What is the true age? Complicated transport history revealed by luminescence dating of multiple buried cobbles.

A significant advantage of rock surface luminescence dating for determining the burial age of cobbles is that well-bleached samples can be identified according to the variation in calculated luminescence age with depth. A plateau in the luminescence age-depth profile extending from the outer surface into the cobble that also gives a lower value than is seen closer towards the interior of the cobble is conclusive evidence of sufficient daylight exposure. The age of this sub-surface plateau represents the time elapsed since the cobble was last exposed to daylight, prior to burial. But there is speculation regarding whether this age of last exposure represents the true age of the formation of the features / landforms of interest for dating. Over 300 cobbles were investigated in this study, originating from different glacial valleys in Tibet. Samples were taken from a range of depositional settings, including different types of moraines and from glaciofluvial sediments. Independent age control and expected ages were provided by cosmogenic nuclide exposure dating and by collection of modern samples. The Ln/Tn signal or equivalent dose were determined using post-IR IRSL protocols. The diverse shapes of luminescence-depth profiles indicate various bleaching / transport histories for different cobbles. Those considered as well-bleached samples could be found from all sampling sites. However, different sites show different proportions of well-bleached cobbles. We found that some cobbles show sub-surface plateaus that were lower than the values toward the centre of the cobble, but the sub-surface ages are apparently older than the independent or expected ages. One explanation of this is that those cobbles were exposed to sufficient daylight in the past to be wellbleached, but not at the time immediately prior to their last deposition which formed the feature or landform sampled for dating, but instead during a previous event since which time they had not been re-exposed to daylight. This means that the age determined for a feature could be overestimated even though a luminescence-depth profile with a low sub-surface dose plateau was obtained from a cobble indicating that the cobble was well-bleached. Unlike traditional luminescence dating of sediment grains, there are practical and logistical reasons why it is unrealistic, if not impossible, to routinely measure a large number of cobbles from every single site and apply statistical models to extract the true age for the feature of interest for dating. The data show us how confident we are in obtaining the true age for a feature or landform when applying luminescence dating on buried cobbles. Recommendations are provided regarding applying rock surface luminescence dating in environments where conditions mean that not all cobbles will have had any previous luminescence signals (including records of previous bleaching events) removed prior to formation of a feature that is subsequently sampled for dating. In such environments, different levels of low sub-surface age plateau recorded in different cobbles from the same feature demonstrate the potential of tracking the pre-depositional history of these sediments by dating multiple cobbles. [ABSTRACT FROM AUTHOR]