Methodology:

Quartz bearing erratic cobbles, which showed evidence of transport by ice, were collected in 2015-16 from rock outcrops adjacent to Pope Glacier in the Amundsen Sea Embayment of West Antarctica by Joanne S Johnson and Stephen J Roberts (British Antarctic Survey), following procedures outlined in Johnson et al., (2020). The top 2.0-5.5 cm of a suite of 9 cobbles were cut at British Antarctic Survey. Quartz mineral separation and purification was performed in the CosmIC Laboratory at Imperial College London largely based upon standard procedures. Samples were initially crushed and sieved to isolate the 250 - 500 micrometre grain size fraction. Further sample processing was based on standard procedures (Kohl and Nishiizumi, 1992; Corbett et al., 2016). Froth flotation was not used to minimise the introduction of contaminant carbon sources (Nichols and Goehring, 2019). Following determination of quartz purity using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) quartz mineral separates (~ 10 g) were shipped to Tulane University, New Orleans, USA for in situ 14C extraction. Extraction of in-situ 14C was performed by Ryan Venturelli using the fully automated Carbon Extraction and Graphitisation System (CEGS) at Tulane University. The first in situ 14C extraction (n = 9) followed the methods of Goehring et al., (2019). The quartz sample was fused with lithium metaborate (LiBO2) flux to ensure total sample melt <1300 degrees C and complete liberation of in-situ 14C (Lifton et al., 2001). First quartz was step heated at 500 degrees C for 1 hour to remove atmospheric 14C before being combusted at 1100 degrees C for 3 hours to liberate in situ 14C (in the form of CO2). Liberated CO2 was cryogenically purified before being collected in a measurement chamber, quantified monometrically and diluted with 14C-free CO2 to ensure a measurable sample size (Goehring et al., 2019). CO2 was graphitized using standard H2 reduction methods over an Fe catalyst (Slota et al., 1987). The configuration of the Tulane extraction line was changed prior to in situ 14C replicate measurements (n = 4). Alterations to the line included a new coil trap, which changes how gas is extracted and trapped following combustion (Lifton et al., 2023) and the introduction of a new mullite tube for 14C extraction due to failure of the previous tube. Mullite tubes often undergo a "break in" period, during which initial 14C blanks are higher but often fall with continued use (Goehring et al., 2014, 2019; Pigati et al., 2010).

14C/12C isotope ratios were measured by accelerator mass spectrometry at the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS) (Woods Hole, USA) using the methods described in Longworth et al. (2015). Stable isotope (delta-13C) analysis was undertaken at the University of California, Davis Stable Isotope Facility. Data reduction (to convert 14C/12C ratios to 14C/C_total) followed methods outlined in Hippe and Lifton (2014) which account for the specific differences of 14C production within a mineral lattice compared to 14C incorporation into organic material. Exposure ages were calculated from in situ 14C concentrations using version 3 of the online calculators: https://hess.ess.washington.edu/math/v3/v3_age_in.html (Balco et al., 2008) with the "LSDn" production rate scaling method for neutrons, protons, and muons following Lifton et al., (2014) and the primary production rate calibration data set of Borchers et al., (2016).

Data collection:

Agilent 5100 SVDV ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometer)

Tulane University Carbon Extraction and Graphitisation System (TU-CEGS)

NOSAMS 3MV Tandetron AMS (Accelerator Mass Spectrometer) system

Version 3 of online calculator at hess.ess.washington.edu (formerly known as CRONUS Online calculator)

Data quality:

Quartz purity was tested using inductively coupled plasma optical emission spectrometry (ICP-OES).

Nine in situ 14C measurements, and four in situ 14C replicate measurements were made. In table AMS_14C.csv aliquot A indicates initial in situ 14C concentrations measured in each sample and aliquot B indicates repeat in situ 14C measurements on the same sample.

A long-term average 14C blank used to correct the initial measurements of in situ 14C (n = 9) is 4.53 +/- 0.24 x 10^4 atoms g-1. For replicate measurements (n = 4), a higher short-term average 14C blank of 7.14 +/- 0.30 x 10^4 14C atoms g-1 was used. A 6 % (1 sigma) uncertainty was assigned to each in 14C measurement concentration reported by AMS before calculating exposure ages. The 6 % uncertainty exceeds the reported analytical uncertainty for all in situ 14C measurements made for this study and reflects the repeatability of measurements of CRONUS-A extracted at Tulane. The in situ 14C concentration of CRONUS-A extracted at Tulane is reported with a long-term value of 6.12 +/- 0.32 x 10^5 (n = 10) (Goehring et al., 2019). The distribution of the long-term 14C measurement background of samples from the Tulane University extraction facility is long-tailed and non-time dependent (Balco et al., 2022).

Blank correction using a long tailed distribution is, therefore, appropriate for low concentration samples such as subglacial bedrock cores (Balco et al., 2023) where blank variability constitutes the dominant source of uncertainty in the in situ 14C measurements (Balco et al., 2022). Blank correction procedures for low concentration samples are discussed at length in Balco et al., (2022) and can be viewed directly via the following link: https://doi.org/10.5194/tc-2022-172-AC2