Carbon isotope systematics of leaf wax n-alkanes in a temperate lacustrine depositional environment

The carbon isotope ratio (δ¹³C) of plant-derived organic carbon preserved in geological archives can be a valuable proxy for the relative abundance of terrestrial plants using C3 and C4 photosynthesis. In certain sedimentary archives, however, mixing of terrestrial C3- and aquatic macrophyte-sourced carbon will result in sedimentary organic matter (OM) δ¹³C signatures that could be misinterpreted as shifts in the abundance of C3 and C4 vegetation. There is potential for this problem to be mitigated using leaf wax n-alkane compound-specific δ¹³C measurements because n-alkane production differs between terrestrial vegetation and aquatic macrophytes. This approach requires an increased understanding of how mixing of terrestrial plant and aquatic macrophyte n-alkane inputs to lacustrine sedimentary archives manifests in the δ¹³C values of different n-alkane homologues in diverse environmental settings. This study examines a Pleistocene lacustrine sequence in southeastern Australia in which the inputs from terrestrial and aquatic macrophytes vary naturally through time, enabling the characterization of the mixing dynamics for different n-alkane homologues. Relative contributions of terrestrial vegetation and aquatic macrophytes were estimated using the relative abundance of mid-chain to long-chain n-alkanes and compared to the δ¹³C values of discrete n-alkane homologues. We find that δ¹³C values of mid- and some long-chain n-alkanes (C₂₃–C₂₉) are strongly impacted by mixing between C3 terrestrial- and non-emergent aquatic macrophyte-derived n-alkanes. In contrast, δ¹³C values of very long chain (C₃₁–C₃₅) n-alkanes integrated in sediments are the least affected by isotopic mixing. These results indicate that aquatic macrophyte inputs can significantly influence C₂₉ isotopic signatures and thus the δ¹³C values of the very long chain n-alkanes (≥ C₃₁) will provide the most robust quantification of n-alkane inputs from terrestrial plants and will be most useful for reconstructing the abundance C3 and C4 vegetation from temperate lake sediments.