137Cs剖面的沉积信息提取——以香港贝澳湿地为例

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137Cs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0.24 cm/a、0.21 cm/a、0.34 cm/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4.8 cm和7 cm。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137Cs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137Cs大气沉降通量记录替代香港地区的137Cs大气沉降通量记录，并没有考虑 137Cs的扩散效应，所以该模型还有待进一步的改进。

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

Three sediment cores, 40～60 cm in depth, extracted at three locations in Pui O Salt marsh of Hong Kong were analyzed for 137Cs content at 1～2 cm intervals. Concentrations of〖KG1〗137Cs were measured γ spectrometrically. The results of the 137Cs analyses shown that there were recognizable 137Cs peak and horizon in the 137Cs profiles in the sediment of the three stations. However, there was only one peak in the 137Cs distribution in the two stations, while two peaks were observed in the other station. The maximums of 137Cs 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 137Cs 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 137Cs 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 137Cs vertical distribution. The difference between the measured and calculated profiles of 137Cs 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 137Cs dating, and when the sedimentation environment was changed from sedimentation to erosion there is some limitations using 137Cs maximum activity to get the sedimentation rate. 