Table S2.

Dose rate data, equivalent doses (D_e), and OSL ages for the CRH 11 samples

Sample	Sample depth (m)	Grain size (μm)	Water/organic content	Environmental dose rate (Gy/ka)				Equivalent dose (De) data			OSL age (ka) §,‡‡
				β-Dose rate*,†	γ-Dose rate†	Cosmic dose rate	Total dose rate‡,§	No. of grains¶,‖	Overdispersion (%)**	CAM De (Gy)§,††
CRH 11-	23.9	125–180	27/7	1.07 ± 0.06	0.81 ± 0.05	0.02 ± 0.01	1.93 ± 0.12	107/1,800	14 ± 5	377 ± 14	195 ± 15
CRH 11-3	24.0	180–212	17/3	1.33 ± 0.07	0.67 ± 0.01	0.02 ± 0.01	2.05 ± 0.08	87/2,200	18 ± 6	425 ± 19	207 ± 13
CRH 11-1	24.3	180–212	26/8	0.73 ± 0.03	0.69 ± 0.01	0.02 ± 0.01	1.47 ± 0.05	77/2,400	20 ± 6	304 ± 14	207 ± 12
CRH 11-2	24.9	125–180	17/2	1.40 ± 0.07	0.85 ± 0.01	0.02 ± 0.01	2.30 ± 0.09	111/2,100	11 ± 7	498 ± 18	216 ± 12

In the fourth column, field water content/mass of organic matter (determined by loss on ignition) is expressed as percent of dry mass of mineral fraction and assigned relative uncertainties of ±10%. β-Dose rate uncertainties were derived from counting statistics for radionuclide measurements made using HRGS. Elemental concentrations obtained from and ICP-MS analyses were assigned relative errors of ±3% to calculate β-dose rate uncertainties. The assigned relative errors for ICP-MS analysis were increased to ±10% when calculating the final γ-dose rate uncertainty of sample CRH 11-4 to accommodate any spatial heterogeneity in the γ-irradiation field. γ-Dose rate uncertainties were derived from counting statistics for elemental measurements made using in situ FGS. Measurements made by in situ FGS using a NaI:Tl detector (CRH 11-1, CRH 11-2, CRH 11-3) or by ICP-MS analysis of dried, ashed, and powdered bulk sediment collected from the main OSL sample position and overlying/underlying deposits at a distance of 15 cm (CRH 11-4). Four replicate FGS measurements were made for each sample and used to derive weighted average elemental concentrations and associated uncertainties using the “energy windows” approach (e.g., ref. 100). Cosmic-ray dose rates were calculated using the approach of Prescott and Hutton (104), adjusted for site altitude, geomagnetic latitude, and thickness of sediment overburden, and were assigned a relative uncertainty of ±10%.

^*Measurements made on dried, ashed, and powdered portions of bulk sediments collected from the OSL sampling positions using either HRGS (CRH 11-1) or ICP-MS (CRH 11-2, CRH 11-3, CRH 11-4).

^†Dose rates were calculated from radionuclide activities and elemental concentrations using the published conversion factors of Adamiec and Aitken (101) and Stokes et al. (102), making allowance for β-dose attenuation (103), long-term water content (100), and mass of organic matter where appropriate.

^‡Includes an assumed internal dose rate of 0.03 Gy/ka based on previously published values for etched quartz grains (e.g., 105) with an assigned relative uncertainty of ±30%.

^§Mean ± total uncertainty (68% confidence interval), calculated as the quadratic sum of the random and systematic uncertainties.

^¶Number of D_e measurements that passed the SAR rejection criteria and were used for D_e determination/total number of grains analyzed.

^‖Quartz grains of 180- to 212-μm diameter were measured in standard single-grain aluminum discs drilled with an array of 300-μm × 300-μm holes. The 125- to 180-μm diameter grains were measured in custom discs drilled with smaller (200 μm × 200 μm) holes to maintain single-grain resolution and avoid any potentially complicating pseudosingle-grain averaging effects (41).

^**The relative spread in the D_e dataset beyond that associated with the measurement uncertainties for individual D_e values, calculated using the CAM of Galbraith et al. (43).

^††D_e uncertainties were derived from net photon counting statistics, an empirically determined instrument reproducibility of 1.6% per OSL measurement and dose–response curve-fitting uncertainties determined in Analyst v3.24 using the Monte Carlo fitting function (106).

^‡‡Total uncertainty includes a systematic component of ±2% associated with laboratory β-source calibration.