二叠—三叠纪界线事件的矿物学、地球化学解译.doc

二叠三叠纪界线事件的矿物学、地球化学解译 地球历史上发生过多次大规模生物集群灭绝事件。其中二叠纪．三叠纪之交P-T的生物大灭绝，摧毁地球上90％以上的海洋生物种，70％的陆生脊椎动物属和大多数的陆生植物，是地史上规模最大的一次生物灭绝。与生物灭绝有关的事件研究成为科学界的热点和前沿领域。对它们的认识有助于理解和把握生命和地球演化史上的一些关键问题，对当代地球环境变化和人类生存状况提供借鉴。P-T界线生物灭绝的特点及地层沉积记录指示，当时可能发生了一系列突发性重大地质事件，包括火山喷发、陨石撞击、海洋缺氧、酸雨、甲烷水合物释放、海平面升降、海洋翻转等。与生物大灭绝之间的成因联系是P-T界线事件研究的关键。目前有两种主要观点火山喷发说和陨石撞击说。但界线地层的标志性沉积记录重复性差，或具有不确定性，使它们之间的争论不曾停止。位于浙江省长兴县的煤山P-T界线地层是国际二叠系-三叠系层型剖面和点GSSP，是国际上关注最多、研究程度最高的界线地层之一。煤山剖面已经发现的异常沉积记录和现象，包括有机碳和无机碳同位素、O、S、Sr同位素、稀土及其他微量元素、分子化石、石膏、富勒烯及其包裹的稀有气体、高温石英、微球粒、Ir等铂族元素、陨石碎片、地外氨基酸等。中国科学院南京地质古生物所金玉玕院士和美国加州理工大学F．Kyte教授组织的“中国煤山二叠-三叠P-T生物灭绝事件国际合作项目”，旨在澄清P-T界线研究的争议，揭示P-T界线事件的本质。他们在煤山D剖面西边150米和550米两处钻孔，穿越P-T界线，取得新鲜的钻孔样品。此外，对煤山剖面关键沉积记录在不同实验室进行盲测对比，特别关注有关陨石撞击的记录和证据。本论文研究是该合作项目的一部分，重点剖析煤山剖面P-T界线地层矿物学和有机地球化学特征，进而揭示它们的科学意义。主要研究内容包括三个方面一是分离界线样品中的元素碳和有机碳，包括多环芳烃PAHs、黑碳BC、纳米金刚石、富勒烯C_60等；二是寻找界线地层中的草莓状黄铁矿，以及其产出特征的研究等；三是寻找与撞击事件相关的矿物学证据，如冲击矿物、纳米金刚石等。本研究的创新点主要体现在两方面1首次在煤山剖面P-T界线地层系统发现细粒草莓状黄铁矿，它是海洋贫氧沉积环境的重要指标；2首次系统分析P-T界线地层中的黑碳BC和燃烧源多环芳烃PAHs，进而提出煤山地区二叠纪末期发生了强烈的大火事件。主要研究成果和认识包括1利用光片原位观察，结合酸溶分离富集的手段，在煤山P-T界线的黄铁矿壳层、25层、26层、28层和29层分别发现大量颗粒细小的草莓状黄铁矿直径平均4.6-87μm，标准偏差1.7-3.5μm。细粒草莓状黄铁矿指示了界线地层贫氧的沉积环境，贫氧事件发生的层位与生物灭绝线一致即25层底部的黄铁矿壳层，表明贫氧事件可能是生物灭绝的直接原因和延迟生物复苏的一个重要因素。2煤山剖面P-T界线地层BC和燃烧源PAHs在MSC23-2、MSC24c、MSC24ec和26层出现四个明显的高值，其中26层峰值特征最明显。BC和PAHs含量从25层底部逐渐上升，至26层最高，到27层后迅速下降到背景值。BC、BC／TOC、δ13C_BC、PAHs等指标反映二叠纪末期出现多次大火事件，大火事件在三叠纪早期出现长期间断。事件层大火对应26层可能具有阶段性，大火的燃烧源除地表植被外，可能还包括泥炭、煤、甲烷水合物等。事件层大火是二叠纪末期的最后一次，也是规模和强度最大的一次燃烧，大火加速了地球表层环境的恶化和生物的灭亡。界线地层中BC和PAHs还表现为长兴组高背景的周期性波动和上部殷坑组低背景的平缓变化，反映了生物灭绝前后陆地生态系统由兴盛到衰败的转变；3在事件层25层中发现高温石英、锆石和铁质球粒等，揭示了界线粘土的火山成因；在黄铁矿壳层25层底部中发现了C_60，与贫氧事件和生物灭绝事件出现在同一层位，可能指示了大火事件。此外，在界线粘土中没有找到纳米金刚石。4综合分析认为，在P-T之交，地球上发生了强烈的火山活动，大规模火山喷发引燃了猛烈的大火，二者共同作用，造成全球气候环境的剧烈变化，并引起贫氧事件的发生。这一链条反应可能是导致生物灭亡和全球生态系统崩溃的主要原因 There were several mass extinction events in the earth\s history, and the Permian-Triassic event is one of the most extensive, which destroyed 90 marine species, 70 land vertebrate genera and most land vegetation. The events related to mass extinction become hot topics of world science. These studies will help to understand the evolution of life and earth, and supply thoughts and s to the current environment changes and living conditions of human being.The P-T mass extinction pattern, together with depositional records suggest a series of great geological events abruptly happened at that time, including Siberia basalt volcanic eruption, bolide impact, anoxic event, acid rain, the release of methane hydrate, the rise and fall of sea level, overturn of oceanic seawater, and so on. The key factor in understanding the P-T mass extinction is the relationship between these events and mass extinction. Up to now, volcanic eruption and bolide impact are two main hypotheses. Though many advances are achieved, contradictions and disputes on the records and boundary events are still left.Meishan section GSSP, Global Stratotype of Permian-Triassic Boundary Section and Point is one of the most