【作者】 冯旭文；

【导师】 金翔龙； 叶瑛；

【作者基本信息】 浙江大学 ， 地质学， 2009， 博士

【摘要】 世界范围内的缺氧河口及近海海域（又称“死亡区域”）自1960s以来呈指数级增加,对海洋生态系统尤其对底栖生态系统造成极大的改变和危害。我国至今尚未对长江口外存在的缺氧现象作专项监测或调查研究。本论文选取长江口缺氧区内外的柱样沉积物为研究对象,在定年的基础上,综合地质和地球化学分析,研究了生源要素（C、N、Si）、碳氮稳定同位素、氧化还原敏感性元素和矿物相等的组成及其变化趋势,结合水体环境的历史和现场调查数据,筛选了表征缺氧的特征参数,推测了富营养化和缺氧发生的年代及发展趋势;讨论了百年来人类活动-尤其是化肥大量使用等与缺氧事件的关系。在²¹⁰Pb定年的基础上,对取自长江口外缺氧区内外的柱样沉积物开展了10种常量元素、13种微量元素和粒度的测定分析,研究了其物源及分布特征。结果表明缺氧区外柱样沉积物主要来源于老黄河口海岸泥沙,大部分元素具有“粒度控制”规律。缺氧区内沉积物主要来源于夏季长江陆源的输入和海洋自生生物死亡后的沉降,部分氧化还原敏感元素（RSE）和亲生物元素不受控于“粒控效应”,其中Mo、Cd、As等氧化还原敏感元素自20世纪60年代末到70年代以来明显富集,分别增加了83%、73%和50%,而Mn出现贫化,指示了缺氧区水体富营养化加剧和底层水体季节性缺氧引起的底层水-沉积物界面氧化还原环境的变化;亲生物元素Ca、Sr、P含量自20世纪60年代末起分别增加了129%、65%和38%,反映了受人类活动影响,近四十年来长江口外水体生产力的提高和生物量的增加。采用X射线衍射（XRD）物相分析表明:两柱样具有稳定但不同的物质来源,百年来方解石含量变化特征与Ca含量的分布特征一致,即均有不同程度递增,进一步佐证了长江口外钙质生物生产力的提高。分析测定了总有机碳（TOC）、总氮（TN）和生物硅（BSi）的含量和有机碳氮稳定同位素(δ¹³ C_org和δ¹⁵N)。研究表明,近百年来缺氧区外柱样样沉积物中BSi%基本不变,TOC%、TN%在1940s～1950s之前、δ¹³C_org在1970s之前不同程度递增,之后则基本不变,反映了近几十年来生物量和水体生产力相对稳定;而缺氧区沉积物中TOC、TN和BSi的含量及其通量自上世纪70年代起均有不同程度增加,其中TOC、TN沉积通量分别增加了约45%、36%;δ¹³C_org值在20世纪前基本不变,平均值为-23.67‰,自1900s开始增大（变重）,1950s之后δ¹³C_org平均值为-22.78‰,尤其自1970s后显著增大;借助简单模式,利用δ¹³C_org估算了陆源和上层水体浮游生物对沉积物输入的贡献,结果表明,1970s以来TOC和TN沉积通量的增加几乎都是因为海洋自生物质输入量增大引起的,且与我国化肥施用量和长江硝酸盐入海通量呈正相关。即陆源物质中的营养盐通过初级生产过程,转换成有机质后,下沉到海底,增加了沉积物中海洋自生物质输入量。说明上世纪尤其是我国改革开放三十多年以来,长江三角洲工农业快速发展,陆源物质排放量的增加,是导致长江口及其邻近海域缺氧发生和水体富营养化加剧的主要因素。上述研究表明,δ¹³C_org、TOC沉积通量、亲生物元素（Ca、Sr、P）和部分氧化还原敏感元素（Mo、Cd、As）可用于反演百年来长江口高生产力区富营养化和缺氧的发生发展,且得出一致的结论:即长江口及其邻近海域20世纪50年代开始出现富营养化,70年代开始加剧引发了底层水体缺氧,并呈进一步加剧的趋势。更多还原

