Excavation

Dhabas 1–3 were excavated under permit from the Archaeological Survey of India (No. F.1/36/2008-EE). All trenches was excavated simultaneously by several teams in 1 × 1 m pits arranged as step trenches down the slope at each locality. Excavation trenches were placed in areas where artefacts were eroding from the slope in high density. Dhaba 1 was excavated in 4 lower steps and 1a was excavated in 12 upper steps covering a total elevation of 9–22 m above river level. Dhaba 2 was excavated in six steps covering a total elevation of 21–28 m above river level. The Dhaba 3 trench is located 25–30 m above river level and was 18-m long. Each pit was excavated in spits of ~10-cm depth, with levels taken after each spit using a line level. All excavated sediments were passed through a 5 mm sieve and all artefacts recovered. The weight of the matrix removed during excavation was recorded and all finds were placed in clip seal plastic bags and labelled with provenance details.

Artefact analysis

All artefacts were washed and taken to the archaeology laboratory in the Department of Ancient History, Culture and Archaeology at the University of Allahabad for analysis. Each artefact was first classified into technological categories such as core, flake, flaked piece and retouched flake and ascribed typological categories where appropriate. All artefacts were assigned individual specimen numbers, weighed, measured with digital callipers and photographed. All information was entered into a relational database along with detailed provenance information for each artefact. Artefact measurement protocols follow those described in Clarkson et al.31. All cores were scanned in three dimensions using a NextEngine laser scanner and a select set of core measurements taken for each31. Select artefacts were illustrated using conventional archaeological illustration techniques and protocols.

IRSL dating

Sediment samples were collected by hammering opaque plastic tubes (5 cm in diameter) into the cleaned section face. The tubes were removed and wrapped in light-proof plastic for transport to the Luminescence Dating Laboratory at the University of Wollongong. Under dim red laboratory illumination, each sample was treated using standard procedures to extract sand-sized grains of K-feldspar51,52. The samples were routinely treated with solutions of HCl acid and H 2 O 2 to remove carbonates and organic matter, respectively, and then dried. Different grain size fractions in the range of 90–212 µm were obtained by dry sieving, and the K-rich feldspar grains separated using a heavy liquid solution of sodium polytungstate with a density of 2.58 g/cm3. The separated grains were etched using 10% HF acid for ~40 min to clean the surfaces of the grains and reduce the thickness of the alpha-irradiated layer around the grain surface. IRSL measurements of the K-feldspar grains were made on an automated Risø TL-DA-20 reader equipped with IR diodes (875 nm) for stimulation, which delivered ~135 mW/cm2 total power to the sample position53. Irradiations were carried out within the reader using a 90Sr/90Y beta source. The IRSL signals were detected using a photomultiplier tube with the stimulated luminescence passing through a filter pack containing Schott BG-39 and Corning 7–59 filters, which provides a blue transmission window (320–480 nm). Aliquots containing several hundred grains (~5 mm in diameter) were prepared by mounting the grains as a monolayer on a 9.8-mm-diameter aluminium disc using “Silkospray” silicone oil as an adhesive.

The dose rates were determined from field measurements of the gamma dose rate, laboratory measurements of the beta dose rate using the sediment samples recovered from each tube hole, and published estimates of the cosmic-ray dose rate and the internal dose rate (due to 40K and 87Rb contained within the K-feldspar grains). The dosimetry data for all samples are summarised in Supplementary Table 2. The gamma dose rates were measured using an Exploranium GR-320 portable gamma-ray spectrometer, which is equipped with a 3-inch diameter NaI(Tl) crystal calibrated for U, Th and K concentrations using the CSIRO facility at North Ryde. At each sample location, 3–4 measurements of 900 s duration were made of the gamma dose rate at field water content. The external beta dose rate was measured by low-level beta counting using a Risø GM-25-5 multicounter system54 and referenced to the Nussloch Loess (Nussi) standard55. These external components of the total dose rate were adjusted for sample water content, assuming a value of 7 ± 2% for all samples (based on the measured (field) water content of each sample, which ranged from 2 to 5%, and making an allowance for collection of samples during the dry season and partial drying out of the exposed sections prior to sample collection); the assigned uncertainty captures the likely range of time-averaged values for the entire period of sample burial. The minor contribution from cosmic rays was estimated from the burial depth and water content of each sample, and the latitude, longitude and altitude of the Dhaba sites56. The internal dose rate was estimated by assuming 40K and 87Rb concentrations of 13 ± 1% and 400 ± 100 p.p.m., respectively57,58,59.

