163-G 1.5/G 1.6

Investigating geologic archives of changes in erosion rates and hillslope processes in response to climate forcing

The principal aim of these two Postdoc projects is to determine how climate shifts and variability influence the dynamics of surface processes and affect landscape evolution over various timescales. Given the demonstrated sensitivity of landscape denudation rates to precipitation along the steep slopes of the eastern margin of the Andean Plateau1, we hypothesize that the generation of thick late Pleistocene sedimentary fill terraces is linked to wetter climate conditions, and possibly also to changes in the variability of rainfall. Modern station observations of weather and sediment transport provide a detailed perspective on how climate variability can affect surface processes, but modern records often span insufficient time periods to characterize aspects of the system that change slowly, yet fundamentally influence the rate at which sediment can be created and mobilized from the Earth’s surface. These aspects include shifts and successional changes in vegetation, the rates at which soil forms and evolves, local sediment storage capacity, and tectonic forcing. By combining data from (1) weather and stream stations, (2) sedimentary archives (lake sediment cores, fluvial fill terraces), and (3) paleoclimate and paleovegetation proxies, we will achieve a multi-temporal perspective on how landscapes respond to climate shifts and changes in climate variability. Parallel efforts to map active structures and their influence on modern river profiles will allow us to identify regions in which recent changes in tectonic forcing is unlikely. These observations will embody a holistic set of empirical observations with which numerical landscape evolution models can be tested and refined. Special attention will be paid to the roles of climate-dependent biological (vegetation) and chemical processes, which are often neglected in considerations of climate-driven landscape dynamics. The first Postdoc project [G 1.5] will focus on comparing modern and paleo-erosion rates derived from cosmogenic nuclides with modern and paleo-climate signals derived from stable isotopes in biomarkers, in each case sampling both modern sediment and sediment from dated fill terraces. The second Postdoc project [G 1.6] will focus on integrating the field data into numerical landscape evolution models, to better understand the effects of different timescales of forcing (e.g., ENSO versus glacial-interglacial forcing) and any changes in surface processes that may explain variations in sediment mobilization and transport (e.g., landslides/debris flows versus soil creep). This work will complement ongoing efforts to understand modern sedimentary dynamics in the intermontane basins of the Argentine Andes, where aggradation of modern rivers has been tentatively been linked to a shift in the amount and variability of rainfall recorded at stream gauge and weather stations in the same areas where the extensive late Pleistocene fill terraces occur. Furthermore, comparisons between the fill terraces and what appears to be a modern analog in the form of an active, 20 km2 alluvial fan provides opportunities to relate processes occurring in the contributing area of the fan (e.g., frequent landslides and debris flows) to those that likely occurred during the late Pleistocene aggradational periods.

Strecker and Bookhagen, 2012