[Zurück]

T. Jabbar, P. Steier, Gabriele Wallner, O. Cichocki, J.H. Sterba:
"Investigation of the isotopic ratio 129I/I in petrified wood";
Journal of Environmental Radioactivity, 120 (2013), S. 33 - 38.

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.

Petrified wood, 129I/127I, 238U, Accelerator mass spectorsopy, instrumental neutron activation analysis, Dating

http://dx.doi.org/10.1016/j.jenvrad.2012.12.010