δ¹³C analyses of individual lignin phenols in Quaternary lake sediments: A novel proxy for deciphering past terrestrial vegetation changes

This study of the sediments from Sacred Lake, Mount Kenya, demonstrates that δ¹³C analyses of individual lignin phenols can be used to assess past changes in the proportion of C₃ and C₄ plants within different plant groups in the lake catchment. Vegetation changes resulting from climatic and atmospheric changes since the last glacial maximum can be deduced from specific lignin phenols. Lignin, a major biopolymer in vascular plants, differs from n-alkyl lipids in (1) its higher specificity and more quantitative representation of vascular-plant input and (2) its major transport mechanism into the sediments by fluvial runoff, as opposed to the greater importance of eolian transport for leaf waxes and pollen. Lignin δ¹³C data support the hypothesis that atmospheric CO₂ concentration is an important factor controlling the vegetation in tropical mountain ecosystems, and provide additional insights into past changes in vegetation structure. Specifically, the δ¹³C values of p-coumaric acid indicate the persistence of C₄ grasses during the late glacial, when the lake level was lower than today, suggesting that aridity also played a role in controlling the abundance of C₄ plants.