The MET-pIRIR procedure29,60,61,62 was applied to determine equivalent dose (D e ) of our samples. The IRSL signals of both regenerative and test doses were measured by increasing the stimulation temperature from 50 to 300 °C in steps of 50 °C. A preheat at 320 °C for 60 s was applied after both regenerative and test doses. At the end of the IRSL measurements for each test dose, a ‘hot’ IR bleach at 325 °C for 100 s was conducted to minimise the residual signal preceding the next measurement cycle. The full experimental procedure is summarised in Supplementary Table 3.

Typical IRSL and MET-pIRIR decay curves and dose response curves (DRCs) for one aliquot of sample Dhab1-OSL4 are shown in Supplementary Fig. 1a, b, respectively. The intensities of the IRSL and MET-pIRIR signals for all the samples are very bright and are on the order of ~105 counts s−1. Different sensitivity-corrected DRCs were observed for the IRSL and various MET-pIRIR signals. These curves were fitted using a single saturating exponential function, which yields characteristic saturation doses of 480, 430, 443, 463, 415 and 308 Gy for the 50, 100, 150, 200, 250 and 300 °C signals, respectively. These results indicate that a natural dose of up to ~800 Gy can be obtained for the Dhaba samples using the MET-pIRIR method.

We tested the applicability of the MET-pIRIR procedure to the Dhaba samples using several routine criteria (e.g., recuperation, recycling ratio, dose recovery, anomalous fading and residual dose)61,62. Tests of the recycling ratio and recuperation (i.e., the ratio between the signal responses from a zero regenerative dose and the natural dose) were investigated based on the construction of DRCs for D e estimation. Recycling ratios for all of the samples fell within the range of 1.0 ± 0.1 and recuperation values were mostly <5%, which are considered acceptable.

For the residual dose test, four aliquots from each of nine samples were bleached by a Dr Hönle solar simulator (model UVACUBE 400) for ~4 h. The residual doses associated with the MET-pIRIR signals were then measured; the results for Dhab2-OSL4 are shown in Supplementary Fig. 1c. The IRSL signal at 50 °C has the smallest residual dose (~2 Gy), which increases as the stimulation temperature is raised. A residual dose of ~18 Gy was obtained for the 250 °C signal, and the highest residual dose (~29 Gy) was observed for the 300 °C signal. The residual doses for the 250 °C signal are summarised for each sample in Supplementary Table 2; the size of the residual dose represents 5–10% of the corresponding D e value of the 250 °C signal for the Dhaba samples. There is no systematic change in the size of the residual dose with D e for our samples, which suggests that the non-bleachable traps associated with the residual signal may have been saturated. A simple subtraction of the residual dose from the apparent D e value may result in underestimation of the true D e value if the residual signal is relatively large compared with the bleachable signal63. To estimate the proportion of residual signal to bleachable signal for our samples, 12 aliquots of Dhab1-OSL2, Dhab1-OSL3 and Dhab2-OSL1 were heated to 450 °C to empty the source traps associated with the residual and bleachable signals. These aliquots were subsequently given different regenerative doses (165, 330 and 496 Gy) and then bleached using the solar simulator for 4 h before measuring the residual signal using the MET-pIRIR procedure. The measured residual signals from the different regenerative doses were compared with the total regenerative signals at the same doses. The residual signal corresponds to only ~5% of the total signal, which is comparable to the residual dose as a proportion of the measured D e . Given the small size of the residual signal relative to the bleachable signal, the simple dose-subtraction approach should give satisfactory results.

