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Investigation of the isotopic ratio 129I/I in petrified wood

2013, Journal of Environmental Radioactivity

In fossil specimens, measurements of the natural isotopic ratio 129 I/I may provide a method to estimate the age of sample. The motivation for measuring the isotopic composition (129 I/I) of petrified wood samples collected from Austria was to check this feasibility. Alkaline fusion together with anion exchange was used to extract iodine from the sample. Typical sample size for this study was 10e90 g. An atomic ratio as low as 10 À14 was determined using accelerator mass spectrometry (AMS). Uranium concentrations measured by instrumental neutron activation analysis (INAA) and a-spectrometry were found to be less than 3 mg kg À1 , therefore the contribution from fissiogenic 129 I was small and an estimation of ages was based on the decrease of the initial ratio (due to decay of the cosmogenic 129 I in a closed system) after subtraction of the fissiogenic 129 I. The value of the prenuclear ratio is crucial for the use of the 129 I system for dating purposes in the terrestrial environment. From the preanthropogenic (initial) ratio of 1.5 Â 10 À12 of the hydrosphere and the results of the present study for the samples from Altenburg (1.05 Â 10 À12) and Fuerwald (6.16 Â 10 À13), respective ages of 8 AE 2.2 and 20.2 AE 2.2 million years were derived. Since samples were collected from a stratum deposited in the Upper Oligocene/Ergerien period (w25e30 million years ago), it can be concluded that these isotopic ratios do not show ages but an elapsed time since fossil wood was isolated from mineral rich water. Paleontological investigation shows that samples from Altenburg had mixed characteristics of old and modern Tertiary plants, thus an origin from a younger stratum re-sedimented with Oligocene cannot be excluded. However, the sample from Drasenhofen reflects that the 129 I/I system might not always be suitable for the dating of petrified wood sample due to fixation of anthropogenic 129 I into surface fractures.

Journal of Environmental Radioactivity 120 (2013) 33e38 Contents lists available at SciVerse ScienceDirect Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad Investigation of the isotopic ratio 129 I/I in petrified wood Tania Jabbar a, *,1, Peter Steier b, Gabriele Wallner a, Otto Cichocki c, Johannes H. Sterba d a Department of Inorganic Chemistry, University of Vienna, Währingerstr. 42, A-1090 Vienna, Austria VERA Laboratory, Faculty of Physics e Isotope Research, University of Vienna, Währingerstr. 17, A-1090 Vienna, Austria c Department of Geology, University of Vienna, Althanstr. 14, A-1090 Vienna, Austria d Atom Institute-Technical University of Vienna, Stadionallee 2, 1020 Vienna, Austria b a r t i c l e i n f o a b s t r a c t Article history: Received 3 August 2012 Received in revised form 19 December 2012 Accepted 23 December 2012 Available online 13 February 2013 In fossil specimens, measurements of the natural isotopic ratio 129I/I may provide a method to estimate the age of sample. The motivation for measuring the isotopic composition (129I/I) of petrified wood samples collected from Austria was to check this feasibility. Alkaline fusion together with anion exchange was used to extract iodine from the sample. Typical sample size for this study was 10e90 g. An atomic ratio as low as 10 14 was determined using accelerator mass spectrometry (AMS). Uranium concentrations measured by instrumental neutron activation analysis (INAA) and a-spectrometry were found to be less than 3 mg kg 1, therefore the contribution from fissiogenic 129I was small and an estimation of ages was based on the decrease of the initial ratio (due to decay of the cosmogenic 129I in a closed system) after subtraction of the fissiogenic 129I. The value of the prenuclear ratio is crucial for the use of the 129I system for dating purposes in the terrestrial environment. From the preanthropogenic (initial) ratio of 1.5  10 12 of the hydrosphere and the results of the present study for the samples