摘要
长江每年携带约4.14亿吨泥沙入海,其中约有60%在河口至东经123°以西扩散并沉积下来。因此,对于长江口及邻近海域沉积速率的比较研究有助于了解潮滩发育特征和近海沉积规律。
基于此,本文利用放射性核素对长江口潮滩及邻近海域柱状样进行沉积速率的测定,并根据表层沉积物的分布特征,分析潮滩的泥沙物源。主要结果包括:
(1)在陆源输入和流场的共同作用下,长江口邻近海域不同的沉积环境,具有不同的沉积速率,位于长江口三角洲的E4柱样,其沉积速率为3.8cm/a;位于浙江沿岸的S5-2柱样沉积速率为1.1cm/a;位于东海外陆架区的E6柱样沉积速率为1.2cm/a。
(2)通过不同核素得到不同时间尺度下的崇明东滩的沉积速率:从高潮滩(14#站位)、中潮滩(27#站位)到低潮滩(45#站位),从过剩~(210)pb得到的平均沉积速率分别为4.6±0.3cm/a、1.2±0.1 cm/a和1.0±0.1 cm/a;从过剩~(228)Th得到的平均沉积速率分别为5.8±0.3cm/a、2.5±0.1 cm/a和2.3±0.1 cm/a。
从~7Be得到的崇明东滩短时间尺度的沉积速率,具有明显的季节特征:在高潮滩(14#站位),夏、冬两季柱样的沉积速率分别为10.0 cm/a和2.8 cm/a;在中潮滩(27#站位),夏季柱样得到的沉积速率为8.4cm/a;在低潮滩(45#站位),不能得到短时间尺度的沉积速率。
(3)受植被覆盖和潮汐水动力等影响,崇明东滩表层沉积物呈现出平均粒径春季大,而秋、冬季小的季节性变化规律;空间变化规律则为:从14#、27#到45#站位,表层沉积物的平均粒径依次增大,沉积物核素活度的空间分布呈现逐渐递减的趋势。
(4)崇明东滩表层沉积物中核素比值~7Be/~(210)Pb_(ex)和~(137)Cs/~(210)Pb_(ex),与徐六泾样品中的对应比值变化趋势相近,与大气核素沉降通量相差较远,表明崇明东滩沉积物主要受陆源影响,且从14#、27#到45#站位,受陆源影响逐渐减弱。
The annual sediment supply from the Changjiang River is 4.14×10~9 tons,in which almost 60%was deposited from the Changjiang River mouth area to 123°E area.The research on the sedimentation rate of the Changjiang Estuay and its adjacent area is of great help to realize the progradation of the tidal flat and the characteristic of the deposition in the adjacent area.
The measurements of deposition rate were carried out in Chongming tidal flat and adjacent area by using multi-radioisotopes.Moreover,the geochemical characteristic and the source of surface sediment of the tidal flat were analyzed by multi-radioisotope.The main results are as follows:
(1)Different sedimentation rates revealed different sampling conditions, associated with the input of terrestrial sediment and transportation across the shelf,the sedimentation of E4、S5-2、E6 were 3.7cm/a,1.1cm/a and 1.2cm/a,respectively.
(2)From the high flat to the low flat of the Chongming tidal flat,the sedimentation rates were decreased.The values were obtained as 4.6±0.3cm/a, 1.2±0.1cm/a and 1.0±0.1cm/a by ~(210)Pb_(ex),respectively;5.8±0.3cm/a,2.5±0.1cm/a and 2.3±0.1cm/a by ~(228)Th_(ex),respectively.
The sedimentation rates obtained by ~7Be presented seasonal variations.In high flat,the sedimentation rate of sediment cores(collected in June and November,2007, respectively)were 10.0 cm/a and 2.8 cm/a,respectively.In the middle flat,only one sediment core's(collected in June,2007)sedimentation rate could be obtained,that is 8.4cm/a.But in the low flat,it is hard to estimate the sedimentation rate for both of the two cores by ~7Be.
(3)Under the influence of the vegetation and hydrodynamics,the sediment grain size in tidal wetland shows temporal and spatial varations.Compared to mean size of surface sediement in autumn and winter,the mean size is much smaller in spring. Sediment grain size exhibited a landward decresing trend,so did the activity of surface sediment.
(4)The ratios of ~7Be/~(210)Pb_(ex)and ~(137)Cs/~(210)Pb_(ex)in Chongming tidal flat all exhibited close to those of Xuliujing,which revealed that the sediment of the Chongming tidal flat was mainly controlled by the supply of the Changjiang River. Owing to the hydrodynamics,the influence of terrial materials to tidal flat of Chongming exhibited an offshore decreasing trend from high flat to low flat.
引文
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