Cosmogenic 10Be-26Al analysis

About 22–30 g of each clean quartz sample was spiked with ~250–300 μg of ⁹Be carrier and dissolved in concentrated HF. Be and Al were extracted by ion exchange chromatography including separation of Fe in an anion column, isolation of Al in an HCl-based cation column and separation of Be in a second cation column set-up using oxalic acid to remove elements other than Be and HNO₃ for Be elution. Be and Al were precipitated as hydroxides and transformed into oxides at 1000 °C and 950 °C, respectively. Measurements of the ¹⁰Be/⁹Be and ²⁶Al/²⁷Al ratios were performed at the ETH Zürich TANDY accelerator mass spectrometer. Measured ratios were normalized to ETH S2007N standard for Be and ZAL94N for Al⁶⁹. A long-term process blank with a ¹⁰Be/⁹Be ratio of (3.5 ± 0.3) × 10⁻¹⁵ (±1σ) was subtracted from the measured ¹⁰Be/⁹Be ratio. The total Al concentration was determined by inductively coupled plasma quadrupole mass spectrometry measurements at ETH Zürich on sample aliquots taken immediately after quartz dissolution. All results are summarized in the Supplementary Table ¹.