The modern sedimentation rate and the distribution character of 7 cores in Hainan Island offshore

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 210Pb 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,B135 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.