Methodology:

Mapping and Stratigraphic section sample collection

Mapping was undertaken in ARC-GIS, with final layouts achieved in Adobe Illustrator v. 26.2.1 or CorelDRAW v. 2020. Lithostratigraphic descriptions and radiocarbon sampling were undertaken at an outer peninsula stratigraphic profile referred to as "new Pingfo II" and a new river section adjacent to the 'Potter Cove section' sampled by Sugden and John (1973). We also sampled terrestrial moss samples for radiocarbon dating from a recently exposed 'Inland outcrop' which is located inside the 1956 CE limit, around 700 m from the active glacier front. The stratigraphic sections were characterised using textural criteria, fabric, composition, sedimentary structures, and grain size analysis to determine the relationships between different depositional units.

Radiocarbon (C-14) dating

Twenty-one Accelerator Mass Spectrometry (AMS) radiocarbon (C-14) ages were obtained from seaweed, marine mollusc shells, penguin and undetermined bones, remnants of terrestrial mosses embedded in stratigraphic profiles and moraine sediments. Calibration of marine sample radiocarbon ages (marine shells and seaweed) was undertaken in Oxcal v. 4.4 using the Marine20 calibration curve (Bronk Ramsey, 2009; Heaton et al., 2020), and a newly recalculated local marine reservoir age offset (delta R) of 666 plus-minus 76 C-14 years (Heaton et al., 2020), which represents the weighted mean delta R of four radiocarbon-dated marine samples collected prior to 1950 CE from the northern Antarctic Peninsula and Signy Island in the online Marine20 database (http://calib.org/marine/). Terrestrial and aquatic moss samples were calibrated using the Southern Hemisphere SHCal20 calibration curve in Oxcal v. 4.4 (Hogg et al., 2020). Post-bomb (more than 1950 CE) ages were corrected according to 13C/12C isotopic ratios from measured pMC with the 'present day' pMC value defined as 107.5 percent (2010 CE) and calibrated using the SHCal13 SH Zone 1-2 Bomb curve in CALIBomb (Reimer and Reimer, 2004; Hua et al., 2013).

Cosmogenic Helium-3 (He-3) nuclide surface exposure dating (CSED).

Five samples were collected for He-3 CSED using a hammer and chisel to remove the upper few centimetres of exposed surfaces. Differential GPS (dGPS) measurements were undertaken using a Trimble Pathfinder ProXH to determine the precise location and altitude of boulders in relation to the landmark DALL 66019M002 (62.23787 degrees S, 58.66455 degrees W, ellipsoidal height 39.376 m) triangulation station located on the Argentine Carlini base, a few hundred metres away from the sampled erratics. dGPS precision is better than 10 cm in all axes, but ellipsoid correction errors are larger. Exposure ages were calculated using the CRONUScalc calculator (Version 2.0; Marrero et al., 2016) with the time-dependent Lal (1991)/Stone (2000) scaling model (Lm) for altitude at Antarctic pressure conditions and the primary calibration data set for He-3 in pyroxene, which yields a long term sea-level high latitude (SLHL) scaled production rate of 122 plus-minus 13 at g-1 a-1 (Borchers et al., 2016). External age uncertainties include production rate uncertainties. Exposure ages determined with other scaling models (e.g., Lifton et al., 2014) vary by up to around 6 percent. We report internal and external uncertainties. Following Balco et al. (2008), external uncertainties are used for comparison with calibrated AMS radiocarbon ages and error ranges.

Grain size analysis

Thirteen samples were dry sieved to separate the fraction larger than 2 mm, placed in an ultrasonic bath for 10 seconds, then placed in a reciprocating shaker and left overnight. Samples were wet sieved to separate the fraction less than 0.063 mm (silt and clay size), and the coarser fraction was dried in an oven at 50 degrees Celsius and dry sieved into sand fractions (more than 1mm, more than 0.5...(11)

Data collection:

Radiocarbon samples were prepared at the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research and British Antarctic Survey. AMS measurements were undertaken at ETH Zurich and Beta Analytical, Miami, and 13C/12C isotope ratios were used to calculate Conventional Radiocarbon Ages.

Data quality:

Pre-bomb calibrated ages have been rounded to the nearest 10 years, and to the nearest hundred years in the manuscript text to reflect realistic total (internal and external) uncertainties. Post-bomb ages have been rounded to the nearest year.