Characteristics Of Diatom Distributions Of The Surfacial Sediments In Tianjin Shallow Sea Area And The Implications For Holocene Geoenvironmental Changes

Taking the intertidal zone, shallow sea and the contiguous coastal lowland, Tianjin section of west Bohai Bay, as the studying area, two major works have been done in this thesis. Firstly, diatom identification, statistics and analysis of 95 surfacial sediment smaples have been systematically carried out. Diatom assemblages, dominant species, their distribution characteristics and relationship with the environmental factors were found out. Transforamtion between dominant species, assemblages and environmental factors were established. Sceondly, based on the abovementioned results, 2 boreholes, drilled in the shallow sea, and 2 oyster reefs, burried in the coastal lowland, have been selected for the case studied in order to reveal the palaeo water depth, salinity and marine influences by using diatom analysis in this thesis. As a result, marine influence curve and diatom proxy were also preliminarily established in the study area.The dominant species in surfacial sediments of the study area consist of Cyclotella striatalstylorum, Paralia sulcata, Actinoptychus undulatus, Coscinodiscus argus, Coscinodiscus perforatus, Coscinodiscus radiatus, Coscinodiscus decrescens, Coscinodiscus subconcavus, Surirella armoricana, Amphora ovalis, Thalassiosira excentrica, Achnanthes delicatula, Nitzschia lanceolata, Thalassionema nitzschioides and so on. The common species are composed of Coscinodiscus oculatus, Diploneis bombus, Deploneis smithii, Grammatophora oceanica, Tryblioptychus cocconeiformis, Actinocyclus ehrenbergii and so on.By using the cluster-analysis for the dominant species, the diatom assemblages of the surfacial sediments have beeen separated into 7 groups (from A to G), which correspond to the various subenvironments from the intertidal zone to shallow sea, respectively. Obviously, the distribution of each group almost parallels to the morden shoreline expect the Group E, which protrudes seawards, influenced by the river discharge.Diatom assemblages and 7 selected non-planktonic dominant species all indicate that water depth is a major factor controlling the diatom vertical distribution. Quantitative relationship of the vetical distributions of different diatom species, assemblages with geoenvironments and water depth were thoroughly discussed. The results indicate that Coscinodiscus decrescens, C. perforatus, C. subconcavus and assemblagesâ… a,â… b,â…¡a are much more sensitively for water depth change.The relationship between the dominant species, assemblages and salinity indicate that salinity is another factor controlling the diatom distribution of the surfacial sediments. Appropriate range of salinity, in which different species and their assemblages lived were also scrutinized.Also, the other environmental factors influencing diatom composition have been studied in this thesis. Comprehansive analyses on different species and assemblages indicate that, except Amphora ovalis and Achnanthes delicatula, grain sizes of the surfacial sediments in the intertidal zone and shallow sea, with water depth <-10m, do not give obvious influences on the diatom distribution.As the second portion of this thesis,4 case-studies, located on the shallow water, northwest Bohai Bay, and the contiguous coastal lowland, were carried out for their diatom compositions in the Holocene strata in order to reconstruct the local palaeoenvironmental changes during the Holocene period.Marine influence curve, established by diatom proxy, indicate that 8 periods of enhanced marine influence were recorded since 7000 cal BP in the study area. And the peak of each diatom-determined period almost corresponds to those warm periods, estimated from the Bond et al. (1997) and Hong et al. (2009) and many others, indicating the peaks of the marine influence curve usually reveal warm climate in the study area. Futhermore, comparision between the diatom-based marine influence curve and the cheniers show that the onset of cheniers was usually synchronous with the increase of marine influence and then gradually ended when the warm period and marine influence diminished. Marine influence fluctuation which was forced by climate changes, controlled shoreline change and cheneir formation in the study area.