STABLE ISOTOPES OF PLANT WAXES IN THE NORTHERN TROPICAL ANDES: A PROXY FOR PALEOLATIMETRY STUDIES IN COLOMBIA?

Hydrogen isotopes of plant waxes (δ²H_w) reflect the isotopic signature of precipitation (δ²H_p and δ¹⁸O_p), which in turn is affected by altitudinal gradients, and thus, when preserved in sedimentary archives, δ²H_w can be used to reconstruct past elevation estimates. The applicability of this proxy in the geologic record is anchored in modern studies, which have revealed that δ²H_w has numerous dependencies that can obscure the δ²H_p and δ¹⁸O_p signal from elevation alone. We present modern calibrations of stable isotopes both in rainfall and in plant waxes from soils at depths of 5-10 cm at 16 sites along altitudinal transects on both the eastern and western flanks of the Eastern Cordillera of Colombia. We aim to disentangle the interrelationships among elevation, rainfall amount, sources of vapor, and other factors on δ²H_w for studies of paleoaltimetry in the Andes. Decreases of mean annual δ²H_p and δ¹⁸O_p values with elevation for the eastern flank follow a predicted trajectory of a Rayleigh distillation process in the region, and regressions yield linear fits with R² of 0.94 and 0.86 for δ²H_p and δ¹⁸O_p, respectively. For the western flank, however, this correlation is absent (R² = 0.11 and 0.04 for δ²H_p and δ¹⁸O_p, respectively), likely because different sources of moisture provide the vapor that precipitates during different seasons of the year, and that introduces complexity that obscures near-linear Rayleigh distillation. Values of δ²H_w from C₂₉, C₃₁, and C₃₃ n-alkanes on the eastern flank decrease with elevation by -11.3, -9.0, and -7.7 ‰ km^-1, with R² = 0.64, 0.53, and 0.54, respectively. For the western flank, however, δ²H_w values show no significant correlation with elevation (R² < 0.25, p-value > 0.2). We conclude that the δ²H_w can be used as a reliable proxy for paleoaltimetry studies only on the eastern side of the Eastern Cordillera, where the water vapor source from the Atlantic and the Amazon Basin encounters its first topographic barrier. Our results caution against the application of this proxy on the western side of the Eastern Cordillera of Colombia.