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.
