The samples were prepared at the Geological Survey of Denmark and Greenland using standard palynological methods involving digestion of 20 g of sediment in hydrochloric and hydrofluoric acid, mild oxidation, and filtering of the organic residues over 11-μm mesh filter (60). Two to three strew slides were mounted per sample, using glycerine gel. To assess the percentage of aberrant forms, all normal and aberrant specimens of LTT- and LCT-spores were counted separately within one strew slide from each sample (tables S2 to S5), using a Leica DM 2000 transmitted light microscope fitted with a differential interference contrast system. During counting, the strew slides were scanned systematically using a ×40 lens (506097, Leica) along transverse transects, until the entire slide was scanned or until ≥100 specimens were counted. Only specimens that exhibit clear signs of aberrancy were counted as aberrant. Because folded, broken, or obscured specimens were always counted as normal, the obtained aberrancy values may be an underestimation. In some samples, statistical significance could not be obtained, and the results were disregarded in the plots in Figs. 3 to 5 and figs. S2 and S4. LCT-spores were generally much less abundant than the LTT-spores. The quantitative data are presented in tables S2 to S7.

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