Carbon isotope systematics of leaf wax n-alkanes in a temperate lacustrine depositional environment. (December 2020)

• Record Type: Journal Article
• Authors: Andrae, Jake W.; McInerney, Francesca A.; Sniderman, J.M. Kale
• Is Part Of: Organic geochemistry. Volume 150(2021)
• Publication Date: 2020-12
• Subjects: Leaf wax -- n-Alkane -- Compound-specific -- Isotopes -- Carbon -- Terrestrial -- Aquatic -- Vegetation; Organic geochemistry -- Periodicals; Biogeochemistry -- Periodicals; Géochimie organique -- Périodiques; 553.205
• DOI: 10.1016/j.orggeochem.2020.104121
• Languages: English
• ISSNs: 0146-6380

Abstract

Highlights: Lacustrine sediments incorporate mixed aquatic and terrestrial plant inputs. Mixing influences n -alkane δ 13 C values differently for different homologues. C27 and C29 reflect mixed sources in a temperate palaeo-lake. n -Alkanes ≥ C31 show the lowest degree of mixing in a temperate palaeo-lake. Very long-chain (≥ C31 ) n -alkanes more robustly reflect the photosynthetic pathway. Abstract: The carbon isotope ratio (δ 13 C) 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) δ 13 C 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 δ 13 C 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 δ 13 C 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 δ 13 C values of discrete n -alkane homologues. We find that δ 13 C 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, δ 13 C 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 δ 13 C 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.