from Altenburg (1.05  10 12) and Fuerwald (6.16  10 13), respective ages of 8  2.2 and 20.2  2.2 million years were derived. Since samples were collected from a stratum deposited in the Upper Oligocene/Ergerien period (w25e30 million years ago), it can be concluded that these isotopic ratios do not show ages but an elapsed time since fossil wood was isolated from mineral rich water. Paleontological investigation shows that samples from Altenburg had mixed characteristics of old and modern Tertiary plants, thus an origin from a younger stratum re-sedimented with Oligocene cannot be excluded. However, the sample from Drasenhofen reflects that the 129I/I system might not always be suitable for the dating of petrified wood sample due to fixation of anthropogenic 129I into surface fractures. Ó 2013 Elsevier Ltd. All rights reserved. Keywords: Petrified wood 129 127 I/ I 238 U Accelerator mass spectrometry Instrumental neutron activation analysis Dating 1. Introduction Petrified wood is a type of fossil that forms in two different geological settings. Trees transported by streams and rivers can become buried in the fluvial sediments of river beds, deltas and floodplains or volcanic ash can bury them (sometimes as still upright trees). First, tissue undergoes decay in a moderately warm environment. Over time, mineral rich water passing through sediment fills all the cavities of the wood structure with silica without altering the overall cellular structure of wood. When this process occurs, and how it proceeds, is a matter of controversy. Buurman (1972) suggested that petrified wood can be formed either by * Corresponding author. Tel.: þ43 1 4277 52623. E-mail address: [email protected] (T. Jabbar). 1 Pakistan Institute of Nuclear Science and Technology, Nilore, Islamabad, Pakistan. 0265-931X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvrad.2012.12.010 replacement or through permineralization (filling), later Sigleo (1978) concluded that silicification is a process of impermeation (void filling) and not of inorganic replacement of organic cell walls (petrifiedwoodmuseum.org/PDF/Permineralization.pdf). Scurfield and Segnit (1984) described that petrification by silica involves penetration of wood via splits or checks caused by the permeation of cell walls via reticulated system. Researchers believe that several millions of years are necessary for the complete formation of solid petrified wood (Akahane et al., 2004). 129 I used for geological dating and as tracer of hydrologic and oceanographic systems has received increased attention in recent years (Fabryka-Martin et al., 1985; Fehn et al., 1994, 2000; Fehn and Snyder, 2003; Moran et al., 1995; Muramatsu et al., 2001; Tomaru et al., 2009). Production of natural 129I (t1/2 ¼ 15.7 Myr) takes place primarily by interaction of cosmic rays with Xe isotopes in the stratosphere and by spontaneous fission of 238U in crustal rocks (induced fission of 235U plays a minor role and can be neglected in most cases). Both of these processes contribute similar amounts of 34 T. Jabbar et al. / Journal of Environmental Radioactivity 120 (2013) 33e38 natural 129I to the surface reservoir (Fabryka-Martin, 1988). In addition to these two natural production modes, large quantities of 129 I have been emitted by anthropogenic activities since the beginning of the nuclear age. The half life of 129I together with the observed input ratios and the detection limit of accelerator mass spectrometry (AMS, 129I/I ¼ 10 14) allow the application of this dating method within a range of approximately 80 Ma, which is beyond the radiocarbon dating limit (Fehn et al., 2007a,b). Although theoretical detection limit of AMS is 2  10 15, lack of the blank material with ratios below 10 14 establishes a practical detection limit of 10 14 (Fehn, 2012). The biophilic nature of iodine suggests that this isotopic system might be particularly useful for the dating of organic material (Fehn et al., 2007a). The isotopic ratio (129I/I) below the zone of bioturbation has been calculated theoretically as 3.5  10 14 by Edwards (1962), 2.2  