^(137)Cs剖面的沉积信息提取——以香港贝澳湿地为例

Sedimentation and Erosion in Pui O Salt Marsh of Hong Kong from ^(137)Cs Distribution in Sediment

通过建立模型,对采自香港大屿山岛贝澳湿地三个站位的沉积样品的137Cs实测剖面与模型计算剖面的比较分析,得出三个站位的沉积速率分别为0.24cm/a、0.21cm/a、0.34cm/a,除P1站位外,P2、P3站位由于受到潮水的作用,表层沉积物受到侵蚀,侵蚀量分别为4.8cm和7cm。该结果与常用的时标方法所的结果进行了比较,P1站位由上述模型所得的结果与常用的时标方法所得的结果一致。可以看出,在稳定的沉积环境下,常用的确定时标的方法可以较准确地计算出沉积速率,而在堆积与侵蚀变化的沉积环境下,常用的确定时标的方法就存在一定的缺陷。该工作为137Cs剖面的沉积信息提取进行了有益的尝试。但上述模型中,由于采用日本东京地区的137Cs大气沉降通量记录替代香港地区的137Cs大气沉降通量记录,并没有考虑137Cs的扩散效应,所以该模型还有待进一步的改进。

Three sediment cores, 40 - 60 cm in depth, extracted at three locations in Pui O Salt marsh of Hong Kong were analyzed for ^137Cs content at 1 -2 cm intervals. Concentrations of ^137Cs were measured r-spectrometrically. The results of the ^137Cs analyses shown that there were recognizable ^137Cs peak and horizon in the ^137Cs profiles in the sediment of the three stations. However, there was only one peak in the ^137Cs distribution in the two stations, while two peaks were observed in the other station. The maximums of ^137Cs activity in the three stations are 2.94 Bq/kg, 4.65 Bq/kg and 6.15 Bq/kg respectively. These values may be compared with that of sediments cores sampled from eastern zone of Hong Kong. Sediment accumulation rates are traditionally calculated by relating the first occurrence of ^137Cs in the sediment sequence to the year 1954 and its maximum activity to 1963. But this method does not make full use of the information stored in the 137Cs distribution in the sediment column and sometimes it is difficult to get the right sedimentation rate in coastal area. To obtain more sedimentation process information, a model was fitted to the ^137Cs vertical distribution. The difference between the measured and calculated profiles of ^137Cs is distinct, accounting for the simplistic model, but the rationality of the model still exists and the difference can be explained. The sediment rates of the three sites were 0.24 cm/a, 0.21 cm/a, 0.34 cm/a according to the model, and some erosion may take place in the surface layer in recent year in P2 and P3 stations. The eroded sediments were about 4.8 and 7cm respectively. And in P1 station the sedimentation rate calculated using the model is coincidence with the result using its maximum activity to 1963. This study indicate that in the coastal area when the sedimentation environment is stead a reliable result can be got using ^137Cs dating, and when the sedimentation environment was changed from sedimentation to erosion there is some limitations using ^137Cs maximum activity to get the sedimentation rate.