渤海湾西北岸埋藏牡蛎礁礁顶上下沉积物中硅藻对“礁泥转换”古沉积环境的重建

渤海湾西北岸大吴庄牡蛎礁是近年来研究程度最高的礁体，天津空港物流中心牡蛎礁则是近年来新发现的礁体。该两处礁体上部的牡蛎个体内部充填的泥、个体之间的泥以及礁体上覆沉积泥层中的硅藻研究表明，礁顶上下（即礁体的上部-顶部和上覆沉积泥层的下部）均富含潮间带环境指示种的Auliscus caelatus，表明礁体发育后期和上覆沉积泥层的初期，均为潮间带环境。尽管如此，从礁顶向上覆沉积泥层转换方式在两处礁体却不相同:（1）大吴庄:Auliscus caelatus种向上逐渐减少，暗示着大吴庄剖面由礁体上部的潮间带中上部、直至上覆沉积泥层下部的潮间带上部、进一步向上覆沉积泥层上部的潮上带转化的趋势。（2）空港:牡蛎礁上覆沉积泥层出现了比例达6%~23%的河口咸水-半咸水种Achnanches brevipes，而下伏礁体内则未见该种，暗示此处泥层受河口作用显著影响。此种不同揭示了礁体停止发育的原因有所不同:在5 520cal BP时，大吴庄礁顶进入潮间带中上部，礁体经常脱离海水而发育停止；在约4 000 cal BP时，空港礁体所在位置因河流作用突然增强，在时间上看很可能是黄河改道在天津入海，古黄河携带大量泥砂入海，掩盖了潮间带的牡蛎礁而导致牡蛎礁发育停止。

Diatom Analysis in the Top and Bottom Muddy Sediments of Buried Oyster Reef for the Reef Mud Conversion Palaeoenvironmental Reconstruction,Northwest Bohai Bay

Dawuzhuang oyster reef is the deeply studied one in the recent years while Konggang oyster reef is a new one on the northwest coast of Bohai Bay.Diatom analysis of the muddy sediments filling into the oyster shells,between the shells and in the overlying muddy layers all contained the Auliscus caelatus which indicated that the late development of the reef and the early stage of the overlying muddy layers are all belonging to the intertidal zone environment.However,the reef-mud conversion was not the same in these two reefs.(1) The Auliscus caelatus in Dawuzhuang oyster reef decreased gradually from the top of the reef to the overlying muddy layers,which indicated an environment change from the middle to upper parts of intertidal zone,upper parts of intertidal zone to supratidal zone.(2) The Achnanches brevipes,indicating estuary environment,with the percentage of 6%~23% were found only in the overlying muddy layers of Konggang oyseter reef.It showed estuary influence in the muddy layers.The above differences revealed the reef-mud conversion was not the same in the two reefs.The Dawuzhuang oyster reef grew up to the middle to upper parts of intertidal zone in the year 5 520 cal BP,and development stopped with the gradual disappearance of the marine influence.The Konggang oyster reef grew up to the intertidal zone in the year 4 000 cal BP and was suddenly buried by the muddy and sandy sediments which maybe from the Yellow River. Therefore,the "reef-mud conversion" periods,which the oyster reefs stopped growing up and began overlaid by the mud layers,are 5 520 cal BP and 4 000 cal BP in Dawuzhuang and Tianjin Konggang,respectively.The seawater didn’t quickly exit when the two reefs stop growing,and then 1.2m thick intertidal zone sediments and 2.9 thick lagoon-intertidal zone sediments respectively deposited in the two locations.The author speculated that the seawater completely withdraw from the two reefs were later than 4 700 and 1 100 years ago.The altitudes of the maximum marine layers are-1.44 m in Dawuzhuang and 0m in Konggang profiles.It was speculated that the relative sea level were 2.95 m under the modern sea level in 4 700a ago and 1.5m under the modern sea level in 1 100a ago.This is due to the crust decline and compaction which indicated that the earlier of the regression,the lower position of the upper limit of the marine layer in the west Bohai Bay. The ancient Yellow River diverged and into the sea from Tianjin in 4 000 years ago.At the same time,the large number of the sediments into the sea and made the coastline constantly advanced seaward which lead to the subsequent oyster reefs developed seaward.