When hillslopes respond to incision triggered by tectonic uplift, there is a competition between chemical and physical processes in shaping the landscape. We are studying a tributary basin of the Middle Folk Feather River (FR) in Sierra Nevada CA, where an incision signal is still propagating throughout the basin. Soils were sampled along 3 hillslope transects: POMD (30% slope at 766m), FTA (56% slope at 673m), and BRC (63% slope at 684m). Given their different elevations, slopes, and proximities to the rejuvenating channel, these hillslopes presumably reflect a wide range of denudation. To capture how the basin’s geophysical and geochemical signals propagate upslope, transects were chosen so that POMD is above the knickpoint, FTA is proximal to the knickpoint, and BRC captures erosion below the knickpoint transition. Surprisingly, the hillslopes--despite their varying rates of denudation--show a constant soil thicknesses along all transects (50cm). Despite this similarity, geochemical differences between the soils do exist, indicating a connection between soil geochemistry and the turnover time of the soils (i.e., soil thickness divided by physical erosion rate). For instance, POMD (with a residence time ~30kyr) visually and chemically had the highest degree of weathering based on soil color (10YR 5/4) and the abundance of pedogenic iron oxides (0.3-0.7%). FTA and BRC, on the other hand, had residence times ~2 & 4kyr, and were less red and less Fe-oxide enriched than the POMD soils (10YR 7/4, ND-0.5%).
Geochemical differences were further shown by Zr enrichments in the fine fractions of POMD and FTA soils. In general, POMD soils show 20% more mass-loss of major elements, such as Fe, Al, Na, and K in the upper horizons. However, it’s important to note that despite this difference, the geochemical profiles of FTA and POMD show strikingly similar levels of element depletion. While we are working on understanding if this similarity is due to weathering rate differences or chemical weathering occurring below the soil-saprolite boundary, the transect chemical differences, combined with grainsize results, indicate that the breakdown of coarse fractions in the regolith (2mm) can be responsible for much of the weathering in the FR basin. This is important because our findings indicate that despite order of magnitude differences in denudation rates and soil residence times, soils with relatively fast residence times can display remarkable similarities to those exhibited by long-residence time soils. These findings are consistent with the view that soils with shorter residence times do occur on steeper hillslopes, however they contrast original expectations that soils would thin and be more physically dominated along hillslopes with steeper gradients. We think, therefore, that another control beside the dynamic chemical and physical processes we report here-- namely tree throw--is a key factor in the soil formation processes in both the older and newly incising areas of FR.
Weinman, B.A., K. Yoo, S.M. Mudd, M.D. Hurst, K. Maher, K. Mayer, C. Andersen. (2010): Integrating Geochemical and Morphologic Evolution of Soil-Covered Hillslopes in a Transient Tributary Basin . Abstract EP23C-04. AGU Fall Meeting, San Francisco, CA, December 13-17.