Most of the OSL ages from the Friedkin site were collected in four vertical sediment columns in 2007, 2008, 2015, and 2016. The 2008 and 2015 columns span the south-north extent of Block A. The 2007 column is in the west part of the block, and column 2016 is in the northeast part of the excavation area (Fig. 1B). In addition, OSL samples were taken from areas of interest that were adjacent to the columns. Figure S1 shows the age estimates with 1 SD error bars for the 2007, 2008, 2015, and 2016 columns. OSL ages from specific depths obtained over multiple years mostly overlap at the 68% confidence intervals. The 2008 and 2015 OSL column of dates are consistently very similar to one another (i.e., similar ages are generated at the same depths). The 2007 column ages are similar to the 2008 and 2015 ages with depth, but there are some discrepancies near the top and base of the 2007 column. The 2016 column of ages yielded ages that are consistently younger than those of the other three columns. This is explained by the upward slope of the deposits reflected in the 2007 column of ages and the downward slope of the deposits in the 2016 OSL record.

Bayesian models were created in OxCal to identify outliers and compute posterior age estimates using a Poisson deposition model (45, 46). An age model was created for the 2007 sequence of ages (fig. S1A and table S2), the combined 2008–2015 ages (fig. S1B and table S2), and the 2016 column of ages (fig. S1C and table S2). All four columns are internally consistent with very good agreement levels. The 2007 column of OSL ages shows agreement indices of 115.4/116.7 (model and overall agreement) with a single outlier (UIC2050). Rerunning the model with the outlier removed increased the agreement indices to 135.9/136.8. The 2008 and 2015 columns were combined into a single Bayesian age model that resulted in model agreement values of 182.3/185.7 with two outliers (UIC2361 and UIC2349). Removing those outliers produced a model with agreement indices of 239/235.2. This combined model was used to produce interpolated ages at approximately 15-mm intervals. Analysis of the 2016 column produced agreement indices of 187.1/179.8 with no outliers.

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