苏北盆地盐城冈西地区全新世海岸带沉积地层年代学及其环境意义

【作者】 刘玉；

【导师】 舒强；

【作者基本信息】 南京师范大学 ， 自然地理学， 2020， 硕士

【摘要】 海岸带是海陆交互作用的地带。全球海岸带地区集中了大量的人口,为人类生存主要聚居地。海岸带记录了海平面变迁,生态环境和沉积环境演变等,是研究人地关系的重要地域。全新世是最年轻的地质时期,也是人类生存的时期,重建该时期海岸带沉积环境和过程,并研究其古气候变化历史,具有深远的科学和现实意义。苏北盆地位于我国江苏北部,常年受东亚季风的影响,能很好的记录气候环境变化。苏北盆地属于典型的海陆过渡带,盆地东部沉积了较厚的海陆交互沉积地层,是研究海平面升降、海岸带地质地貌、海陆变迁的理想场所。而对研究区进行较为准确的定年是重建沉积过程的前提和基础。众多学者对苏北盆地进行了年代研究,然而海侵时期所得年代结果与实际地层年代沉积顺序有所差异。本文根据团队多年所获得的利用全样有机质和原地埋藏同生贝壳AMS¹⁴C测年结果与利用光释光测年技术（OSL）获得的测年结果为基础,探讨不同测年材料和方法在海岸带的适用性和可靠性,进而为海岸带地区的古气候环境演化、海陆变迁以及沉积环境演变等提供较为可靠的年代学支撑。主要结论如下:（1）通过对冈西剖面10个光释光样品进行条件检验实验（等效剂量测试前的预热坪实验和剂量恢复实验）,结果表明石英光释光测年在本研究中具有较好的适用性,研究区沉积环境与长三角类似,样品晒退程度较好。样品剂量率的计算充分考虑了含水量、U、Th、K元素和α系数对结果的影响并赋予一定误差值,最终计算出较为可靠的OSL年龄。（2）通过将全样有机质和原地埋藏同生贝壳样的AMS¹⁴C测年结果和OSL测年结果的对比可以得出:湖相沉积物中全样有机质¹⁴C测年结果较为可靠;而海侵层沉积物由于多次搬运再沉积,用全样有机质测得的年代则难以代表地层沉积的年代;而海侵层中的贝壳由于是地层沉积时被埋入沉积物中的生物活体,其AMS¹⁴C年代能够较好的代表沉积物的埋藏年代;冈西剖面不同沉积相沉积物中的细颗粒（4-11μm）石英非常适合OSL测年,能够揭示地层沉积的年代。（3）通过不同材料、不同方法测年结果的对比检验,建立了冈西剖面较为精准的年代框架。由于不同深度段间沉积环境的巨大差异,整个剖面的年代可以划分为三段:深度500-426 cm之间的淡水湖相沉积,年代为13-7.6 ka BP,平均沉积速率为0.012 cm/a;深度426-150 cm之间的潮滩-浅海沉积,年代为7.6-6.5 ka BP,平均沉积速率为0.251 cm/a;深度150-55 cm之间的淡水湖相沉积,年代为6.5-1.4 ka BP,平均沉积速率为0.018 cm/a。（4）根据所得年代结果,讨论了冈西剖面区域环境演化过程。阶段Ⅰ:13-7.6ka BP（深度500-426 cm）,气温变暖,降水增多,湖面逐步扩张阶段。晚更新世末期至全新世早期,气温变暖,海平面上升,但对本区影响较小,沉积速率慢,为淡水湖相沉积;阶段Ⅱ:7.6-6.5 ka BP（深度426-150 cm）,气温和降水持续升高、增多,海平面上升,发生海侵,7 ka BP左右海平面达到最高。随着海平面的不断上升,水流搬运能力逐步加强,层段内出现潜穴、海水贝壳碎片和咸水硅藻,沉积速率加快,指示此段为潮滩-浅海相沉积;阶段Ⅲ:6.5-1.4 ka BP（深度150-55 cm）,海平面达到最高之后,海水逐渐退出研究区域,研究区沉积速率降低,重新发育淡水湖泊。更多还原

