Paper No. 130-8
Presentation Time: 9:50 AM
STABLE ISOTOPES OF PLANT WAXES IN THE NORTHERN TROPICAL ANDES: A PROXY FOR PALEOLATIMETRY STUDIES IN COLOMBIA?
Hydrogen isotopes of plant waxes (δ2Hw) reflect the isotopic signature of precipitation (δ2Hp and δ18Op), which in turn is affected by altitudinal gradients, and thus, when preserved in sedimentary archives, δ2Hw 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 δ2Hw has numerous dependencies that can obscure the δ2Hp and δ18Op 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 δ2Hw for studies of paleoaltimetry in the Andes. Decreases of mean annual δ2Hp and δ18Op 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 δ2Hp and δ18Op, respectively. For the western flank, however, this correlation is absent (R2 = 0.11 and 0.04 for δ2Hp and δ18Op, 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 δ2Hw 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, δ2Hw values show no significant correlation with elevation (R2 < 0.25, p-value > 0.2). We conclude that the δ2Hw 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.