渤海湾西北岸大吴庄牡蛎礁测年与古环境变化

Age and Paleoenvironmental Change of Dawuzhuang Oyster Reef on the Northwest Coast of Bohai Bay

对位于渤海湾牡蛎礁平原东部的大吴庄牡蛎礁一个完整剖面的系统14C年代测定与校正表明,该礁体的持续时间约为1 600年(7 200～5 600 cal BP).以壳体生长纹层氧、碳同位素获得的礁体平均建造速率1 cm/yr来计算礁体的持续时间,结果仅为14C计年历时的三分之一.14C测年与壳体同位素计年结果的对比表明,礁体的每一水平层平均占据了一定的时间(约200年),这一时间段是不适宜礁体正常建造的'缓变型地质环境恶化期'.礁体正常建礁层及水平夹层的时空分布与地区性气候变化曲线对比结果表明,水平夹层与正常建礁层的转换与地区性气温波动相对应.水平夹层代表的'缓变型地质环境恶化期',对应着地区性气温偏冷时期.

The results of radiocarbon dating of the fifteen shells in the section 1 of Dawuzhaung Oyster Reef, which is located in the Northwest Coast of Bohai Bay, show that the duration of this reef is about 1 600 yrs (from ca. 7 200-5 600 cal BP). The whole thick of this reef is about 6 m. And if we calculate with the mean building-up rate of 1 cm/yr to this oyster reef, which is based on both the analysis to stable isotopes of the live and fossil oyster shells and count to the amount of concave up and concave down ridges on the hinge, the duration of it is only one third of the results given by radiocarbon dating. The difference between the two is attributed to the horizontal layers in the oyster reef. Each horizontal layer lasts a relative long period of ca. 200 yrs, which is not suitable for oyster growing and its reef buildinng-up. And we name this as chronic geo-environmental change recorded in the oyster reef during the middle Holocene in Northwest Coast of Bohai Bay, China. Comparing between the time-space of horizental layers and normal layers in the Dawuzhuang reef body with the local paleoenivronmental curve inferred from the peat cellulose and ice core has shown that the interchanges of horizental with normal layers match the flunctuations of the regional paleoenvironment changes. The normal layers represent the warmer periods and the horizontal layers were for the cooler periods during the Middle Holocene.