| Long-chain n-alkanes are extensively utilized terrestrial plant biomarkers that serve as paleoenvironment and paleoclimate proxy records. However, robust proxy interpretations require robust calibrations, and a major goal of this work is to ground-truth the interpretative framework for molecular and isotopic compositions of fossil n-alkanes. To evaluate how n-alkane chain lengths in modern plants relate to plant type, we compiled new and published data to test whether particular n-alkane chain lengths predominate in and represent given plant groups. With the exception of Sphagnum and aquatic plants, we find that chain-length distributions are highly variable within plant groups such that chemotaxonomic distinctions between grasses and woody plants are difficult to make, and that chain length instead may be responding to environmental drivers. To test the influence of temperature on chain-length distributions, we sampled soils and plants in a transect from Minnesota to Texas, spanning >20°C mean annual temperature. We find that n-alkane chain length correlates with temperature, which may explain the increase in average chain length observed globally at the onset of the Paleocene-Eocene Thermal Maximum and may also apply to other ancient climate perturbation events.;To examine the combined influence of diagenesis, landscape, and flora on fossil biomarkers, we assessed paleosol lipids for 36 sites along a ∼4km long transect through a late Cretaceous landscape (Big Cedar Ridge, Wyoming) where the plant community is well-preserved. We correlated leaf wax n-alkanes, hopanes and other aliphatic lipids, and compound-specific carbon and hydrogen isotope ratios with floral cover, soil composition, and topography. Results show excellent lipid preservation and the influence of multiple drivers, including plant community, allochthonous carbon inputs, and preservational biases across the landscape. Lastly, we applied the stable carbon isotope ratios of fossil leaves and seeds as indicators of canopy position using fossils from an early Miocene site (Mush Valley, Ethiopia). Our findings indicate that fossil isotope ratios can be interpreted as representing individual morphotypes rather than as a more homogeneous signal of otherwise organic-rich sediments, and they provide evidence of plant community structure and depositional environment in the reconstruction of an extinct African rainforest. |