We also tested the validity of the dose-subtraction correction and performance of the MET-pIRIR procedure using a dose recovery test. Four aliquots of sample Dhab2-OSL4 were first bleached by the solar simulator for 4 h and then given a dose of 220 Gy, which was measured as an ‘unknown’ dose using the MET-pIRIR procedure. The ratios of measured dose to given dose for the IRSL and MET-pIRIR signals are shown in Supplementary Fig. 1d. After correcting for the residual doses shown in Supplementary Fig. 1c, dose recovery ratios of ~0.9 were obtained for the 50 and 100 °C signals, and ratios of 1.02 ± 0.02, 1.03 ± 0.02, 1.02 ± 0.02 and 1.01 ± 0.03 for the 150, 200, 250 and 300 °C MET-pIRIR signals, respectively. The results of this dose recovery test suggest, therefore, that the combination of MET-pIRIR and simple dose-subtraction procedures can recover a dose consistent with the known dose given to our samples, so we adopted these procedures to estimate the final D e values and ages for the Dhaba samples.

Previous studies of pIRIR signals have shown that the anomalous fading rate (g-value) depends on the stimulation temperature, with negligible fading rates observed for MET-pIRIR signals at 200 °C and above29,60,61,62. No fading correction is therefore required for ages estimated from the high-temperature MET-pIRIR signals. To directly test the absence of significant fading for the samples studied here, we conducted anomalous fading tests on K-feldspar grains from samples Dhab2-OSL1 and Dhab3-OSL1 using a single-aliquot measurement procedure similar to that described by Auclair et al.64, but based on the MET-pIRIR measurement procedure in Supplementary Table 3. The g-values calculated for the IRSL and MET-pIRIR signals (Supplementary Fig. 1e) show that the fading rate is highest for the 50 °C IRSL signal (3.2 ± 0.4 and 4.1 ± 0.7% per decade for Dhab2-OSL1 and Dhab3-OSL1, respectively) and decreases as the stimulation temperature is raised. The fading rates for the 200 °C signal are <1% per decade and are consistent with zero for the signals measured at 250 and 300 °C, suggesting that negligible fading or non-fading is achieved at the two highest stimulation temperatures.

Based on the above performance tests, the MET-pIRIR procedure was used to measure the D e values for all samples. The D e values obtained for each of the MET-pIRIR signals are plotted against stimulation temperature (D e –T plots) for each of the samples from Dhaba 1, 2 and 3 in Supplementary Figs. 2–4, respectively. We also applied a fading correction65 to the D e values based on the g-values in Fig. 1e. The fading-corrected D e values are shown as red squares in Supplementary Figs. 2–4. After applying the fading correction, the fading-corrected D e values for the 150 and 200 °C signals are consistent with those obtained at higher temperatures (>200 °C), which have negligible fading rates. This further supports our proposition that the MET-pIRIR procedure can access a non-fading signal for the samples studied here and, hence, the D e values and ages obtained from the elevated temperature signals should be reliable. More importantly, since the signals measured at different temperatures are bleached at significantly different rates (Supplementary Fig. 1c), the consistency in D e values across a wide range of stimulation temperatures (i.e., 150–300 °C) indicates that our samples had been sufficiently bleached prior to deposition. At lower stimulation temperatures (50 and 100 °C), the D e values are underestimated, even after correcting for fading, which is consistent with the dose underestimation observed at 50 and 100 °C in the dose recovery test (Supplementary Fig. 1d).

Given the much lower residual dose of the 250 °C signal compared with the 300 °C signal (Supplementary Fig. 1c), we consider the D e values obtained using the 250 °C signal as the most reliable for the Dhaba samples. The final ages were, therefore, based on the D e values and associated uncertainties estimated from the 250 °C MET-pIRIR signal (Supplementary Table 2).

Reporting summary

Further information on research design is available in the Nature Research Reporting Summary linked to this article.