Geomorphology

Landforms were mapped in the field using differential global positioning satellite and satellite imagery. This formed the basis for detailed work on sediment morphology, lithology, weathering and cosmogenic isotope analysis. Selected areas, such as complex debris accumulations, were mapped with a laser scanner as well as high-resolution vertical aerial photographs taken from an unmanned aerial vehicle. The British Antarctic Survey Polarimetric-radar Airborne Science Instrument ice-sounding radar was used to image deeper englacial reflections (data collected during an airborne survey of the Institute Ice Stream, austral summer 2010/2011 (ref. 36)). A differential global positioning satellite system, mounted on a snowmobile, traversed along the ice margin of both massifs to provide a reference surface from which to normalise sample elevations. Topographic control on the ice margin introduces variability in elevation and therefore uncertainties in the normalization process are estimated at±15 m. Supplementary Fig. 1 shows the general ice flow configuration around the southern Heritage Range. Supplementary Fig. 2 shows a geomorphologic map of the Marble Hills massif. Supplementary Fig. 3 shows sample locations from the Patriot Hills. Supplementary Fig. 4 illustrates the nature of the glaciated upland surface and the type and distribution of weathered debris.

Cosmogenic nuclide analysis

The cosmogenic nuclide data are presented in Supplementary Tables 1–3 and Supplementary Figs 5–6. All exposure ages discussed are based on 10Be ages because the production rate is better constrained; 26Al and 21Ne are used to constrain exposure histories. The measurement of 21Ne was completed because, as a stable isotope, it gives a measure of the total exposure time (assuming no erosion) irrespective of subsequent burial, and because it can potentially record longer periods of exposure than is generally possible with radioactive isotopes.

The sampling strategy for cosmogenic nuclides was designed to reduce the chance of nuclide inheritance, and exclude the possibility of nuclide loss through erosion. We targeted subglacially derived clasts with striated surfaces and subangular to subrounded shapes. We sampled the freshest appearing, quartz-bearing, brick-sized clasts resting on flat bedrock to minimize problems of post-depositional movement and self-shielding. It is crucial to be convinced that the exposure ages reflect the time since deposition of a freshly exposed clast rather than a signal inherited from the past. To test whether clasts emerging on the glacier surface in blue-ice areas have no inherited cosmogenic nuclides, we analysed seven clasts on the present ice margin. All had negligible amounts of both 10Be and 26Al, implying that they were first exposed to cosmic rays when they emerged at the ice-surface. In view of the similar lithologies, and thus origin, of quartz-rich erratics at higher elevations, it is reasonable to argue that they too were deposited with no significant pre-exposure. This is reinforced by the clustering of multi-isotope exposure ages from each sampled site. Thus, we conclude that the cosmogenic nuclide concentration in the erratics accurately reflects their exposure history since deposition.

Laboratory and analytical techniques

Whole-rock samples were crushed and sieved to obtain the 250–710 μm fraction. Be and Al were selectively extracted from the quartz component of the whole-rock sample at the University of Edinburgh’s Cosmogenic Nuclide Laboratory following established methods37,38. 10Be/9Be and 26Al/27Al ratios were measured in 20–30 g of quartz at the Scottish Universities Environmental Research Centre Accelerator Mass Spectrometry (AMS) Laboratory in East Kilbride, UK. Measurements are normalized to the NIST SRM-4325 Be standard material with a revised39 nominal 10Be/9Be of 2.79 × 10−11 and half-life of 1.387 Ma (refs 40, 41), and the Purdue Z92-0222 Al standard material with a nominal 26Al/27Al of 4.11 × 10−11, which agrees with the Al standard material of Nishiizumi42 with half-life of 0.705 Ma (ref. 43). Scottish Universities Environmental Research Centre 10Be-AMS is insensitive to 10B interference44 and the interferences to 26Al detection are well characterized45. Process blanks (n=6) were spiked with 250 μg 9Be carrier (Scharlau Be carrier, 1,000 mg l−1, density 1.02 g ml−1) and 1.5 mg 27Al carrier (Fischer Al carrier, 1,000 p.p.m.). Samples were spiked with 250 μg 9Be carrier and up to 1.5 mg 27Al carrier (the latter value varied depending on the native Al-content of the sample). Blanks range from 3.3 to 9.3 × 10−15 [10Be/9Be] (<1% of total 10Be atoms in all but the ice-margin samples); and 1.6–7.5 × 10−15 [26Al/27Al] (<1% of total 26Al atoms in all but the ice margin samples). Concentrations in Supplementary Table 1 are corrected for process blanks; uncertainties include propagated AMS sample/lab-blank uncertainty and a 2% carrier mass uncertainty and a 3% stable 27Al measurement inductively coupled plasma optical emission spectrometry uncertainty.

