Methodology:

The sediments were recovered with a mega-corer (MC), box corer (BC), kasten corer (KC) or jumbo gravity corer (JGC) on the Amundsen Sea continental shelf in the vicinity of Thwaites Glacier and Pine Island Glacier. Sediment cores were collected during cruises NBP1902 (Jan-Mar 2019) and NBP2002 (Jan-Mar 2020) with RV/IB Nathaniel B. Palmer.

The clay fraction (less than 2 micrometers) was separated from the bulk sediment in settling tubes, and 40 mg clay suspension was mixed with 1 ml of an internal standard (0.04 percent MoS2 suspension). The samples were then mounted as texturally oriented aggregates by rapidly filtering the suspension through a membrane filter (0.20 micrometer pore width). The filter cakes were dried at 50 degrees Celsius, fixed on aluminium tiles and exposed to ethylene glycol vapour at a temperature of 60 degrees Celsius for at least 18 h immediately before the X-ray analyses. The measurements were conducted on a diffractometer system Rigaku New Miniflex with Cobalt K-alpha radiation (30 kV, 15 mA). First, the samples were X-rayed in the range 3-40 degree2theta, with a step size of 0.02 degree2theta and a measuring time of two seconds per step. Then, the range 27.5-30.6 degree2theta was X-rayed with a step size of 0.01 degree 2 theta and a measuring time of four seconds per step in order to better resolve the (002) peak of kaolinite and the (004) peak of chlorite. The X-ray diffractograms were evaluated using the interactive ''MacDiff'' software (Petschick 2010, Petschick et al. 1996). The main clay mineral groups were identified by their basal reflections at 16.5 Angstrom (smectite, after glycolisation), 10 and 5 Angstrom (illite), 14.2, 7, 4.72 and 3.54 Angstrom (chlorite), and 7 and 3.58 Angstrom (kaolinite) after the diffractograms had been adjusted to the MoS2 peak at 6.15 Angstrom. For semi-quantitative evaluations of the clay mineral assemblages, empirically estimated weighting factors on the integrated peak areas of the individual clay mineral reflections were used (Biscaye 1964, 1965).

Data collection:

A diffractometer system, Rigaku New Miniflex with Cobalt-K-alpha radiation (30 kV, 15 mA), was used for the X-raying.

The diffractograms were analysed with the interactive ''MacDiff'' (v. 4.2.5) software (Petschick 2010).

Data quality:

By preparing, analysing and evaluating samples six times, a standard deviation of c. 1 percentage was obtained for the concentrations of smectite, chlorite and kaolinite, and of c. 1.5 percentage for illite.