摘要
利用多道α能谱仪，对2005年8～9月在海南岛近海采集的7个沉积岩芯进行了^210Pb的沉积速率测定，探讨了海南岛近海陆架上现代沉积速率的区域性分布特征，结果表明：位于港湾内的B1168站位由于沉积物供应充足，有最高的沉积速率，达2．9cm／a；位于河口海湾附近且受沿岸流影响的B289站位，有很高的沉积速率．可达1．6cm／a，沉积环境较稳定；位于西南海底沙脊区北缘且靠近昌化江河口的B97、B135、B10站位也有较高沉积速率，分别达到1．0、0．89和0．47cm／a，在表层都出现了^210Pb放射性活度倒置的现象，表明所处区域有较强混合作用；处于西南外陆架的C4站位受北部湾环流影响，沉积速率为0．6cm／a；位于东部外陆架的B377站位处于上升流区，沉积速率较低，为0．21cm／a。可见，海南岛近海陆架上的现代沉积速率存在着明显的区域分布：在物质来源丰富的沿岸流作用区和河口区附近，现代沉积速率很高；在陆架环流沉积作用区，现代沉积速率也较高；在水深较大的外陆架上，由于沉积物供应相对匮乏，沉积速率一般较低；在近岸潮流沙脊区，由于水动力很强，无法形成现代细粒沉积。同时，在陆架上，沉积速率有随着水深的增加而降低的趋势。由此可见，海南岛近海海域的沉积速率与该区的物质供应、水动力条件和海底地形等因素有密切关系。
Based on the ^210Pb dating method,sedimentation rates of 7 cores gathered in 2005 summer in Hainan Island offshore were measured. The different distribution model of ^210 Pb reflected different sedimentation environment in different area. The core B1168 in Yangpu Harbor was under the influence of river,wave and tide, with fluctuant sediment granularity. Due to the abundant supply of sediment, the modern sedimentation rate was as high as 2.9 cm/a. The other 6 cores were located on the continental shelf. The core B289 was not far from the Hailing Bay of southeast of Hainan Island,where deposited a lot of sediment carried by rivers and alongshore current. The modern sedimentation rate can reach 1.6 cm/a, and the distribution model of ^210Pb was normal,showing a stable sedimentation environment;B97,13135 and B10 located near the north periphery of the sea bottom sand ridge area in southwest Hainan Island offshore,and were not far from the estuary of Changhuajiang,which is the largest estuary in west Hainan Island. The fine sediment from the big river and the sediment which can not deposit in the sand ridge area can gather in this zone,which fits the hydrodynamic condition,and it is not strange that the modern sedimentation rates of them are 1.0,0.89 and 0.47 cm/a respectively. From the distribution models of 210pb,we found strong mixing in the surface sediment of these 3 cores,and the turbidity data show a high turbidity in this sedimentation area,which indicates the post-transit of sediment;C4 was located on the southwest outer continental shelf of Hainan Island, the sediment carried by rivers in southwest Hainan Island and by near shore seasonal flow was under the influence of the North-Bay circumfluence,the modern sedimentation rate here is 0.6 cm/a;B377 was located on the east outer continental shelf of Hainan Island,with the largest water depth 94 m in the 7 cores,the up flow of seawater here is important to the deposit of suspended matter,also due to low supply of sediment,the modern sedimentation rate is 0.21 cm/a.
The sediment supply and the hydrodynamic condition are vital to the modern sedimentation rate. There are many rivers on Hainan Island transporting terrestrial matter into sea,though some of them are small and short,the gradient of the rivers is high. In the tropic rainy season, the water flow can take a lot of terrestrial matter into sea and the fine grade sediment was able to move a rather long distance and form effective sedimentation on the continental shelf. Except the matter from Hainan Island,some of the fine sediment from the south China continent controlled by the seasonal current and North-Bay circumfluence will transfer surrounding the Hainan Island offshore and often deposits in the area where the hydrodynamic effect is low.
On the continental shelf of Hainan Island, different place has different modern sedimentation rate. The modern sedimentation rate is high in the estuary zone and the alongshore current influenced zone where the sediment supply is abundant. The modern sedimentation rate is also high in the place where there is shelf current. But on the outer shelf where is short of sediment supply,the sedimentation rate is usually low. And even no modern fine sediment can be formed in the strong tidal controlled zone for the very high hydrodynamic condition. On the continental shelf, along with the water depth increase, the sedimentation rates trend to decrease.
There are 2 muddy sedimentation zones due to the existing of alongshore current on the north continental shelf of South China Sea,where will be the focused zones of future ^210Pb research.
出处
《海洋学研究》
北大核心
2008年第3期9-17,共9页
Journal of Marine Sciences
基金
“908”专项资助项目(908-01-CJ19)
