Aridity and vegetation composition are important determinants of leaf-wax δD values in southeastern Mexico and Central America

Leaf-wax hydrogen isotope composition (δDwₐₓ) is increasingly applied as a proxy for hydroclimate variability in tropical paleoclimate archives, but the factors controlling δDwₐₓ in the tropics remain poorly understood. We measured δDwₐₓ and the stable carbon isotope composition of leaf-waxes (δ¹³Cwₐₓ), including both n-alkanes and n-alkanoic acids, from modern lake sediments and soils across a marked aridity gradient in southeastern Mexico and northern Central America to investigate the importance of aridity and vegetation composition on δDwₐₓ. In this region the estimated hydrogen isotope composition of meteoric water (δDw) varies by only 25‰, and variability in δDw does not explain the relatively large variance in δDwₐₓ (60‰). Instead, the aridity index, defined as the ratio of mean annual precipitation to mean annual potential evapotranspiration (MAP/PET), explains much of the variability in the hydrogen isotope fractionation between leaf-waxes and meteoric water (εwₐₓ/w). Aridity effects are more evident in lake sediments than in soils, possibly because integration of leaf-waxes across a broad catchment masks small-scale variability in εwₐₓ/w that is a consequence of differences in vegetation and microclimates. In angiosperm-dominated environments, plant ecology, inferred from δ¹³Cwₐₓ, provides a secondary control on εwₐₓ/w for n-alkanoic acids (εₙ ₋ₐcᵢd/w). Low δ¹³Cₙ ₋ₐcᵢd values are associated with high εₙ ₋ₐcᵢd/w values, most likely reflecting differences in biosynthetic hydrogen isotope fractionation between C₄ grasses and C₃ trees and shrubs. A similar relationship between δ¹³Cₙ ₋ₐₗₖₐₙₑ and εₙ ₋ₐₗₖₐₙₑ/w is not observed. These results indicate that changes in either aridity or vegetation can cause large variability in δDwₐₓ that is independent of the isotopic composition of precipitation, and these effects should be accounted for in paleoclimate studies.