广西近岸西部中全新世显著海侵及气候特征

The Significant Transgression during Middle Holocene and the Climatic Characteristics in Western Coast of Guangxi

关键词:  广西近岸  中全新世  显著海侵  气候

Key words:  coastal area  middle Holocene  significant transgression  climate

摘要:

[目的]北部湾北部是冷暖空气的交汇带,气候复杂多变,而断裂活动导致的沉降差异又是海岸变迁的重要驱动因素,为了探究该区域环境变迁的规律,研究其中全新世显著海侵和气候特征。[方法]以广西西部近岸为典型,于防城港湾口外水下斜坡进行沉积物岩芯取样和粒度、元素、矿物、微体古生物、¹⁴C年龄等地质地球化学要素综合测试,分析对比要素的变化特征及其气候联系。[结果]孢粉组合反映植被以山地热带、亚热带种属居优。沉积物组分为粘土质粉砂、砂质粉砂和粉砂质砂。体现出元素分段变化,碎屑矿物含量变异性较高的分布规律;化学元素等基本以3.00~2.80 m深度为界,前后分布有明显环境差异,显示气候变干变冷之特征。该转折点亦是大规模海侵的起始阶段,主要表现为绿石明显富集,Sr/Ba显著增高,有孔虫、硅藻、钙质和硅质生物碎屑随之快速增加;该转折点在时间和要素变化特征方面与邻区相近,揭示这并非偶然现象,与中全新世6.00 ka B.P.前后夏季风减弱、冬季风加强的大尺度气候变化相联系。[结论]中全新世广西西北部近岸出现了大规模海侵,可能与区域乃至全球气候变化有关。

Abstract:

[Objective] The northern part of Beibu Bay is the intersection of cold and warm air, the climate is complex and changeable, and the settlement difference caused by the faulting activity is also an important driving factor of the coastal change. In order to explore the rule of environmental change in that area, the significant transgression and climate feature were studied in Holocene age.[Method]The western near shore area in Guangxi was chosen as a typical case. Sediment cores were recovered on the underwater slope around the port of Fangcheng. Some geological and geochemical parameters of the core sediments such as grain sizes, elemental compositions, mineral compositions, micropalaeontological compositions and 14C ages were determined and their features and relationships with climate were discussed.[Results] The 14C dating in bottom of above sediments core showed middle-Holocene age and there was an apparent underlay weathering transition. Pollen also showed tropical and subtropical species mainly.Grain parameters changed vertically and geochemical elements, detrital minerals and bio-sediment and so on changed basically in two stages from depth of 3.00~2.80 m, revealing colder climate and also the initiation of significant transgression which was related with 6.00 ka B.P.event of monsoon's recession and high sea level decreasing in south region and their peak's transmission to northern coast.[Conclusion] There is a large-scale transgression in northwestern coast of Guangxi, which may be related to regional and global climate change.