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Sterba, Johannes H.
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article
Investigation of the isotopic ratio I-129/I in petrified wood
Abstract
In fossil specimens, measurements of the natural isotopic ratio <sup>129</sup>I/I may provide a method to estimate the age of sample. The motivation for measuring the isotopic composition (<sup>129</sup>I/I)of petrified wood samples collected from Austria was to check thisfeasibility. Alkaline fusion together with anion exchange was used toextract iodine from the sample. Typical sample size for this study was10–90 g. An atomic ratio as low as 10<sup>−14</sup> was determined usingaccelerator mass spectrometry (AMS). Uranium concentrations measured byinstrumental neutron activation analysis (INAA) and α-spectrometry werefound to be less than 3 mg kg<sup>−1</sup>, therefore the contribution from fissiogenic <sup>129</sup>I was small and an estimation of ages was based on the decrease of the initial ratio (due to decay of the cosmogenic <sup>129</sup>I in a closed system) after subtraction of the fissiogenic <sup>129</sup>I. The value of the prenuclear ratio is crucial for the use of the <sup>129</sup>I system for dating purposes in the terrestrial environment. From the preanthropogenic (initial) ratio of 1.5 × 10<sup>−12</sup> of the hydrosphere and the results of the present study for the samples from Altenburg (1.05 × 10<sup>−12</sup>) and Fuerwald (6.16 × 10<sup>−13</sup>),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 UpperOligocene/Ergerien period (∼25–30 million years ago), it can beconcluded that these isotopic ratios do not show ages but an elapsedtime since fossil wood was isolated from mineral rich water.Paleontological investigation shows that samples from Altenburg hadmixed characteristics of old and modern Tertiary plants, thus an originfrom a younger stratum re-sedimented with Oligocene cannot be excluded.However, the sample from Drasenhofen reflects that the <sup>129</sup>I/I system might not always be suitable for the dating of petrified wood sample due to fixation of anthropogenic <sup>129</sup>I into surface fractures.