Neon isotopes were measured in ∼250 mg of leached quartz (250–500 μm). Samples were wrapped in aluminium foil and loaded into a Monax glass tree and evacuated to<10−8 torr for 48 h before analysis. Samples were successively heated for 20 min to 1,200 °C in a double-vacuum resistance furnace with a tungsten heating element and a molybdenum crucible. The extracted gas was cleaned on two hot SAES TiZr getters. The heavy noble gases (Ar, Kr and Xe) were absorbed onto a charcoal trap cooled with liquid nitrogen. Neon was then absorbed on to a charcoal trap at −228 °C for 20 min, and the residual He was removed by a turbomolecular pump. The Ne was released from the charcoal at −173 °C, and the isotopic composition analysed using a MAP 215–50 noble gas mass spectrometer. All Ne isotopes were measured in 11 peak jumping cycles using a Burle channeltron electron multiplier operated in pulse-counting mode. Neon abundances were determined by peak height comparison with Ne from 95.2±0.5 μcc STP air. The reproducibility of Ne abundances was better than ±1.5%, and isotopic ratios of replicate calibrations were better than ±0.5%. Interference corrections and detailed analytical procedure is presented elsewhere46. The 20Ne blank at 1,200 °C was typically ∼1 × 10−11 ccSTP and were indistinguishable from the atmospheric isotopic composition after correction for interfering species. Consequently no blank correction is made to the data in Supplementary Table 2. The consistency of procedures is demonstrated by the reproducibility of the cosmogenic 21Ne concentration in replicate analyses of MH12–27 (Supplementary Table 2). In all samples Ne isotope compositions are consistent with binary mixture of air and cosmogenic Ne. The 21Ne concentrations in Supplementary Table 2 include a correction for nucleogenic 21Ne (that is, non-cosmogenic 21Ne) of 7.7±2.4 × 106 at g−1. This is the value estimated by Middleton et al.47 for Beacon Sandstone; we use this value as a best estimate based on the similar lithology and thermal history of the rocks. This value is close to the mode of the range of nucleogenic 21Ne measured in Antarctic rocks (see Balco and Shuster48 for a review). However, there is likely variability in the nucleogenic 21Ne concentrations that could impact the youngest samples. The conclusions are insensitive to uncertainty in burial time.

Exposure age calculations

For exposure age calculations we used default settings in Version 2.0 of the CRONUScalc programme49. This is the product of the CRONUS-Earth collaboration that allows for all commonly used nuclides to be calculated using the same underlying framework, resulting in internally consistent cross-nuclide calculations for exposure ages, erosion rates and calibrations. The CRONUS-Earth production rates50 with the nuclide-dependent scaling of Lifton-Sato-Dunai51 were used to calculate the ages presented in the paper. Sea level and high latitude production rates are 3.92±0.31 atoms g−1 a−1 for 10Be and 28.5±3.1 atoms g−1 a−1 for 26Al. However, the use of Lal/Stone26,52 scaling does not change the conclusions of the paper despite the ∼3 and 8% older exposure ages for 10Be and 26Al, respectively. Rock density is 2.7 g cm−3 and the attenuation length used is 153±10 g cm−2. No corrections are made for rock surface erosion or snow cover and thus exposure ages are minima. Finally, we make no attempt to account for production rate variations caused by elevation changes associated with glacial isostatic adjustment of the massif through time53. This is justified because the samples have been exposed for multiple glacial cycles and thus any variations in elevation associated with ice loading and unloading, which has been of similar magnitude (maximum elevation difference 170 m), are likely to have been averaged out to the point of being smaller than other sources of uncertainty.

The CRONUScalc code for 21Ne was modelled after the existing code for 3He and only includes spallation production. The 21Ne production rate is tied to the total CRONUScalc 10Be production rate (assuming 1.5% production from muons49) with a 21Ne/10Be ratio of 4.08±0.37 (ref. 48), resulting in a 21Ne production rate of 16.26±1.96 atoms g−1 a−1 at sea level, high latitude scaled according to nuclide-dependent Lifton-Sato-Dunai48,50,51. There are several other alternative 21Ne production rates (all converted to be consistent with Lifton-Sato-Dunai scaling): 14.5 (Amidon et al.54, 18.0 (Vermeesch et al.55) and 18.9 (Niedermann et al.56). The Balco and Shuster48 rate was used because it is based on ratios tied to 10Be instead of 26Al, it uses a relatively large dataset compared with other 21Ne studies, it was performed using Antarctic samples, and the resulting rate falls in the middle of the production rate range. The differences in age using the other production rates given above range from 12% older to 14% younger than those given in the paper. While these changes are significant, the exposure ages are consistently similar or older than the corresponding 10Be and 26Al ages so the exact choice of 21Ne production rate does not affect the conclusions presented in the paper. For comparison, Lal/Stone26,52 scaling in CRONUScalc was used in conjunction with the production rate from Balco and Shuster48and produced 21Ne exposure ages that were approximately 3% younger than those produced using the nuclide-dependent Lifton-Sato-Dunai scaling scheme with the Balco and Shuster48 production rate.

Supplementary Figs 5 and 6 show plots of the isotopic ratios of 26Al/10Be and 21Ne/10Be. Samples should plot within the erosion island if they have been continuously exposed and eroding, and within the complex zone if they have been buried for a significant period of time, long enough for the shorter lived nuclide to preferentially decay. The 26Al/10Be system should be more sensitive to recent burial than the 21Ne/10Be system because of the shorter half-life of 26Al (0.705 ka). In our samples, the burial signal implied by the 21Ne/10Be ratios is greater than that implied by 26Al/10Be ratios. There are a few possible explanations. First, this may partly reflect the uncertainties on 21Ne production rates as discussed above. Second, it is possible that the samples contain additional nucleogenic 21Ne that has not been corrected for. A final explanation is that 21Ne, which is stable, is recording a period of exposure that is not evident in the 26Al/10Be system. At present it is not possible to discriminate between the above scenarios. In any case, our conclusions are not sensitive to these minor discrepancies.

Till analysis for marine traces

Scherer (R. Scherer, personal communications, 2014) examined four till samples from both current and elevated blue-ice moraines and found no evidence of diatoms or biogenic silica.