## 3 Materials and Methods  
Sampling for OSL dating was performed on homogeneous sand lenses within the dated alluvial deposits (Figure 2) by hammering steel cores. In the Luminescence Laboratory of the University of A Coruna, the dry-sieved sand grains (180-250 \(\upmu\)m) from the central part of the core were treated with HCl and H\({}_{2}\)O\({}_{2}\). The dried feldspars and heavy minerals were removed by density separation using sodium polytungstate solutions with densities of 2.62 g/cm\({}^{3}\) and 2.70 g/cm\({}^{3}\). The obtained quartz was HF-etched (40%) to remove any remaining feldspars. Quartz multigrain aliquots were checked with infrared (IR) stimulation to ensure the absence of contamination by minerals other than quartz.  
Luminescence measurements were performed on small multigrain aliquots (100 grains) mounted on stainless steel discs in a Rise DA-15 automated TL/OSL reader equipped with blue light emitting diodes (LEDs) (470 \(\pm\) 30 nm) for stimulation, a Hoya U-340 filter and a 9235 QA photomultiplier. A \({}^{90}\)Sr/\({}^{90}\)Y beta source of 0.120 \(\pm\) 0.003 Gy/s was used for irradiation. The single-aliquot regenerative dose (SAR) protocol [12] was used to estimate the \(D_{e}\)s. Preheat tests were carried out to choose the appropriate preheat treatment, and recovery tests were performed after assessing the \(D_{e}\)s. The OSL signal was stimulated for 40 s at 125 \({}^{\circ}\)C. The early background (EBG) [13] from 0.4 s to 4 s natural OSL decay curve were subtracted from the fast component (first 0.4 s) for signal integration. Descriptive statistics were used on the \(D_{e}\)s obtained from individual aliquots using Abanico plots and normality tests (Kolmogorov-Smirnov (Lillier.); Shapiro-Wilks if n \(<\) 50) were performed. Dose-variation test as a function of aliquot size [14] were performed, from 10, 30, 100, 300 and \(>\)700 grains.  
Coarse K feldspar grains (180-250\(\upmu\)m) and fine grains (\(<\)63 \(\upmu\)m) were also obtained and used for IRSL dating [15]. The IRSL\({}_{50}\) and post-IR IRSL\({}_{290}\) signal was measured on both grain sizes. The IRSL signals were stimulated for 200 s at 50 \({}^{\circ}\)C and 290 \({}^{\circ}\)C, integrating both the first 5 s and using the last 25 s for background subtraction [16]. However, it was not possible to obtain \(D_{e}\)s from the IRLS signals due to the low amount of both coarse (250-90 \(\upmu\)m) and fine (\(<\)63 \(\upmu\)m) feldspar grains obtained. The scarce grains obtained provided saturated IRSL signals [11].  
The \(D_{r}\) was estimated to use a Camberra-XTRA gamma detector (Ge-Intrinsic) during 46-68 h and counting. [PERSON]'s conversion factors were used [17]. The alpha contribution was neglected for quartz dose rates, and the beta dose rate was corrected due to HF etching [18]. Moisture and water saturation values were assessed for all samples to estimate average water content. The cosmic dose rate was calculated in accordance with Prescott and Hutton [19].  
The concentration of potassium (and therefore \({}^{40}\)K) was also estimated in the fine (\(<\)63 \(\upmu\)m) and coarse (63-1000 \(\upmu\)m) grains of bulk sediments for each sample by X-ray fluorescence spectrometry (XRF). The grain size distribution of samples was assessed by laser diffraction (\(<\)250 \(\upmu\)m) using a Saturn-DigiSizerII laser detector and dry-sieving (250-2000 \(\upmu\)m). Thus, it was possible to weigh the concentration of K on the silt (2-63 \(\upmu\)m) and sand (63-2000 \(\upmu\)m) grains for each sample.