Sedimentary Environmental Evolution And Optical Stimulated Luminescence(OSL) Chronology Of Sediments Since MIS 5 In The Modern Yellow River Delta Area

Many marine transgression and regression events had taken place in eastern coastal plain of China and shelf shallow sea area due to climate change since Late Quaternary. The area is influenced by strong land-sea interaction, which is sensitive to climate change and sea level change. The sedimentary strata was composed of marine deposits and terrestrial deposits alternately, which contains abundant climate and environmental information.The Bohai Sea is an inland sea of China that connects to the North Yellow Sea(NYS) via the Bohai strait. Water depth in the Bohai is mostly less than 30 m, and the average depth is only 18 m. The modern Yellow River delta is located in the south of Bohai, and it is an important region for the marine-continental variety cycle studying since which influenced by climate since the Late Pleistocene.Recently, the sedimentary evolution in modern Yellow River delta and its response to the sea level fluctuation has attracted many scholars’ attention. Previous studies have revealed the three large scale transgressions since the Late Pleistocene, that is, the Cangzhou transgression, Xianxian transgression and Huanghua transgression. Their marine sedimentary beds are the third,the second and the first marine beds in ascending order, which are formed during the Marine Isotope Stage 5( MIS 5), Marine Isotope Stage 3( MIS 3) and Marine Isotope Stage 1(MIS 1) respectively. But the time of marine sedimentary beds are measured by paleomagentism and 14 C. There are many debates about the chronostratigraphic division due to the limit of dating technique. According to the 14 C dating method, the samples which are more than 20 ka BP are probably younger. Some scholars dated the marine sedimentary beds by Optically Stimulated Luminescence( OSL) and hold the view that the second marine sedimentary bed was developed during MIS 5.In this paper, three borehole cores YRD-1401, YRD-1402 and YRD-1403 are recovered in the Yellow River Delta area. Based on analyses of sedimentary characters, in conjunction with OSL and AMS 14 C dating, grain-size analysis, and benthic foraminifera identification, the borehole core YRD-1402 can be divided into four depositional units, DU 1 to DU 4 in descending order. Our results show that DU 4 was deposited in a shallow subtidal zone to littoral environment and corresponds to the third marine bed; and that DU 3, DU 2 and DU 1 were deposited in a fluvial environment, coastal marsh to coastal or shallow shelf, and the Yellow River Delta during AD 11~1048, respectively. DU 2 and DU 1 correspond to the first marine bed. The borehole core YRD-1401 can be divided into five depositional units, DU 1 to DU 5 in descending order. DU 5 was deposited in a fluvial environment which formed during MIS 6; DU 4 was deposited in a tidal flat to shallow subtidal zone, fluvial environment and tidal flat to shallow subtidal zone, which formed from MIS 5 to early MIS 3; DU 3, DU 2 and DU 1 were deposited in a fluvial environment, coastal marsh to coastal or shallow shelf and the Yellow River Delta after AD 1855. The borehole core YRD-1403 can be divided into four depositional units, DU 1 to DU 4 in descending order. DU 4 was deposited in a tidal flat to coastal environment, fluvial environment and tidal flat to coastal environment, which formed from middle to late MIS 5 to the middle of MIS 3; DU 3, DU 2 and DU 1 were deposited in a fluvial environment, tidal flat to coastal shallow shelf environment, prodelta deposit during AD 11~1048, ocean reconstruction layer during AD 1048~1855 and Yellow River Delta after AD 1855.The characteristic of OSL chronology shows that all the samples have a standard growth curve. According to the preheating experiments and dose recovery test, we think the SAR protocol combined with SGC is appropriate for De determination in the study area.According to other cores in the Bohai Sea area, we find that the T-2 and T-3 are hard to distinguish. Because of the sea lever change, sediment supply and Neotectonic movement, there are too many differences among the sedimentary thickness, phase sequence and stratigraphic completeness. Our research shows that the upper marine sedimentary beds are formed during MIS 1, MIS 3 and MIS 5 respectively. OSL dating suggests the second marine sedimentary bed was formed at early and middle MIS 3 instead of 24~39 ka in the previous studies.The results show that there are three marine sedimentary beds in the study area since Late Pleistocene. The second marine sedimentary bed which formed during early and middle MIS 3 would have been eroded by the fluvial deposit during the middle MIS 3 to Holocene in some borehole cores.