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Reducing empiricism in luminescence geochronology: Understanding the origins of luminescence from individual sand grains

Periodic Reporting for period 4 - RELOS (Reducing empiricism in luminescence geochronology: Understanding the origins of luminescence from individual sand grains)

Período documentado: 2020-03-01 hasta 2020-12-31

Sediments preserve a history of the evolution of the Earth’s surface and its response to a changing climate – a history that can only be read reliably if we know the age of the sediments. Luminescence dating plays a major role in geology and archaeology, and is applicable to almost all sediments from the last 500,000 years – it dates the last time the sediment grains were exposed to daylight. The total amount of energy (dose) absorbed by the mineral grains in the sediment from natural radiation is divided by the rate of energy absorption (dose rate) to give the age. The purpose of RELOS was to improve the reliability of luminescence dating by determining the sources of unexpected spread (over-dispersion) in measured doses derived from sand-sized grains of quartz and feldspar, the two most common mineral in sediments.
We first hypothesized that grains do not remain electrically neutral while absorbing energy during burial and measurement (as is normally assumed). We found that there is a significant buildup of charge imbalance, with some grains charging negatively, and others positively depending on the size of the grain and the range of the radiation. Despite the fact that the effect of charge imbalance was observed experimentally and is clearly considerable, we were unable to make the link to the dispersion observed in natural dose distributions.
RELOS also asked how variation in the rate at which energy accumulates in one mineral grain compared to another has an effect on over-dispersion. We found that the size, distribution and geometry of the mineral grains in the sediment play a role, as well as the distribution of radioactivity inside and outside of the grains. A mathematical model was developed to account for these effects. For feldspar, we were unable to observe the expected relationship between luminescence and calculated dose, questioning some of the fundamental assumptions underpinning feldspar luminescence dating; these observations remain unexplained and require further research.
In our effort to develop measurement procedures that give the same luminescence response per unit dose as in nature, we discovered that the global terrestrial reference site for the last 2.5 million years contains major hiatuses of several tens of thousands of years. These age gaps were shown to correlate with regional and global climate phenomena.
In summary, RELOS has significantly improved our knowledge of the fate of charge in natural dosimeters, of charge transport and of calibration of luminescence signals in terms of dose. However, we were unable to directly link our findings to the cause of over-dispersion in single-grain data and more research is needed to solve this important long-standing question. Our results have been published in 20 peer-reviewed articles.
RELOS was carried out by a team largely made up of physicists and geoscientists. All the work described below was presented at international conferences as oral or poster presentations and unless otherwise mentioned, stars (*) indicate that the work was subsequently published in the peer-reviewed international literature.
A new electron detector for our luminescence instrument was developed that is capable of not only measuring photons (luminescence) but also electrons emitted from grain surfaces. This detector is unique in the world in its design and performance (*). This detector is now available to staff and visiting scientists to the luminescence research group of the Technical University of Denmark.
Through computer modelling and laboratory experiments we investigated the effect on luminescence production of the build-up of (a negative) electrical charge in sand-sized mineral (quartz and feldspar) grains. This charge buildup, its effect on luminescence and the ultimate fate of the excess charge had never been considered before and new insights were gained in these phenomena (*). Some of the experimental data could be explained using the models developed in the project (*) but other results were inconsistent (*). This work was presented multiple times at international conferences and resulted in a PhD thesis and four peer-reviewed articles.
We undertook a checking and recalibration of our analytical facilities used to measure radionuclide concentrations (*) and developed an alternative facility (*). A new method was developed which allows us to measure dose rate with improved precision (<3%) (*). The improved calibration of the existing facility, the new facility and the new method are now routinely used by the luminescence research group and its international visitors.
The effect of granularity on dose rate was also modelled and it was concluded that the mean dose rate to dosimeter grains in a granular matrix is dependent on the grain-size distributions of the source grains, and of the bulk sediment, as well as on the grain size of the dosimeters and the spatial distribution of radioactivity (*).
We were unable to observe the expected relationship between K concentration, grain size and dose in feldspar. These findings question a long-held fundamental assumption in feldspar dosimetry (*). RELOS also showed that the apparent beta dose rate is significantly different from that expected (*). The expected correlations between the spatial distribution of luminescence with features in optical and concentration (Na, K) images were investigated using consolidated samples and it was found that different infra-red stimulated luminescence signals are derived from different mineralogical regions (*).
RELOS set out to measure natural and laboratory-generated dose response curves on sand-sized K-rich feldspar extracted from reference site loess sections in China. We found that the sedimentary record at one reference site is incomplete and cannot be used for constructing natural dose response curves (*). Data from the other reference site has been presented internationally and publication is in progress.
We also contributed to investigations on the effect of grain size on the shape of the quartz optically stimulated luminescence (OSL) dose response curve (ERC Starting grant INTERTRAP; *).
RELOS confirmed that existing and widely-used rejection criteria for quartz single-grain data do not result in a reduction in over-dispersion (*), and showed that in partially-bleached environments single-grain quartz and feldspar distributions show similar degrees of over-dispersion suggesting an unexpected similarity in the degree of bleaching (*). We contributed to the testing of a software package for uncertainty calculations on OSL ages (*) and to a state-of-the-art primer on quartz luminescence dating (*).
The first coupling of a radiation transport model to a luminescence production model.
A demonstration that the fundamental assumptions of feldspar dosimetry are unreliable.
A demonstration that the sedimentary record at the terrestrial reference site is incomplete.
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