Leaf-wax hydrogen isotope composition (δD wax) is increasingly applied as a proxy for hydroclimate variability in tropical paleoclimate archives, but the factors controlling δD wax in the tropics remain poorly understood. We measured δD wax and the stable carbon isotope composition of leaf-waxes (δ 13C 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 δD wax. In this region the estimated hydrogen isotope composition of meteoric water (δD w) varies by only 25‰, and variability in δD w does not explain the relatively large variance in δD wax (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 (ε 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 ε wax/w that is a consequence of differences in vegetation and microclimates. In angiosperm-dominated environments, plant ecology, inferred from δ 13C wax, provides a secondary control on ε wax/w for n-alkanoic acids (ε n-acid/w). Low δ 13C n-acid values are associated with high ε 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 δ 13C n-alkane and ε n-alkane/w is not observed. These results indicate that changes in either aridity or vegetation can cause large variability in δD wax that is independent of the isotopic composition of precipitation, and these effects should be accounted for in paleoclimate studies.
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We are grateful to Bessie Oliva, Roger Medina Gonzalez, Mary Beth Day and Alice Douglas for assistance with field collections. Thanks to Gerard Olack and Dominic Colosi for assistance with isotope measurements. All isotope measurements were made in the Yale Institute for Biospheric Studies Earth Systems Center for Stable Isotope Studies. Thanks to Brett Tipple and Hagit Affek for helpful discussions. Thanks to Joseph Richards for advice on statistical methods. Dirk Sachse, Julian Sachs, Nemiah Ladd and one anonymous referee provided constructive reviews that greatly improved the manuscript. Dirk Sachse also provided data that was valuable in preparing plots for this manuscript. P.D. gratefully acknowledges the support of a NSF Graduate Research Fellowship, a Geological Society of America Graduate Student Research Grant, a Yale Institute for Biospheric Studies Center for Field Ecology Pilot Grant, a Lewis and Clark Fund for Exploration and Field Research Grant, and the Josef Albers Travelling Fellowship.
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