10 12 by Edwards and Rey (1968), 1 10  10 13 by Burger (1980) and 3 30  10 13 by Fabryka-Martin (1984) (all as cited by Fabryka-Martin, 1984) assuming iodine exchanges rapidly between all surface reservoirs such as the oceans, biosphere and atmosphere to produce constant preanthropogenic 129I/127I ratio. Once the iodine is removed from well mixed surface reservoir, the initial ratio will decrease due to continuous decay of 129I. The ratio of 1.5  10 12  10% has been verified experimentally by AMS measurements of marine sediment samples off the coast of Carolina in the Atlantic Ocean and later in a series of samples from other marine locations and is used as the initial ratio for age determinations using 129I in hydrosphere (Fehn et al., 1986; Schink et al., 1995). Although the average pre-anthropogenic ratio measured in marine sediments from Cape Hatteras is approximately 1.2  10 12, there is a significant variation in the samples measured, with a range of 0.7e1.48  10 12 (Moran et al., 1998). The observation of 129I/127I as low as 0.2  10 12 in the depth profiles from the Easter islands and Indian ocean and the results obtained by Ernst et al. (2003) of pre-nuclear iodine preparations of (0.2  1.4  1)  10 12 shed some doubt on previously proposed natural equilibrium isotopic ratio of 1.5  10 12 (Daraoui et al., 2012; Ernst et al., 2003). However, this issue is still not settled and ages are being calculated using isotopic ratio of 1.5  10 12. Up till now dating with 129I mainly focused on marine samples (hydrates, brines and salt domes) with high concentrations of iodine. There are only two reports on dating of terrestrial samples because of the low iodine concentration and the uncertainty of the pre-nuclear ratio (10 11 10 13) (Szidat et al., 2000; Luo et al., 2011; Zhou et al., 2010). In this context, we have tried to date lignite, however, the lignite did not represent a closed system with respect to iodine (Wallner et al., 2007). Following our previous attempts we now investigated petrified wood. In general petrified wood is found in the silicified form, though this is not the only possible matrix. Because silicified wood is far more prevalent and probably better preserved (Kuczumow et al., 1999), it was adopted as an object of our research. Weathering products of rocks are probably the source of the non crystalline (or amorphous) form of hydrated silica (SiO2$nH2O). Continued crystallization and water losses transform the opal into chalcedony. Over time, re-crystallization occurs to convert chalcedony to quartz, the hardest form of silica (Suneson, 2010). The present study aims to investigate pre-anthropogenic ratios and the possibility of dating petrified wood samples based on the decay of the cosmogenic 129I component (after subtraction of the fissiogenic 129I). Acid leaching with HF (Gao et al., 2007) and alkali fusion (Brown et al., 2005; Bienvenu et al., 2004; Chai and Muramatsu, 2007; Date and Stuart, 1988) are commonly used methods for separating trace iodine from silicate minerals. The drawback of acid digestion includes loss of iodine due to formation of volatile iodine compounds, and incomplete sample decomposition leading to the doubts as to whether the 129I was completely released from the solid phase. Therefore we investigated the alkaline fusion method in more detail. Different salts have been used in the fusion process; we choose NaOH as fluxing agent which is applicable in samples containing silicates. The content of stable iodine in our samples was determined with instrumental neutron activation analysis (INAA). In order to estimate the amount of fissiongenic 129I derived from decay of 238U, wood samples were also analyzed for their uranium content by alpha counting and INAA. Finally, the 129I/I isotopic ratio was determined by AMS. In addition, we checked the chemical composition of samples. 