Investigating C4 Grass Contributions to N-alkane Based Paleoclimate Reconstructions

Abstract:

Plant wax n-alkanes are long-chain, saturated hydrocarbons contained within the protective waxy cuticle on leaves. These lipids are pervasive and persistent in soils and sediments and thus are ideal biomarkers of ancient terrestrial organic matter. In ecosystems dominated by C₃ plants, the relationship between the carbon isotopic value of whole leaves and lipids is fairly well documented, but this relationship has not been fully investigated for plants that use C₄ photosynthesis. In both cases, it is unclear if the isotopic relationships are sensitive to environmental conditions, or reflect inherited characteristics. This study used a natural climate gradient on the Kohala peninsula of Hawaii to investigate relationships between climate and the δ¹³C and δ²H values of n-alkanes in C₃ and C₄ plants.

δ¹³C of C₃ leaves and lipids decreased 5 ‰ from the driest to the wettest sites, consistent with published data. Carbon isotope values of C₄ plants showed no relationship to moisture up to 1000 mm mean annual precipitation (MAP). Above this threshold, δ ¹³C values were around 10‰ more depleted, likely due to a combination of canopy effects and C₄ grasses growing in an uncharacteristically wet and cold environment. In C₃ plants, the fractionation between leaf and lipid carbon isotopes did not vary with MAP, which allows estimations of δ¹³C leaf to be made from alkanes preserved in ancient sediments.

Along this transect, C₃ plants produce around twice the quantity of n-alkanes as C₄ grasses. C₄ grasses produce longer carbon chains. As a result, n-alkanes in the geologic record will be biased towards C₃ plants, but the presence of alkanes C₃₃ and C₃₅ indicate the contributions of C₄ grasses_.  In both C₃ and C₄ plants, average chain length increased with mean annual precipitation, but the taxonomic differences in chain length were greater than environmental differences.

Hydrogen isotopes of n-alkanes show no trends with MAP, but do show clear differences between plant functional types and sampled taxa. That there is no strong trend between fractionation and aridity supports the use of δ²H_lipid as a record of environmental waters at the time of lipid synthesis, provided that the relative contributions of each plant functional type can be identified using other proxies.