用 137 Cs计年法确定湖泊沉积物沉积速率研究进展

Abstract

The vertical distribution of ~(137)Cs in sediment profiles is present in accordance with the record of ~(137)Cs fallout variations in the atmosphere. Therefore,~(137)Cs can be used to infer a geochronology for sediment profiles and sediment accumulation rates can be estimated by measuring vertical distribution of ~(137)Cs in sediment profiles. But some factors must be taken into account for dating horizon in sediment profiles more accurate and precise. The time lag between the time of atmospheric deposition of ~(137)Cs and the time of deposition of ~(137)Cs to sediment profiles should be calculated. For some sediment profiles ~(137)Cs concentration per unit of clay should be explained. In order to prove the accurate of ~(137)Cs dating, sediment accumulation rates estimated by ~(137)Cs dating should also be compared with that measured at same site by other methods, such as standard sediment survey method, ~(210)Pb dating, sedimentary lamination, pollen and so on. Then with several depositional horizons marked by ~(137)Cs the depth (cm) or the mass (g/cm~(2)) of sediment for different time periods can be calculated. And it can be made to compare sediment accumulation rate in depth (sediment accumulation rate) and in mass (sediment flux) for different time periods. Because of compaction effect on sediment the sediment flux can well and truly reflect sedimentation rate. Based on the sediment flux and the area of lake bed measured by GIS, the annual gross accumulation of sediment can be calculated and internal load of pollutants in the lake sediment, lake evolution and environment changes of the lake basin can be studied further. Therefore, ~(137)Cs dating has been extensively used for evaluation of sediment accumulation rate of lake, estuary, and ocean. However, there are still some problems under resolve. Firstly, how thickness of sampling should be divided up to meet both the needs of ~(137)Cs dating precision and the propriety of measurement work. Secondly, attention should be paid to the lag time between ~(137)Cs deposition in atmosphere and in sediment relates to the transport time from soil loss in field to lake. Thirdly, it should be also carefully taken into account whether diffusion and infiltration movement of ~(137)Cs in sediment profiles occurred and what their mechanisms are.