Using Multi-Homologue Plant-Wax Carbon Isotope Signatures to Reconstruct
Tropical Vegetation Types
Abstract
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 forests 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 n-alkane:
-34.5 +0.9/-0.6 ‰ ; Suess-effect corrected) allowing for the detection
of even minor incursions of savanna into rainforests (13C-enriched).
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 homologues, thereby enabling the
differentiation of these vegetation types. The same multi-homologue
isotope analysis can be extended to differentiate African shrublands
from savannas.