Influence of plant growth form, habitat and season on leaf-wax n-alkane hydrogen-isotopic signatures in equatorial East Africa

Abstract

Leaf-wax n-alkanes are produced by terrestrial plants, and through long-term preservation in sediments their stable hydrogen-isotopic signature (δ²Hwax) provides useful information on past hydrological variation for paleoclimate reconstructions. However, gaps remain in our understanding of the relationships between the isotopic signatures of leaf waxes and the plants’ source water. In this study, we investigated the influence of plant growth form, habitat and season on the distribution patterns and δ²Hwax values of 14 plant species (among which are two grasses, five trees and seven shrubs) sampled during four successive dry and wet seasons in three distinct habitats around Lake Chala in equatorial East Africa. Variation in δ²Hwax was analyzed with linear mixed-effect models and compared with the associated values of xylem water (δ²Hxylem), leaf water (δ²Hleaf) and biosynthetic hydrogen fractionation (εbio). Our results show that plant growth form was the most important driver of modern-day δ²Hwax variability in the study area, and that differences in δ²Hwax among habitats to a large extent reflect how each major growth forms is represented in those habitats. Individual plant species appear to express substantial species-specific isotopic fractionation that cannot be attributed to the tested external factors but rather seem to depend on intrinsic (e.g., plant phenological and biosynthesis-related) factors. For the purpose of calibrating δ²Hwax signatures against vegetation types, it is thus crucial to analyze representative samples of the plant communities present in the study area. Our results further indicate that paleohydrological studies in regions receiving rain from multiple moisture sources must take into account possible seasonal bias in the δ²Hwax signature relative to annual rainfall, due to unequal use of those moisture sources by the plants. Finally, the strong influence of plant growth form on δ²Hwax values argues for δ²Hwax variation in paleo-records being evaluated in conjunction with independent proxy data on changes in vegetation composition. Differences in n-alkane distribution patterns among trees, shrubs and grasses (e.g., average chain length, carbon preference index and C31/(C29+C31) ratio) may provide such proxies, and can be produced from the same leaf-wax n-alkane dataset used to determine δ²Hwax.