Erosional response to climate variability in NW Argentina
Abstract
Changes in climatic or tectonic forcing alter the efficiency and intensity of geomorphic processes and drive punctuated variability in rates of erosion as landscapes adjust to changing boundary conditions. Changes in climatic forcing, however, generally occur more frequently and over significantly shorter timescales than variations in tectonic forcing; often resulting in landscapes that are in a continuous transient state, constantly adjusting and responding to variations in climatic conditions. Therefore, quantifying the erosional response to climate change is fundamental to understanding how landscapes evolve. The intermontane basins on the eastern flanks of the southern central Andes, within the Eastern Cordillera and northern Sierra Pampeanas of NW Argentina, are characterized by repeated episodes of widespread basin aggradation followed by fluvial excavation. Although basin geometry in this region is controlled by tectonic convergence and reactivation of older structures, the cycles of valley aggradation and excavation demarcate the transient response of the landscape to climate variability and the intensity of moisture delivered to this region. Although this cyclical pattern of intermontane basin filling and re-excavation is widespread throughout the Andes, and similar patterns have been observed in many other mountain ranges around the world, the links between climate variability, rates of erosion, and sediment transport or storage within the fluvial system are still not well understood. We develop a long-term erosional history for the eastern flanks of the southern central Andes, and then compare this history with both modern climate patterns and paleo-climate proxies from marine and terrestrial records in order to quantify the influence of past climate change on rates of erosion and sediment transport within this region. Modern erosion rates were derived from cosmogenic radionuclide (CRN) inventories within sediment from active channels, whereas paleo-erosion rates were determined from CRN concentration from sediment stored within the extensive aggradational terraces that have been preserved within the intermontane basins. The age of the aggradational terraces were determined by dating interbedded ash layers, radiocarbon, and OSL measurements collected proximal to the CRN sampling sites. The precise age control on the aggradational terraces allows us to accurately account for the CRN decay within the detrital samples and to temporally correlate the measured paleo-erosion rates with paleo-climate records. The extensive preservation of aggradational terraces within this currently arid region allows us to develop an erosional history that extends from modern rates back to ~800 ka. We argue that periods of intensified precipitation enhance rates of erosion on steep hillslopes and high peaks and cause aggradation of valley fill by overwhelming the transport capacity of the low-gradient fluvial networks within the valley bottoms. Conversely, as precipitation decreases or the highlands adjust to a new climate regime, erosional fluxes decrease and allow trunk streams to incise into the aggraded fill and ultimately re-excavate valley bottoms.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMEP52A..06C
- Keywords:
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- 1105 GEOCHRONOLOGY / Quaternary geochronology;
- 1616 GLOBAL CHANGE / Climate variability;
- 1637 GLOBAL CHANGE / Regional climate change;
- 1815 HYDROLOGY / Erosion