2. Material and methods 2.1. Description of samples Three petrified wood samples used in this study were surface picked from northern Lower Austria in 1985 (Table 1). The petrified wood was found scattered over the sites and partially buried in the ground. Samples were mixed with remains of gravel layers deposited by Tertiary rivers, which also had erased the former floodplain forests and sedimented the stems for petrification. Two samples collected from Waldviertel were a semiring-porous wood (evergreen Mediterranean Oak type-Fuerwald) and a semiringporous wood (hornbeam type-Altenburg). Both had been resedimented in the course of time and were preserved in a layer associated with the Upper Oligocene/Ergerien period, w25e30 million years ago. The third piece stems from Drasenhofen, Weinviertel and is a ring-porous Oak type (modern deciduous Oak) as it still grows in Austria. It was found in a layer from the Pliocene period >3 million years ago and obviously was re-sedimented. As the paleontology of the Altenburg species spectrum shows mixed characteristics of old and modern Tertiary plants, the sample investigated might have been re-sedimented (condensed) into the layer where the Fuerwald sample was found (Cichocki, 1998, 1992). The samples were sliced into 1-cm pieces with a diamond saw (water-cooled) perpendicular to the trunk axis, washed with Millipore water to remove surface contamination and air dried. The crushed and grinded samples were screened through a sieve. The fraction finer than 315-mm was used for further analysis. Composition of samples was checked using INAA and EDX (Energy Dispersive X-ray spectrometry). The results of elemental analysis are shown in Table 2. 2.2. Extraction method Iodine isotopes were extracted by alkaline fusion following procedures from literature (Nishiizumi et al., 1983; Schmidt et al., 1998) after some modifications. About 10e90 g of sample was mixed with NaOH (1:3) as fluxing agent in a porcelain crucible, spiked with 2.5e5 mg of iodide (in-house standard Vienna-NaI-78, isotopic ratio 10 14 from Isotrace Laboratory) and was fused at 300  C. The fused solid was then extracted with 100e200 mL of hot water and the residual was dissolved using a mixture of 1 mL of concentrated H2SO4 and 1-mL 1-M NaHSO3 solution. In the beginning of the acidification of the filtered leachate, a large amount of precipitate were produced, however, with the addition of acid, most of these precipitates can be dissolved (Hou et al., 1999). Remaining steps are the same as given in literature (Jabbar Table 1 Sampling site specifications. Sampling site Latitude (N ) Longitude (E ) Altenburg Fuerwald Drasenhofen 48.64 48.68 48.78 15.58 15.53 16.64 T. Jabbar et al. / Journal of Environmental Radioactivity 120 (2013) 33e38 Table 2 Elemental analysis of petrified wood samples. Element Concentration (mg kg-1) Altenburg Drasenhofen Fuerwald Sia Na K Ca Mn Al Sc Cr Fe Co Ni Zn As Rb Sr Zr Sb Cs Ba La Ce Nd Sm Eu Tb Yb Lu Hf Ta W Th 420  39 68.23  4.09 n.d. 393  70 20.07  0.30 6804  34 0.180  0.003 14.59  0.72 1478  38 0.87  0.03 2.48  0.16 12.4  0.5 0.40  0.03 n.d. 3.79  1.70 23.80  3.09 0.06  0.01 n.d. 483  9 2.37  0.09 2.76  0.08 n.d. 1.54  0.07 0.27  0.01 0.14  0.01 0.50  0.03 0.08  0.01 0.57  0.02 n.d. 0.84  0.10 0.11  0.01 467  23 74.54  4.47 n.d. 507  91 16.15  0.32 6328  53 0.180  0.003 14.92  0.72 1459  38 0.71  0.02 3.86  0.24 481.9  13.4 0.81  0.04 n.d. n.d. 90.75  6.57 0.29  0.01 n.d. 1793  48 10.10  0.30 13.57  0.40 11.70  2.80 2.54  0.12 0.56  0.01 0.31  0.01 0.67  0.09 0.11  0.01 2.85  0.08 n.d. 0.26  0.09 0.15  0.01 450  40 67.97  4.07 n.d. 283  45 7.51  0.15 1983  40 0.170  0.003 13.43  0.68 1396  36 0.68  0.02 3.99  0.76 460.7  12.8 0.34  0.01 n.d. n.d. 80.01  3.52 0.29  0.01 n.d. 1671  45 0.45  0.02 12.93  0.50 10.94  2.73 0.39  0.02 0.53  0.01 0.28  0.01 0.57  0.07 0.031  0.002 2.67  0.07 n.d. 3.79  0.34 0.15  0.01 n.d. (not detected). a mg/g. et al., 2011, 2012). The clear solution was loaded onto the column packed with already conditioned 5e40 g of Dowex 1  8 (100e200 mesh Cl form) and was washed with 50-mL 0.1-M HNO3. Iodide was eluted with 200 mL of 0.5-M HNO3. Subsequently, iodine was precipitated as AgI by addition of 4-mL of 0.01-M AgNO3. Silver iodide was thereafter isolated by centrifugation. The precipitates were washed