6 ± 0.1	12,423 ± 14	3.7 ± 0.2	1.025 ± 0.004	177.7 ± 0.6	
Poly 1-3	863.1 ± 0.3	230.0 ± 0.2	3.7 ± 0.3	1.030 ± 0.001	11.8 ± 3	
Poly 1-4	1,098.2 ± 0.3	158.8 ± 0.1	1.6 ± 0.2	1.031 ± 0.001	21.8 ± 8	
Poly 1-5	780.3 ± 0.3	294.0 ± 0.2	-0.3 ± 0.3	1.030 ± 0.001	8.4 ± 1.6	
Poly 1-6	740.5 ± 0.3	319.5 ± 0.2	0.3 ± 0.3	1.035 ± 0.001	7.3 ± 1.3	
Poly 1-7	901.4 ± 0.3	187.7 ± 0.2	3.7 ± 0.3	1.031 ± 0.001	15.1 ± 4.7	
Poly 1-8	1,034.1 ± 0.3	103.5± 0.1	-221.1 ± 0.2	1.031 ± 0.001	31.5 ± 17.5	
Boti 881-5	33,474 ± 19	926.6 ± 15.7	13.80 ± 0.6	1.048 ± 0.008	115.7 ± 9	
Boti 881-6	45,879 ± 18	422.5 ± 6.7	13.61 ± 0.4	1.045 ± 0.006	347 ± 56	
Boti 881-7	73,632 ± 20	131.3 ± 2.2	15.52 ± 0.3	0.895 ± 0.004	1,533 ± 826	
Boti 881-8	52,773 ± 19	149.1 ± 2.4	16.71 ± 0.4	1.050 ± 0.005	1,136 ± 530	
Boti 881-9	45,069 ± 19	1,441 ± 24	15.53 ± 0.4	1.087 ± 0.006	103.9 ± 5.2	
Boti 881-10	29,655 ± 22	1,018 ± 20	15.44 ± 0.8	1.050 ± 0.010	93.3 ± 7.7	
1 All errors are 2σ standard deviation.	
2 d234Um = {[(234U/238U)/(234U/238U)eq]-1}×103 measured where (234U/238U)eq is the atomic ratio at secular   
  equilibrium;  λ234 = 2.8262×10-06, λ238 = 1.55125×10-10, λ 230 = 9.158×10-6 (Chen et al. 2000)	
3 Activity ratio calculated by dividing 230Th/238U by the 230Th/238U atomic ratio at secular equilibrium.	

visible in Boti 881, which may be a palaeoclimate proxy (McDermott 2004).
Stalagmite Poly 1 from Polytechnika cave is 220 mm long, with a ~50 mm diameter. It is pristine calcite (two XRD samples), except for the lowermost part and the edge, where clay particles are incorporated. The crystals are in the form of radial columnar palisades. Both stalagmites are dense and finely laminated. Boti 881 is white and shows weathering at the edge, whereas Poly 1 shows a brown color and shows no secondary alteration.

Geochronology
From both stalagmites presented here, 6-8 samples for each stalagmite were taken for U/Th dating. The sample size required was calculated after determination of the 238U concentration on 3-4 samples (~100mg each) on a Finnigan ELEMENT ICP-MS. To control the samples for detrital thorium 230, the activity ratio 230Th/232Th is used. If this ratio is bigger than 100, the detrital thorium fraction is assumed to be negligible. As it turned out, the entire material discussed here is older than ~500,000 years. This is based on the 230Th/238U activity ratio found to be >1 (tab. 1), which represents secular equilibrium of the parent and daughter isotopes and hence the limit of the U and Th method (Edwards et al. 1986/87). The smaller errors found for the activities for samples Poly 1-3 to Poly 1-8 reflect the fact that we use a larger sample mass and also we improved the blank value. We are not able to explain the 3%-excess above the secular equilibrium number with our internal or external precision. We are establishing U/Pb dating techniques using MC ICP-MS analysis, which hopefully will also help us to understand the activity values found. Fortunately, stalagmite Boti 881 shows very high 238U concentrations (tab. 1, fig. 2). We use the ratios of stable lead isotopes to determine the age of the sample (Tera & Wasserburg 1972). So far, we are able to implement a column separation procedure to obtain lead and uranium fractions with minimal amount of calcium. This method will allow dating these surprisingly old stalagmite samples. The low 238U concentration in Poly 1 (fig. 3) will probably not allow U/Pb age determination.
Further we found that the 238U concentration trend in Boti 881 shows similarities to the d18O curve, although the number of samples is not sufficient for detailed comparison. Similar observations have been reported by Kuang et al. (2002) and were interpreted as related to organic matter in soil above the cave. Thus, the 238U concentration may be another palaeovegetation indicator. We cannot clearly observe the same trend in Poly 1, probably because of a lack of more samples. 

