长江口水下三角洲137Cs最大蓄积峰的分布特征

在长江口水下三角洲地区,用 137Cs进行沉积速率的测定时,一般认为 137Cs最大值所在层位为1963年沉积层。本文通过建立模型,对此进行了计算验证。模型中利用日本东京地区的 137Cs年平均大气沉降通量,再通过东京与上海两地的降水量进行校正,并根据不同的沉积速率和取样间隔分别计算出 137Cs剖面中的最大值范围,再将理论计算值与实测值进行对比。通过模型的计算,发现理论上沉积物中出现 137Cs最大值的层位均为1963年沉积层,将理论最大值范围与本研究所测11根柱样以及前人在本区域所测6根柱样 137Cs最大值的对比,发现其中有9根柱样的最大值在理论计算值范围之内。总体上看,理论值与实测值有较好的对应关系,理论值范围基本可作为衡量实测最大值是否为1963年沉积物所在层位的评判标准之一。本文对利用大气沉降 137Cs信息计算沉积物中 137Cs的分布进行了有益的尝试。

DISTRIBUTION OF THE 137CS MAXIMUM OF THE SUBAQUEOUS DELTA IN THE YANGTZE RIVER ESTUARY

A series of sediment cores,using the gravity coring equipment,were collected along the Latitude 31.0°N from longitude 122.0°E to 122.5°E in April & November 2006 from the subaqueous delta of Yangtze River Estuary.The length of the cores varied from 100cm to 235cm.Following visual inspection for geological parameters and photography of the cores,the cores were cut at 2-cm intervals for the entire length of the cores.Small aliquots of the sub-samples were selected for grain size distribution measurements,using a Master-sizer 2000(Malvern Instruments,UK).The activities of 137 Cs were measured by γ spectrometry.The counting system is equipped with a 40 ％ efficiency HP Ge detector(GMX30P-A,ORTEC,USA)interfaced to a digital γ ray spectrometer(ORTEC DSPEC Plus TM).The sediment samples were sealed in a plastic cup and were counted for 72ksec(about 20 hours).In the subaqueous delta of Yangtze River Estuary,it is generally believed that the position of 137 Cs maximum represents the year 1963 when we determine the deposition rates using 137 Cs dating.In this paper,we validate whether the position of 137 Cs maximum represents the year 1963 in this area by establishing a model.In this model,we used annual 137 Cs deposition at Tokyo and precipitation of Tokyo and Shanghai to carry out the annual 137 Cs deposition in Yangtze River Estuary,and then,we calculated the maximum activity in the 137 Cs profile in the sediment which depended on the deposition rate and the sub-sampling interval. We compared the calculated values with the measured ones. By calculating,we found that the 137 Cs maximum activity represents the year 1963 in the calculated 137 Cs profiles.The 137 Cs maximum decreases with the increasing of the sedimentation rate and the sub-sampling interval. By comparing the 137 Cs maximum in the 17 profiles(11 profiles in this work,and 6 from published data in the subaqueous delta of Yangtze River Estuary)with the calculated values,9 cores gave the results,which showed that the maximum of 137 Cs calculated from the model consistent with that from the measured date.Whereas,it was observed in 6 cores providing with higher measured values than the calculated ones,in these cores the sedimentation rates were high and much additional land-origin source of 137 Cs may be absorbed. We made a meaningful test on calculation the distribution of 137 Cs in sediment in Yangtze River Estuary by using atmospheric deposition of 137 Cs,and provided a meaningful method on judging the reliability of the measured 137 Cs maximum which represented the deposition year of 1963.As for atmospheric deposition is the annual average data,the 137 Cs input from the Yangtze River was not to be considered,which made the model remain to be further improved.