Geomorphology

Landforms were mapped in the field using differential GPS and satellite imagery. This formed the basis for detailed work on sediment morphology, lithology, weathering and cosmogenic nuclide analysis. Selected areas, such as complex debris accumulations were mapped with a terrestrial laser scanner as well as high-resolution vertical aerial photographs, taken from an unmanned aerial vehicle35,36. A differential GPS system, mounted on a snowmobile, traversed the ice margin of all three massifs to provide a reference surface from which to normalize 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 an overview image of the Marble, Independence and Patriot Hills in the southern Heritage Range and the sample locations. Supplementary Fig. 2 illustrates the nature of the glaciated upland surface with photos of individual samples.

Cosmogenic nuclide analyses

The cosmogenic nuclide data are presented in Supplementary Data 1–3, and Supplementary Figs 1–4. The data relating to the older, weathered deposits in the Patriot and Marble Hills were the subject of a previous publication28; however, we include the 26Al and 10Be data set here. In addition, we also include in our data set 11 samples previously reported from the same bedrock spurs7; the exposure ages of the latter have been recalculated to be consistent with the present study.

The sampling strategy for cosmogenic nuclides was designed to reduce the chance of nuclide inheritance, and exclude the possibility of nuclide loss through postglacial erosion. We targeted sub-glacially derived clasts with striated surfaces and sub-angular to sub-rounded 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 relatively low amounts of cosmogenic 10Be and 26Al, equivalent to 0–0.8 ka and in two cases 1.5 ka of pre-exposure. 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 pre-exposure of a similar magnitude. On the Independence Hills medial moraine ridge, we collected four large, broad-based limestone and sandstone boulder samples to determine the age of this feature.

The presence of striations on most sampled rock surfaces validates our assumption of negligible rock surface erosion. However, it is worth exploring the sensitivity of our results to this assumption. The effect of incorporating even an exceptionally high Antarctic erosion rate of 2.5 mm ka−1 would be to increase LGM ages by ∼4% and Holocene ages by 2%. The high end of most Antarctic sandstone erosion rates is ∼1 mm ka−1; applying this rate would increase LGM ages by 2% and Holocene ages by 1%. Therefore, we argue the results are insensitive to our assumption of zero erosion.

In this study, we favour the youngest exposure age at each altitude to best represent the elevation of the ice surface at the time (Fig. 4; Supplementary Fig. 3). This is a common approach in Antarctica where problems of pre-exposure/inheritance dominate processes that might cause ages to be too young. For example, we are less concerned with shielding of a sample from cosmic radiation by snow, soil, loess or vegetation in a polar environment. Other forms of shielding such as exhumation through till, or sample self-shielding by rolling of clasts, can be avoided by sampling clasts situated on flat bedrock, as in the present study. One feasible process that could cause erroneously young exposure ages would be in areas of stranded, ice-cored tills. Here a clast could melt out to become exposed after the ice sheet thinned. We avoid this by sampling from bedrock ridges rather than embayments where ice-cored tills are more common. Our seven ice-margin samples act as geological blanks. These suggest that inheritance is low, but could account for up to 1.5 ka of pre-exposure even before being incorporated into the blue-ice moraine. Blue-ice moraines are ice-marginal, supraglacial features and it is reasonable to assume there may be a period of exposure within the blue-ice moraine before the clast is eventually deposited on the mountainside as the ice sheet thins (Supplementary Fig. 2). This is likely to explain much but not the entire observed scatter in the age versus elevation plots, and it reinforces our decision to favour the youngest samples.

