Using Multi-Homolog Plant-Wax Carbon Isotope Compositions to Reconstruct Tropical Vegetation Types.

The stable carbon isotope composition (δ¹³C) of plant components such as plant wax biomarkers is an important tool for reconstructing past vegetation. Plant wax δ¹³C is mainly controlled by photosynthetic pathways, allowing for the differentiation of C₄ tropical grasses and C₃ forests. Proxy interpretations are however complicated by additional factors such as aridity, vegetation density, elevation, and the considerable δ¹³C variability found among C₃ plant species. Moreover, studies on plant wax δ¹³C in tropical soils and plants have focused on Africa, while structurally different South American savannas, shrublands and rainforests remain understudied. Here, we analyze the δ¹³C composition of long-chain n-alkanes and fatty acids from tropical South American soils and leaf litter to assess the isotopic variability in each vegetation type and to investigate the influence of climatic features on δ¹³C. Rainforests and open vegetation types show distinct values, with rainforests having a narrow range of low δ¹³C values (n-C₂₉ alkane: -34.4+_-0.7^+0.9 ‰ (Q₂₅⁷⁵); Suess-effect corrected). This allows for the detection of even minor incursions of savanna (δ¹³C n-C₂₉ alkane -29.2_-2.1+^3.7 ‰) into rainforests. While Cerrado savannas and semi-arid Caatinga shrublands grow under distinctly different climates, they can yield indistinct δ¹³C values for most compounds. Cerrado soils and litter show pronounced isotopic spreads between the n-C₃₃ and n-C₂₉ alkanes, while Caatinga shrublands show consistent values across the two homologs, thereby enabling the differentiation of these vegetation types. The same multihomolog isotope analysis can be extended to differentiate African shrublands from savannas. [ABSTRACT FROM AUTHOR]