Abstract 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.

中文翻译：

---

温带湖相沉积环境中叶蜡正构烷烃的碳同位素系统学

摘要 地质档案中保存的植物源有机碳的碳同位素比 (δ13C) 可以作为利用 C3 和 C4 光合作用的陆生植物相对丰度的有价值的代表。然而，在某些沉积档案中，陆地 C3 和水生大型植物来源的碳的混合将导致沉积有机质 (OM) δ13C 特征，这可能被误解为 C3 和 C4 植被丰度的变化。使用叶蜡正烷烃化合物特定的 δ13C 测量有可能缓解这个问题，因为陆地植被和水生大型植物之间的正烷烃产量不同。这种方法需要更多地了解陆生植物和水生大型植物正烷烃输入到湖泊沉积档案的混合如何体现在不同环境设置中不同正烷烃同系物的 δ13C 值中。这项研究检查了澳大利亚东南部的更新世湖相序列，其中来自陆地和水生大型植物的输入随时间自然变化，从而能够表征不同正烷烃同系物的混合动力学。陆地植被和水生大型植物的相对贡献是使用中链到长链正构烷烃的相对丰度估算的，并与离散正构烷烃同系物的 δ13C 值进行比较。我们发现中链和一些长链正构烷烃 (C23-C29) 的 δ13C 值受到 C3 陆生和非水生大型植物衍生正构烷烃之间混合的强烈影响。相比之下，沉积物中集成的超长链 (C31-C35) 正构烷烃的 δ13C 值受同位素混合的影响最小。这些结果表明，大型水生植物输入可以显着影响 C29 同位素特征，因此极长链正构烷烃 (≥ C31) 的 δ13C 值将提供陆生植物输入正构烷烃的最可靠量化，并将最有用从温带湖泊沉积物中重建丰度 C3 和 C4 植被。沉积物中集成的超长链 (C31-C35) 正构烷烃的 δ13C 值受同位素混合的影响最小。这些结果表明，大型水生植物输入可以显着影响 C29 同位素特征，因此极长链正构烷烃 (≥ C31) 的 δ13C 值将提供陆生植物输入正构烷烃的最可靠量化，并将最有用从温带湖泊沉积物中重建丰度 C3 和 C4 植被。沉积物中集成的超长链 (C31-C35) 正构烷烃的 δ13C 值受同位素混合的影响最小。这些结果表明，大型水生植物输入可以显着影响 C29 同位素特征，因此极长链正构烷烃 (≥ C31) 的 δ13C 值将提供陆生植物输入正构烷烃的最可靠量化，并将最有用从温带湖泊沉积物中重建丰度 C3 和 C4 植被。