【作者】 杨雪英；

【导师】 龚一鸣；

【作者基本信息】 中国地质大学 ， 古生物学与地层学， 2013， 博士

【摘要】 本文提出了不同成因莓状黄铁矿的矿物学判别依据,使一直模糊的莓状黄铁矿成因学更加清晰、明朗。发现并详细论述了莓状黄铁矿演化成自形晶、莓体粒径变大、被氧化或被其他矿物交代所需的环境因素,为莓状黄铁矿的形成和演化提供了证据和理论基础。通过广西桂林杨堤泥盆系剖面莓状黄铁矿与碳、硫循环关系的讨论,认为其生物大灭绝可能与脉动式的海底热液活动有关,海底热液活动造成深部和中部水体升温,水体中的H2S气体由于升温而发生去气作用上涌至表层,消耗了表层水体中大量的氧,导致浅海生物因缺氧和有毒气体的伤害而灭绝,为泥盆纪生物大灭绝提供了新的证据。莓状黄铁矿是由形态和大小一致的黄铁矿微晶组成的球状体,是黄铁矿在沉积岩中最普遍存在的形态之一,表明其有着比较宽广的形成条件。虽然莓状黄铁矿至今仍有生物与非生物成因的争论,但是不同特征莓状黄铁矿的成因和成莓机理可能应归因于不同的生物和非生物过程。莓状黄铁矿的形成和形态对氧化还原条件很敏感,其粒径经常被用来指示沉积过程中古氧相,区分上覆水柱中的静海环境(厌氧环境)和有氧环境,并且其物理及化学特征提供了地质历史时期海洋对环境变化响应的重要记录以及与全球铁、硫、碳和氧循环的联系。本次研究主要以华南泥盆系地层与化石中的莓状黄铁矿为对象,对不同载体如围岩、实体化石及遗迹化石Zoophycos中莓状黄铁矿进行扫描电镜扫描并结合能谱分析,并且将不同载体中莓状黄铁矿的矿物学特征进行综合对比及系统分析,提出莓状黄铁矿的成因矿物学识别标志,为生物成因和非生物成因的莓状黄铁矿提供关键判据和成莓机理。并且发现了莓状黄铁矿存在向不同方向演变的中间状态,通过分析在演变过程中周围环境对它的影响,为莓状黄铁矿的演化揭示环境控制因素。以杨堤剖面为例,通过测量全岩微量元素、全岩无机碳碳同位素数据及碳酸岩晶格硫酸根硫同位素数据,结合莓状黄铁矿的赋存状态、讨论了莓状黄铁矿与碳、硫循环的关系,为泥盆纪生物大灭绝提供了可靠证据。通过刘不同载体如围岩、实体化石和Zoophycos中莓状黄铁矿的扫描电镜结合能谱分析,计算每个载体中莓状黄铁矿的莓体直径D、微晶直径d、以及莓体直径与微晶直径之比D/d,莓体中的微晶个数、微晶形态、微晶间的杂质情况得出了以下结果：非生物成因莓状黄铁矿与生物和非生物共同作用形成的莓状黄铁矿差别不大,莓状黄铁矿间的差别仅和其所形成微环境的无机化学条件有关,即氧化还原速率、成核时间以及微晶生长速率有关;生物成因莓状黄铁矿的莓体直径和微晶直径变化范围较小且值也都偏小,莓体直径变化有限,不受环境氧化还原条件影响,仅与所形成的微环境有关,莓体直径/微晶直径(D/d)大多小于10,且微晶为立方体的莓状黄铁矿极有可能是生物成因莓状黄铁矿,有“膜”莓状黄铁矿为生物成因莓状黄铁矿；而生物和非生物共同作用形成莓状黄铁矿及非生物成因莓状黄铁矿莓体直径和微晶直径变化范围较大,莓体直径能反映环境的氧化还原条件,莓体直径／微晶直径(D/d)大多大于10,未见立方体微晶,莓体表层无“膜”。zoophycos中莓状黄铁矿的形成是主要是微生物活动的结果,微生物富集围岩中的Fe质,形成胞外及胞内莓状黄铁矿,zoophycos中深色蹼层比浅色蹼层更具还原性,且微生物活动更强烈。此外,我们还发现莓状黄铁矿在不同的环境条件下,可能会向不同的方向演化。在碱性氧化条件下,莓状黄铁矿可能会被氧化交代,形成磁铁矿、赤铁矿、闪锌矿或黄铜矿等莓状体假晶；在酸性弱氧化条件且[Fe][S]饱和度较高的条件下,莓状黄铁矿中黄铁矿微晶的粒径会慢慢长大,从而导致整个黄铁矿莓状体的粒径也随之变大；在酸性氧化程度较低甚至较缺氧且[Fe][S]饱和度较低的条件下,莓状黄铁矿会逐渐向自形方向演化,首先是有微晶的自形黄铁矿直至完全重结晶融合成黄铁矿自形单晶,如立方体、八面体或五角十二面体单晶。通过对广西桂林杨堤泥盆系剖面弗拉期-法门期(F-F)事件层及其附近的莓状黄铁矿、微量元素、C-S同位素变化关系进行分析(其中C为全岩无机碳同位素、S为碳酸岩晶格硫酸根CAS硫同位素),我们发现F-F之交研究区36层-39层C-S同位素表现出同步变化的耦合关系：在事件层即40层C、S同位素整体上都呈现出正漂移的特征,但各自又分别表现出几次不太同步的振荡现象;在F-F界限之后C-S同位素先表现出不同步变化的不耦合关系,即C同位素还在继续正漂移,至41层顶部才开始出现慢慢回落的趋势,而硫同位素已经开始负漂移了。而后即从41层顶部开始C-S同位素表现为同步负漂的耦合关系。莓状黄铁矿在36-39层数最很少,到40层开始增多,但总体上含量还是比较少,粒径都不大,小于10μm,有少数几层粒径达到15μm,到40层顶部粒径开始慢慢变大,大多都达10μm,到41层莓状黄铁矿数最开始急剧增多,到42层粒径甚至达到30um以上。通过分析微量元素数据及莓状黄铁矿的赋存状态,我们排除了火山、陆源等因素,认为C、S同位素的短暂振荡可能与脉动式的海底热液活动有关,其生物大灭绝也可能与脉动式的海底热液活动有关,海底热液活动造成深部和中部水体升温,水体中的H2S气体由于升温而发生去气作用上涌至表层,消耗了表层水体中大量的氧,导致浅海生物因缺氧和有毒气体的伤害而灭绝,随后深部热水也上升至表层,导致事件层S同位素出现快速负漂而后又快速恢复。热液活动还可能造成研究区的有机物热解和氧化还原特征的变化,这些都是C、S同位素出现短暂振荡的原因。这与事件层出现的莓状黄铁矿最大粒径及黄铁矿的溶蚀和氧化状况相一致,同时也与微最元素所显示的氧化还原状态相一致,即总体上处于弱氧化环境,但存在弱氧化到还原环境之间的来回波动。在F-F界线之上硫同位素开始负偏,而碳同位素继续正偏而后才开始慢慢回落,这可能与当时海水的氧化还原状态有关即氧化性逐渐增强有关。氧化性增强表层海水中含硫细菌或其他硫化物被氧化生成具低δ34s值的硫酸根进入海水中,使得δ34SCAs负偏,同时氧化性增强海水生态系统得到修复,微生物活动减弱,宏体生物开始复苏,可能也有些陆源物质的输入,莓状黄铁矿的含量增大可以说明这一点,此时海洋初级生产力减小,这些因素使得事件层之上硫同位素负偏,C同位素正偏之后也开始负偏。更多还原

