硅藻组合指示的渤海湾西北岸两个牡蛎礁体生长环境的差异

REVEALING PALEO-ENVIRONMENT DIFFERENCE OF TWO OYSTER REEFS BY DIATOM ASSEMBLAGES IN THE NORTHWEST COAST OF BOHAI BAY

渤海湾西北部沿海平原分布众多牡蛎礁体,通过对岭头和大吴庄牡蛎礁体的详细研究,岭头埋藏牡蛎礁体及礁体上覆泥质沉积物剖面中共发现硅藻18属32种,自下而上划分了3个硅藻带(I—III),显示礁体在向上建礁至被掩埋的过程中受海水作用逐渐减弱。对相距约20km的大吴庄礁体硅藻结果(不含上覆泥层)的再研究,及与岭头硅藻结果(不含上覆泥层)的对比显示:大吴庄礁体硅藻种相对丰富,潮间带河口种和沿岸种含量多,说明礁体生长在潮间带或浅海区靠近河口的位置,建礁过程中受河流与海洋的双重影响,潮间带河口种C.stylorum和C.striata在礁体中垂向含量的波动变化,指示了建礁过程中河流影响强弱的变化。相反,岭头礁体硅藻种相对单一,浅海与浮游种含量大,生长位置可能远离河口或为逐渐封闭的泻湖或海湾环境,建礁过程中主要受海洋影响,浅海种P.sulcata和浮游种T.nitzschioides垂向含量的波动变化反映了礁体受海洋影响强弱的变化。

Among the oyster reefs on the northwest plain of the Bohai Bay, the Lingtou and Dawuzhuang are the two being carefully studied. 32 species in 18 genera diatoms were identified from the Lingtou oyster reef and the overying muddy sediments. From bottom to top, three diatom zones have been identified indi eating that the marine influence had faded out during the oyster reef growing. The re-study of the diatom assemblage of the Dawuzhuang oyster reef, 20 km away form Lingtou, and the comparison of the diatom assemblages between the two reefs indicates that the Dawuzhuang oyster reef contains more diatom species with a big percentage of the intertidal-estuarine and coastal species. It suggests that the reef was then living in an intertidal zone or a shallow sea next to the estuary. It was under the influence of both the river and the sea, and the percentage of C. stylorum and C. striata indicates the change in river influence. On the contrary, the Lingtou oyster reef has fewer diatom species and a big percentage of marine and plankton species. It means that the reef maybe lived a little far away from the estuary or in a closed lagoon or a semiclased bay. It was influenced by sea only, and the percentage changes in P. sulcata and T. nitzschioides indicate the fluctuation of marine influences.