Mixed Signals From the Stable Isotope Composition of Precipitation and Plant Waxes in the Northern Tropical Andes

We evaluate the efficacy of the stable isotope composition of precipitation and plant waxes as proxies for paleoaltimetry and paleohydrology in the northern tropical Andes. We report monthly hydrogen (delta H-2(p)) and oxygen (delta O-18(p)) isotope values of precipitation for an annual cycle, and hydrogen isotope values of plant waxes (delta H-2(wax)) obtained from modern soils along the eastern and western flanks of the Eastern Cordillera of Colombia. delta H-2(p), delta O-18(p), as well as the unweighted mean delta H-2(wax) values of n-C-29, n-C-31, and n-C(33)n-alkanes in the eastern flank show a dependence on elevation (R-2 = 0.90, 0.82, and 0.65, respectively). In stark contrast, the stable isotope compositions of neither precipitation nor plant waxes from the western flank correlate with elevation (R-2 < 0.23), on top of a negligible (p-value >0.05) correlation between delta H-2(wax) and delta H-2(p). In general, delta H-2(wax) values along the eastern flank of the Eastern Cordillera seem to follow the trend of a simple Rayleigh distillation process that is consistent with studies elsewhere on the eastern side of the Andes in South America. Neither delta H-2(p) nor delta O-18(p), and therefore delta H-2(wax), offer reliable estimates of past elevations in the western flank, due perhaps to water vapor source mixing, evaporation overprint, contrasting plant communities, and/or differences in evapotranspiration. Thus, delta H-2(wax) is only reliable for paleohydrology and paleoaltimetry reconstructions on the eastern flank of the Andes, whereas interpretations based on delta H-2(p) and/or delta O-18(p) west of the highest point of the Eastern Cordillera need to consider mixing of moisture sources in addition to precipitation amount. Plain Language Summary As air rises over mountainous terrain, heavy isotopes of hydrogen and oxygen preferentially enter the condensate and rain out during ascent leaving remaining vapor depleted in the heavy isotopes. Previous studies have shown that stable isotope values of both precipitation and plant waxes correlate with elevation, and therefore, can be used to reconstruct past surface elevations. This correlation with elevation is clear on the eastern flank of the Eastern Cordillera of Colombia in the northern tropical Andes. On the western flank, however, no correlation was found for either precipitation or plant waxes. Complex atmospheric circulation and the topographic configuration of inter-Andean valleys pose a challenge for simple isotope signals. It follows that geologists carrying out paleoaltimetry and paleohydrology studies using stable isotopes should be wary of mixed moisture sources and/or complex evaporation paths and be more confident in paleoelevation and paleoclimate estimates if moisture sources are simple.