with 25% NH3 and twice with Millipore water in order to dissolve other halides that may have co-precipitated with AgI. The contamination introduced in the process was determined by the preparation of blank samples processed in the same way as real samples and was found insignificant. The percentage recovery of iodine was also investigated as a function of concentration of NaOH and fusion temperature. For this purpose, petrified wood samples were placed in crucibles, 1 mL of iodine carrier (2.5 mg mL 1) was added and different amounts (2mL, 3-mL, 4-mL, 5-mL and 6-mL) of saturated NaOH solution were mixed with the samples. The fusion time was varied from 300- C for 5 h (Schmidt et al., 1998), 450  C for 4 h (Chao et al., 1999) and 550  C for 1 h (Brown et al., 2005) after 30 min heating at 150- C in each case. The maximum recovery of iodine was obtained for fusion with NaOH (1:3) at 300- C for 5 h as determined by inductive couple plasma mass spectroscopy (ICP-MS) after filtration. One drawback of alkali fusion is that a large amount of flux to sample (3:1) is required which is not suitable for analyzing a big sample. The advantages of this method are its simplicity and that it does not require any special equipment. 2.3. Total stable iodine The iodine content of petrified wood was determined by INAA. Typically 200-mg of powdered samples were sealed in a polythene 35 vial. The samples were irradiated with pneumatic transport system of the TRIGA Mark II research reactor at the Atominstitut (Tech. Uni. Vienna) at a thermal neutron flux density of 1  1012 cm 2 s 1 for 2 min, followed by 10 min decay and then counting for 10 min. The irradiation and counting period are limited due to the short half-life of 128I (t1/2 ¼ 25 min). Any chemical treatment which could lead to a contamination of the sample was avoided as well as the surface of the sample container was cleaned after irradiation to get rid of any surface contamination. The 443 keV gamma line of 128I was used for measuring the iodine content with a 151 cm3 HPGe detector, a lossfree counting system and a PC (Hou et al., 1998). The resolution of this system is 1.8 keV for the 1332 keV gamma line of 60Co and the relative detection efficiency is 50.1%. The advantages of INAA are high sensitivity, good accuracy and precision, and only small amounts of samples are needed. 2.4. 129 I/I ratio For the determination of the 129I/I ratio, the dried AgI precipitates were mixed with silver powder (AgI: Ag 1:1 by weight) and pressed into copper targets for AMS measurements carried out at the Vienna Environmental Research Accelerator (VERA). Negative ions from the Cesium sputter source were mass analyzed, stripped at the terminal of 3 MW Tandem Accelerator and 4þ charge state was chosen for the measurement, where 127I4þ was measured as current using a Faraday cup and 129I was measured using gas ionization detector. To avoid cross contamination from high ratio reference materials, the samples were normalized to our in-house standard Vienna-AgI-111, which was calibrated to be (1.8  0.2)  10 13 in previous measurements. 2.5. Uranium concentration Samples were homogenized by grinding, dried at 105- C to constant weight and ashed at 450- C overnight to destroy organic matter. Samples were then digested by the radiochemical procedure given in (Eichrom Technologies, 2005) with slight modifications. The samples were leached in 10-mL 14-M HNO3 and 5-mL 12M HCl in a Teflon beaker, and 232U tracer (13.85  0.20 mBq) was added. After boiling the mixture for 3 h, the sample was centrifuged for 30 min at 4000 rpm (relative centrifugal force (RCF) is 1646). The residue was leached twice with 20-mL 14-M HNO3 and 15-mL 40% HF for 3 h and was then rejected. The combined solutions were evaporated till dryness, and fumed three times with 5-mL boric acid (c ¼ 5 g/100 mL), 5-mL 12-M HCl and 5-mL 14-M HNO3 respectively. The residue was dissolved in 80-mL of 8-M HCl and uranium was separated by anion exchange and measured a-spectrometrically after micro precipitation with neodymium fluoride (Wallner et al., 2007). Uranium was also determined by INAA. The samples were irradiated at a thermal neutron flux density of 1  1013 cm 2 s 1 for 36 h, followed by cooling for 5 days and then counting for 30 min. Here the 277.7 keV photopeak of 239Np (half-life 2.4 days) produced by neutron capture of 238U was used (El-Ghawi et al., 2005). 