渤海湾西北岸4ka BP前后古环境重建

More and more high resolution paleoclimatic records confirm that a number of subcentennial climate abrupt change events were exit in the Holocene. Such abrupt climate change events had a very close relationship with the social and economic development of human. 4.2 ka BP event are recorded in Asia, North America, the Mediterranean, Southern Europe and other places which indicated that this event were global in extent. However, the record of this evnent has not been adequently substantiated in the coastal zone, especially in northern China muddy coastal zones. Sea level change is an important part and comprehensive expression of climate change. Paleo sea level reconstruction, espescially the sea level change research in the climate abrupt period is very important. The buried oyster reefs are good geological bodies that contain the information of the Holocene geo-environmental and sea level changes on the coast of Bohai Bay. This application is intended to choose zengkouhe, biaokou and the newly discovered oyster reefs on the coast of Bohai Bay as the study objects, use the sedimentology, chronology, paleontology and isotope geochemistry methods to reconstruct the paleo-environment and seal level curve which recorded by these reefs. Then, we want to further explore the possible geological record and response mechanism of 4.2 ka BP event among these records.

越来越多的高分辨率古气候记录证实全新世存在一系列数十至百年尺度的快速波动，而这种突然的气候变化与人类社会经济发展有着非常密切的关系。4.2ka气候突变事件在亚洲、非洲和北美洲，以及地中海和南欧等地均有记录，表明了它的全球性特征。但是，在海陆交互的海岸带、特别是我国北方泥质海岸带地区，该事件的记录迄今未得到充分证实。海面变化是气候变化重要组成部分和综合表现形式。古海面重建、特别是气候异常期的海面响应研究非常重要。渤海湾全新世埋藏牡蛎礁是记录全新世地质环境与海面变化的信息载体。本申请拟以渤海湾第VI礁群中的罾口河、俵口及二者之间新发现的牡蛎礁体为研究对象，通过沉积学、年代学、古生物学和同位素地球化学等手段，重建礁体记录的古环境与古海面信息、探寻可能存在的4.2ka事件的地质记录与响应机制。

按照研究计划，项目组以渤海湾西北岸俵口南部、罾口河和岭头牡蛎礁体为研究对象，通过对钻孔和礁体剖面的系统取样及样品沉积学、AMS 14C年代学、古生物学和同位素地球化学分析测试，重建了礁体记录的渤海湾西北岸4ka BP前后古环境，并探寻了可能存在的“4.2ka BP”事件记录及响应机制。取得如下主要成果：（1）俵口南部礁体高精度的高程测量显示牡蛎礁体顶板埋深-1.82~-3.316m，平均埋深-2.737m。底板埋深-5.578~-8.775m，平均埋深-7.684m（以上均为黄海85高程）。（2）AMS 14C年代学显示，俵口南部礁体发育年代为6178~4046 cal BP，岭头和罾口河礁体起讫年代分别是4378~4234 cal BP和7123~4212 cal BP，均归属于渤海湾西北岸牡蛎礁群III-1。（3）俵口南部、岭头和罾口河3处埋藏牡蛎礁体的古环境重建结果显示：全新世以来，研究区依次经历了早全新世潮上带盐沼至潮间带——中全新世潮下带-潮间带中下部-潮间带中上部——晚全新世沼泽-盐沼低地沉积环境的海进-海退过程。在约7ka cal BP以来，俵口南部、岭头和罾口河3处牡蛎礁体先后出现。礁体初期生长在低潮线以下的浅水环境，随着礁体的向上建造水深逐渐变浅，海水影响减弱，约5~4.3ka cal BP时，礁体进入潮间带中下部，并在约4.2~4ka cal BP时被上覆泥层覆盖，停止发育。之后，逐渐脱离海水影响，由潮间带中上部进入沼泽-盐沼相沿海低地沉积环境并相继成陆。（4）岭头、罾口河和俵口南部礁体的AMS 14C年代学、古生物学和同位素地球化学的分析结果均表明，研究区在5~4ka cal BP期间发生了一次气候波动过程：约5~4.3ka cal BP期间研究区气候出现变冷变干的趋势，4.3ka cal BP后逐渐转暖，气候变化事件的转折点发生于约4.3ka cal BP。这一明显的环境变化是我国北方泥质海岸带地区对“4.2ka BP事件”的响应。