Convective influences on paleoelevation proxies: Implications for the uplift history of the Central Andean Plateau

Abstract:

The surface uplift history of the Central Andean Plateau remains one of the most important unresolved issues in the tectonics of South America. Several studies have suggested that the region was rapidly uplifted during the Miocene as an isostatic response to delamination of a crustal root, but other studies have disputed these interpretations. A wealth of geological data have been brought to bear on the problem, but stable isotope paleoaltimetry is particularly relevant for understanding the surface uplift history of an orogen because it is based on the relationship between surface elevation and precipitation isotopic composition. To the extent that the altitude effects on isotopic composition can be disentangled from other atmospheric influences on proxy records, the technique can provide unique constraints on the surface elevation history of a region.

Changes in water vapor sources, evaporation, and air parcel trajectories have all been recognized as influencing paleoelevation proxies. Precipitation in the Central Andean Plateau is affected by the South American Monsoon, where atmospheric deep convection can exert a very large influence on water vapor isotopic composition. Recent theoretical advances and observational studies have provided new constraints on these influence, with important implications for isotope-based paleoaltimetry studies.

Two important processes that are active in South America can induce substantial deviations from the traditional Rayleigh paradigm. Subcloud evaporation and vapor recycling during continental convection can lower water vapor dD values by several 10s of per mil below that predicted by Rayleigh distillation, and mixing between moist and dry air parcels can raise water vapor dD by a similar amount above that predicted by Rayleigh distillation. Furthermore, small shifts in the position of the Bolivian high can induce very rapid, large changes in the lateral distribution of precipitation d values. We use isotopic data from the Tropospheric Emission Spectrometer (TES) to show that the magnitude of these changes is comparable to those induced by surface uplift of the Central Andean Plateau, suggesting that the history of monsoonal and mixing influences in the region must be better constrained for successful paleoaltimetry interpretations.