3. Results and discussion The energy dispersive X-ray spectrometry and INAA revealed the main composition of the samples. Silicon dioxide was the most prevalent component, with other elements (Al, Fe, Ba, Ca, Zn, Na) at low levels but clearly noticeable (Table 2). The respective Si and Fe concentrations were similar in all samples, while Ca gave differing values for all three samples investigated; lowest Al concentration was observed in Fuerwald, whereas Zn and Ba were considerably lower in the Altenburg sample. Differences in sample composition 36 T. Jabbar et al. / Journal of Environmental Radioactivity 120 (2013) 33e38 from Fuerwald and Altenburg were unexpected as they were collected from a single stratum of limited geographic extent. On the other hand, compositional similarity between Fuerwald and Drasenhofen is not surprising since similar characteristics in specimens from different localities have also reported (Kuczumow et al., 1999; Mustoe, 2008). We observed that the samples from Altenburg and Fuerwald were easier to cut and crush than the sample from Drasenhofen though organic matter as measured by loss on ignition was found to be nearly the same (0.05e0.07%) in all three samples. However, the fact that the highest concentration of Zr was detected in the Drasenhofen sample supports our observation (Debsikdar and Sowemimo, 1992). Our samples showed iodine concentrations (mg kg 1) of 5.5  0.4 (Altenburg), 9.7  0.4 (Fuerwald) and 0.32  0.03 (Drasenhofen) as detected by INAA (Table 3). The concentration of iodine in the first two samples is much higher than the average iodine concentration of 0.12 mg kg 1 in bulk continental crust (Muramatsu and Wedepohl, 1998) and higher than the range of 0.1e0.5 mg kg 1 found in rock-forming minerals and in igneous rocks (Fabryka-Martin, 1984). Our first measurements of the 129I/I ratio with only 10 g sample mass gave too high values on the order of 10 10, which we thought might be due to contamination. The presence of small quantities of recent material could cause a substantial shift in isotopic ratios as shown in Fig. 1. For excess 129I, there are three potential sources of contamination (two anthropogenic and one natural): 129 I introduced from air during sample preparation I introduced to fissures or fractures of samples from (modern) atmospheric precipitation and from ground moisture on the collection spot (iii) Fissiogenic 129I built up in the sample itself or incorporated after leaching from nearby U bearing geologic formations before the end of petrification (i) (ii) 129 Since contamination is an important problem for low level iodine analysis, some chemical blanks were prepared by taking 5 mg of iodine carrier (NaI) and carrying out the entire sample preparation for AMS analysis. The average amount of 129I in these blanks was 5.25  2.54  10 16 g which is one order of magnitude higher than NaI precipitated as AgI without sample preparation steps. This may be related to presence of 129I in some chemicals. Still the concentration of 129I found in different blanks prepared by varying amount of chemicals has same order of magnitude and would not affect isotopic ratio considerably. Considering surface contamination, we washed all samples with Millipore water, but excess 129I might have accumulated in open voids and structural fractures. This sort of contamination could be overcome by washing the surface of the samples and by increasing the sample size. At the same time we decreased the amount of iodine carrier, thus enhancing the 129I/I ratio measured. We continued to increase the sample mass until we got constant 129I/I ratios below preanthropogenic level for the samples Fuerwald and Altenburg. However, this did not work for the sample from Drasenhofen. Thus, Fig. 1. Correlation between