Relative terrestrial exposure ages inferred from meteoric 10 Be and NO 3-1 concentrations in soils along the Shackleton Glacier , Antarctica 2

Modeling studies and field mapping show that increases in ice thickness during glacial periods were not uniform 17 across Antarctica. Rather, outlet glaciers that flow through the Transantarctic Mountains (TAM) experienced the greatest 18 changes in ice thickness. As a result, ice-free areas that are currently exposed may have been covered by ice at various points 19 during the Cenozoic, thereby providing a record of past ice sheet behavior. We collected soil surface samples and depth 20 profiles every 5 cm to refusal (up to 30 cm) from eleven ice-free areas along the Shackleton Glacier, a major outlet glacier of 21 the East Antarctic Ice Sheet (EAIS) and measured meteoric Be and NO3 concentrations to calculate and estimate surface 22 exposure ages. Using Be inventories from three locations, calculated maximum exposure ages range from 4.1 Myr at 23 Roberts Massif near the Polar Plateau to 0.11 Myr at Bennett Platform further north. When corrected for inheritance of Be 24 from prior exposure, the ages (representing a minimum) range from 0.14 Myr at Roberts Massif to 0.04 Myr at Thanksgiving 25 Valley. We correlate NO3 concentrations with meteoric Be to estimate exposure ages for all locations with NO3 depth 26 profiles but only surface Be data. These results indicate that NO3 concentrations can be used in conjunction with meteoric 27 Be to help interpret EAIS dynamics over time. We show that the Shackleton Glacier has the greatest fluctuations near the 28 Ross Ice Shelf while tributary glaciers are more stable, reflecting the sensitivity of the EAIS to climate shifts at TAM 29 margins. 30 31 https://doi.org/10.5194/esurf-2020-50 Preprint. Discussion started: 8 July 2020 c © Author(s) 2020. CC BY 4.0 License.


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Exposed terrestrial surfaces in Antarctica have previously been used to elucidate glacial history and assess ice sheet 33 stability during warm periods (Balco, 2011;Denton et al., 1993;Mackintosh et al., 2014). While Antarctica is thought to 34 have had a permanent ice sheet since the Eocene, both the East and West Antarctic Ice Sheets (EAIS and WAIS, 35 respectively) have fluctuated in extent and thickness throughout the Cenozoic (Barrett, 2013;DeConto and Pollard, 2016; 36 Huybrechts, 1993). The WAIS has been drastically reduced in size during interglacial periods and there is evidence from

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The collapse of the WAIS during the Pliocene contributed ~5 m to sea level, but Pliocene sea levels were at least 25 41 m higher than today, indicating additional water sources, likely from the EAIS and Greenland Ice Sheet (GIS) (Dwyer and

Sample collection
During the 2017-2018 austral summer, we visited eleven ice-free areas along the Shackleton Glacier: Roberts Peak and Mt. Wasko, represented by only one sample) with a plastic scoop and stored in Whirl-Pak™ bags. One sample was 129 collected furthest from the Shackleton Glacier or other tributary glaciers (within ~2,000 m) in a transect to represent soils 130 that were likely exposed during the Last Glacial Maximum (LGM) and previous recent glacial periods. A second sample was 131 collected closer to the glacier (between ~1,500 and 200 m from the first sample) to represent soils likely to have been 132 exposed by more recent ice margin retreat.    Table S1. Since there is a strong grain size dependence of meteoric 142 10 Be where very little 10 Be is carried on coarse (>2 mm) grains (Pavich et al., 1986), the gravel portion of the sample was not 143 included in the meteoric 10 Be analysis. The remaining soil (<2 mm) was ground to fine powder using a shatterbox.