Stable Isotopes
For stable isotope measurements, more than 550 sub-samples for Poly 1 and more than 280 sub-samples for Boti 881 were recovered. Multiple Hendy-tests were carried out to test for isotopic fractionation in individual growth layers (Hendy 1971). In fig. 2 and 3 the isotope ratios, d13C and d18O, against growth axis, as well as the Hendy-test plots are given. Stable oxygen isotope ratios range of Boti 881 from -11.2 ‰ to -14.1 ‰ (all isotope values are given against Vienna Pee Dee Belemnite (VPDB) standard). The uncertainty in the Hendy-tests has been considered to be negligible (with the variation in the calcite Δ 

Fig. 2 Stable isotope ratios, Hendy-test plots and 238U concentrations of stalagmite Boti 881 (Botovskaya cave). The 3-point running average is given in red, the 7-point running average in blue. The uranium concentration shows similarities with the δ18O record. 

δ18Oc = 0.2 < 2 σ standard deviation (= 0.2 ‰ - 0.3 ‰)). Therefore, isotopic equilibrium during stalagmite growth is assumed (Linge et al. 2001, Breitenbach 2004).
The δ13C values vary from +1.8 ‰ to -1.6 ‰. Here the 2 σ error for the measurements is <0.2 ‰. δ13C values of this range are commonly associated with C3 vegetation (McDermott 2004, Baker et al. 1997). Nonetheless, since we find both, aragonite and calcite, in Boti 881, interpretation of isotopic signatures remains difficult because of possible fractionation between both phases (Breitenbach 2004).
The stable oxygen isotope ratios of Poly 1 range from -11.7 ‰ to -14.2 ‰, with 2σ standard deviation of <0.3 ‰. Five Hendy-test traverses were taken, none of which shows significant correlation between δ13C and δ18O. Δ δ18Oc is as big as 0.77 ‰, but is caused by sampling error (for details see Breitenbach 2004). A tolerance of 0.5 ‰ in Δ δ18Oc is assumed following Linge et al. 2003. Equilibrium conditions are likely for the time of precipitation of sampled growth layers.
The δ13C values vary between -3.9 ‰ and -7 ‰ and 2σ = 0.16 ‰. These values may be related to shifts in the vegetation above the cave between C3 (forest) and C4 (grasses) photosynthesis (McDermott 2004).

Fig. 3 Stable isotope ratios, Hendy-test plots and 238U concentrations of stalagmite Poly 1 (Polytechnika cave). Error-bars of the U samples show the range one sample integrates. In blue the 7-point running average, and in red a 3-point running average is presented.
Conclusions
The petrography of both stalagmites supports palaeoclimatic analysis, because the samples consist of dense laminated and not secondary effected calcite and aragonite material. The use of the U/Pb dating method will allow us for the first time to determine palaeoclimate proxies older than 500,000 years from stalagmites from the continental Siberia. Furthermore, the high uranium concentrations in Boti 881 may be another proxy itself for soil and/or vegetation cover on the surface (Kuang et al. 2002). The stable isotope signals in both stalagmites shows large variations and are therefore useful for climatic interpretation. Further investigation will show detailed analysis of various proxies of these extraordinary old records from a region not well explored yet. Comparison with records from lake Baikal will enable us to understand the climate of the past of this continental Asian region.

Acknowledgements
Palaeoclimatic research was funded on DFG travel support (grant: 436 RUS 18/3/03 H. Oberhaensli) and INTAS grant 03-51-4152. Logistic support came from Speleoclub ARABIKA, Irkutsk, RF. XRD analysis was kindly carried out by R. Naumann (GFZ Potsdam).

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