晚更新世末北黄海中部硬质粘土层的形成及其古环境意义

FORMATION AND PALEO-ENVIRONMENTAL IMPLICATIONS OF HARD CLAY IN THE CENTRAL NORTH YELLOW SEA DURING THE LATE PERIOD OF PLEISTOCENE

北黄海中部晚更新世末以来低海面时期发育的硬质粘土层是古环境变化信息的重要载体。文章运用沉积学、孢粉学和年代学等方法，从样品中提取出了可指示北黄海硬质粘土层成因的孢粉、藻类化石，并揭示了其记录的古环境和古气候特征。研究表明，该硬质粘土层的形成年龄介于12602～10357cal．aB．P．之间，年代上与发生在12．9～11．6ka期间的末次冰消新仙女木气候回冷事件相吻合，与不同沉积环境中形成的北黄海泥炭层为同一时期的沉积，可作为新仙女木事件在北黄海陆架响应的一个重要证据。高含量淡水藻类（环纹藻、盘星藻）与香蒲的存在表明北黄海硬质粘土层的形成环境主要为淡水水域，而硬质粘土层中藜科-蒿属的存在指示其形成期间间或受到海水的影响，其上下层段海生沟鞭藻含量的增加说明硬质粘土层沉积初期及形成后期海水作用的增强，淡水藻类与陆生植物孢粉此消彼长的变化规律反映了硬质粘土层形成的阶段性特征。硬质粘土层中蒿属-香蒲-松-禾本科-单缝孢和松-蒿属-禾本科-单缝孢-香蒲孢粉组合表明研究区当时处于寒冷而湿润的环境，低地为以河流湿地为主的平原草甸，周边山地有针阔叶混交林分布。北黄海晚更新世末期形成的硬质粘土层与长江三角洲地区同期形成的硬质粘土层孢粉藻类组合中均出现高含量的淡水藻类化石，而差异性体现在前者存在一定比例的蒿属藜科植物花粉，主要由两者所处的纬度不同所致。

Borehole DLC70-2 （38°27.00＇N, 122°24. 15＇E; 70.6m in length, 52.8m water depth） was drilled from the eentral North Yellow Sea （NYS）. Sediment cores were split and described in detail, including eolor, sedimentary texture and structure, and stratigraphieal boundaries before sampling. There exists a hard elay of 4.92m in thickness （from 5.10m to 10.02m in Core DLC70-2）. In order to reveal its origin and environmental implieations, sediment samples from the interval of 4.45- 10.84m in Core DLC70-2 were colleeted for grain size, palynologieal and dating studies, seventy samples with roughly 10-em interval for grain size analysis, thirty-six samples with about 20-era interval for palynologieal studies and two samples from the underlying and overlying strata of the hard clay for AMS ^14C dating. AMS ^14C age data together with lithology analysis suggest that the hard clay was formed from about 12602ca1.a B.P. to 10357cal. a B. P. , in the similar period with the peat layer of the NYS deposited in different sedimentary environment, which is highly coincident with the age of the Younger Dryas event that was the most significant cold climatic event during the last deglacial warming and occurred between 12.9 - 11.6ka. The coincidence indicates that the formation of hard clay in the central NYS possibly related to the Younger Dryas event. The high abundance of freshwater algae （Concentricystis and Pediastrum） and Typha indicates that the hard clay is a freshwater deposit. The presence of Chenopodiaeeae-Artemisia in the hard clay indicates that the study area was occasionally affected by seawater during the formation of hard clay. Dinoflagellate occurs in increasing abundance in the upper and lower section of hard clay suggesting the study area had growing marine influence during the early and late stage of the formation of hard clay. The reciprocal variation law between freshwater algae and land-flora shows that the hard clay was formed in several stages. The sporo-pollen assemblages （Artemisia- Typha-Pinus-Poaceae-Monoletes and Pinus-Artemisia-Poaceae-Monoletes-Typha） from hard clay show that the study area was in a cold and humid environment during the formation of hard clay, and characterized by meadow present in the riverine wetlands with coniferous and broadleaved mixed forest. There is high abundance of freshwater algae in the hard clay during the late period of Pleistocene both in the central NYS and the Changjiang River delta, while the former contains a certain number of pollen Chenopodiaceae Artemisia, which is largely caused by their different latitude.