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

The hydrogen (delta D-wax) and carbon (delta C-13(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 delta D-wax and delta C-13(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 delta C-13(wax) and delta D-wax. This transect has three advantages: existence of long-term precipitation isotopic composition (delta D-r) records, a single wet season potentially reduces variability due to seasonality, and abandoned Byzantine period (similar to 300-600 AD) agricultural terraces that reduce modern and ancient soil mixing and provide age constraints. We find that soil delta C-13(wax) is constant (0.4 parts per thousand, 1 sigma) across a 500- to 1,300-mm/year rainfall gradient and appears insensitive to rainfall amount, unlike bulk plant delta C-13. The absence of a rainfall effect suggests that delta C-13(wax) may be better suited to reconstructing C-3/C-4 plant ratios than bulk delta C-13. Homologue average soil delta D-wax significantly correlate with delta D-r, and the offset between delta D-r and soil delta D-wax (epsilon(app)) correlates with growing season relative humidity. The seasonality of leaf production accounted for at most similar to 10% of total plant delta D-wax variability. Lastly, soil delta D-wax and delta C-13(wax) variability is reduced by similar to 80% relative to plant delta D-wax and delta C-13(wax) variability. Our results show that soil delta D-wax and delta C-13(wax) faithfully record delta D-r and landscape C-3-C-4 plant contributions and thus support the utility of these proxy data in paleosols and archaeological sites.