2.     Material and Methods (1-2 pages)
 
 
To constrain the latest Pleistocene and early Holocene activity of ice sheet and glacier movement in the Petermann Gletscher region, rock samples from erratic and moraine boulders were collected across Washington Land and the islands within Nares Strait (Figure 1). We present 10Be surface exposure dates from seventy quartz-bearing erratic boulders collected from fourteen different isolated plateaus across Washington Land, from three islands within Nares Strait, from the landscape adjacent to the left margin of the Petermann Gletscher (dashed box in Figure 1), and from a right-lateral moraine of the Humboldt Gletscher sector of the GrIS.  Erratic lithogies such as granites, gneisses, and sandstones lacking any local outcrop were found throughout Washington Land, indicating that the boulders were deposited by glacial movement. 
To investigate a land-terminating sector of the GrIS, thirty samples were collected from fourteen isolated plateaus across the interior of Washington Land with sample elevations ranging from 428-695 m (mean = 541 m), and had boulder heights ranging from 60-250 cm (mean = 102 cm).  Erratic boulders sampled across this 1950 km2 area (nearly 20% of the entire Washington Land area) were collected from surfaces with similar high degrees of weathering containing permafrost features characteristic of formerly glaciated upland terrains (e.g. Briner et al., 2006).  10Be exposure ages from erratic boulders on perched on plateaus across Washington Land will give geochronological constraints on the timing and rate of GrIS retreat from Nares Strait during deglaciation. 
Bracketing Washington Land, the GrIS has two marine-terminating ice margins; Petermann Gletscher to the north and Humboldt Gletscher to the south.  Exposed erratic and moraine boulders sampled proximal to the lateral margins of these glaciers will provide insight to the sensitivity of ice to documented oceanic and atmospheric temperature fluctuations since the LGM.  Twenty-two samples collected in a 15 km2 area proximal (within 4 km of modern ice) to the left margin of the modern Petermann Gletscher (dashed box on Figure 1) will give insight to the timing of retreat of this marine-terminating outlet glacier since the LGM.  Samples from this ice proximal area ranged in elevation from 107-415 m (mean = 198 m), and ranged in boulder height from 30-200 cm (mean = 84 cm).  Twenty of these Petermann-proximal samples came from erratic boulders resting directing on bedrock or weathered surfaces, and two of these samples came from the crest of a left lateral moraine of Petermann Gletscher.  To investigate the response of the Humboldt Gletscher sector of the GrIS, six samples were collected from a right lateral moraine of the Humboldt Gletscher located 0.5 km outboard of the modern ice position and 12 km up-ice from the modern 90 km-wide calving margin where Humboldt Glacier meets Kane Basin.  The samples were collected from boulders ranging in elevation of 128-138 m (average = 131 m) and the boulder heights ranged from 30-50 cm (mean = 38 cm).
To constrain the timing of initial deglaciation and the break-up of the Innuitian and Greenland Ice Sheets across Nares Straits, twelve samples were collected from three islands within Nares Strait and from two sites coast of Washington Land (Figure 1).  The samples ranged in elevation from 173-544 m (mean = 332) and the height of the boulders ranged from 25-100 cm (mean = 53 cm).  Boulder samples from all the sites consist of ~1 kg of quartz-bearing lithologies collected from the top 2 cm of the flat upper-most surface of boulders.  A handheld GPS receiver with a horizontal uncertainty of ~2 m was used to record sample coordinates, with elevations extracted from the 2-m resolution ArcticDEM (https://www.pgc.umn.edu/data/arcticdem/). 
Samples underwent physical and chemical preparation at the University of Wisconsin-Madison Cosmogenic Isotope Laboratory using procedures modified from (Kohl and Nishiizumi, 1992).  Samples were crushed and sieved to separate the 250-710 mm size fraction, magnetically separated, and etched in dilute HCl and HF/HNO3 acid solutions.  Chemical frothing was performed on all non-magnetic grains in order to separate quartz from feldspar.  The remaining quartz grains were treated with additional HF/HNO3 etches until the desired quartz purity was achieved.  Quartz purity was measured by inductively coupled plasma optical emission spectroscopy at the University of Wisconsin-Madison Water Science and Engineering Laboratory.
Beryllium was isolated from quartz following procedures adapted from the University of Vermont Cosmogenic Nuclide Laboratory (Corbett et al., 2016).  Samples were spiked with ~193 mg of 9Be carrier solution prepared from raw beryl (OSU White standard; 9Be concentration of 251.6 ± 0.9 ppm).  We used anion and cation exchange columns to isolate Be, and BeOH was precipitated in a pH 8 solution.  The BeOH gels were converted to BeO by incineration, mixed with Nb powder, and packed into stainless steel cathodes for accelerator mass spectrometry (AMS) analysis.
