Sedimentation and Erosion in Pui O Salt Marsh of Hong Kong from ~(137)Cs Distribution in Sediment
Sedimentation and Erosion in Pui O Salt Marsh of Hong Kong from ~(137)Cs Distribution in Sediment
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作者：
Liu Zhi-yong
在香港贝澳盐沼的三个地点提取了三个深度为40～60厘米的沉积物岩心，以1～2厘米的间隔分析了137Cs的含量。 137Cs 的浓度通过 γ 光谱测量。 137Cs分析结果表明，三个站沉积物的137Cs剖面均存在可识别的137Cs峰和层位。然而，两个站的 137Cs 分布只有一个峰，而另一个站则观察到两个峰。三个站点的 137Cs 活度最大值分别为 2.94 Bq/kg、4.65 Bq/kg 和 6.15 Bq/kg。这些值可以与香港东部地区沉积物岩心采样的值进行比较。传统上，沉积物堆积速率是通过将沉积物序列中137Cs的首次出现时间与1954年联系起来，并将其最大活动时间与1963年联系起来。但这种方法没有充分利用沉积物柱中137Cs分布所存储的信息，有时很难得到沿海地区正确的沉积速率。为了获得更多的沉积过程信息，建立了模型拟合 137Cs 垂直分布。 137Cs 的测量剖面和计算剖面之间的差异明显，说明模型过于简单化，但模型的合理性仍然存在，并且差异可以解释。根据模型计算，三个站点的沉积速率分别为0.24 cm/a、0.21 cm/a、0.34 cm/a，P2、P3站近年来表层可能发生了一定的侵蚀。侵蚀沉积物厚度分别约为4.8和7厘米。并且在P1站，利用该模型计算的沉降速率与利用其最大活度至1963年的结果相吻合。本研究表明，在沿海地区，当沉积环境稳定时，利用137Cs测年可以得到可靠的结果，而当沉积环境从沉积转变为侵蚀时，利用137Cs最大活度计算沉降速率存在一定的局限性。
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 γ-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.