Methodology:

Chronostratigraphic data

Radiocarbon (C-14) dating:

Moraines adjacent to the BIC were mapped and interpreted from field observations and satellite images (DigitalGlobe, Catalogue ID: 1030010020C0C900; Google Earth, 2006 and 2011). Contour lines are derived from the Antarctic Digital Database with elevation data (±5 m) obtained from the Instituto Antartico Uruguayo (1997). Radiocarbon ages from moraines and stratigraphic sections were obtained by Accelerator Mass Spectrometry (AMS) dating of marine mollusc shells, terrestrial mosses and seaweed layers embedded in sediments and represent maximum ages for BIC glacier readvance Calibration of marine sample radiocarbon ages (marine shells) 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 (deltaR) of 666±76 C-14 years, 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 (>1950 CE) ages were corrected according to 13C/12C isotopic ratios from measured pMC with the 'present day' pMC value defined as 107.5% (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):

Large boulders >50 cm in diameter on the NW Bellingshausen Ice Cap (BIC) glacial foreland were surveyed and classified. Three samples were collected for He-3 CSED using a hammer and chisel to remove the upper few centimetres of exposed surfaces. Surface shielding due to snow cover was minimised by sampling from wind-exposed localities (cf. Johnson et al., 2012, 2017, 2020; Glasser et al., 2014; Lindow et al., 2014). Laboratory analysis for 10Be cosmogenic nuclide surface exposure dating followed the procedures of Kohl and Nishiizumi, (1992) and Binnie et al., (2015)

Code, data, all packages used, and package references, can be found at: https://github.com/stever60/Fildes_Peninsula

Data collection:

1.1 Cosmogenic data: Quartz purity was determined prior to dissolution by ICP-OES

1.2 Radiocarbon data: 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:

1.1 Cosmogenic data: Differential GPS (dGPS; WGS84 ellipsoid) data for erratic boulders were obtained using a GPS Trimble Pathfinder ProXH. As geodetic reference, we used the landmark DALL 66019M002 (-62.237870; -58.664545;, ellipsoidal height 39.376 m) at the Argentine Carlini base, c. 17 km from the sampled erratics. Post-processed uncertainties for the samples were less than 0.1 m in altitude and in the horizontal. Sample concentrations are relatively low but the maximum blank subtraction was <6%. Analytical uncertainties for the 10Be concentrations were derived by summing in quadrature the uncertainty in the mass of Be added during sample processing (estimated to be 1% at 1 sigma) and the AMS measurement uncertainties of both the samples and blank.

1.2 Radiocarbon data: 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.