【Abstract】 This paper has proposed identifying criteria of different genetic pyrite framboids in mineralogy. The research makes the faint genesis of pyrite framboids clearer and obviouser. At the same time, we find and detailedly discuss environmental conditions when pyrite framboids evolve into euhedral crystals, or framboid diameter is bigger, or pyrite framboids are oxidized into other minerals. These results provide evidences and theoretical basis for the formation and development of pyrite framboids. By studying pyrite framboids, trace element, the coupling of the high-resolution C-S isotope records across the Frasnian-Famennian boundary in Guilin, Guangxi, we considered that the Frasnian-Famennian mass extinction can be related to pulsatile hydrothermal activity. Because hydrothermal activity leaded to deep and middle water heated, degasification of heated water made H2S removed from water and upwell to surface water, the gas H2S consumed plenty of oxygen in surface water. Thus hypoxia and toxic gas leaded to mass extinction in shallow continental shelves, these provided credible evidences for Frasnian-Famennian mass extinction.Pyrite famboids are spheroidal or sub-spheroridal aggregates composed of pyrite microcrystals with same shape and uniform size:They are widely found in sedimentary rocks and other geologic carriers. So pyrite framboids can form under wider geochemical environment. Although formation mechanism of pyrite framboids exists controversy of biogenesis and abiogenesis. Different characteristic pyrite framboids can attribute to different biotic and abiotic action. The shape of pyrite framboids is very sensitive to redox conditions, Framboid diameter usually is used to trace paleoxygenation facies, discriminating the anaerobic (enxinic) and aerobic environment in overlying water column, The physical and chemical characteristics of pyrite framboids provide important record that sea responsed to environmental change in the geological history, and the relation with iron, sulfur, carbon and oxygen cycle of the globe. This study largely based on pyrite framboids in the Devonian strata and fossil, South China, by comparing and analyzing mineralogical characteristics of pyrite framboids in different carriers, these carriers include Devonian carbonatites and body fossils (Brachiopodas and bryozoans and corals) from South China, and other surrounding rocks and body fossils reported by predecessors as well as those synthesized in inorganic environment by predecessors, We has proposed identifying criteria of different genetic pyrite framboids in mineralogy. It was surprising that pyrite framboids can evolve to different directions under different environments, by analyzing the influence of environment during the course of evolution. The factors controlling the development of pyrite framboids are revealed. Taking Yangdi section as an example, by measuring trace elements and inorganic carbon isotope of total rocks and S isotope of carbonate-associated sulfate (CAS), as well as the occurrence state of pyrite framboids, The paper discusses the relation of pyrite framboids and C-S cycle, and provides the credible evidence for Devonian mass extinction.Scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS) was used to compare and analyze these pyrite framboids in different carriers such as surrounding rock、entity fossil、 Zoophycos, The items of analysis and comparison include framboid diameter(D), microcrystal diameter(d), D/d, the quantity and