天津平原罾口河牡蛎礁剖面中的硅藻及其记录的古环境

DIATOM IN ZENGKOUHE BURIED OYSTER REEF,TIANJIN PLAIN AND ITS PALEOENVIRONMENTAL IMPLICATIONS

对天津平原罾口河HD303牡蛎礁剖面礁体中各层牡蛎壳内泥质充填物及礁体上覆泥层做硅藻分析,显示剖面礁体部分自下至上为潮间带中下部(Ⅰ带)、潮间带下部至潮下带(Ⅱ带)以及受风暴潮或强波浪影响较强、环境频繁变化的潮间带中上部(Ⅲ带);礁体上覆泥层自下至上为澙湖潮间带上部(Ⅳ带)、平均大潮高潮线附近的潮上带(Ⅴ带)和潮上带(Ⅵ带)的环境变化过程。另外,硅藻分析还显示上覆泥层受河流影响较礁体有所减弱,上覆泥层下部(Ⅳ带和Ⅴ带)形成时海水盐度比礁体大,推测在礁体发育后期,礁体所处位置封闭成潟湖及其河流作用减弱导致盐度的增加等因素是牡蛎礁停止发育的主要原因;而礁体上部水平夹层发育的原因是礁体发育后期,水体变浅导致礁体上部经常出露水面,易受风暴潮或强波浪作用,造成牡蛎倒伏所致。罾口河牡蛎礁HD303剖面的海相层上限的高度约为礁顶上1.3m(海拔1.15m)处,代表了海退时的平均大潮高潮线的高度。

Diatom was used to reconstruct the palaoenvironment of the muddy sediment infilling and the muddy layers overlying Zengkouhe oyster reef along the section HD303, Tianjin plain. Three diatom zones, namely I , II and III from bottom to top, have been identified in the oyster reef, corresponding to the environments of the middle-lower intertidal zone, the lower intertidal zone to the subtidal zone, and the upper-middle intertidal zone often influenced by storm surge or strong waves respectively. There are also three diatom zones, IV, V and VI from the bottom to the top in the overlying muddy layers. Diatom assemblage indicates that they were deposited in the upper part of the intertidal zone of a lagoon, the intertidal zone near the mean high level of a spring tide and the supratidal zone successively. The influence of river on the overlying layers was weak and the salinity in the zones of IV and V was higher than that required by the oyster reef. It is speculated that the closing of the lagoon and the reduction of river influence were the reason to stop the growth of the oyster reef. The shallower water depth made the reef environment more vulnerable to storm and/or strong wave. The reworked clastic layers consisting of shell fragments appeared in the upper part of the reef provided the evidence for the evolutionary history. And the mean water level of the spring tide could be as high as 1.3 m above the reef top then.