extensive studied P-T boundary sections in the world. Abnormal depositional records, including carbon isotope, oxygen isotope, sulphur isotope, strontium isotope, trace elements and rare earth elements, biomarker, gypsum, fullerene, noble gas, high quartz, microspherule, iridium and other platinum group elements, meteorite fragments, have been searched and studied for many times.The international cooperative item “Cooperative Investigation of Permian-Triassic Events PTEs on new core samples from Meishan, South China“, led by Prof. Jin from Nanjing Institute of Geology 2 First analysis of BC and combustion-derived PAHs in the P-T boundary, and pointing out intensive wildfires occurred at the end of Permian in Meishan. The detailed results list as below.1 Sizes and abundances of pyrite framboids were determined from acid residues and in situ on polished sections of the Meishan P-T boundary samples. Abundant fine pyrite framboids occur in pyrite lamina, Beds 25, 26, 28 and 29 in the Meishan PTB with the mean size of 4.6-8.7μm, reflecting the dysoxic depositional environment. The dysoxic event coincided with the mass extinction line the pyrite lamina, i.e. the bottom of bed 25. Thus, the dysoxic event probably was the main reason for the mass extinction and prolonged recovery of biomass. 2 There exist large quantity of BC and combustion-derived PAHs in the samples of MSC23-2, MSC24c, MSC24ec and bed 26 in the Meishan P-T boundary section. The indicators of BC、BC/TOC、δ13C_BV、PAHs revealed frequent wildfires occurred at the end of Permian, most extensive wildfire coinciding with event beds beds 25 and 26, and long time of wildfire gap in the early Triassic. Event beds wildfire was the last and most extensive at the end of Permian. It was likely a long combustion and may have different phases. The sources of fire probably included land vegetation, peat, coal and methane hydrate. Event beds wildfire accelerated the environment deterioration and the mass extinction. It most likely resulted from the abrupt climate and environment change inducing by violent Siberia basalt volcanic eruption. Abundance with periodical fluctuation of BC and PAHs in Changshing ation vs deficiency with sooth change in upper Yinkeng ation showed the flourish and decline of land ecosystem before and after the mass extinction.3 The discovery of high quartz, zircon and iron spherule in the event beds bed 25 discloses the volcanic origin of boundary clay. The discovery of fullerene in the pyrite lamina the bottom of bed 25, coinciding with the occurrence of dysoxic event and mass extinction, probably indicates the intensive wildfire. Furthermore, nanodiamond didn\t extract from Meishan boundary clay.4 In summary, violent volcanic activities occurred during P-T transition resulted in global environmental changes and intensive wildfires, and consequently dysoxic event occurred. The biomass of land and ocean may be seriously destroyed in the interactive chain.;