Sediments targeted for OSL dating are clay-rich distal overbank sediments in close association with artifact-bearing levels. Because these sediments did not contain a sand (2000 to 63 μm) fraction or appreciable coarse silt (63 to 44 μm), we focused on OSL dating the 4- to 11-μm quartz fraction, which had yielded previously accurate ages (11, 13). Samples for optical dating were taken by hammering 10- to 12-cm-long, 2.6-cm copper tubing into profile walls at close vertical intervals (Figs. 1 to 3). Under safe-light conditions (Na-vapor indirect illumination) in the Geoluminescence Dating Research Laboratory at Baylor University, the outermost 1.0-cm length of sediment inside the tube was removed, leaving an interior segment with zero ambient light exposure and which was acceptable for OSL dating. Organic matter and carbonate content were removed from the sample by soaking in H202 for 24 hours and reaction with HCl (12%), respectively. Fine-grained (4 to 11 μm) quartz was extracted by suspension settling, following Stokes’ law. Subsequently, quartz grains for this fine fraction (4 to 11 μm) were isolated by digestion in hydrofluorosilicic acid (with prior silica saturation) for 6 days (34). The mineralogical purity of this fine-grained fraction was checked by an assay with a Raman spectrometer, with a 1-μm beam width. The optical purity of quartz separates was tested by exposing aliquots to infrared excitation (1.08 W from a laser diode at 845 ± 4 nm), which preferentially excites feldspar minerals. If these tests indicated feldspar contamination, then the hydrofluorosilicic acid soaking was repeated. All resultant samples showed weak emissions (<400 counts/s) with infrared excitation at or close to background counts, and the ratio of emissions from blue to infrared excitation of >20, indicating a spectrally pure quartz extract (35).

Single aliquot regeneration (SAR) protocols (36, 37) were used in this study to estimate the equivalent dose (De). This analysis was completed on the quartz fraction (4 to 11 μm) for 32 sediment samples and for 30 to 50 separate aliquots for each sample (tables S1 and S2). In the laboratory, the sediment was adhered to a 1-cm-diameter circular aluminum disc as a thin coating (<20 μm). This thin layer of sediment was achieved by suspending the fine quartz in methanol (1 cm3) within a 2-cm3 flat-bottom tube, with an aluminum disc at the tube bottom. The evaporation of the methanol over 2 to 3 days in total darkness results in the fine-grained fraction adhering to the disc.

An Automated Risø TL/OSL-DA-15 system (38) was used for SAR analyses. Blue light excitation (470 ± 20 nm) was from an array of 30 light-emitting diodes that deliver ~15 mW/cm2 to the sample position at 90% power. Optical stimulation for all samples was completed at an elevated temperature (125°C) using a heating rate of 5°C/s. All SAR emissions were integrated for the first 0.8 s of stimulation out of 40 s of measurement, with background emissions integrated for the last 10 s of data collection, for the 30- to 40-s interval. The luminescence emission for all quartz fractions showed a dominance of a fast component [see (36)] with >90% diminution of luminescence after 4 s of excitation (fig. S2), with blue light and with corresponding “fast ratio” of >20 (39).

A series of experiments was performed to evaluate the effect of preheating at 160°, 170°, 180°, 190°, 200°, 210°, 220°, 240°, and 260°C on isolating the most robust time-sensitive emissions and thermal transfer of the regenerative signal before the application of SAR dating protocols [fig. S3; see 36, 37)]. These experiments entailed giving a known dose (2 to 10 Gy and 31 Gy) and evaluating which preheat temperature resulted in recovery of this dose. There was concordance (±10%) with the known dose for preheat temperatures between 160° and 200°C with a used initial preheat temperature of 180°C for 10 s in the SAR protocols (fig. S3). A second preheat at 180°C for 10 s was applied prior to the measurement of the test dose. A final heating (hot wash) at 260°C for 40 s was applied to minimize carryover of luminescence to the succession of regenerative doses for the equivalent dose analyses (table S1 and fig. S3). A test for dose reproducibility was also performed following procedures of (36) with the initial and final regenerative dose of 4.2 Gy (fig. S2), with the resultant recycling ratio of 1.00 ± 0.05.

The SAR protocols were used to resolve equivalent dose for 32 samples (table S1). The statistical significance of an equivalent dose population was determined for 27 to 45 quartz aliquot per sample (table S1). Aliquots were removed from the analysis if the recycling ratio was not between 0.90 and 1.10, the zero dose was >5% of the natural emissions, or the error in equivalent dose determination was >10%. De distributions were log normal and exhibited overdispersion values ≤20%, except for sample DF16-05 (table S1 and figs. S1 and S2). An overdispersion percentage of a De distribution is an estimate of the relative SD from a central De value in the context of a statistical estimate of errors (40, 41). A zero overdispersion percentage indicates high internal consistency in De values with 95% of the De values within 2σ errors. Overdispersion values <20% are routinely assessed for small aliquots of quartz grains that are well solar reset, such as far-traveled eolian and fluvial sands [e.g., (42, 43)], and this value is considered a threshold metric for calculation of a De value using the central age model in (41).

The determination of the environmental dose rate is a needed component to calculate an optical age. The dose rate is an estimate of exposure to ionizing radiation for the dated quartz grains. This value is computed from the content of U and Th, 40K, Rb, and cosmic radiation during the burial period (table S1). The U and Th content of the sediments, assuming secular equilibrium in the decay series, 40K, and Rb, was determined by inductively coupled plasma mass spectrometry by ALS Laboratories (Reno, NV). A significant cosmic ray component between 0.24 and 0.29 mGy/year was included in the estimated dose rate, taking into account the current depth of burial (44). Moisture content (weight percent) during the burial period was derived from instrumented, nearby pedons whose floodplain soils were similar to those that formed along a higher-order tributary, such as Buttermilk Creek (11). Moisture content for these clay-rich sediments (>50% clay) varied between 30 and 38% with an error of 3%. The datum year for all OSL ages is AD 2000.

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