USING FALLOUT RADIONUCLIDES TO IDENTIFY THE EROSIONAL IMPACT OF LAND USE IN SOUTHWESTERN CHINA

Southwestern China was subjected to major deforestation from 1950 to 1997. In 1998, the Chinese government banned agriculture and logging on steep slopes through the Returning Farmland to Forest Initiative to prevent flooding and downstream erosion. By analyzing the activity of the fallout radionuclides ²¹⁰Pb_ex and ¹³⁷Cs in detrital river sediments from Southwestern China, we are able to quantify the effects of deforestation and subsequent reforestation on short-term erosion rates. We hypothesize that deforestation increased surface erosion rates from the 1950s through the early 1990s, while reforestation since the late 1990s has slowed erosion rates.

Using ²¹⁰Pb_ex and ¹³⁷Cs concentrations in detrital sediment, we are able to qualitatively estimate the depth and timing of erosion in the upstream watershed. ¹³⁷Cs is a tracer for erosion of surface material that was exposed during the time of atmospheric nuclear weapons testing in the 1950s-1960s. Samples that contain ¹³⁷Cs must result from minimal and shallow erosion since at least the 1950s. ²¹⁰Pb_ex is a tracer for current surface material based on its atmospheric development through the ²³⁸U decay series. Therefore, highly eroded watersheds affected by deforestation will have little to no detectable ¹³⁷Cs, while reforested watersheds with slow erosion will have ²¹⁰Pb_ex but may not have detectable ¹³⁷Cs. The separation of sediment into fine and medium grain sizes allows these nuclide concentrations to be measured reproducibly because of documented sediment grain-size-dependencies of surface adhered nuclides.

Preliminary data on medium sand fractions show that about 30% of samples have detectable ²¹⁰Pb_ex while few samples have detectable ¹³⁷Cs, suggesting that erosion rates were high during the time of deforestation but have slowed since. The data we have collected from multiple sites within Southwestern China suggest reforestation likely decreased erosion rates of previously cultivated land.