Timing and Mode of Landscape Response to Glacial-Interglacial Climate Forcing From Fluvial Fill Terrace Sediments: Humahuaca Basin, E Cordillera, NW Argentina

Abstract:

Fluvial fill terraces provide a record of changes in sediment production and/or transport in response to external forcing. The N-S striking Humahuaca intermontane basin (Eastern Cordillera, NW Argentina) parallels the eastern margin of the Puna Plateau and is known for frequent landslides/debris flows during the wet season and protracted past wet periods. Fill terraces along tributaries (with 20-1100 km² catchments) to the trunk stream are dated with OSL to between ~30 and 120 ka. Aggradation phases on the west side of the basin correlate with past wet periods, and those on the east side with dry periods. The difference may arise because the river-network geometry of eastern catchments promotes sediment storage, resulting in delayed sediment delivery to the trunk stream and/or a higher threshold to erosion and sediment transport. Cosmogenic nuclide (¹⁰Be) concentrations of sand (<0.7 cm) and pebbles (1-3 cm) reveal that in modern stream sediments, (1) denudation rates from sand (<0.1 mm/yr) overlap with bedrock erosion rates, (2) denudation rates from pebbles are 1.2 to 4x higher, which could reflect the importance of mass movements, and (3) eastern catchments yield lower rates. From wet-phase (west side) terraces, denudation rates are higher than those from modern streams, while highly scattered pebble denudation rates of 0.1-10 mm/yr may reflect an increased frequency of mass movements in past wetter periods. In contrast, dry-phase (east side) terrace pebble and sand denudation rates overlap with modern rates, ²⁶Al/¹⁰Be ratios are low, and a sample from the sediment storage zone has a relatively high pebble denudation rate of 0.3 mm/yr. We interpret the patterns to imply that mass movements are triggered throughout the valley during wet climate phases and induce aggradation, but slow re-excavation of stored sediments from eastern catchments leads to increased ¹⁰Be concentrations and delayed sediment delivery to the main valley. Such behavior at an orogen scale could attenuate or mask landscape responses to climate forcing.