Extended Data Table 3 GISP2 bedrock 26Al data
From: Greenland was nearly ice-free for extended periods during the Pleistocene
- Sample and measurement details for the 26Al samples. ‘BLK’ denotes process blanks, that is, 27Al carrier that underwent a chemical procedure identical to that of the respective samples. Natural 27Al was determined from two independent splits taken from the fully digested quartz sample by ICP-OES analysis, calibrated with in-house, gravimetrically prepared 27Al standards cross-checked with commercial standards.
- 1The mean of the corresponding BLK values was subtracted from the sample to give the corrected value.
- 2To better account for memory effects during the AMS measurements, we performed a time-progressive background correction: 26Al/27Al measurements at PRIME laboratory are done in one-minute cycles and we directly subtracted BLK counts (counts per minute) closest to the measurement cycle of the corresponding sample.
- Both background corrections yield very similar results, and we use the data based on the time-progressive background correction. See Methods for additional details. GISP2/3/4 and corresponding BLKs were analysed with AMS standards at PRIME mixed with niobium, while the other samples were measured versus AMS standards mixed with silver. The 26Al measurement of the lowest sample, GISP8/9, given in small italics, was an initial sensitivity test: we added 27Al carrier to the Al fraction after the cation column separation, so that any aluminium loss during the geochemical processing would go undetected; also, the ‘count-rate-based blank correction’ for this measurement was substantial and the measured 26Al/27Al ratio fell into the 10−15 range, which is near the detection limit of the AMS analysis and thus challenging; taken together, the 26Al measurement of GISP8/9 is less reliable and not directly comparable to those of GISP5/6/7 and GISP2/3/4, and we base our discussion on these two upper samples.
