Dawuzhuang Oyster Reef: Its Reef-Mud Transformation And The Geoenvironmental Changes On The Northwest Coast Of Bohai Bay

Oyster reefs with the overlying muddy sediments form the common binary phase structure on the northwestern coast of Bohai Bay. Such special geological phenomenon, ie. reef?mud transformation, showed 9 times on the Oyster Reef Plain. This paper chooses The Dawuzhuang buried oyster reef as the study object which located in the northwestern shore of Bohai Bay. Its thickness and area are both larger, and it has the higher study level by far in many reefs.The samples were taken for the chronology, sedimentology and micropaleontology analyses. The dating result of the tree branch on the top of oyster reef was 5520 cal BP. Comparing to the dating result of the oyster shell samplesD-12at the top of the oyster reef, 5652 cal BP, the two only has a difference of 130 years only.OSL dating show age inversion near the top of the reef. It probably because the sediment of samples around are not homogeneous, and some content of U, Th and K in the oyster reef disturbed the sediment result. The concentrated ages are probably because of the faster accumulation. Comparing to the AMS 14C results of the shell on the top of the reef, there may be a sedimentary break at the top of the reef.The LOI difference of the mud between within the reef body and above the reef is possibly because the oyster growth requires the maintenance of organic matter, as well as their decay can increase the organic matter content. The grain size had no significant difference no matter whether they contain shell debris layer. From the reef to the overlying soil layer, the parameters of the grain size are changed gradually. Changes in magnetic characteristics may have a great relationship with the sediment source. Oyster reef grows on the northwestern coast of the Bohai Bay, the substances supply is from Jiyun River and Luanhe River, but the Yellow River is the largest source of the sediment supply into the Bohai Sea. Its frequent diversion may also cause the obvious changes in the magnetic susceptibility.Diatom analysis shows that the entire section, from 0.45m at the top of oyster reef up, including the overlying soil layer, is divided into four belt, and the influence by sea water gradually weakened, the mud?reef transformation occurred gradually, no sudden environmental changes. Pollen result shows that when the gradient occurs , the theropenedrymion was growing , and the climate had frequent small fluctuations (presumably small changes in both temperature and humidity). Foraminifera analysis shows that sedimentary environment changed from the lower intertidal zone to the upper intertidal zone. There were 21 individuals died in the season from spring to hot summer, which selected from 27 reef shells at the top of the Dawuzhuang reef, indicating that oyster reefs may belong to normal death.Summary, from the construction of oyster reefs to die, the influence of the sea water gradually smaller. In the late Holocene, the relative sea fall and because the oyster live long stationary, the reef constructed in the vertical space, when the reef reach to the top position at sea level, that is middle intertidal zone, they were exposed to the air half of time one day, so the reefs stop constructing, oysters also stop to grow up. Meanwhile, after the mid-Holocene, the coastline continued to advance, as reefs relatively unchanged, so they get closer to the shoreline. As the coastline continued to advance, oyster reefs gradually stop growing, the environment is gradually inappropriate for the growth of oysters, oyster decreased. As time goes on, all the oysters gradually perish. Mud sediment accumulated rapidly, all the oyster reefs are covered by the mud eventually.