African C3 grass n-alkane distributions & plant chemotaxonomy

Reconstructing past vegetation can elucidate the timing, climate forcings, and biotic mechanisms of ecosystem change. Plant macro- and microfossils are traditionally used to study past vegetation but suffer from production and taphonomic biases, such as underrepresentation of important herbaceous vegetation components. Geochemical proxies can fill this gap, but carbon isotopes (δ¹³C) in isolation are unable to distinguish between structurally different C₃ habitats, such as forests and grasslands. Thus, new geochemical methods to identify grassy C₃ ecosystems are necessary. We present n-alkane chain length distributions of 209 plant specimens from two Kenyan C₃-dominated ecosystems, representing a wide range of plant functional types (PFTs). We find that C₃ PACMAD grasses produce exceptionally high abundances of long chain C₃₃ and C₃₅ n-alkanes (ACL = 32.7, mean C₃₃ + C₃₅ relative abundance = 0.69), unlike other C₃ PFTs which produce low abundances of C₃₃ and C₃₅ (ACL = 28.9–30.3, mean C₃₃ + C₃₅ relative abundance = 0.0–0.21). This finding highlights the importance of measuring and reporting the C₃₅ n-alkane. Our data further demonstrate that n-alkane distributions can serve as a proxy for some African C₃ PACMAD grasses, offering a new paleoecological tool for distinguishing C₃ vegetation types.