Abstract: Analyzing Hillslope asymmetry using erosion rates calculated with radioactive Cesium-137


My name is Korede Olagbegi and I am rising senior here at William & Mary. I am a geology major at the college and I play soccer here as well. I will work on research this summer along side professors James Kaste and Gregory Hancock of the Geology department.

Over the summer and for my senior thesis I will be working with asymmetric hillslopes in the coastal plain, with the College Woods as the main study area. Hillslope asymmetry occurs when opposite facing hillslopes (i.e when the north and south facing or east and west facing hillslopes) have different average slope angles. This indicates an aspect variability in hillslope evolution, that is there may be different processes active or not active that are shaping the landform depending on the direction a hillslope may be facing. 

The major research question to be assessed deals with what is causing asymmetric hillsopes to develop in the coastal plain. Do the erosion rates differ on slopes, meaning one slope would have to be steeper to achieve this equal erosion rate,  or is  that there’s a difference in the effectiveness of active processes on the opposing slopes. In order to tackle this question erosion rates along the hillslopes will be calculated with measurements of Cesium-137. 

Cesium-137 makes a good sediment tracer because there are no natural sources and it s virtually only moved by physical processes. As a result of nuclear testing, measurable amounts of Cesium-137 were deposited in soils beginning in 1954. Assuming a uniform deposition of Cs-137 across the landscape and with knowledge of the base input, comparing the vertical distribution of Cs-137 in sediments with the temporal and spatial distribution of fallout Cs-137 from the atmosphere, sediment transport can be mapped and erosion rates calculated. At sites where soil erosion is occurring, total Cs-137 in the soil profile should be less than the input from the fallout ( and at sediment deposition sites, total Cs-137 should be greater than the input). These calculated erosion rates can be compared  to tell us something about what is actually happening (varying processes) to shape the landscape and  whether slope angle may be an indicator of erosion rates.

Hillslope asymmetry is widespread and these processes I’ll explore apply to regions across the globe. These are processes that overtime shape the landscape we all live on. Understanding the rates at which soil is eroded for instance is extremely important. Soil is an important resources providing us with a myriad of essential services including slowing water down which has implications both agriculturally and with environmental hazards.