Figure 5 explores the effect of being wrong in our assessment of the scatter of exposure ages in terms of pre-exposure and delayed deposition. Rather than preferentially selecting the youngest ages, we treat the observed scatter as a result of both geological processes making some exposure ages too young, and inheritance making some exposure ages too old; in doing so we can derive an average thinning rate using all the available data. In this way, we produce an average thinning rate for each individual massif, and for all three sites together under the assumption that thinning occurred concurrently, for example, refs 13, 31. We follow the approach of Johnson et al.13 in modelling 10,000 linear regressions through all 10Be exposure ages <10 ka on each massif. For each iteration, the age for each point is randomly chosen from within the uncertainty bounds on that sample and then a linear regression is fit to those points, removing any results with negative slopes (negligible: <0.001%). Uncertainties are calculated statistically so that 68% of the resulting slope values fall within the given range. The results suggest average thinning rates of 20.9±2.7 cm a−1 at Marble Hills, 6.7±0.4 cm a−1 at Patriot Hills, and 5.1±0.2 cm a−1 at Independence Hills (1σ), but these rates would be higher following the youngest exposure age approach. When considering all sites together, the modelling suggests a lower and linear thinning rate of 8.8±0.2 cm a−1, but initiated a little earlier at about 8.5–9 ka. The exercise demonstrates that our conclusion of a mid-Holocene pulse of thinning, which was complete by 3.5 ka, is not sensitive to our interpretation based on the youngest exposure. In Supplementary Fig. 4, we increase the range to include all 10Be exposure ages <15 ka, and then with three clear outliers (3σ) removed to obtain a linear regression through all deglacial exposure ages. The model suggests the onset of initial deglaciation was at 9–10.5 ka, with a lower average thinning rate of 8.1±0.2 cm a−1. The onset of thinning implied is similar to that inferred from the youngest exposure age at Supplementary Fig. 4c, 10 ka.

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 (SUERC) Accelerator Mass Spectrometry (AMS) Laboratory in East Kilbride, UK. Measurements are normalized to the NIST SRM-4325 Be standard material with an assumed39 10Be/9Be of 2.79 × 10−11, 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 Nishiizumi40. SUERC 10Be-AMS is insensitive to 10B interference41 and the interferences to 26Al detection are well characterized42. 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 3.3–9.3 × 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 Data 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 (ICP-OES) uncertainty.

Cl was extracted from three carbonate samples at the University of Edinburgh following a similar procedure to those outlined in Marrero et al.43. A 35Cl carrier (ORNL batch 150301, ∼1.6 mg 35Cl) was added to the samples before dissolution. The sample was dissolved using trace analysis quality nitric acid and Cl was precipitated as AgCl. The AgCl was purified, pressed and then measured at the SUERC AMS Laboratory. The sample was blank-corrected individually for 36Cl and total Cl concentrations (∼2% each) in the process blanks for that batch of samples. Full sample chemistry and measurement information can be found in Supplementary Data 2.

Exposure ages

For exposure age calculations we used default settings in Version 2.0 of the CRONUScalc programme44. 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 exposure ages were calculated using a 10Be half-life of 1.387 Ma (refs 45, 46), an 26Al half-life of 0.705 Ma (ref. 47), and a 36Cl half-life of 0.301 Ma. The CRONUS-Earth production rates44,48,49 with the nuclide-dependent scaling of Lifton–Sato–Dunai50 were used to calculate the ages presented in the paper. Sea level and high latitude production rates are 3.92±0.31 atoms per g per a for 10Be, 28.5±3.1 atoms per g per a for 26Al, 56.0±4.1 atoms per (g Ca) per a−1 for 36Cl–Ca and 759±180 neutrons per (g air) per a−1 for 36Cl–P f (0). The sample had very low Cl (15.6 p.p.m.), so the choice of P f (0) does not change the age. The use of Lal/Stone51,52 scaling does not change the conclusions of the paper. If Be ages are calculated using the Lal/Stone scaling model, most ages (>1 ka and <1 Ma) range from 1.8 to 3.8% older, with an average of 2.8%. Older samples (>1 Ma) average 4.6% older. For Al samples, most ages range from 5.3 to 8.7% older, with an average of 6.9%; older samples are 11.5% older. The Cl age is 11% older using Lal/Stone scaling. Rock density is assumed 2.7 g cm−3 for quartz-bearing erratics and 2.5 g cm−3 for limestone; 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 minimal. Finally, we make no attempt to account for production-rate variations caused by elevation changes associated with glacial isostatic adjustment of the massif through time. Any glacial isostatic uplift would cause the exposure ages to be too young. However, we note that glacial isostatic adjustment in this area is low27, and the effect would be minimal on the youngest ages.

Data availability

The authors declare that the data supporting the findings of this study are available within the article and its Supplementary Information and data files. Any further data or information is available on request from the corresponding author (A.S.H.).