Sedimentary Records Since Last Deglaciation And The Formation Of Modern Sedimentary Pattern In Eastern Beibu Gulf

To understand the distribution, sediment source, sediment transport path,deposition rate, and the formation mechanism of mud sediment in eastern Beibu Gulf,1522surface samples were analyzed in grain size and421surface samples wereanalyzed in elementary geochemistry, and210Pb profiles of16cores were discussed.In order to reveal the general change of sedimentary environment of eastern BeibuGulf since Last Deglaciation, and to understand the influence of circulation tosedimentation in Beibu Gulf, granularity, calcium carbonate content,C/N and rareearth elements parameters of15cores were analyzed, while11cores were dated byAMS14C.The results show that there are3discontinuous sandy areas from north to southin eastern Beibu Gulf: the northeastern corner of the gulf (Ⅰ1), Qiongzhou Strait (Ⅰ2),and southwestern Hainan Island near shore (Ⅰ3). But the muddy area is continuouslydistributed off sandy area with falcate shape, from north to south, there are: outerQinzhou Bay (Ⅱ1), west Leizhou Peninsula offshore (Ⅱ2, with2parts, Ⅱ2-1andⅡ2-2), west Hainan Island offshore (Ⅱ3) and southwest Hainan Island offshore (Ⅱ4).This mud belt lay on late Pleistocene layer and generally with thickness less than2meters and was mainly deposited after last transgression in Holocene. Relict sedimentappeared around Bailongwei Island in central gulf and was mixed with but not totallyoverlapped by recent sediment.Chemical weathering index (CIA), Σ LREE/Σ HREE,(La/Yb)N, δEu and δCecan be used in distinguishing different sediment sources. The sediment ofⅠ1and Ⅰ3both came from coastal erosion, while the sediment source of Ⅰ2was controlled bybottom erosion in Qiongzhou Strait and sediment of Ⅲ mainly came from paleo-RedRiver delta, but generally reformed by modern sediment. The sediment source ofmuddy area is complex and the contribution of near source and remote source differsin area. The main source of Ⅱ1should be the Guangxi coastal rivers. Differently,remote source controlled Ⅱ2, mostly contributed by the erosion products fromQiongzhou Strait sediment, while more complex sources existed in Ⅱ2-1. Ⅱ3andⅡ4were hardly influence by Pearl River or Red River, and mainly controlled by localHainan Island source material.In the eastern part of the gulf, the relatively high sedimentation rates distributedin the area near Qinzhou Bay with water depth less than20m and the inner continentalshelf west to Weizhou Island-west Hainan Island with water depth between20m and 50m. It seems that there is a sediment conveyor belt driven by the anti-clockcircumfluence on the east side of Beibu Gulf which transports fine sediment south tonorth, and the sediment mainly come from Hainan Island, Qiongzhou Starit andbroader water outside the gulf. The converging of north going current with westentering water from Qiongzhou Starit lead to unloading of suspended material andformed the deposition center of fine sediment near west mouth of Qiongzhou Strait.The movement of rest sediment divided into two parts, one pointed to northwest andthe other turned to northeast and deposited off west Leizhou Paninsula. It seems thatthe west border of the sediment conveyor belt at south of Qiongzhou Strait is near50m isobaths while at north of Qiongzhou Strait is near109°E. In winter, this borderprobably moves westward a little.Along with the sea level rose during Last Deglaciation, the sedimentaryenvironment of Beibu Gulf changed from oceanic to terrestrial, and the calciumcarbonate content of sediment increased by degrees but C/N decreased by degrees.Thetypical pattern of calcium carbonate content and C/N curve can be used in comparisonand division of strata. The rare earth elements records also had response to sea levelvavriation and witnessed the construction of modern subaquatic delta.The transport and aggradation of sediment were controlled by the circulation inBeibu Gulf which probably formed at about7ka BP, and the composition of sedimentstayed stable since middle Holocene. The distribution of depositional thickness sincemiddle Holocene was homologous with modern sedimentation rates, indicating thatthe transport pattern expirenced little change since the formation of the circulation inBeibu Gulf in middle Holocene.