## 5 Discussion  
The OSL signals of all the studied samples are dim, not different in quartz grains of sediments of either the metasedimentary or granitoid areas of the Mero basin. A higher concentration of uranium, thorium and potassium radioisotopes [22; 23] would be expected in sediments of the granitoid area, as observed on the gamma radiation map of the area [24] (although these measurements were taken at 50 cm above the ground). Such a high cor  
Figure 4: Potassium distribution and overdispersion of metasedimentary samples. (**a**) Potassium distribution in sand fraction (f: fine; m: medium; c: coarse) and silt-clay fraction. (**b**) Proportion of potassium weighted by the proportion of silt and sand fractions. (**c**,**d**) Linear correlation of samples with high \(OD\): (**c**) \(D_{r-beta}\) vs. K in silts; (**d**) \(OD\) vs. K in silts. (M: Mero basin samples).  
relation between high gamma radiation and concentration of radioisotopes in granitoid lithologies has been frequently observed in the region [1,25], ranging the \(D_{s}\)s between 3 and 5 Gy/ky for fluvial deposits with bright OSL signals. In contrast, the estimated \(D_{s}\)s for the Mero basin is low and similar among samples, including for the sample located in the granitoid area (Mero-8) (Table 1). The used quartz grains highly come from Precambrian metasediments [9,10] and Variscan granitoids [26]. The alluvial deposits studied [4] correspond to a late stage of relief dismantling at the end of the Pleistocene, during which runoff acted on a highly weathered rocky substrate that provided clays, sands and fragments of quartz and quartzite. The brief transport that they suffered justifies the bad selection of the sediments and the low rolling of the quartz and quartzite fragments. They correspond to a wet periglacial stage of the Late Pleistocene [27]. It is possible that such circumstances could lead to radioisotope leaching (above all for U, Th and K), and this could explain the low \(D_{s}\)s obtained [11]. Moreover, no correlation or dependence is observed between signal intensities and \(D_{s}\)s in the samples.  
The characteristics of the OSL signal from quartz grains have been related to the original lithology [28,29] or their degree of weathering [30], as well as the sedimentary transport [31] at a local or even regional scale [32,33]. In the present work, no differences were observed in the signal characteristics and the type of quartz according to its lithologic origin (metasediments or granitoids). The response of quartz grains from sediments deposited downstream (Mero-2 or Mero-4) is similar to that of quartz grains upstream (samples Mero-1 or Mero-6) (Figure 1). The quartz grains analysed in a nearby basin from similar Hercynian granitoids [2] also show low sensitivity, unlike quartz grains from other basins in the region [1]. Both the dim signals and OSL signal sensitivity hindered the use of single-grain for dating, which provides uncertainty for each grain [34]. However, for all samples, the \(D_{s}\)s of the most sensitive aliquots (with brighter OSL signals, higher normalisation ratios and a lower relative error) were found to be similar to the central estimators (median, arithmetic mean or weighted mean of the CAM) [11]. Thus, the CAM is considered for assessing the \(D_{s}\)s, provided that the data fit a normal type distribution (symmetrical and unimodal) [21,35]. In this sense, sample Mero-7 shows a normal-type distribution and low overdispersion value (13%). Thus, we can take this sample age as a reference age to assess the age estimates of the other samples using the CAM. Due to the high \(OD\) values observed (Table 1) for the other samples, we compared and discussed the possible factors that hypothetically contribute to such \(OD\)[36].