We used TCN surface-exposure dating and OSL analyses to constrain the timing of deglaciation in the former MnIS flow path. Samples for TCN analysis were taken from glacially transported boulders within a relatively small area (i.e., 102 to 104 m2) and were assumed to be contemporaneous (i.e., with no evidence to suggest otherwise, adjacent boulders are assumed to have been deposited at broadly the same time during ice sheet wastage) (52). We report a total of 26 new cosmogenic 10Be surface exposure ages from seven carefully chosen sites situated at various key points along the flowline of the former MnIS. An additional 13 10Be ages, from two other sites, can be used to constrain the MnIS retreat chronology; these have been reported previously (53, 54) but are presented here with recalculated exposure ages. The combined dataset of 39 10Be samples is presented in table S2. Other published 10Be ages from three other sites are shown in Fig. 1 as supporting evidence of ice sheet retreat in NW Scotland. These were recalculated, as above, from previous studies (55, 56). These ages are not presented in tables S2 to S12 in data file S1 or used in the Bayesian modeling analysis as they lie outside the main flow path of the ice stream. OSL dating of proglacial and ice-marginal sediments has been used successfully elsewhere to constrain ice sheet margin behavior and the timing of deglaciation around the British Isles (57, 58). We report three new OSL ages from two carefully chosen sites to augment our 10Be TCN dataset (table S5). OSL samples were taken from glacigenic sediments directly relating to the margin of the former MnIS.

Field descriptions and site-specific details of each of the TCN and OSL dating sites are not presented here for reasons of brevity but can be accessed on the project data depository or obtained from the authors upon request. Samples for TCN analysis were taken from the uppermost surfaces of glacially transported boulders using a battery-operated angle-grinder and/or hammer and chisel by experienced field scientists wearing the appropriate protective clothing. We chose boulders resting on level surfaces, where possible, >100 m away from slopes (>5°); preference was given to large boulders with b axis > 1 m to minimize the potential for disturbance and snow cover. We sampled flat or gently rounded surfaces and avoided fractured or obviously weathered/degraded surfaces to minimize the potential for variable “erosion rates.” Care was taken to avoid any unnecessary surface damage to the boulders during sampling; sharp and prominent edges were removed after sampling to minimize the visual impact of sampling. Photographs were taken before and after sampling in most cases. Boulder dimensions were measured in the field to the nearest 0.1 m. Sample locations and elevations were recorded on a hand-held GNSS (Global Navigation Satellite System) unit (with GPS and GLONASS satellite acquisition). Positional data were checked against UK Ordnance Survey 1:25,000 maps and in a GIS database for accuracy. Any natural shielding from surrounding topography was measured in the field using a compass and clinometer and corrected for using the CRONUS-Earth online calculator (59) accessed on 01 October 2017 (http://stoneage.ice-d.org/math/skyline/skyline_in.html).

Sample thickness was measured using digital calipers. Individual rock fragments were measured from which a mass-weighted average thickness was calculated for each sample. All new samples reported here were crushed and washed at the University of Glasgow (School of Geographical and Earth Sciences). Following sample crushing and washing, quartz was separated from the 250- to 500-μm fraction using standard mineral separation techniques (60) and purified by ultrasonicating in 2% HF/HNO3 to remove remaining contaminants (mainly feldspars) and meteoric 10Be. The purity of the leached samples was assessed by measuring the aluminum content using flame atomic absorption spectrometry with bulk Al content considered a proxy for presence of feldspars. Be extraction was carried out at two independent laboratories housed at the Scottish Universities Environmental Research Centre (SUERC): the NERC Cosmogenic Isotope Analysis Facility and the SUERC Cosmogenic Nuclide Laboratory, using established procedures (61). The 10Be/9Be ratios of all samples (new and previously reported) were measured on the 5 MV accelerator mass spectrometer (AMS) at SUERC (62). Locational and analytical details for all samples are given in table S2. NIST27900 [10Be/9Be = 2.79 × 10−11] standardization was used in the AMS measurements. Measured ratios were converted to concentrations of 10Be in quartz (atoms g−1). Blank corrections ranged from 1.2 to 10.3% and were propagated in quadrature with attendant AMS analytical uncertainties. Analytical, chemistry, and blank data are included in table S2. See the Supplementary Materials for more information on age calculations, choice of production rates, and other external factors taken into consideration.

We sampled for OSL using opaque 30-mm-diameter tubes hammered into the chosen sand-grade sediment facies to prevent exposure to sunlight during field sampling. Samples were capped and labeled and kept in dark conditions at all times during analysis. External beta dose rates were determined for OSL dating using inductively coupled plasma mass spectrometry (ICP-MS) and ICP atomic emission spectroscopy (ICP-AES), while the external gamma dose rates were determined using in situ gamma spectrometry in the field (table S5). Water contents of 23 ± 5% were estimated considering the field and saturated water contents and environmental history for each sample; these are expressed as a percentage of the mass of dry sediment. Samples were taken from depths of 0.7 m (T8SKIG01), 9.0 m (T8SKIG02), and 2.0 m (T8GABB01); cosmic dose rates were determined accordingly (63).

To isolate coarse-grained quartz for OSL analysis of equivalent doses (De), each sample was first treated with a 10% (v/v) dilution of 37% HCl and with 20 (v/v) of H2O2 to remove carbonates and organics, respectively. Dry sieving then isolated the 212- to 250-μm-diameter grains, and density separation using sodium polytungstate provided the (quartz-dominated) fractions (2.62 to 2.70 g cm−3). The quartz grains were etched for 1 hour in 40% hydrofluoric (HF) acid to remove the outer portion of the quartz grains that was affected by alpha irradiation and also remove any contaminating grains of feldspar. After the HF etching, grains were washed in a 10% solution of HCl to remove any fluorides that may have been produced. Grains were finally mounted into 10 by 10 grids of 300-μm-diameter holes in a 9.8-mm-diameter aluminum single-grain disc for analysis.

All luminescence measurements were performed using a Risø TL/OSL DA-15 automated single-grain system equipped with a 90Sr/90Y beta source (64). A green laser was used to stimulate the grains for 1 s, and the OSL signal was detected through a 2.5-mm-thick U-340 filter and convex quartz lens placed in front of the photomultiplier tube. A preheat plateau test performed on 5-mm aliquots of sample T8SKIG02 was used to determine the preheat temperature (220°C for 10 s) used with a cut-heat of 160°C for the single-aliquot regenerative dose (SAR) protocol (65). OSL stimulation was performed at 125°C for 1 s, and the initial and background OSL signals were summed over the first 0.1 s and final 0.2 s, respectively. Typical decay curves and a dose-response curve measured for a single grain of quartz from sample T8SKIG02 are shown in fig. S6. Dose-recovery experiments performed on samples T8SKIG02 (ratio of 0.96 ± 0.03 and overdispersion of 15 ± 1%) and T8GABB01 (ratio of 1.01 ± 0.03 and overdispersion of 0 ± 0%) suggest that the SAR protocol was appropriate for OSL dating. The overdispersion values determined from dose-recovery experiments provided estimates of the amount of scatter caused by intrinsic sources of uncertainty that is beyond measurement uncertainties incorporated into a natural De distribution (66).

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