River incision drives hillslope topography and this topography affects the chemical properties of the soil catena. However, the linkages between these two fundamental landscape properties as they evolve in response to basal channel incision, have not yet been quantitatively studied. This project integrates hillslope geomorphology based onsediment budgets with the spatial varioation in the geochemistry of soil. Modeling, field, and laboratory approaches are used to examine two key hypotheses. The first hypothesis states that at a given hillslope position, the rate of soil chemical weathering is limited by the rates of mineral supplies via soil transport and soil production. The second hypothesis states that the rate of soil chemical weathering is determined by the mean age and age distribution of minerals in the soil. Two independent models of hillslope soil mass balance and mineral grain tracking will be developed and combined with toposequence measurements of soil and bedrock elemental chemistry and soil production rates and Airborne Laser Swath Mapping. The study will be conducted at three hillslopes with increasing rates of basal channel incision within a tributary basin to the Feather River in the western Sierra Nevada, California. The outcomes from this study will include the temporal and spatial dynamics of soil chemical weathering rates within hillslopes experiencing different history and rates of basal channel incision rates.

Landscapes are often characterized as repeating units of stream channels and hillslopes. This study will investigate the complex coupling between the formation of stream channels and soils. Both of these processes are relevant to managing land use effectively, maintianing sound agricultural practices, and evaluating how soils and landscapes change in response to a variety of natural and human-induced influences. The processes will be investigated in the field as well as in the laboratory and through the development of a sophisticated numerical model. The project will include science teachers in a local middle school and bring soil-stream interactions plus real-world scientific research into these classrooms.

Project Report

The shape and chemical compositions of land surface is constantly changing. This dynamics is ultimately governed by the interactions between tectonics and hydrological cycles. As tectonical force pushes up landmasses, streams and rivers cut into the lands. The result is the hills and mountains. Scientists hypothesized that mountain building in the past, such as Himalayan uplift, caused global ice ages by sequestering atmospheric carbon dioxide. This suggestion is based on observations that erosion from steep slopes further exposes minerals to interact with meteoric water that is acidic due to dissolved atmospheric carbon dioxide. The consequence is mineral weathering that consumes carbon dioxide. However, we know little how such process unfolds in the landscapes that are becoming steeper due to increasing rate of tectonically derived channel incision. Here we, a team of scientists from soil science and geomorphology, characterized and quantified how chemistry of lands responds to actively down-cutting channels. We conducted a series of field works at Northern Sierra Nevada where granitic plateau has been dissected by the Middle Fork Feather River over the past ~5 million years. The relief between the plateau and the river is about 1 km. Tributary channels, trying to keep up with the incision of the Feather River, become steeper toward the river and have waterfalls in the middle of the tributary basins. Above the waterfalls unfold gentle and rounded hillslopes. In contrast, extremely steep hillslopes and cliffs provide challenges to hikers below the waterfalls. Using a radio-isotope, 10Be, which slowly accumulates in the soils that are surface exposed to cosmic rays, we determined soil erosion rates which were found to be greater below the waterfalls by more than 10 folds. However, such dramatic increase in erosion rate translated to only about two-fold increase in chemical weathering rate, indicating that incision-to-weathering conversion is not as efficient as we previously thought. Though most of scientific focus on chemical weathering in eroding landscape has been directed to soils, we also discovered that most of chemical weathering and its reactions to increasing channel incision occur within the chemically decomposed rock, called saprolite, that underlies the soils. Another surprising finding is that soils in the Feather River maintain their thickness at a constant level over the wide range of erosion rates determined by 10Be. Our contemporary paradigm states that soil thickness declines with greater erosion rate. This concept is challenged by our intensive measurements of soil thickness with different methods. When we examined the extent of soil mixing or disturbance with a tracer called optically stimulated luminescence that measures the time length since quartz particles had been exposed to surface and thus to sunlight, soil disturbance presumably by tree throws is largely invariant over the area. Therefore, we conclude that the magnitude and frequency of soil disturbance by tree throws are similar over the erosion gradient and that this disturbance is capable of disrupting saprolite already weakened by chemical weathering at our sites. During the earlier part of the study, we analyzed detailed topographic data derived from laser scanning of the land surface from airplane. When the 3-D mapping data was merged with sophisticated mathematical models, the results enabled us to scale up what we had found from a tributary basin to the entire (~200km^2) basins. This approach was also used to understand how watersheds with distinct rock types – granite vs. metavolcanic – evolve differently in their reactions to tectonically-provoked channel incision. This was the first quantitative field scale illustration of the role of rocks in landscape evolution. We are working on using these techniques to scale up the findings on chemical weathering and soil production to larger scales. Progresses of the part of the project were shared with 7th and 8th grade students at the Newark Charter School in Delaware. Students listened to the challenges and funs of doing science in the remote field sites and in laboratory settings. Significance of soils in ecological and environmental issues was discussed in their classes. Soil and rock samples and field gears were also provided for student’s hands on experience. Likewise, the outcomes of the study were introduced in the researchers’ undergraduate and graduate courses in soil science and geomorphology. Both doctoral student and postdoctoral researcher trained through the project have secured permanent jobs in Geological Survey and in Academia. In conclusion, this study, which is the first attempt to zoom in geochemical behaviors of landscapes experiencing dynamic channel incision, leads us to an ability to reveal previously-unidentified but crucial components of chemical evolution of the landscapes and to quantify the changing processes and rates of chemical weathering and soil production. These outcomes will bring us closer to ultimate understanding of tectonic-climate coupling.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Paul Cutler
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University of Minnesota Twin Cities
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