【作者】 陈勇；

【导师】 雷怀彦；

【作者基本信息】 厦门大学 ， 海洋地质， 2019， 硕士

【摘要】 天然气水合物作为极具潜力的新型能源之一,日益受到科学界和各国政府的热切关注。而其保存需要稳定的温压条件,海平面下降、洋壳构造运动、海底滑塌等因素都可能造成水合物的失稳分解并释放出甲烷。上逸的甲烷在垂向运移过程中与海水中的硫酸盐等电子受体发生甲烷厌氧氧化反应,改变着海洋沉积物中碳、硫、铁等元素的赋存形态。而铁系矿物作为沉积物磁性的主要载体,其赋存形态的改变,最终会在磁学上留下“印记”。因此,对天然气水合物赋存区表层沉积物铁的赋存形态与磁性特征及其演化规律进行研究,不仅有助我们理解甲烷厌氧氧化对海洋沉积物铁的不同赋存形态、沉积物磁学特征的影响,还可以探究早期成岩过程中沉积物磁学特征与活性铁组分之间的耦合关系,为判识深层天然气水合物藏失稳分解提供新的依据,从而为我国海底天然气水合物勘探开发事业提供更多的基础数据及参考资料。本文以探寻水合物赋存区浅层沉积物的铁的赋存形态与磁学特征演化规律及其耦合关系为目标,选取了南海北部水合物潜在区神狐海域的SH1和A27站位的沉积物柱状样为研究对象,开展了沉积学、环境磁学以及地球化学等研究,获得以下主要认识:(1)南海北部神狐海域SH1 A27站位沉积物以悬移质的半深海沉积物为主,粉砂为主的粒级特征有利于沉积物有机质的吸附与保存。AMS14C的测年结果显示研究区平均沉积速率处于较高水平(29.48 cm·ka-1),结合该区域的构造、温压及其气源条件等背景资料,表明研究区具备水合物形成的物质基础条件。(2)两个站位的粒度分布频率曲线的双峰指示了沉积物的多重来源,但以珠江和台湾来源为主。粒径上以240cm附近为分界点,表现出下粗上细的两段式特征和沉积速率在纵向剖面自下而上递减的变化,共同揭示了研究区17 ka cal.B.P.以来海进使得陆架出露减少,与珠江口距离延长,使沉积物趋细,沉积速率放缓。(3)SH1和A27站位的沉积物环境磁学研究结果显示两个站位的磁学均受到了早期成岩作用和物源变化的共同影响,导致各深度段内磁性矿物组合特征发生改变。两个站位沉积物的磁性贡献主要来自磁铁矿,但含量上磁性矿物在上层沉积物中以针铁矿、赤铁矿等铁(氢)氧化物为主,随着黄铁矿矿化过程的不断进行,黄铁矿逐渐成为深层的主要磁性矿物。而A27站位深部由于受陆源输入改造明显,使得铁(氢)氧化物与黄铁矿共同成为了深层沉积物主要磁性矿物形态。(4)SHI站位10～]30cm、A27站位10～90cm层位磁性下降主要受控于微生物介导的铁的异化还原(DIR)引起磁铁矿的溶解。SHI站位140～380cni、A27站位100～260cm层位磁性降低主要是硫酸盐氧化有机质(OM-SR)所致。两个站位在230cm和260cm之下磁性受到了硫酸盐氧化有机质(OM-SR)和陆源输入的共同影响。A27站位较SH1站位更靠近岸线,因而其深部磁性受陆源改造主导。(5)通过环境磁学及地球化学手段识别出SH1站位的硫酸盐-甲烷转换带(SMTZ)的上界在380cm附近。A27站位在430cm之下受到了深层硫酸盐驱动甲烷厌氧氧化(AOM-SR)的改造,指示了A27站位较浅的甲烷-硫酸盐转换带(SMTZ)。(6)表层深度段内磁性特征与Femag含量随深度从快速下降至逐渐平缓的变化趋势可以用于识别铁异化还原(DIR)过程。而硫酸盐氧化有机质作用(OM-SR)可以通过χ、SIRM、χARM、Femag、Feox的下降并伴随Fepy含量上升来识别。且通过磁学手段可以识别以上两个过程的界限。硫酸盐-甲烷转换带(SMTZ)上部χ、SIRM、χARM的明显下降、Feox和Femag含量谷值的出现及Fepy含量明显升高,可能指示了下覆天然气水合物的分解。更多还原