All 10Be/9Be ratios were measured at the Purdue University Rare Isotope Measurement Laboratory (PRIME Lab) and normalized to standard 07KNSTD3110, which has an assumed 10Be/9Be ratio of 2.85 x 1012 (Nishiizumi et al., 2007).  Sample ratios were corrected using batch-specific blank values that ranged from 1.22 x 10-15 to 4.53 x 10-15 (n = 10).  AMS uncertainty ranged from 1.43 to 6.56%.
10Be exposure ages (Table 1) were calculated with online CRONUS-Earth exposure age calculator version 3 (http://hess.ess.washington.edu/math/index_dev.html; Balco et al., 2008) using the regionally calibrated Baffin Bay/Arctic 10Be production rate (Young et al., 2013) and Lal/Stone time-varying scaling scheme (Lal, 1991; Stone, 2000).  This production rate has been used extensively to calculate exposure ages in Greenland (Kelley et al., 2015; Lesnek and Briner, 2018; Beel et al., 2016; Larsen et al., 2016; Sinclair et al., 2016) and is similar to the independently-derived 10Be production rates from other regions (Balco et al., 2009; Putnam et al., 2010).  Because few constraints on Holocene relative sea level change exist for the broader Washington Land region, uncertainties about the elevation history of our samples and the relatively minor effect of estimated elevation adjustments (<5%) [EC2] lead us to report our 10Be exposure ages without elevation corrections.  We made no corrections for snow shielding or erosion.  Samples were collected from windswept topographic highs and we found no correlation between boulder height and 10Be age, suggesting boulders have not experienced significant snow cover.  We observed glacial polish on exposed bedrock in Washington Land and the islands within Nares Strait, which implies minimal erosion of rock surfaces since deglaciation.  Therefore, we assume erosion on sample surfaces to be negligible. 
3.     Results (1 page; 1 table)
 
We present 10Be ages from boulders in geographic (N-S) order in the different subregions and geomorphologic feature that they were sampled from in Washington Land (Table 1).  Starting with the islands within Nares Strait and along the Washington Coast, the 10Be ages range from 25.11 ± 0.79 to 8.28 ± 0.31 kys.  Excluding the ages that are not within error of other ages, there are three different populations of samples.  The oldest has a mean of 20.44 ± 0.7 ky (n = 4), the middle group has a mean of 16.86 ± 0.50 (n = 3) and the youngest group has a mean of 9.49 ± 0.45 (n = 2).  A correlation with elevation exists, with the youngest group of ages being from elevations of 173 m and 180 m from two different islands (Joe Island and Crozier Island) located ~100 km apart. The oldest group of samples have elevations ranging from 544 to 389 m (n = 3) from the site on the Washington Land coast.  The middle group of samples are both from Franklin Island and have elevations of 443 m and 444 m.
The 10Be ages of the samples across the interior of Washington Land present the largest spread in ages, from 85.37 ± 1.25 ky to 6.77 ± 0.31 ky (n = 30).  Focusing on the younger samples, a group of five samples are within error of each other with a mean of 7.98 ± 0.30 (n = 5) from higher elevation isolated plateaus spanning a distance of ~45 km.  The next group of ages that are within error of each other have a mean of 14.24 ± 0.43 (n = 4) from higher elevation sites that span an area of ~1200 km2.  Following that group, the next group of ages that are within error have a mean of 18.02 ± 0.53 ka (n = 5).  The remaining samples have an even spatial and temporal distribution stretching back to 85.37 ± 1.25 ky.     
In total, the 22 samples from an elevational transect carried out within 4 km of the modern Petermann Gletscher have 10Be ages from 20.12 ± 0.75 ky to 7.12 ± 0.23 kyrs.  The oldest two ages can be removed from the population as they are older than independent records of deglaciation.  A group of 3 samples are within error of one another but are not within error of the remaining 17 samples.  The older group has a mean of 12.66 ± 0.51 kyrs (n = 3) and the younger group has a mean of 8.53 ± 0.40 kyrs (n = 17).  Two of this group are from boulders on the left lateral moraine of Petermann Gletscher (n = 2), which have a mean age of 8.25 ± 0.54 kyrs, but it should be noted that these two samples are not within error of one another (ILL samples on Table 1).  Boulders on the right lateral moraine of Humboldt Gletscher (n = 6) have an average of 9.26­­­ ± 0.44 kyrs, with an error-weighted mean of 8.49 ± 0.33 kyrs.