shape of microcrystal, the impurity among microcrystals, Our study initially obtained the following results:there are no great differences between abiogenic pyrite framboids and those by biological and abiological processes. Differences between pyrite framboids only depend on inorganic chemical conditions of the environment, where pyrite framboids formed, for example redox rate, nucleation time and microcrystal growth rate. Characteristics of biogenic pyrite framboids include that framboid diameter(D) and microcrystal diameter(d) of biogenic pyrite framboids are low and vary slightly, framboid diameter isn’t affect by redox conditions of depositional environments, only in relation with micro-environment, Most D/d is less than10, and pyrite framboids made up of cubic microcrystal are very likely formed by biological action, pyrite framboids with membrane wrapping the outer surface are biogenic. Characteristics of abiogenic pyrite framboids include that framboid diameter (D) of abiogenic framboid vary widely, framboid diameter can reflect redox conditions of depositional environments. Most D/d is more than10, no cubic microcrystal, no membrane wrapping the outer surface. The pyrite framboids in Zoophycos were the result of biological action, microbe enrich Fe in the surrounding rock, formed pyrite framboids, intracellular and extracellular. The reduction degree in deep spreiten of Zoophycos was more than in light spreiten, and microbial activity is stronger.Besides that, we discovered pyrite framboid can develop to different directions under different environments. Under the conditions of alkaline oxidation, Pyrite framboid can be oxidized and replaced to form pseudomorphs framboid of magnetite, hematite, sphalerite or chalcopyrite, etc; Under the conditions of acidic weak oxidation and high [Fe][S] saturation, microcrystals in pyrite fromboids can slowly grow up, microcrystal diameter is bigger, so framboid diameter is bigger with microsrystal; Under the conditions of acidic weaker oxidation even anoxia and low [Fe][S] saturation, pyrite framboids gradually developed to euhedral crystal, first formed euhedral pyrite made of microcrystal, then microcrystal recrystallized and fused, last formed single euhedral pyrite, such as cube, octahedron, or pentagonal dodecahedron crystal.By analysizing and researching for pyrite framboid, trace element, the coupling of the high-resolution C-S isotope records across the Frasnian-Famennian boundary in Guilin, Guangxi. The C isotope means whole-rock inorganic C isotope, the S isotope means sulfur isotopic composition of carbonate associated sulfate (CAS). We discover that C-S isotope records showed coupled change in synchronization from No.36layer to No.39layer, the C-S isotope record showed the characteristic of positive excursion as a whole in the event layers, but the C isotope and S isotope records displayed unsynchronized vibration, respectively. Above Frasnian-Famennian transition, firstly, C-S isotope records didn’t show coupled change in synchronization, that was, C isotope