Tectonics, climate, and landscape evolution of the southern-central Andes revealed by leaf wax stable isotopes

Abstract:

Orogenic plateaus and their flanking ranges strongly impact regional and global hydrology, vegetation, and erosion patterns. In the last decade, reconstructions of the topographic evolution of plateaus and mountain belts have increasingly relied on leaf wax hydrogen isotope data (δD_wax), a paleo-hydrology proxy obtained from organic material in sedimentary rocks. Today, the isotopic composition of precipitation and leaf waxes is influenced by orography, and sedimentary strata deposited during topographic growth in the geologic past may record this relationship. We use a multi-isotope-proxy approach with δD_wax and δ¹³C_wax from the 4-km-high Andean Plateau (Pastos Grandes Basin - PGB, 24°38' S, 66°40' W) and a 2-km-high intermontane basin (Angastaco Basin, 25°41' S, 66°04' W) located farther east in the E Cordillera to separate potential influences and to decipher spatiotemporal patterns of (eco) hydrological changes during Tertiary topographic growth.

The PGB hosts folded sediments that span 9 to 2 Ma, and it has been an integral part of the Andean Plateau since at least 15 Ma. δD_wax values in the PGB sediments record a decrease from -125 to -203 ‰ between 8.5 and 7.6 Ma, which coincides with the disappearance of red beds and evaporites. A similar, but less pronounced D-depletion from -107 to -144 ‰ is recorded by the Angastaco Basin sediments over the same time interval. We suggest that this regional signal may reflect the onset of moisture transport into this inherently dry region via the South American Low-Level Jet. Concurrently, δ¹³C_wax values on the plateau increased from -32.5 to -27.5 ‰ and reflect (eco) hydrological changes, while no such changes are observed in the Angastaco Basin. Subsequent folding of PGB strata documents continuous intrabasinal deformation until 2 Ma, with potential elevation gain within the plateau. δD_wax values from the plateau increase from -218 to -152 ‰ over this time interval. Our observations suggest enhanced aridification in the plateau realm, whereas the Angastaco Basin and the eastern flank of the Andean Plateau transitioned into humid conditions, and subsequently back to semi-arid conditions and reduced moisture transport toward the orogen interior due to orographic barrier formation starting from ca. 6.5 Ma.