晚更新世末期以来黄河口古地理环境的演变

PALAEOGEOGRAPHIC ENVIRONMENTAL EVOLUTION IN THE HUANGHE RIVER ESTUARY SINCE LATE PLEISTOCENE

文章通过老黄河口海域CB20A孔岩芯的粒度、碎屑矿物、有孔虫、介形虫、瓣鳃类、腹足类、孢粉、植物化石和14C测年等指标的分析鉴定结果,探讨并研究黄河口区古地理环境的演变。从晚更新世末期至今,本区地理环境演变经历了6个主要阶段,即全球最末一次冰期的早期、间冰期、晚期和冰后期的早期、中期及晚期。其间出现明显的气候冷暖交替、海平面升降及岸线进退的变化,多次出现沧海桑田的景观。在这6个变化阶段中还有一些规模较小的海平面升降,如最末一次冰期早期气候寒冷,海平面大幅度下降,然而在这一时期的沉积物中却发现了气候变暖的海相沉积层,反映海平面曾经上升。而在最末一次冰期的间冰期,气候变暖,但是它并非始终温暖如一:其下部沉积物中的化石反映的是气候较冷海面较低的滨海沼泽特征;中部反映的是气候温暖的浅海环境;而上部反映的是海面明显下降的潮坪环境。这些气候和海面波动现象在冰后期仍然存在。

The palaeogeographic environmental evolution are studied and discussed by analyzing the CB20A Core obtained analysis, in the offshore area of the ancient Huanghe River estuary, including grain size analysis, detrital mineral palaeontological researches of foraminifera, ostracoda, lamellibranchia, gastropoda, pollen and spores, and megafossil plants, and ^14C dating as well. The palaeogeographic environmental evolution from the Late Pleistocene up to now can be divided mainly into 6 stages, i.e. the early stage of the last glaciation, the interglacial stage of the last glaciation, the late stage of the last glaciation, the early stage of the postglacial age, the middle stage of the postglacial age, and the late stage of the postglacial age. In those stages the palaeoclimates alternated along with the risings and failings of the sea level, which are corresponding respectively to the shore-line invading and retreating. The landscape features also changedaccordingly from sea to land. In addition to the above 6 major stages, some short-term sea level risings or failings are also observed. For example, the general climate was cold and the sea level fell during the early stage of the last glaciation. However a marine deposit sublayer indicating a warmer period has been found in the sediments corresponding to that stage, which reflects a sea level rising. Furthermore, although on the whole the climate was warm in the interglacial stage of the last glaciation, fossils evidence indicates a climate variation for that stage as follows ： the lower part of the sediments was deposited in a sea marsh environment of a colder climate with a lower sea level ; the middle layer of a warmer climate in a shallow-sea environment ; and the upper layer in a tidal-flat environment with the lowest sea level. A similar climatic variation and sea level fluctuation existed also in the postglacial age. The data of the variations of palaeoclimate, sea level and shoreline obtained from the CB20A Core are helpful for studying palaeogeographic environmental evolution history of the estuary. This study can also be used as a scientific foundation for oil and gas exploitation in this area.