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Reference curves of CH4 vs. δ18Oatm color-coded for age. (A) Reference curve based on NGRIP (121.1–105 ka; 12) and NEEM (128.2–119.9 ka; 6) CH4 and δ18Oatm data. (B) Analyzed sample sections plotted as squares, color-coded for δ18Oice on the reference curve from A.