【Abstract】 The coastal zone is the transition zone between sea and land.The coastal zone of the world has a large population,which is the main settlement for human.The coastal zone records the changes of sea level,the evolution of ecological environment and sedimentary environment,which is an important area to study the relationship between man and land.Holocene is a critical period for the formation and development of human society.It is of far-reaching scientific and practical significance to reinforce the research on the Holocene climate change and the reconstruction of sedimentary environment in the coastal area.The northern Jiangsu basin is located in the eastern part of china.The climate in this region is dominated by the East Asian monsoon variations.In the eastern part of the basin,there are thick interactive sedimentary strata that deposited during the land-sea interactions,which is an ideal place to study sea-level fluctuations,coastal geology and land changes.The reliable chronology is of crucial importance for the reconstruction of sedimentary environment in this area.Many researchers have carried out chronological studies in the northern Jiangsu basin,but the results of the transgression period are different from the actual chronological studies.In the study,the AMS¹⁴C dating and optically stimulated luminescence dating method were used to set up a robust chronological framework for the sedimentary sequence in the northern Jiangsu basin.The applicability and reliability of the two methods were discussed for different dating materials in the study area.The main conclusions are as follows:（1）A series of test measurements on the samples in Gangxi section indicated that the applied SAR protocol is suitable to date the quartz in the Gangxi section.The sedimentary environment in the study area is similar to the Yangtze River Delta,and the samples shall be well bleached before burial.In the calculation of external dose rate,the influence of water content,Uranium,Thorium,potassium andαefficiency is fully considered,and a certain error value is given.（2）By comparing OSL ages and the AMS¹⁴C results of the whole organic matter sample and the in-situ buried syngenetic shell samples,we conclude:The ¹⁴C ages of organic matter are reliable in lacustrine sediments;However,the age of transgressive sediments dated by the whole organic matter can not represent the age of deposition because of the sediment reworking;In the transgressive phase,the in-situ buried syngenetic shell was a living organism buried in the sediment when the stratum was deposited,which can better represent the age of strata.The fine-grained（4-11μm） quartz in the sediments of different sedimentary facies of GX profile is very suitable for OSL dating,which can reveal the age of deposition.（3）Based on the comparative test of the results of different materials and methods,a robust age framework of Gangxi section is established.Due to the significant variation in depositional environment at different depth,the whole section can be divided into three sections units:The depth between 500 cm and 426 cm is freshwater lacustrine deposit,the age is between 13-7.6 ka BP,and the average deposition rate is 0.012 cm/a;The depth between 426 cm and 150 cm is tidal flat shallow sea deposit,the age is between 7.6-6.5 ka BP,and the average deposition rate is 0.251 cm/a;The depth of 150-55 cm is fresh water lake facies sediment with ages between 6.5-1.4 ka BP,and the average deposition rate is 0.018 cm/a.（4）Based on the chronology,the evolution process of the sedimentary environment in the Gangxi section is discussed.Stage I:in the period of 13-7.6 ka BP（depth:500-426 cm）,the temperature warmed,the precipitation increased,and the lake surface expanded gradually.From the end of late Pleistocene to the early Holocene,the temperature was warming and the sea level was rising,but it had little influence on this area,and the deposition rate was low,which was freshwater lake facies deposition;Stage II:in the period of 7.6-6.5 ka BP（depth:426-150 cm）,temperature and precipitation continued to rise and increase,sea level was rising and reached the highest level at about 7 ka BP,and transgression occured.With the rise of sea level,the hydrodynamic conditions are enhanced,and there are burrows,fragments of sea shells and diatoms in the interval and the deposition rate was accelerated,which indicate that this segment is tidal flat shallow sea facies deposition;Stage III:in the period of 6.5-1.4 ka BP（depth:150-55 cm）,sea level reached the highest level,then gradually retreated from the study area,the sedimentation rate decreased,and the fresh water lake was developed again.更多还原