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

Leaf-wax hydrogen isotope composition (dDwax) is increasingly applied as a proxy for hydroclimate variability in tropical paleoclimate archives, but the factors controlling dDwax in the tropics remain poorly understood. We measured dDwax and the stable carbon isotope composition of leaf-waxes (dCwax), 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 dDwax. In this region the estimated hydrogen isotope composition of meteoric water (dDw) varies by only 25&, and variability in dDw does not explain the relatively large variance in dDwax (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 (ewax/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 ewax/w that is a consequence of differences in vegetation and microclimates. In angiosperm-dominated environments, plant ecology, inferred from dCwax, provides a secondary control on ewax/w for n-alkanoic acids (en-acid/w). Low d Cn-acid values are associated with high en-acid/w values, most likely reflecting differences in biosynthetic hydrogen isotope fractionation between C4 grasses and C3 trees and shrubs. A similar relationship between dCn-alkane and en-alkane/w is not observed. These results indicate that changes in either aridity or vegetation can cause large variability in dDwax that is independent of the isotopic composition of precipitation, and these effects should be accounted for in paleoclimate studies. 2012 Elsevier Ltd. All rights reserved.