records continued positive excursion, to the top of No.41layer, C isotope records gradually went back, but S isotope records began already negative excursion, C-S isotope records showed coupling of negative excursion in synchronization above the top of No.41layer. The quantity of pyrite framboid was very rare from No.36layer to No.39layer, increased till to No.40layer, but the quantity still was low as a whole, framboid diameters were not big, below10μm, framboid diameters reached15μm only in a few layers, framboid diameters began to largen at the top of No.40layer, most pyrite framboids were up to10μm, the quantity of pyrite framboids began to increase greatly, framboid diameters increased, too, even exceeded30μm in No.42layer. By analysizing the data of trace elements and the occurrence state of pyrite framboids, we exclude these factors of volcanism and errigenous input and consider transient fluctuation of C-S isotope records could be primarily related to pulsatile hydrothermal activity in the bottom of ocean, The Frasnian--Famennian Mass Extinction could be also related to hydrothermal activity, because hydrothermal activity leaded to deep and middle water heated, H2S in those water happened degassing owing to heated water and upwelled to surface water, consumed plenty of oxygen, leaded to mass extinction in shallow continental shelves due to damage from hypoxia and toxic gas, the deep hot water upwelled to surface water soon afterwards, leaded to S isotope records showing quickly negative excursion then recovered quickly. At the same time, hydrothermal activity could lead to organic matter pyrolysis and variation of redox in the study area, all those were the cause of transient fluctuation of C-S isotope records in the event layers. The result was consistent with the occurrence state of pyrite framboids such as framboid diameter mostly below10μm, up to15μm only in a few layers, corrosion and oxidation of some pyrites across the Frasnian-Famennian boundary, meanwhile, accorded with redox state by analyzing trace metals, that was, The event layers were under weak oxidation environment as a while, but had fluctuation between weak oxidation and reduction environment. Above the Frasnian-Famennian boundary, S isotope records began negative excursion, but C isotope records continued positive excursion, then began to fall slowly, these could be related to gradually increasing oxidizability. When oxidizability in surface water increased, the thiobacterias or sulfur-containing compounds were more easily oxidized to SO42-with low δ34S entering into seawater, So δ34SCAS value began negative excursion, at the same time, increased oxidizability maked marine ecosystem gain ecological repair, microbe activity weakened, macro-organisms started to recover, probably terrestrial inputs added, the increased quantity of pyrite framboids could account for this, at the moment primary productivity decreased, those factors leaded to sulfur isotope showing negative excursion,δ12C value showing positive excursion then negative excursion.更多还原