Precipitation and erosion gradients within and across the southern central Andes

The tectonic and climatic evolution of the broken foreland and the intra-orogenic Puna Plateau of the southern central Andes in NW Argentina have resulted in strong contrasts in elevation, rainfall, and surface-process regimes. A pronounced E-W rainfall gradient has existed since the late Miocene. This involves the wet, windward eastern flanks of the orogen (~2-3 m/yr) to progressively drier western leeward basins and ranges (~0.1 m/yr) in the Eastern Cordillera, which borders the Altiplano-Puna Plateau and receives <0.1m/yr rainfall. In this study, we analyze the impact of spatiotemporal climatic gradients on surface erosion: First, we present >40 new basin-averaged erosion rates derived from cosmogenic nuclide inventories to derive spatial erosion patterns. Second, we reevaluate paleoclimatic records from the Calchaquíes Basin (66°W, 26°S), a large intermontane basin bordered by high (> 4.5 km) mountain ranges, to demonstrate temporal variations in erosion rates associated with past climate change. We report on two key observations that emphasize the importance of climatic parameters regarding the efficiency of surface processes in space and time: (1) We identify an intrinsic channel-slope behavior in different climatic compartments of the southern central Andes. Channel slopes in dry areas (< 0.25 m/yr rainfall) are slightly steeper than in wet areas (>0.5 m/yr) with equal drainage areas, thus compensating lower amounts of discharge with steeper slopes. (2) Erosion rates can vary by an order of magnitude between presently dry (~0.05 mm/yr) and wetter (~0.5 mm/yr) Pleistocene conditions within the same area, emphasizing the strong climatic impact on erosion in this threshold environment. Analogous to present-day ENSO forcing associated with intensified rainfall and sediment production in these dry highlands, we suggest that protracted wet conditions in the past caused rainfall to reach areas with steeper channel slopes in the orogen interior, ultimately leading to an intensification of mass transport and transient storage of sediment in lower sectors of the drainage basin. In contrast, drainage basins in the Puna Plateau reacted by oscillating lake levels and sustained internal drainage conditions, thus protecting the orogen interior and the plateau as a geomorphic entity.