INCORPORATING EVAPORATION HISTORIES INTO PALEOALTIMETRY RECONSTRUCTIONS: AN ANDEAN CASE-STUDY

Stable-isotope paleoaltimetry exploits the observed relationship between elevation and the isotopic composition of precipitation to reconstruct past surface elevations from proxy materials. Proxy materials used in stable-isotope paleoaltimetry, such as pedogenic carbonates, are assumed to form in isotopic equilibrium with soil waters. In turn, soil waters at depth are often assumed to reflect the annual average meteoric water isotopic composition. In arid regions, surface and soil waters experience significant amounts of evaporation that drive isotopic compositions to less negative values. Therefore, a key uncertainty in constraining the evaporative history of major orogens is to understand the degree to which proxy materials are recording the isotopic composition of meteoric water, and therefore, how robustly elevations are recorded.

In many high-elevation regions, such as the Central Andes, isotopic data linking modern precipitation and surface waters are sparse, making it difficult to assess how well surface waters record the isotopic composition of precipitation. To address this, we sampled modern precipitation and surface waters in the Central Andes and show that streams along the eastern flanks of the Altiplano preserve the local isotopic composition of precipitation whereas streams on the Altiplano have experienced significant evaporation, causing surface waters to be isotopically enriched relative to precipitation. Cenozoic proxy data and paleoclimate models both suggest that strong surface evaporation on the Altiplano has been pervasive throughout its history. Using a Monte Carlo method to calculate paleo-meteoric water values, we estimate that evaporative enrichment has likely led to an underestimation in stable-isotope based elevation reconstructions of pre-Miocene Andean elevations by at least 1.1 km. Furthermore, by assuming that past surface waters were exposed to evaporative fractions similar to modern waters, we calculate the likely range of precipitation δ¹⁸O and demonstrate that both slow, protracted uplift and pulsed periods of intense uplift are consistent with proxy data. Combined with sedimentological constraints suggesting that the Andes had reached at least 25% of their modern elevation by 25 Ma, Neogene uplift of the Altiplano is limited to no greater than 2.7 km.