Climatic Indicators And Sedimentary Environment Studies Inferred From Transgressive And Regressive Sediments Of Core LZ908,South Bohai Sea

Quaternary glacial and interglacial climate change caused alternating fluctuationsin global sea level, and deposited transgressive and regressive interaction strata incoastal area. Coastal sedimentary environment changes frequently, so its sedimentrecords a wealth of information on these changes. Therefore, exploring the evolutionof the sedimentary environment of coastal areas, not only is important forreconstructing the history of regional environmental change and understandingland-sea interaction, but also can provide a scientific basis for predicting the futureevolution of the environment. LZ908core of this study is located in the Laizhou Bay,Bohai Sea, on the basis of relatively reliable age framework, we study transgression,regression changes of the ancient marine environment by sediment foraminiferacharacteristic in southern Bohai Sea; have principal component analysis about eachparameter change of sediment color reflectance, discuss the main factors affectingtransgressive and regressive sediments color change; analysis sediment grain size,color reflectivity, magnetic susceptibility and pollen indicators, to explore variousindicators environmental implications, cyclical changes and possible influencingfactors; contrast climate change indicated by these indicators with global climatechange or sea level changes, get regional climate variations in Laizhou Bay area.Preliminary conclusions are as follows:1. The foraminifera assemblages from a new borehole LZ908in the southerncoast of Laizhou Bay, Bohai Sea were studied, and then the transgressive strata wereindentified. Combined with accelerator mass spectrometry radiocarbon14C (AMS14C)and optically stimulated luminescence (OSL) ages, the occurrence time of thesetransgressions were re-determined. The result showed that three major large-scaletransgressions occurred separately at the beginning of marine isotopic stage7(MIS7), the last interglacial period (MIS5) and the Holocene. In addition, a small-scaletransgression occurred in the mid-MIS6, and the corresponding transgressive stratumwas deposited. The transgressive deposition of MIS3was also discovered in this study.However, the characteristics of the foraminifera indicated the environment during thisperiod was colder than that in the MIS5.2. By comparing the brightness and calcium carbonate content of LZ908core，found that the change of L*values and calcium carbonate content was very consistent,indicating a strong consistency between the two indicators, so the L*value used asalternative indicator of calcium carbonate content was feasible. Factor analysis on thefirst derivative of the color reflectance factor was conducted by great variance rotationmethod. By comparing factor load curve and the first derivative curve of thecomponent known to sediment, concluded that factor1associated with sedimentorganic content, factor2associated with clay mineral content of the sediments, andfactor3associated with the goethite content of the sediments.3. Through the principal component analysis of the sediment grain size, found<32μm fine fraction content of LZ908core changed similarly or basically consistentwith65o N summer solar radiation and δ18O value of Shennongjia Sambo CaveStalagmite. This showed that in the vast region of China, climate change wassynchronized, and <32μm fine fraction content of LZ908core could serve as a goodindication of the East Asian monsoon intensity index. By combining magneticsusceptibility and pollen data, the results showed that the magnetic susceptibility wasa good climate indicator, high susceptibility indicating warm and humid climate, lowmagnetic susceptibility reflecting dry and cold climate. Compared magneticsusceptibility curves of LZ908core with deep-sea oxygen isotope curve, highmagnetic susceptibility values corresponded to low oxygen isotopes, as interglacialenvironment, low magnetic susceptibility corresponding high oxygen isotope valuesfor the glacial environment. Brightness values and CaCO3content higher layersroughly corresponded to a period of high sea level, which indicating warm and humidenvironment. Conversely, lower brightness value and CaCO3content layers indicatedthat climate is cold and dry during low sea level period. In LZ908core sedimentary sequence, the red value a*mainly reflected oxidizing or deoxidizing state of thedepositional environment. Floodplain environment for the glacial period, lower sealevel and oxidizing condition, high iron oxides and hydroxides increased, sedimentcolor is reddish-brown or brown, a*value is larger. Between shallow or coastalenvironment, interglacial sedimentary environment is deoxidizing environment, highiron oxides and hydroxides turn into low iron oxides and hydroxides, sediment coloris dark gray, taupe or brown-green, a*value is small.4. By analyzing sediment grain size, magnetic susceptibility and red values a*ofLZ908, these indicators demonstrated that the climate in the southern Bohai Searesponded to the orbital forcing at the precession, obliquity and eccentricity bandsrespectively.