【Abstract】 Natural gas hydrate is a new type energy with great potential,and it is receiving increasing attention from the scientific community and governments.However,it requires stable temperature and pressure conditions to preserve,geological events like sea level decline,oceanic crust structure movement,seabed slip may cause the hydrate to decompose and release methane.The anaerobic oxidation of upward methane with the electron acceptors like sulfate in seawater changes the phases of carbon,sulfur and iron in marine sediments.The changing of iron-based minerals,as the main carrier of sediment magnetics,will eventually lead to a "imprint" on magnetism.Therefore,the study of the existing states of iron and magnetic characteristics of the surface sediment and their evolution in the natural gas hydrate-bearing area will not only help us understand the influence of AOM(Anaerobic oxidation of methane)reaction in magnetism and phases of iron.And the study can also explore the coupling relationship between the magnetism and the existing states of iron in the early diagenesis process.This study can provide a new basis for the identification of the instability decomposition of deep gas hydrate reservoirs,and provide more basic data.In this paper,we aim to explore the existing states of iron and the evolution of magnetic characteristics and their relationship in shallow sediments in the hydrate-bearing area.We chose the sedimentary column samples SH1 and A27 in the Shenhu area of the hydrate potential area in the northern continental sloop in South China Sea as the research objects,to proceed a series of sedimentary,environmental magnetic,and geochemical researches.The main understandings are as followings:(1)The sediments of SH1 and A27 sites in the Shenhu sea area in the northern South China Sea are mainly made up with suspended semi-deep sea sediments.The silty component can provide the adsorption and preservation of sediment organic matter.The dating results of AMS14C show that the average deposition rate in the study area is at a high level(29.48 cm·ka-1),combined with background data such as structure,temperature and pressure and gas source conditions,indicating that the study area has the basic condition for gas hydrate formation.(2)The double peaks of the particle size distribution of the two stations indicate multiple sources of sediment,but are mainly from the Pearl River and Taiwan.From bottom to top,the characteristics of finer grain size and slower deposition rate reveal that the sea level of the study area has been raised from 17 ka cal.BP,which made the distance from the Pearl River estuary prolonged,so that the sediment was thinner and the deposition rate is slower.(3)Environmental magnetic studies at SH1 and A27 cores show that the magnetic properties of the two cores are affected by the early diagenesis and source changes,resulting in changes in the magnetic mineral and characteristics in each depth segment.The magnetism of the two cores are dominated by magnetite,but the content of magnetic minerals in the upper sediments is mainly iron(hydrogen)oxides such as goethite and hematite.Along with the process of pyrite mineralization,pyrite has gradually become the main magnetic mineral in the deeper layer.The deep layer of the A27 core is obviously influenced by the source changing,so that the iron oxide/hydroxide and the pyrite together become the main magnetic mineral form of the deep sediment.(4)The magnetic decline of the SH1 core at 10～130cm and the A27 core at 10～90cm is mainly controlled by the microbial-mediated dissimilation iron reduction(DIR).The magnetic reduction of the SH1 core 140-380cm and the A27 core 100～260cm layer are mainly caused by oxidation of organic matter-sulfate reduction(OM-SR).The magnetic properties of the two stations below 230 cm and 260 cm are affected by the combination of OM-SR and source changing.The A27 core is closer to the shoreline than SH1,so its magnetism is dominated by source changing.(5)The upper boundary of sulfate-methane transition zone(SMTZ)has been identified by the environmental magnetism and geochemical method in the SH 1 core is around 380 cnm.The sediment of A27 core below 430 cm was influenced by methane anaerobic oxidation of methane-sulfate reduction(AOM-SR),indicating a shallow methane-sulfate transition zone(SMTZ)at the A27 site.(6)The characteristics that magnetic susceptibility and Femang decrease from rapid to gradual of the surface can be used to identify the dissimilation iron reduction(DIR)process.OM-SR can be identified by the decrease of χ,SIRM,χARM,Femag,Feox and the increase of Fepy content.And the boundaries of two processes above can be recognized.The significant decrease of χ,SIRM,χARm,the contents of Feox and Femag,with the increasing of Fepy content,can be used to figure out the upper sulfate-methane transition zone(SMTZ),which also indicate the decomposition of natural gas hydrate.更多还原