【作者】 李贞；

【导师】 李珍；

【作者基本信息】 华东师范大学 ， 自然地理学， 2010， 硕士

【摘要】 本文对广西海岸带14个表层沉积环境剖面和8个柱状岩芯进行粒度、孢粉分析和210Pb测试。利用由表层沉积剖面获得的孢粉组合特征与沉积环境之间的关系,对柱状沉积物的孢粉组合进行沉积环境解译,在210Pb年代框架中获得广西海岸带近百年来沉积环境演变特征及影响机制。对广西北仑河口区,珍珠湾,钦州湾和英罗湾地区的不同海岸带14个剖面的79个样品进行孢粉分析和粒度分析。结果表明孢粉分布特征与海岸带沉积环境密切相关。红树林区和堤坝内陆缘的孢粉浓度较高,而裸滩特别是水动力作用强的砂质滩,孢粉浓度极低反映出水动力对孢粉分布的控制作用。孢粉组合中总体上蕨类孢子在海岸带浓度较低,向海方向具有增加的趋势,主要原因是孢子易于浮于水面,经水动力的分选运移,使其在远岸区得到富集；Pinus花粉主要来源于海岸带附近的山区,其分布往往表现出水力与风力共同作用的特征,陆上风力或河流的远距离携带及水体中悬浮运移的特点,使其明显比其它内陆花粉含量高；热带雨林和季雨林主要建群种的花粉类型含量极低；Poaceae花粉的分布与距其主要区内堤的距离有关,近堤处含量较高；人工种植防护林的主要类型Casuarinaceae花粉的百分含量呈由岸向海逐渐减少的趋势。红树林海岸带的孢粉分布总体呈现原位沉积特征,内陆植物孢粉主要通过水流携带作用沉积下来,而局地孢粉主要受控于红树林主要建群类型,以优势植物花粉为主,另外还受植物自身特点(如花粉传媒方式)和沉积物类型、高程和潮汐动力作用等外部条件影响。非红树林区的红树林花粉含量均在小于10%。通过对表层孢粉组合与沉积环境的分析结果,初步选用与海岸带不同沉积单元密切相关的6个指标：孢粉浓度、双气囊类花粉、蕨类孢子、木麻黄花粉、红树植物花粉和沉积物平均粒径作为沉积环境的替代指标,确定不同海岸带及沉积单元的指标阈值,获得柱状样沉积环境记录的解译依据。对取自广西珍珠港、钦州湾、北海和英罗湾海岸带的8个柱状沉积物进行210Pb比活度测定,以确定年代框架。F11孔因强烈扰动作用,未计算其沉积速率,钻孔F14,LM01,Q24,Q32,Q37,C11和018平均沉积速率分别为0.67 cm／a,0.61 cm／a,0.25 cm／a,1.68 cm／a,0.63 cm／a,0.70 cm／a和0.44 cm／a,计算获得底部年代分别约为公元1888年,1846年,1727年,1956年,1864年,1916年和1816年。对8个柱状沉积物进行孢粉和粒度分析,在210Pb年代框架控制下,利用表层沉积剖面研究建立起来的沉积环境指标阈值及孢粉分布控制机理,解释孢粉及环境替代指标记录的沉积环境信息。总体上,近百年以来,广西海岸带具有明显的陆向迁移特征,十年-年级尺度上具有地域差异性。在210Pb年代框架下,珍珠港20世纪以来,表现出岸线略向陆地迁移的形势。钦州湾地区总体表现出西向扩张,东向缩减的趋势。20世纪初期西海岸沉积物中红树林花粉Rhizophora和Aegiceras锐减,反映出红树林滩衰退的特征；20世纪60和70年代,钦州湾西海岸线明显向内陆迁移,高潮滩环境向低潮滩演化,高潮滩向内陆迁移演化的空间被人类限制(修筑堤坝等),使滩涂面积具减少。而20世纪80年代始,东海岸线由于钦江流域来沙的迅速堆积,沉积速率较高,岸线向海推移明显,沉积物中木本植物花粉和禾本科均有所增加,内陆记录增强,而围垦修堤的人类活动也加速了低潮滩向高潮滩演化。北海20世纪中叶以来,高潮滩环境向低潮滩环境演变,岸线向陆迁移明显。英罗湾20世纪70年代以后,红树林滩衰退明显,沉积环境向低潮滩演化。同时,二十世纪以来,Poaceae>40μm花粉的普遍增多反映出人类种植活动加剧,反映了人类对海岸带的开发活动增强。更多还原

