北部湾北部近岸全新世沉积环境变化及对8.2 ka小冷期的响应

The Coastal Sedimentary Environmental Changes and Their Response to Early 8.2 ka Cold Event along in the Northern Beibu Gulf since Holocene

关键词:  北部湾北部  全新世  8.2ka小冷期  区域响应

Key words:  northern coast of Beibu Gulf  Holocene  8.2 ka cold event  regional response

摘要:

[目的]北部湾北部是东亚夏季风、印度季风以及蒙古冷高压影响的交汇处,对气候变化敏感,该区沉积物要素在全新世的分布与变化,可反映该区气候波动特征和海岸格局演进之影响因素。[方法]在北部湾北部广西水域依次钻取3孔岩芯并进行粒度、矿物、元素、微体古生物测试鉴定以及¹⁴C测年,综合分析对比沉积物要素的垂直分布、变化以及响应特征。[结果]全新世孢粉组合为热带-亚热带山地组合。岩芯粒度波动旋回变化,经历了自河床向浅海湾的变迁,沉积物具有陆源性,元素等几乎与沉积旋回细组分同步变化。早期三角洲盆地钻孔显示为盐沼环境;相对晚更新世或者末期,全新世早期沉积物颗粒粗减细增,有机质增幅超过700%。碎屑矿物种类增加,孢粉和硅藻化石逐步丰富,硅藻属种数也保持增加趋势,沉积速率为全新世平均值的1.36~1.85倍,具有风化较强、水沙丰沛、沉积快、物质分配活跃、生物较为繁盛的特点,与邻区全新世中早期气候适宜期相符合。早期气候仍然波动,可辨认出8.2 ka小冷期事件的影响,Sr/Ba比值偏低。[结论]研究区作为东亚低纬度季风区海陆过渡界面,其环境变化与气候变化相联系。

Abstract:

[Objective] The northern part of Beibu Gulf is not only in the area where East Asian summer monsoon meets Indian monsoon but also in the Mongolian cold high pressure zone. It is sensitive to climate changes.The distribution and variation of some indexes in the core sediments from this area since Holocene reflect the characteristics of climate fluctuation and the factors controlling the coastal evolution.[Methods] In the northern Beibu Gulf, three cores were drilled from the area around Guangxi Province. The grain size, mineral and elemental compositions were determined, and paleontological identification and 14C dating were conducted. The vertical distribution, variation of some indexes and their response characteristics to the climate event were compared and discussed.[Results] In Holocene the sporopollen assemblage was characterized by the species of tropical and subtropical mountains. The fluctuation of the grain sizes along the cores represented a sedimentary cycle, indicating the gradual change from the river bed deposits to the shallow bay deposits. The sediments were of terrigenous origin and the elemental compositions were almost synchronous with the fine components in the sedimentary cycle. Boreholes from the early Delta basin showed that the sediments deposited in the salt marsh environment. Compared with the late Pleistocene or the end of the Late Holocene, in the sediments deposited during the early Holocene the coarse particles reduced and the fine particles increased, so the contents of organic matters increased by more than 700%. The types of detrital minerals also increased. The sporopollen and diatom fossils were abundant, and the number of diatom species also increased. The deposition rate was 1.36-1.85 times of the average rate in the whole Holocene. These features indicated the enhanced weathering, abundant sediment supplying, high deposition rates, and high biological productivity in this period, which was accommodated to the climate conditions. Although the climate still fluctuated, the effect of the small cold event in 8.2 ka was remarkable and the Sr/Ba ratio was low.[Conclusion] As a land-sea interaction zone in the low altitude monsoon region of East Asia, the environmental changes of the study area were associated with the climate changes.