Fortymile River Cosmogenic Isotope and Luminescence Data Collected 2016-2019

Cosmogenic isochron burial dating - A fully referenced description of this method appears in (Bender et al., 2020). The isochron method requires sampling quartz-bearing sedimentary material archiving a range of pre-burial isotope concentrations at a single depth horizon (indicative of common burial history) buried by several meters of sediment, to minimize post-burial production of each isotope. In the high Fortymile River terrace gravel,samples were collected from four shallow pits (sample horizons 2-3 meters deep) except one sample suite (about 25 meters deep) taken from a deep quarry. The middle Fortymile River terrace gravel was sampled in three shallow pits (sample horizons 2-3 meters depth). Samples were prepared at the University of Vermont. The Aluminum-26/Aluminum-27 and Beryllium-10/Beryllium-9 ratios were measured via accelerator mass spectrometry at the Purdue Rare Isotope Measurement Laboratory in several runs from 2017-2019 against standards listed in Bender and others (2020). Each isotope measurement was corrected for background isotope levels by subtracting the average of all blank measurements (Bender et al., 2020).

Basin averaged erosion rate - A fully referenced description of this method appears in (Bender et al., 2020). We quantified basin-averaged erosion rates using 10Be concentrations measured in quartz from modern river sand (250-850 μm) at ten tributary outlets on the Fortymile River. Be-10 accumulates in quartz at Earth’s surface to depths commensurate with cosmic ray e-folding length (~0.6 m in rock), at rates determined largely by latitude and altitude and inversely proportional to erosion rate. Dividing the 10Be production depth by a given measured erosion rate thus estimates the duration of erosion at the measured rate. Given similar rock type and durability throughout the Fortymile River basin and minimal lithologic variation within the sampled tributary catchments, we assume that quartz sampled at the 250-850 μm grain size range is well- mixed and represents erosion throughout the tributary catchment upstream. We use a MATLAB likelihood estimation routine to compute the most likely elevation and latitude from pixel values within each sample catchment and use the CRONUS online calculator (https://hess.ess.washington.edu/) to assess erosion rates based on Be-10 concentrations measured in quartz sand from each outlet. Topographic shielding minimally impacts Fortymile River Be-10 production rates because topography is relatively open in the basins we sample, with average hillslopes of 5 to 20 degrees. Similarly, the mean winter snow depth of <50 cm measured at Fortymile Basin SNOTEL sites 1275 and 1189 (https://wcc.sc.egov.usda.gov/nwcc/) confers shielding effects equivalent to a 1-3% reduction in Be-10 production. Consequently, we assume minimal snow and topographic shielding effects on Be-10 production rate, and report uncertainties computed by CRONUS that reflect these assumptions (i.e., shielding factor = 1).

Infrared luminescence dating - A fully referenced description of this method appears in (Bender et al., 2020). All analyses performed at the Utah State University (USU) Luminescence Lab, North Logan, Utah. Samples are opened and processed under dim amber safelight conditions. Sample processing for feldspar infrared stimulated luminescence (IRSL) dating followed standard procedures involving sieving, HCl and bleach treatments, heavy liquid mineral density separation at 2.58 g/cm3 with no HF pre-treatment, to isolate the potassium-rich feldspar component of a narrow grain-size range, 125-212 micrometers. The lab follows the latest single-aliquot regenerative-dose (SAR) procedures for potassium feldspar dating in which the IRSL signal is measured at 50 degrees Celsius. SAR protocol includes tests for sensitivity correction and brackets the equivalent dose (DE) the sample received during burial by irradiating the sample at five different doses (below, at, and above the DE, plus a zero dose and a repeated dose to check for recuperation of the signal and sensitivity correction). The resultant DE data are fit with a saturating exponential curve. The IRSL age is calculated by correcting for fading (loss of signal with time) using the fading method of Auclair and others, 2003 and the age correction model of Huntley and Lamothe, 2001. The cumulative DE and age data are reported at 2-sigma error were determined using a Central Age Model (CAM) or the Minimum Age Model (MAM), depending on the distribution of DE results and evidence for partial bleaching. In cases in which the samples have significant positive skew and overdispersion, ages are calculated based on MAM. Dose-rate calculations were determined by chemical analysis of of the U, Th, K, and Rb content using ICP-MS and ICP-AES techniques by ALS Chemex in Elko, NV, using conversion factors from Guerin and others, 2011. Internal grain beta dose rate was determined assuming 12.5%K and 400 ppm Rb attenuated to grain size using Mejdahl, 1979. Alpha contribution to dose rate determined using an efficiency factor, 'a-value', of0.086 +/- 0.0009. The contribution of cosmic radiation to the dose rate was calculated using sample depth, elevation, and latitude and longitude. Dose rates are calculated based on water content, sediment chemistry, and cosmic contribution.