Sedimentation and Erosion in Pui O Salt Marsh of Hong Kong from 137Cs Distribution in Sediment〖HT5SS〗

Key words: 
• 137Cs /
• Pui O Salt marsh /
• model /
• sedimentation rate

Abstract: 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. 