The Palae-environment Evolution And Its Background In The Bohai Bay Since The Late 5000aBP

Qualitative and quantitative analysis on the benthic foraminifer fauna are based on 168 surface sediment samples and three short piston cores(A435,B83,B166),which were deposited from the north of Bohai Bay. Considering the distribution of benthic foraminifer,AMS14C dating and Q-mode analysis, which discuss the palae-environment evolution and its background in the Bohai Bay since the late 5000aB.P.Based on the analysis, no planktonic foraminiera is found in the region. The benthic foraminiferal fauna are dominated by euryhaline littoral, shallow water and brackish water species. Narrow salinity and deep water species are rare which can be found at the water depth of 15～20m.Q-mode factor analysis is employed on 162 samples and 40 benthic foraminiferal species and four varimax factors with accounts for 93.247% recognizes as follows: AssemblageⅠ(Ammonia beccarii vars.～Quinqueloculina akneriana rotunda ) represents the combination of coastal and shallow waters in shallow water with low salinity; AssemblageⅡ(Protelphidium tumberculatum) represents the combination of environment of the larger depth and higher salinity; AssemblageⅢ(Ammonia beccarii vars.～Elphidium magellanicum) represents the alongshore shallow sea environment; AssemblageⅣ(Textularia foliacea) represents the water depth over 20m of the eastern of the region.The sediment of core B83 deposited as early as 5000a. Q-mode factor analysis is employed on 99 samples and 23 benthic foraminiferal species and we recognize four varimax factors. Factor 1 is Protelphidium tumberculatum assemblage; factor 2 is Textularia foliacea assemblage; factor 3 is Ammonia beccarii vars. assemblage; factor 4 is Elphidium magellanicum assemblage. The distribution of factors load accords with the character of benthic foraminifer community.The sediment of core A435 deposited as early as 4000a. Q-mode factor analysis is employed on 53 samples and 23 benthic foraminiferal species and we recognize four varimax factors. Factor 1 is Protelphidium tumberculatum assemblage; factor 2 is Quinqueloculina akneriana rotunda assemblage; factor 3 is Elphidium magellanicum assemblage; factor 4 is Ammonia beccarii vars. assemblage. The distribution of factors load accords with the character of benthic foraminifer community. The results indicate the more affect of freshwater to core A435 from Haihe,Jiyunhe,Yellow river than core B83.Thus, based on the benthic foraminiferal fauna character and Q-mode factor analysis, there would be divided into 4 periods as early as 5000a in the north of Bohai Bay:The first period, 5000～4000aB.P. It corresponded to shallow sea of inner shelf environment. During this period, the climate was warm and dry. The sea level was steady.The second period, 4000～2500 aB.P.,It corresponded to alongshore shallow sea of low temperature and low salinity environment. During this time period, the climate fluctuated. As early, the climate was warm, soon afterwards it became colder.The third period, 2500～1000 aB.P. It corresponded to the alongshore shallow sea with cooler water environment. The climate was cool and dry in this time.The forth period, 1000aB.P. to present. It corresponded to the offshore environment. During this time period, the climate fluctuated. At the early stage, the climate became dry. The climate precipitous declined. Nowadays, the climate has changed warmer .