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

Autor(es)

Haggi, C. ; Bertassoli Jr, D. J. ; Akabane, T. K. ; So, R. T. ; Sawakuchi, A. O. ; Chiessi, C. M. ; Mendes, V. R. ; Jaramillo, C. A. ; Feakins, S. J.

• Tipo de documento: Artigo Científico
• Fonte: JOURNAL OF GEOPHYSICAL RESEARCH-BIOGEOSCIENCES; v. 129, n. 4, p. 23-pg., 2024-04-01.

Resumo

The stable carbon isotope composition (delta 13C) of plant components such as plant wax biomarkers is an important tool for reconstructing past vegetation. Plant wax delta 13C 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 delta 13C variability found among C3 plant species. Moreover, studies on plant wax delta 13C in tropical soils and plants have focused on Africa, while structurally different South American savannas, shrublands and rainforests remain understudied. Here, we analyze the delta 13C 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 delta 13C. Rainforests and open vegetation types show distinct values, with rainforests having a narrow range of low delta 13C values (n-C29 alkane: -34.4-0.7+0.9 ${-}34.{4}_{-0.7}<^>{+0.9}$parts per thousand Q2575 $\left({Q}_{25}<^>{75}\right)$; Suess-effect corrected). This allows for the detection of even minor incursions of savanna (delta 13C n-C29 alkane -29.2-2.1+3.7 ${-29.2}_{-2.1}<^>{+3.7}$parts per thousand) into rainforests. While Cerrado savannas and semi-arid Caatinga shrublands grow under distinctly different climates, they can yield indistinct delta 13C 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 multi-homolog isotope analysis can be extended to differentiate African shrublands from savannas. The reconstruction of past vegetation dynamics is key for the understanding of the impact of future climate variability on ecosystems. One of the most widely used tools to reconstruct past vegetation from sediment deposits are plant waxes - comparably stable molecules that form the wax coating of leaves. The ratio of heavier and lighter carbon isotopes preserved in plant waxes can be used to differentiate between rainforest and tropical savanna vegetation. This method has been frequently applied in African vegetation types. Other tropical regions such as South America, which have different vegetation structure, remain understudied. In our study, we characterize the plant wax carbon isotope composition of the major tropical South American vegetation types. One of the complications of the method in both African and South American vegetation types is that (semi-) arid shrublands and savannas show similar plant wax carbon isotope values. To further differentiate between arid shrublands and savannas, we show that the comparison of the carbon isotope values from different plant waxes can be useful both in Africa and South America. n-Alkane and fatty acid carbon isotope compositions were studied to provide proxy endmembers for tropical South American vegetation types Forest vegetation types show a narrow carbon isotope range facilitating the detection of minor savanna incursions into rainforest The offset of the carbon isotope composition of different n-alkane homologs can be used to differentiate savannas and shrublands (AU)

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