Landscape response to late Pleistocene climate change along the Puna Plateau margin: Sediment flux and cosmogenic landslide signatures modulated by basin geometry

Abstract:

Along the steep flanks of the southern Central Andes (eastern margin of the Puna Plateau), fluvial fill terraces preserve archives of landscape response to climate change over millennial timescales. These archives record information about past erosion and aggradation rates, erosion processes, and even paleoclimate. In the Humahuaca Basin of NW Argentina, our 29 new optically stimulated luminescence ages of late Pleistocene fill terrace sediments demonstrate that past river aggradation occurred over different intervals on the west and east sides of the valley. On the west side, aggradation coincided with periods of increasing intensity of the South American Monsoon System and the South Atlantic Convergence Zone (increasing precipitation), while on the east side, aggradation coincided with periods of decreasing intensities of both systems (decreasing precipitation) or with more variable conditions. Denudation rates and grain-size dependencies from 70 new cosmogenic ¹⁰Be analyses reveal that landslides were more important during periods of increasing precipitation compared to today. On the west side of the valley, a sudden pulse of sediment led to aggradation near the intersection with the trunk stream. In contrast, on the east side, the pulse of sediment likely blocked the narrow bedrock gorges that characterize those catchments, leading to temporary sediment storage in upstream perched basins; sediment evacuation into the main valley occurred preferentially during periods of decreasing precipitation and fewer landslides. Different levels of fluvial connectivity to the trunk stream for the western and eastern catchments within the Humahuaca Basin produces heterogeneity in the locus of aggradation and the timing of sediment movement through the system. Hence, for larger basins that integrate sub-basins with differing geometries or degrees of connectivity, sedimentary responses to climate forcing are likely to be attenuated.