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Meteoric 10 Be (Table 2) was extracted and purified at the NSF/UVM Community Cosmogenic Facility following 145 procedures originally adapted and modified from Stone (1998). First, 0.5 g of powdered soil was weighed into platinum 146 crucibles and 0.4 g of SPEX 9 Be carrier (with a concentration of 1,000 μg mL -1 ) was added to each sample. The samples were fluxed with a mixture of potassium hydrogen fluoride and sodium sulfate. Perchloric acid was then added to remove 148 potassium by precipitation and later evaporated. Samples were dissolved in nitric acid and precipitated as beryllium 149 hydroxide (Be(OH)2) gel, then packed into stainless steel cathodes for accelerator mass spectroscopy isotopic analysis at the 150 Purdue Rare Isotope Measurement Laboratory (PRIME Lab). Isotopic ratios were normalized to primary standard 07KNSTD 151 with an assumed ratio of 2.85 x 10 -12 (Nishiizumi et al., 2007). We corrected sample ratios with a 10 Be/ 9 Be blank ratio of 8.2 152 ± 1.9 x 10 -15 , which is the average and standard deviation of two blanks processed alongside the samples. We subtracted the 153 blank ratio from the sample ratios and propagated uncertainties in quadrature.

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We developed a mass balance using the fluxes of meteoric 10 Be in and out of Shackleton Glacier region soils to 162 calculate the amount of time which has passed since the soil was exposed (Pavich et al., 1984(Pavich et al., , 1986. The model assumes 163 that soils that were overlain by glacial ice in the past, and are now exposed, accumulated a lower surface concentration and 164 inventory of 10 Be than soils that were exposed throughout the glacial period (Fig. 3). The concentration of meteoric 10 Be at 165 the surface (N, atoms g -1 ) per unit of time (dt) is expressed as a function (Eq. 1), where the addition of 10 Be is represented as 166 the atmospheric flux to the surface (Q, atoms cm -2 yr -1 ) and the removal is due to radioactive decay, represented by a 167 disintegration constant (λ, yr -1 ) and erosion (E, cm yr -1 ) with respect to soil density (ρ, g cm -3 ).

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However, this function is highly dependent on dz, which represents an unknown value of depth into the soil column 170 which is influenced by meteoric 10 Be deposition and removal. We can account for this uncertainty and other uncertainties regarding 10 Be migration in the soil column by calculating the inventory (I, atoms cm -2 ) of the soil (Eq. 2), assuming that Q 172 has not changed systematically over the accumulation interval (Graly et al., 2010;Pavich et al., 1986).
If we know the inventory of meteoric 10 Be in the soil profile, the concentration at the surface, and soil density, and 175 use published values for erosion and 10 Be flux to the surface, we can combine Eq. (1) and Eq.

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Equation (3) provides a maximum exposure age assuming that soil profile did not have meteoric 10 Be before it was exposed to the surface (N0 = 0). Since our exposure age dating technique relies on the number of atoms within the sediment 180 column (I), any pre-existing 10 Be atoms in the soil (N0 ≠ 0) cause the calculated age to be an overestimate (    Thanksgiving Valley to 0.14 Myr at Roberts Massif (Table 3). These corrected ages are minimum ages.  (Table S2).

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We used the relationship between NO3and 10 Be to estimate 10 Be concentrations for all seven soil profiles ( . However, our inferred maximum estimated ages also indicate that, similar to the more northern locations, the samples 331 collected closest to the glacier are likely younger and were more recently exposed due to ice retreat (Fig. 8).

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Tributary glaciers in the Shackleton Glacier region appear to behave differently than the Shackleton Glacier itself.

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This is best demonstrated by the Bennett Platform samples, collected near the tributary Gallup Glacier. Bennett Platform is 334 unique in being the only location we sampled with large lateral moraines and several nearby medial moraines (Fig. 2c). The 335 surface concentration of meteoric 10 Be is lower at Bennett Platform than what would be expected from regression models 336 relating concentration with elevation and distance from the coast (Fig. 5). The lower concentrations of 10 Be, in turn, result in relatively lower calculated and estimated exposure ages ( Fig. 8; Table 3). Specifically, the exposure ages suggest that glacier 338 retreat following termination of the last glacial period was delayed at Bennett Platform.

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We argue that the younger than anticipated exposure age is due to differing glacial dynamics between tributary and