ADVANCES IN MOLECULAR BIOMARKER ANALYSES OF TERRESTRIAL AND MARINE SEDIMENTS FOR RECONSTRUCTING NEOGENE HOMINID ENVIRONMENTS

Geochemical fingerprints in sediments and fossils have made significant contributions over the last 25 years to understanding vegetation, climate, and mammalian diets at Neogene hominid sites. Arguably, the use of carbon isotopes to determine the proportions of C₃ and C₄ vegetation on the landscape and in mammal diets has been the most valuable contribution from geochemistry to the study of hominid ecology and paleoenvironments. This includes recent studies of carbon isotopes of plant waxes from marine and terrestrial sequences. We briefly review some early isotopic studies and assess their impact on our current understanding of climate and human evolution.

We then turn to new methods that have the potential to make contributions to the next 25 years of paleoenvironmental research at hominid sites. We focus on two organic geochemical methods for evaluating vegetation structure and composition. Both methods are independent of isotopes, enabling us to evaluate ecosystem evolution in East African across the Neogene, and specifically, prior to the spread of C₄ grasses beginning at 10 Ma, where less is known about vegetation structure and type.

First, we explore a method for reconstructing woody cover from plant wax abundances, or n‑alkane molecular distributions, in terrestrial and marine sediments. Our preliminary calibration data from modern soils and plants suggest plant functional type (e.g., grasses and woody vegetation) can be distinguished by molecular distributions. We apply this method to n‑alkane distribution data from marine and terrestrial sediments to evaluate changes in woody cover across the Neogene. The second method we explore uses a class of triterpenoid biomarkers, PTMEs, that are produced primarily by Poaceae (grasses), as proxy for grass abundance. We analyze PTME concentrations in marine sediments to reconstruct grass abundance through the Neogene. Our woody cover data track the expansion of C₄ grasslands over the past 10 Myr, but also reveal key periods where woody cover decreased in the middle and early late Miocene. PTME data also support the presence of C₃ grasses prior to the spread of C_{4 ­}grasses. These new approaches, while especially useful in the first part of the Neogene, are also complimentary to isotopic methods for vegetation and hydroclimate reconstructions.