Tropical Leaf Wax n-Alkane and n-Alkanoic Acid Reflect δD of Precipitation During Early Stages of Leaf Growth: Insights From an Isotope Labeling Experiment

The hydrogen isotopic compositions of leaf-wax n-alkanes (delta D-alk) and n-alkanoic acids (delta D-acid) reflect ambient climatic conditions (including precipitation delta D-values, delta D-precip). However, the understanding of climatic conditions of exactly which period (i.e., early or entire period of the leaf's lifespan) these biomarkers preserve is still evolving. Previous studies on the delta D-alk and delta D(acid )values, done only in extra-tropical regions, mostly indicate that delta D-alk values are biased toward the early growing season whereas delta D-acid values are not biased toward any season. To decipher the seasonal bias in delta D-alk and delta D-acid records from tropics, we conducted a long-duration experiment wherein deciduous and evergreen species were grown using normal water (delta D = -2 parts per thousand) during early stages of the leaf growth and later using isotopically labeled water (delta D = 1,000 parts per thousand). Our experiment revealed (a) in deciduous and evergreen species, delta D-alk and delta D-acid values reflect delta D-precip during early stages of the leaf growth, (b) synchronous synthesis of n-alkanes and n-alkanoic acids, and (c) in deciduous species, minor incorporation of the previous year's photosynthates in leaf wax pool of the current year's mature leaves. Our study suggests that delta D(alk )and delta D-acid records in the tropics are biased toward the climatic conditions prevailing during early stages of leaf growth. This bias should be considered while comparing the delta Dprecip values generated from leaf wax proxy records and isotope-enabled atmospheric circulation models.