Leaf Wax δD and δ¹³C in Soils Record Hydrological and Environmental Information Across a Climatic Gradient in Israel

The hydrogen (δD_wax) and carbon (δ¹³C_wax) isotope compositions of long-chain alkanes derived from plant waxes record hydrological and environmental conditions. However, the integration of plant n-alkanes into the sedimentary cycle, the variability of δD_wax and δ¹³C_wax in soils, and the paleoclimate applicability in paleosols and archaeological sediments are poorly constrained. We sampled plants and soils across a steep climate transect in Israel to understand how plant type and environmental parameters shape δ¹³C_wax and δD_wax. This transect has three advantages: existence of long-term precipitation isotopic composition (δD_r) records, a single wet season potentially reduces variability due to seasonality, and abandoned Byzantine period (~300–600 AD) agricultural terraces that reduce modern and ancient soil mixing and provide age constraints. We find that soil δ¹³C_wax is constant (0.4‰, 1σ) across a 500- to 1,300-mm/year rainfall gradient and appears insensitive to rainfall amount, unlike bulk plant δ¹³C. The absence of a rainfall effect suggests that δ¹³C_wax may be better suited to reconstructing C₃/C₄ plant ratios than bulk δ¹³C. Homologue average soil δD_wax significantly correlate with δD_r, and the offset between δD_r and soil δD_wax (ε_app) correlates with growing season relative humidity. The seasonality of leaf production accounted for at most ~10% of total plant δD_wax variability. Lastly, soil δD_wax and δ¹³C_wax variability is reduced by ~80% relative to plant δD_wax and δ¹³C_wax variability. Our results show that soil δD_wax and δ¹³C_wax faithfully record δD_r and landscape C₃-C₄ plant contributions and thus support the utility of these proxy data in paleosols and archaeological sites.