DOI: 10.6084/m9.figshare.25559664.v1
title: Table S6-Single-grain luminescence ages of the LS profile from NE China.xlsx

description: This dataset provides the single-grain luminescence age obtained from a black soil profile in Northeast China.<b>Sample collection</b> Luminescence sampling was carried out by hammering a 20 cm long and 3.5 cm diameter stainless steel tube vertically into the cleaned-off soil profile. Both ends of the sample tube were sealed with tin foil, then wrapped in a black plastic bag to avoid exposure to light and water evaporation. A total of 10 luminescence samples were collected at 20 cm intervals from a depth of 20 cm down to 200 cm. Considering the fine texture of the soil, a parallel sample was taken at each sampling depth to ensure sufficient coarse-grained quartz and feldspar particles. These parallel samples were subsequently mixed together in the luminescence laboratory. <b>Sample processing</b> In the red-light luminescence laboratory, materials from both ends of the tube, which may have been exposed to light, were scraped out and retained for moisture content determination and environmental dose rate measurements. The remaining materials in the center of the tube were then wet-sieved to obtain grains larger than 38 μm. This initial sieving was carried out because fine grains were out of scope in this study, and there was no need to include them in further treatment for time efficiency consideration. Grains larger than 38 μm were soaked with 30% H2O2 and 10% HCl to remove organic matter and carbonates, respectively. Samples were re-sieved to select grains in the range of 38-63 μm and 63-90 μm. Solutions of sodium polytungstate with densities of 2.58, 2.62, and 2.75 g/cm3 were used to extract potassium feldspar (K-feldspar, &lt;2.58 g/cm3) and quartz (2.62-2.75 g/cm3) mineral grains. Feldspar grains separated from this procedure were directly used for luminescence measurement. Quartz particles were further etched for 1 hour in 40% hydrofluoric acid (HF) to remove feldspar remains and to eliminate the effect of outer alpha-irradiated skin. Subsequently, they were immersed in a 10% HCl wash to remove fluorite precipitates. The purity of the quartz separates was monitored using IR stimulation (Duller, 2003).<b>Dose rate determination</b> Dose rates were measured using the light-exposed end-sections of the sample tube. Samples were first dried to estimate their water content and then homogenized by grinding. A high temperature of 450 °C was applied for 10 hours to remove organic matter. The samples were then mixed with wax and cast in cups to provide a constant counting geometry. These cups were stored for about three weeks to allow for the establishment of secular equilibrium between 222Rn and 226Ra (Madsen et al., 2010; Breuning-Madsen et al., 2017). The concentrations of 40U, 232Th and 238K were measured using the Risø sodium iodide gamma spectrometer at the Luminescence Dating Laboratory of Nanjing Normal University. The external radionuclide concentrations were converted to dose rates using conversion factors from Adamiec and Aitken (1998). The water content measured in the laboratory was used to correct for attenuating effects on dose rate. This was calculated as the weight percent of water compared to dry sediment using gravimetric methods. The contribution of cosmic radiation was also estimated to calculate the total dose rate of a sample. This was determined using the formula of Prescott and Aitken (1994), which takes into account the depth of the sample below the ground surface, along with the latitude, longitude, and altitude of the sample location. For internal components, a potassium concentration of 12.5 ± 0.5% and rubidium content of 400 ± 50 ppm was used (Huntley and Hancock, 2001). <b>Equipment and De measurement set-up </b>Luminescence measurements were performed at the Luminescence Dating Laboratory of Nanjing Institute of Geography and Limnology using a Risø TL/OSL DA-20 automated single-grain system equipped with a 90Sr/90Y beta source. The luminescence signal was detected through a blue filter pack (BG-39, BG-3) placed in front of the photomultiplier tube when measuring K-feldspar and through U-340 when measuring quartz. Single grains were placed onto a specific single-grain disc, which is 9.8 mm in diameter with 100 sample holes (150 μm in diameter, 150 μm in depth) distributed evenly on the surface. Single-grain measurements of quartz were carried out on NL-2546 (60 cm) and NL-2550 (140 cm). Since the amount of quartz grains in the 63-90 μm range was limited, a grain-size range of 38-63 μm was used instead. This particle size range made it more difficult to ensure individual grain in each hole. Whereas, considering the relatively low proportion of light-emitting grains of quartz (Duller, 2008), we assume that it is still approximated as a single-grain measurement. A single aliquot regenerative-dose protocol (SAR; Murray and Wintle, 2000) was used to determine De, and dose recovery tests (DRT) were conducted on both samples to verify the reliability of these measuring protocols. The luminescence signal was integrated from the first 0.1 s of stimulation minus a background estimated from the last 0.2 s of stimulation. For feldspar single-grain measurements, grain size range of 63-90 μm was used. To deal with anomalous fading of feldspar (Huntley and Lamothe, 2001), the protocol of post-IR IRSL was applied (Thomsen et al., 2008). To determine a suitable pair of preheating and stimulation temperatures, a preheat plateau test was carried out on NL-2550 (140 cm). Preheat temperatures were held at 180, 200, 220, 250, 280 and 320 °C for 60 s, respectively, followed by post-IR IRSL measurements at elevated temperatures that tracked the preheat temperature by ~30 °C (Riedesel et al., 2018). All post-IR IRSL measurements were preceded by an IRSL measurement at 50 °C, irrespective of preheat conditions. Consequently, luminescence signal stimulated with 225 °C (pIR50IR225) was used to determine De of feldspar, as De calculated with this stimulation temperature located in a preheat plateau and it is convenient for future comparison with other studies (e.g., Buylaert et al., 2009). DRTs for pIR50IR225 were applied to NL-2546 (60 cm) and NL-2550 (140 cm), with 300 grains for each sample, to verify the reliability of the protocol. To evaluate the effect of partial bleaching, feldspar grains were first exposed to the sunlight simulator for 11 hours, and residual dose was measured on 300 grains for each sample. The initial and background luminescence signals were calculated by summing the first 0.08 s and final 0.2 s of stimulation, respectively. Luminescence data was screened using criteria following Peng and Li (2017), and analysis was performed through the R package "numOSL". Grains that did not meet the following criteria were discarded: (1) the relative error of Tn is greater than 20%; (2) the recuperation ratio is greater than 5%; (3) the recycling ratio is greater than 10%; (4) the upper limit of figure-of-merit (FOM) exceed 10%; (5) the reduced chi-square (RCS) is greater than 5