【Abstract】 In this paper, grain size analysis, palynological analysis and 210Pb measurement are carried out on surface sediments from 14 sections and core sediments from 8 cores in the Guangxi coastal zone. According to the results from surface sections, we discuss the relationship between palynological records and sedimentary environment, and further to the mechanism on palynological distribution. Then, the extracted proxies are used to interpret the palynological record, grain size and Pb from core sediments, reconstruct the evolution of sedimentary environment in the coastal zone of Guangxi within the last hundred years on the 210Pb chronological frame, and indicate the potential factors on the evolution.Analysis of Grain size and palynology is carried out on 79 surface sediments from 14 sections in the coastal zone of Beilun estuary, Zhenzhu bay, Qinzhou bay and Yingluo bay. The results show that the palynological distribution is highly correlated with the sedimentary condition in coastal zone. High palynological concentration occurs in the mangrove forest and inner dyke, while low palynological concentration is in the bare tidal flat which is in a much stronger hydrodynamic system. This reflected palynological distribution is controlled by hydrodynamic force, especially the hydrodynamic sorted action similar to sediment materials. In General, fern spore content is low in the coastal zone, and tend to increase seaward, which is associated with its enrichment by floating transport and hydrodynamic sorting action. From mountainous area near the coastal zone, Pinus pollen obviously prevails over other terrestrial pollen, ascribing to the relative enrichment by wind and fluvial transports in a long distance and a floating way in sea. Pollen of species dominated in tropical rainforest and seasonal rain forest occur in very low contents. Poaceae pollen seems to be associated with the distance from dyke, indicated by its higher contents near dykes. Pollen content of Casuarinaceae dominant in planted shelterbelt trends to decrease seaward with an increase in distance from the dyke. Mangrove pollen are clearly characterized by in-situ deposition in mangrove tidal zone, where terrestrial palynological grains are mainly transported by water and deposit in very low content, whereas mangrove pollen of dominant species in communities prevail over others in very high content. Besides compositions of communities, palynological distribution in mangroves is also affected by characteristics of plant itself (e.g. pollination way), hydrodynamic conditions and elevation, etc. Content of mangrove pollen in non-mangrove tidal areas is generally less than 10%, which could be a boundary value indicating an allochthonous record. According to the relationship between palynological records and sediment units obtained from surface sections,6 proxies including palynological concentration, disaccate pollen, Casuarinaceae pollen, mangrove pollen, fern spore, and mean grain size of sediment, are selected as indicators on the sedimentary environment. Threshold values of indicators are established for the coastal various sediment zones in order to reconstruct the paleo-sedimentary environment recorded in cores.210Pb specific activities are measured on samples from 8 sediment cores to determine the chronological information. Core F11 is not calculated sedimentation rate due to strong disturbance on samples. The average sedimentation rates in cores F14, LM01, Q24, Q32, Q37, C11 and 018 are 0.67 cm/a,0.61 cm/a,0.25 cm/a,1.68 cm/a,0.63 cm/a,0.70 cm/a and 0.44 cm/a, respectively, and the ages at the bottom of each cores are about 1888 AD,1846 AD, 1727 AD,1956 AD,1864 AD,1916 AD and 1816 AD, accordingly.Results from palynological analysis and grain size analysis on 8 sediment cores reflect the evolution sedimentary environment within last several hundred years, under the 210Pb chronological framework, on the basis of threshold values of indicators and the mechanism on palynological distribution obtained from surface sections.In general, the coastal line tends to landward migrate within the last hundred years However, regional differences on coastline migration can be recognized on decade to annual time scales. Coastline of Zhenzhu bay had slightly migrated landward since 20th century. Qinzhou bay shows the trend of westward expansion and eastward reduction. In the western coast area, a sharp decline of Rhizophora and Aegiceras pollen content in western coastal area reflects a degradation of mangrove in the early 20th century. The western coastline obviously migrates landwards, and high tidal zone evolutes into low tidal zone. The area of tidal flat reduces due to the restricted space by human activities such as building dykes in 1960s and 1970s. The east coastline has obviously migrated seaward, corresponding to the rapid deposition with more materials from Qinjiang River since 1980s. Terrestrial influence is intensified, shown by an increase in arboreal pollen and Poaceae pollen. This perhaps is associated with human activities, such as reclamation and diking, which accelerate the evolution from low tide zone to high tidal zone. The coastline of Baihai area has migrated landward since the middle of 20th century, resulting in an evolution from low tidal zone to the high tidal zone. Yingluo bay is characterized by an evolution to low tidal zone with a declination of mangrove beach after 1970’s. In addition, an increase in Poaceae pollen (>40μm) content reflects the intensified human cultivated activity for the exploit in the coastal zone after about 20th century.更多还原