Sedimentary Characteristics Of The Mud Depositional Body In The North Of The Yellow Sea

Based on interpretation of high-resolution sub-bottom profiles from the center and the west of the South Yellow Sea, and on magnetic susceptibility, lithology, grain-size, AMS 14C isotope dating, UK37– sea surface temperature analysis of the core YE-2, ZY-2 and ZY-1 from the central South Yellow Sea, Late Quaternary stratigraphic sequence and its spatial distribution of the South Yellow Sea, evolution of the East Asian winter monsoon recorded by environmental sensitive grain-size component, sea surface temperature of the central South Yellow Sea and its paleo-environmental meaning are studied. And the impact of different pre-treatment on grain-size analysis result is also studied using the sediment of the core ZY-2.Late Quaternary strata covered by the survey line in the South Yellow Sea comprise four stratigraphic units: 1) High sea level mud deposition in the Holocene, whose bottom surface is smooth and conformity, and without erosion trace, core YE-2, ZY-2 and ZY-1 show the unit is mainly composed of clayey silt with sand content <1%; 2) Transgressive deposition in the last deglaciation, whose bottom surface is smooth, and with erosion trace, core YE-2 shows the unit is mainly composed of sandy silt; 3) Filling deposition in the last glaciation, whose bottom surface is drastically undulated and eroded, unconformity with underlying Late Pleistocene stratum, core YE-2 shows the unit is mainly composed of silt with drastically fluctuated sand content; 4) Late Pleistocene deposition which is mainly composed of clayey silt shown by core YE-2.Based on sub-bottom and core data, and referred to previous reaches, it is suggested that the so-called mud area in the North Yellow Sea (or mud area in the center of the North Yellow Sea), mud area in the center of the South Yellow Sea, mud area in the east to the Haiyang, and the Shandong Peninsula mud wedge (or Yellow River's subaqueous delta, or subaqueous clinoform off the Shandong Peninsula) is a spatial connected depositional body, which is named"the mud depositional body in the north of the Yellow Sea". It starts from the Miaodao Island, extends eastward along the northern Shandong Peninsula, then turns southward at the eastern tip of the Shandong Peninsula, even reaching to 80 m isobaths. The mud area off the old Yellow River mouth of northern Jiangsu and the mud area in the east of the South Yellow Sea, western Korea Peninsula are not included in the mud depositional body in the north of the Yellow Sea. It is calculated that volume of the mud body is about 700 km3, on the assumption that dry density of the sediment is 1.2 g/cm3, the mass of mud body is about 840×109 t.The mud depositional body in the north of the Yellow Sea is thin at the place where its surface sediment is silty clay, but thick at clayey silt position, the thickest position is where its surface sediment is sand– silt– clay, representing the coarse grain-size position of the mud body is thick, fine grain-size position of the mud body is thin. Thickness and sediment of the mud body are different respectively at the different place of the mud body, because of the difference of marine hydrodynamic environment and sediment supply upon the mud body.Pretreatment method of clayey silt sediment in core ZY-2 from the central South Yellow Sea impacts its grain-size analysis result. The results are much different between pretreating the sediment with acid and hydrogen peroxide and with only hydrogen peroxide. The mean grain-size of the sediment pretreated with acid and hydrogen peroxide is finer than that with only hydrogen peroxide. The result by the pretreatment with hydrochloric acid and hydrogen peroxide is similar as that with acetic acid and hydrogen peroxide, content of <1μm component in the sediment pretreated with hydrochloric acid is little less than that with acetic acid, because of the hydrochloric acid dissolving clay mineral which is mainly enriched in fine component. So that, pretreatment method before grain-size analyzing is determined.Evolution of the East Asia winter monsoon in the last 6 ka which is recorded by 9.3 ~ 120.7μm environmental sensitive grain-size component of the sediment in core ZY-2 comprises five phases: 1) 6000 ~ 4200 aBP, strong East Asia winter monsoon; 2) 4200 ~ 3500 aBP, weak East Asia winter monsoon; 3) 3500 ~ 1800 aBP, steady East Asia winter monsoon; 4) 1800 ~ 800 aBP, weak East Asia winter monsoon; 5) 800 ~ 200 aBP, strengthening East Asia winter monsoon. And there are several abrupt strengthening East Asia winter monsoon events in each phase.As shown as the UK37– SST records of core YE-2, the SST is low in 8.4 ~ 6.9 kaBP, about 14.2℃on average. Continuous increasing of the SST from 6.9 kaBP to 6.5 kaBP indicate intrusion of the Yellow Sea Warm Current, after the intrusion of the Yellow Sea Warm Current, the SST in central South Yellow Sea increases by 2℃on average. This is the first continuous paleo-temperature record in the study area.