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. O°N from longitude 122. O°E to 122. 5oE in April & November 2006 from the subaqueous delta of Yangtze River Estuary. The length of the cores varied from lOOcm 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 137Cs 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 137Cs maximum represents the year 1963 when we determine the deposition rates using 137Cs dating. In this paper, we validate whether the position of 137Cs maximum represents the year 1963 in this area by establishing a model. In this model,we used annual 137Cs deposition at Tokyo and precipitation of Tokyo and Shanghai to carry out the annual 137Cs deposition in Yangtze River Estuary, and then, we calculated the maximum activity in the 137Cs profile in thesediment 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 137Cs maximum activity represents the year 1963 in the calculated 137Cs prof'iles. The 137Cs maximum decreases with the increasing of the sedimentation rate and the subsampling interval. By comparing the 137Cs 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 137Cs 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 137Cs may be absorbed.We made a meaningful test on calculation the distribution of 137Cs in sediment in Yangtze River Estuary by using atmospheric deposition of 137Cs,and provided a meaningful method on judging the reliability of the measured 137Csmaximum which represented the deposition year of 1963. As for atmospheric deposition is the annual average data ,the 137Cs input from the Yangtze River was not to be considered,which made the model remain to be further improved.
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