sample size and isotopic ratio (log scale) for petrified wood samples. Dotted line shows preanthropogenic value of 1.5  10 12. the very hard and compact sample from Drasenhofen with a ratio of 6.5  10 12 might contain fissiogenic 129I taken up from the environment during the period of petrification. Re-crystallization and physical deformation of the fossil wood from the weight of overlying sediments or from tectonic events may lead to brittle fissures that may allow for deposition of minerals at a later stage (Trostle, 2009). These processes may add fissiogenic 129I or natural iodine (with the above given initial 129I/I ratio) to the samples long after the end of petrification. So the 129I/I ratios of the samples with high stable iodine concentration (Fuerwald and Altenburg) were found below the measured pre-anthropogenic input ratio of 1.5  10 12 for the hydrosphere indicating the absence of anthropogenic contributions, while the Drasenhofen sample with low stable iodine concentration had a ratio above the prenuclear threshold and therefore was rejected. In Fig. 2 we compared our results with other data from the literature. Iodine chemicals from the nitrate deposits in Atacama Desert, Chile, which had been stored from times before the onset of the nuclear age, and iodine bearing minerals had clearly lower average 129I/I ratios of approximately 0.2  10 12 (Fehn et al., 2007b). Similarly for the first time in soil sample (at depth of 10 cm) from Eastern Islands, isotopic ratio below 10 12 was observed (Daraoui et al., 2012). Assuming 1.5  10 12 as pre-nuclear Table 3 Iodine concentration and isotopic ratios of petrified wood samples. Sample Altenburg Drasenhofen Fuerwald Amount (g) 127 90 32 60 5.5  0.4 0.32  0.03 9.7  0.4 I (ppm) 129 127 I/ 10 I 12 1.15  0.19 6.49  0.89 0.63  0.12 Correcteda 129I/ 127 I  10 12 1.06  0.11 6.13  0.70 0.62  0.03 a After substraction of the fissiogenic 129I, considering only spontaneous fission of U as given in (Fabryka-Martin, 1988). 129I (atoms/g) ¼ 1470  238U concentration (ppm). 238 Fig. 2. Comparison of present results with published data. 37 T. Jabbar et al. / Journal of Environmental Radioactivity 120 (2013) 33e38 Robs ¼ Ri e l129 t Where Robs ¼ observed 129I/I ratio, Ri ¼ initial 129I/I ratio of 1.5  10 12, l129 ¼ decay constant of 129I (4.41  10 t ¼ years since iodine-129 deposition. 8 y 1 ), and Obviously, the age values calculated strongly depend from the assumed initial 129I/I ratio. With older samples like Fuerwald, however, at least the order of magnitude of the derived age should be correct. According to the standard decay equation, Fuerwald and Altenburg reflect the respective end of the petrification process at about 20.2  2.2 and 7.95  2.20 million years ago (Fig. 3), whereas the depositional formation that contained the samples belongs to Upper Oligocene/Ergerien period, (w25e30 million years ago). We must consider that the duration of the mineralization process of the silicified wood must be added to get the real age of the wood, and that also later processes influencing the surrounding strata and resulting in deformation and alteration of the wood may have an influence on the 129I/I ratio and therefore on the derived age. Considering the paleontological determination and hardness of the trunk pieces collected from Altenburg it is not surprising that the sample from 1,8 Initial ratio 1,6 Altenburg 1,4 Fuerwald I x 10 1,2 1 I/ value, the nitrate deposits as well as the minerals might have been formed about 3 half-lives of 129I ago. On the other hand, the values measured for the petrified wood from Fuerwald and Altenburg are much lower than the reported values of 7  10 12 and 1  10 10 (5e 66 times higher than pre-nuclear value) of a thyroid gland powder from USA, 1943 and of soil from Moscow, 1910 respectively (Szidat et al., 2000). The uranium contents and isotopic ratios 234U/238U measured in petrified wood samples are presented in Table 4. The 234U/ U activity ratios of the samples were close to unity, suggesting that the material has not been strongly leached (Kanai et al., 2006). The uranium concentrations measured by INAA are also shown. These values were used further, considering INAA results are more reliable as no chemical separation is involved. 