PhD ceremony: mw. H.J. van Es,
13.15 uur, Academiegebouw, Broerstraat 5, Groningen
Thesis: Thermoluminescence dating of sediments using mineral zircon
Promotor(s): prof. H.W. den Hartog, prof. R.J. de Meijer
Faculty: mathematics and natural sciences
The interest in Quaternary geochronology increases rapidly, because high-resolution histories of recent Quaternary events are important issues in e.g. sea level studies. The carbon-14 method is a major standard method for recent times but it can only be used for dating carbon-bearing material of 60 ka age or younger. Younger sediments are often dated with quartz by measuring the light intensity originating from recombination of defects, which were created in the crystal lattice during irradiation. However, the results of this method are strongly affected by variations in the surrounding sediment.
Exactly because of this, dating with zircon (ZrSiO4) has major advantages: (1) the build-up of the age of the sample takes place inside the grains, because they are irradiated internally by alpha particles from U and Th, elements which are incorporated in the crystal lattice. (2) The dose rates are 2-3 orders of magnitude higher than for quartz and (3) it is a common mineral in most sediments. In this thesis we will demonstrate that the known problems with zircon dating are solved by, among others, selecting the most transparent grains, which are suitable for dating in complete darkness.
What is thermoluminescence (TL)? Thermoluminescence is the thermally stimulated emission of light. When irradiated (zircon crystal) grains are heated to a few hundred ºC, the damage in the crystal lattice is repaired and the grains emit visible light: thermoluminescence. The higher the radiation dose the more light is emitted. Below 150°C the TL spectra feature two narrow peaks due to light emitted by an impurity (Dysprosium ions: Dy) and an unknown broad band at about 400 nm. Above 200°C, the two peaks and the band have disappeared as a result of annealing and the spectrum shows six narrow lines due to the emission of light by another impurity (Terbium ions: Tb). Exposure of the zircon grains to sunlight also resets the dating clock to zero, while the irradiation by U and Th continues. Therefore, the dating “clock” is started from 0 after the last exposure to sunlight.
We have solved the problems with zircon dating by: (i) Selecting the most homogeneous grains of the highest optical quality. Dark grains are not suited since they show scattering or absorption of light. In addition, the resetting in transparent grains is more complete. (ii) Investigating zircon during irradiation, thermal treatment, optical bleaching and long-term storage in the dark to deal with the problem of the decrease of the TL signal after laboratory irradiation (fading), while further improvement was obtained by selecting the most stable component of the TL spectrum for dating. (iii) Developing a model, which describes the defect system in irradiated zircon. The model is used as a tool to optimize the dating procedure.
The most important achievement of our research project is that we have developed standard procedures for optical dating of young coastal sediments with zircon TL. To substantiate our claim we have carried out a full analysis of each step in the dating procedure of a 175-years-old, age-controlled, historical sediment sample from the coast of the Dutch Island of Ameland. The results of these experiments have provided us with detailed information on the behavior of zircon in each step of the dating protocol. Experiments on zircons from several places around the world proved that our method is effective.
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