A comparison on the accumulation characteristics of 7Be and 137Cs in lake sediments and surface soils in western Yunnan and central Guizhou, China

Abstract

The uplifting of the Himalayas has resulted in the development of a three-tiered landform in Southern China, the Yunnan–Guizhou Plateau is the middle tier of this landform on the eastern slope of the Himalayas. Lake Hongfeng and Lake Baihua in central Guizhou are about 1000 km away from Lake Erhai in western Yunnan. Sediment cores were collected from Lakes Erhai and Lugu in west Yunnan and Lakes Hongfeng and Baihua in central Guizhou, along with 28 surface soil cores from the watersheds of these lakes. The accumulation characteristics of 7xxxxBe and 137yyyyCs show that: (1) 137Cs activities in the soil cores of central Guizhou were higher than that in the western Yunnan. The activities and maximum penetrative depth of 7Be were similar for the two regions. The activity ratio of 7Be/ 137Cs in surface soil reached 100–1000 in western Yunnan, but only 10–100 for central Guizhou region. (2) 7Be inventories in soil cores at most sampling sites of central Guizhou were low, reflecting severe soil erosion. 7Be inventories in the soil cores of Lake Lugu watershed in summer–autumn were higher than that in Lake Hongfeng/Baihua watershed and also higher than that in Lake Erhai watershed. This could be related to high 7Be precipitation in summer and altitude. (3) 7Be inventories in sediment cores of Lake Erhai, Lake Lugu and Lake Baihua were 237±73, 322±19 and 783±44 Bq m −2, respectively. Based on activity values that were corrected to deposition, prior to 1986, 137Cs inventories in these lakes were 519±26, 937±13 and 3704±56 Bq m −2, respectively. (4) Modeling indicated that 7Be and 137Cs accumulations in Lake Hongfeng and Lake Baihua were dominated by watershed erosion, and controlled by the retention factor in Lake Erhai and Lake Lugu, but the 137Cs inventory ratios obtained in the sediment core and from direct atmospheric fallout were similar, and the ratios were smaller for 7Be. (5) 7Be inventories from direct atmospheric fallout in Lake Erhai, Lake Lugu and Lake Baihua were 0.07±0.02, 0.29±0.02 and 0.08±0.01 Bq m −2, respectively. Prior to 1986, 137Cs inventories were 0.11±0.01, 0.22±0.01 and 0.37±0.01 Bq m −2, respectively, showing that 137Cs deposition has a regional difference. This phenomenon may reflect that uplift of the Himalayas has a screening effect on the fallout of globally distributed atmospheric pollutants in western Yunnan, China.