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

Leaf-wax hydrogen isotope composition (delta D-wax) is increasingly applied as a proxy for hydroclimate variability in tropical paleoclimate archives, but the factors controlling delta D-wax in the tropics remain poorly understood. We measured delta D-wax and the stable carbon isotope composition of leaf-waxes (delta C-13(wax)), 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 delta D-wax. In this region the estimated hydrogen isotope composition of meteoric water (delta D-w) varies by only 25 parts per thousand, and variability in delta D-w does not explain the relatively large variance in delta D-wax (60 parts per thousand). 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 (epsilon(wax/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 epsilon(wax/w) that is a consequence of differences in vegetation and microclimates. In angiosperm-dominated environments, plant ecology, inferred from delta C-13(wax), provides a secondary control on epsilon(wax/w) for n-alkanoic acids (epsilon(n-acid/w)). Low delta C-13(n-acid) values are associated with high epsilon(n-acid/w) values, most likely reflecting differences in biosynthetic hydrogen isotope fractionation between C-4 grasses and C-3 trees and shrubs. A similar relationship between delta C-13(n-alkane) and epsilon(n-alkane/w) is not observed. These results indicate that changes in either aridity or vegetation can cause large variability in delta D-wax that is independent of the isotopic composition of precipitation, and these effects should be accounted for in paleoclimate studies. (C) 2012 Elsevier Ltd. All rights reserved.