Application of a 137Cs fingerprinting technique for interpreting responses of sediment deposition of a karst depression to deforestation in the Guizhou Plateau, China

Deforestation to reclaim land often triggers severe soil erosion in the Guizhou Karst Plateau. 137Cs dating of the deposited sediments in the karst depression bottom was used to estimate soil losses by surface erosion since deforestation started in 1979 on hillslopes in the Shirenzhai catchment, Puding County, Guizhou Province. The catchment has a drainage area of 0.054 km2. The average 137Cs contents of the top and peak layers in five cores of the depression bottom (with an area of 2652 m2), collected in 2009, were 2.35 and 7.25 Bq kg−1, respectively. The medium depths in the depression (which ranged between 84 cm and 113.5 cm with a mean value of 92.1 cm) showed the presence of sediments deposited in 1979. The total volume and weight of the deposited sediments since 1979 were estimated to be 1965 m3 and 2496 t, respectively. The depression bottom can be treated as a temporary impoundment and its sediment trapping efficiency was estimated to be 0.7. The relevant average soil erosion rate on the hillslopes was 2315 t km−2 yr−1 since 1979. The mean 137Cs inventory of the five cores was 7693 Bq m−2, which was ∼10 times the local reference inventory of 782 Bq m−2. The total 137Cs activity of the sediments in the bottom was 20.4×106 Bq, and the relevant 137Cs inventory loss from the hillslopes was 358 Bq m−2 (since 1954), which accounted for 45.8% of the reference inventory. As soil erosion was not severe before and after the period of deforestation and following cultivation in 1979–1990, the erosion rates on the hillslopes could potentially reach 7000 t km−2 yr−1.