【Abstract】 Seasonal hypoxia areas （dead zones） in the estuaries and coastal oceans have spread exponentially since the 1960s and have serious consequences for ecosystem functioning especialy changed significantly the benthic ecosystem. As far no special monitoring or observating programmes focused on the seasonal hypoxia in the Changjiang Estuary in China. The purpose of this study was to reconstruct the hypoxia history by the core sediments in the Changjiang Estuary. So two cores were selected for comparison study, one within the region of seasonal hypoxia and the other nearby but outside the hypoxia region. The geological and geochemistrical analyzing methods were used to （a） reveal the distribution characteristic of the biogenic elements, stable organic carbon and nitrogen isotopic ratios, redox sensitive elements （RSE） and the mineral phase; （b） select several hypoxia marked indicators combined with historic and in-situ data on watershed environment; （c） deduce the onset of eutrophic and hypoxia in the Changjiang Estuary as well as the developing trend; （d） discuss the relations between the hypoxia anthropogenic activities especially as like the fertilizer application.The grain size and elements, including ten kinds of major elements and thirteen kinds of trace elements were determined on the ²¹⁰Pb-dated sediment cores. The two core sediments had different resources and represented different element distribution characteristics. Major sources of the core sediment outside the hypoxia region were from the old Huanghe estuary, and the distributions of most elements were controlled by "grain size effects". The sources of the core sediment within the hypoxia region were mainly from the riverine input of the Changjiang River in summer and the burial fluxes of the dead marine biology. The distribution of some redox sensitive elements （RSE） and biogenic elements did not conform to "grain size effects". The Mo, Cd, As et al. have been enriched significantly, which concentrations increased about 83%, 73% and 50% respectively since the late 1960s to 1970s. Conversely the Mn has been depleted since 1970s. The results indicate the redox environment of the bottom water-sediment interface has been changed which might be caused by the eutrophication and hypoxia in the Changjiang Estuary. The concentrations of biogenic elements as well as Ca, Sr, P have also increased about 129%, 65% and 38% respectively since the late 1960s, which reflected the increasing of productivity and biomass influenced by the anthropogenic activities during the last forty years. With the X-ray diffraction （XRD） method the mineral phase characteristics were studied and implied the two cores sediments had the stable but different sources. The contents of calcite increased in some degree over the last 100 years in the two cores, which distribution consisted with the Ca’s. The results demonstrated the increasing of calcinosis organism productivity in the Changjiang Estuary and its adjacent area.The total organic carbon （TOC）, total nitrogen （TN）, biogenic silica （BSi）, stable carbon and nitrogen isotopic ratios (δ¹³ C_org andδ¹⁵ N) were determined on the ²¹⁰Pb-dated sediment core. The results showed the concentrations of BSi were almost the constant, the concentrations of TOC and TN increased before 1940s～1950s andδ¹³ C_org increased before 1970s in the core sediments outside the hypoxia region, which reflected the biomass and the productivity hardly changed in the past several decades. For the core sediments within the hypoxia region, the concentrations of TOC, TN, BSi as well as their sedimentation fluxes have increased in some degree since 1970s. TOC, TN fluxes increased about 45%, 36% respectively. The averageδ¹³C_org value of the core was -23.67‰which remained nearly constant before the twentieth century. Theδ¹³ C_org values increased after 1900s, two marked increases were observed from 1950s and 1970s. A simpleδ¹³C_org model was used to estimate the contribution of terrestrial and marine organic matter inputs for the sediment, which indicated the increase in accumulation since 1970s was almost exclusively marine. The increasing of marine organic matter accumulation （TOC, TN and BSi） was corresponding with the increasing of fertilizer consumption and the NO₃-N budgets from the Changjiang River. The riverine runoff of fertilizers and nutrients stimulated the algae blooming. Enhanced primary production resulted in an enrichment of organic matter in the sediment. These data support the hypothesis that anthropogenic nutrient loading has been a significant factor on the eutrdphication in Changjiang Estuary.The results showed that the contents of carbon and nitrogen stable isotope, fluxes of TOC, biogenic elements as well as Ca, Sr, P and some RSEs just like Mo,Cd, As in sediments could be used to trace or reconstruct history of the coastal eutrophication and hypoxia in the high productivity zone in the Changjiang Estuary over last 100 years. These parameters gave the same conclusions consistently: the estuary eutrophication in the Changjiang Estuary and its adjacent region began in 1950s and accelerated in 1970s, then the enhanced eutrophication has caused and developed hypoxia since 1970s which has been fueled and showed the increasing trend.更多还原