129I can also be produced by spontaneous fission of 238U, with minor production by neutron induced fission of 235U depending on rock types. However, the uranium concentrations of 1 and 2.2 mg kg 1 are rather low in our samples, so the amount of fissiogenic 129I built up in the already petrified wood is negligible (the fissiogenic 129I from 1 mg kg 1 uranium corresponds only to 2% of the 129I/I ratios, considering 50% of the atoms have already decayed in case of Fuerwald). But, as already mentioned, potentially complicating the interpretation is the migration of fissiogenic 129I (from nearby geological formation) dissolved in mineral rich water during the silication process that might lead to fixation of iodine within the wood structure together with scavenging of Fe, Al oxides and organic matter (a removal of iodine might also be possible, but would not alter the isotopic ratio). Assuming that (stable as well as radioactive) iodine was fixed in the petrified wood, the decay of 129I provides an elapsed time since the end of the petrification process: 0,8 0,6 0,4 0,2 0 0 5 10 15 20 25 Time (Ma) Fig. 3. Correlation between isotopic ratios and estimated elapsed time. Altenburg is much younger than Fuerwald and might have its origin from a younger stratum mixed with the underlying Oligocene. 4. Conclusions and perspective The ratio 129I/I as well as stable iodine and uranium concentrations were measured in three samples of petrified wood with AMS and INAA, respectively. The sample from Drasenhofen did not yield isotopic ratios independent from sample size and gave a ratio higher than the pre-anthropogenic level (1.5  10 12) used as the initial ratio for age determinations; we think that this sample does not represent a closed system. The other two samples, however, gave ratios smaller than this value, and so we were able to estimate a time elapsed since the end of the petrification process by using the decay of the cosmogenic 129I (the amount of fissiogenic 129I was calculated from the uranium concentration and was subtracted from the total 129I measured). Both samples (Altenburg and Fuerwald) had been collected from the same stratum deposited during Upper Oligocene/Ergerien period (w25e30 million years) and gave post mineralization ages of 8  2.20 and 20.2  2.2 million years, respectively. These differences in ages may be explained by paleontological determination of species giving a hint to a possible resedimentation of the “younger” sample on the underlying older layer. Taking into account that several millions of years might be necessary for the complete formation of silicified wood, our data show a reasonably good correspondence to the ages of the surrounding geological stratum. Still the ages calculated are nominal due to ambiguity in initial isotopic ratio and discrepancy in mineralization process of the silicified wood. So for the first time a research protocol for dating silicified wood from the isotopic ratio 129 I/I has been established. However, due to funding constraint, only a representative sample from each location was analyzed. A much more extensive study regarding larger numbers and masses of subsamples, analysis in fractions (outer and inner rings) and site characterization (129I/I and uranium concentrations in underground water) is suggested. Table 4 Uranium concentration of petrified wood samples. Sample Altenburg Method 238 INAA 2.24 3.7 0.32 0.5 1.1 1.4 a-Spectrometry Drasenhofen Fuerwald INAA a- Spectrometry INAA a-Spectrometry U (ppm)       0.13 0.1 0.02 0.1 0.1 0.1 234 U/238U 1.2  0.1 1  0.1 1  0.06 Acknowledgment Authors would like to thank Mag. Siegfried Fürtauer, Institute of Inorganic chemistry/material chemistry and Mag. Dr. Stephan Puchegger, Faculty Center for Nanostructure Research, University of Vienna for their help in sample preparation and Silicon analysis by EDX spectrometry. 38 T. 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