【作者】 张欣；

【导师】 刘健； 赖忠平；

【作者基本信息】 中国地质大学 ， 第四纪地质学， 2021， 博士

【摘要】 晚第四纪以来,全球海平面变化剧烈,大规模的海侵–海退过程在中国东部海岸带–陆架区形成了以海相与陆相交替出现为特征的沉积地层,并记录了丰富的古环境演化信息。目前中国东部陆架及海岸带地区晚更新世以来的地层研究已经取得了丰硕成果,但大多数研究其时间尺度集中在全新世以来。由于可靠测年数据较少,以及不同地区之间地层对比的不足,使全新世以前（尤其是100～40 ka）的地层其沉积学和年代学研究较为薄弱,阻碍了我们对该地区晚更新世以来的海侵历史和末次冰消期以来海平面上升在陆架区的沉积记录的认识。本文系统研究了中国东部海岸带–陆架区的9个钻孔的岩性和有孔虫等特征,并以光释光测年（Optical Stimulated Luminescence,OSL）为主,结合加速器质谱（accelerator mass spectrometry,AMS）年代对每个钻孔分别建立了可靠的年代框架。同时,本文对浙江近岸泥质区的23个柱状样进行²¹⁰Pb年代学分析,得到了该区域的现代沉积速率分布特征。基于上述9个钻孔的岩性特征与年代学框架,并与已搜集的研究区内其他研究程度较高的钻孔资料进行地层对比,本文对中国东部海岸带–陆架区近20万年以来的地层结构和沉积环境演化进行了划分与探讨。钻孔测年数据表明,25 ka以来的AMS ¹⁴C年龄和OSL年龄具有较好的一致性。当超过25 ka（尤其是40 ka）后,AMS ¹⁴C年龄趋向饱和,而OSL年龄（尤其是长石OSL年龄）仍随地层深度的增加而增大;同时,在沉积动力较为复杂的地区（如扬子浅滩地区）,AMS ¹⁴C年代序列容易出现年代倒置现象,而OSL年代序列更为符合地层层序律,表明了OSL在晚更新世早期以来地层测年中具有良好的应用前景。各钻孔的岩性特征与年代学框架表明:MIS 7～MIS 6期间,浙江沿岸地区从陆到海方向以河流沉积环境到潮坪和浅海沉积环境为主,南黄海西部陆架和江苏海岸带地区主要为陆相和潮坪相地层（远岸区沉积了较厚的浅海相地层）,渤海地区在该时期受海侵影响程度较弱,以河流沉积环境和受潮汐影响的河流沉积环境为主;MIS 5时期,研究区内普遍发育海相沉积,但其发育时间并不同步,且各钻孔的沉积记录有所不同,出现滨岸–潮坪相、浅海相和河流相地层的波动;MIS4时期全球海平面下降,海水退出中国东部陆架区,此时河流相地层发育;MIS 3早期,海水再次入侵陆架区,但此次海侵强度较小,渤海海岸带发育滨岸–潮坪沉积,此时南黄海西南部由于大量河流沉积物的输入,发育三角洲沉积;MIS 3中晚期至MIS 2时期,研究区内河流相地层发育,同时普遍存在不同程度的沉积地层间断;MIS 1至今,气候变暖,海平面以阶段式上升为特征,此时研究区内自海向陆发生沉积环境的转变,由陆相逐渐过渡为海相沉积环境,并发育了潮流沙脊和沿岸泥质体等沉积体系。通过与前人研究资料进行对比,本文有以下几点认识:（1）渤海西岸第二海相层形成于MIS 3早期,该时期的海平面高度在-26.8～-19.87 m之间。（2）11.5 ka以来,扬子浅滩地区从河口湾环境转变为受潮汐影响的陆架环境,并发育潮流沙脊持续至约7.0 ka。（3）浙江沿岸泥质区多个柱状样的现代沉积速率分析,结合表层沉积物粒度和水体环境要素分布特征,揭示了该区上升流的沉积记录,为泥质体的形成机制提供了新的证据。更多还原

【Abstract】 Sea level has fluctuated dramatically throughout the late Quaternary,and the coastal to shelf areas of eastern China have been subjected to alternations of marine and continental environments during the transgressive and regressive progresses.These sedimentary strata contain records of past environments that have the potential to inform our understanding of the evolutionary history of paleoenvironment.The stratigraphy sequence of the coastal to shelf areas of eastern China since the late Pleistocene have received widespread research focus.However,most of these studies focused on the Holocene.Weakness in chronological study and limited correlation in the coastal to shelf areas hinder us to understand the transgressive history since late Pleistocene and sedimentary evolution in response to the post-glacial sea level rise.In this study,nine cores were collected from the coastal to shelf areas of eastern China.In addition to the lithological investigations of these cores,the chronological framework for these cores were also established by combining OSL and AMS ¹⁴C methods.Moreover,23 vibrocores were collected from the muddy area along the Zhejiang coast to analyse their sediment accumulation rates over the past 100 years using ²¹⁰Pb geochronology.According to the lithological and chronological characteristics from all the cores（including some well-studied cores in previous studies）,a detailed division and discussion on the sedimentary structure and evolution in the coastal-shelf areas of eastern China since 200 ka have been made.The dating results show that the ages of AMS ¹⁴C are consistent with that of OSL in the last 25 ka.A serious underestimate of ¹⁴C age is always found in old samples（>25 ka）,while the OSL dating technique can acquire longer timescale chronological framework and have the potential to elucidate the sedimentary evolution since the Late Pleistocene.Moreover,¹⁴C age reversals are common in complex depositional environments because of the reworking.The OSL ages generally follow the chronological order.Combining chronological and lithological characteristics from all cores,the sedimentary evolution in the coastal to shelf areas of eastern China can be summarized as follow:（1）From MIS 7 to MIS 6,the sedimentary environments in the Zhejiang coastal areas were dominated by fluvial environments in the nearshore areas and tidal flat to littoral environments in the offshore areas.Fluvial and tidal flat environments prevailed in the western South Yellow Sea and Jiangsu coastal plain,and the environment of the Bohai Sea region had been mainly fluvial and tide-fluvial.（2）In MIS 5,the marine sedimentary beds occurred widespread and not appeared simultaneously,reflecting that fluctuations of global sea level.（3）The sea level fall and fluvial sedimentary environment developed in the study area in the period of MIS4.（4）A weak transgression occurred and tidal flat deposits were developed in the modern Yellow River delta in early MIS 3 stage.A large amount of delta strata occurred in the coastal area of Jiangsu and western Yellow Sea due to lots of river-derived sediments input during this period.（5）From middle MIS 3 to MIS 2,fluvial sedimentary environments prevailed and the sedimentary hiatus are common in the study area.（6）During the post-glacial rise in sea level,the sedimentary environments in the coastal-shelf region were transition from fluvial deposition to neritic deposition since MIS 1,forming a series of depositional systems such as tidal sand ridges and coastal muddy deposits.Finally,some major conclusions can be summarized as follows:（1）The MIS 3transgression in western Bohai Sea occurred in early MIS 3 stage and the sea level was estimated between-26.8 and-19.87 m.（2）The sedimentary environment in Yangtze Shoal changed from an estuary to a tidal shelf due to the abrupt sea level rise at～11.5ka,leading to the development of tidal sand ridges until 7.0 ka.（3）The sedimentary signals of the upwelling along the Zhejiang coast were revealed by the modern sediment accumulation rates,grain sizes of surface samples and distributions of Chlorophyll in surface water,and new evidence of the formation of the muddy deposit was presented.更多还原