【作者】 胡庆；

【导师】 冯庆来； 黄俊华；

【作者基本信息】 中国地质大学 ， 地质学， 2011， 硕士

【摘要】 碳稳定同位素(δ13C)的研究已在地层划分、对比、古环境、地质事件的研究中得到广泛的运用。由于δ13C的变化是对环境变化和生命活动的直接响应,其组成在地层单元中的波动变化,可以直接或间接反映全球碳循环的变化。613C异常偏移通常与影响生命演化进程和作用的重大地质历史事件紧密相关,如海平面升降、火山活动、海洋缺氧事件、小行星撞击等,而且往往具有大范围甚至全球等时性和一致性。二叠纪—三叠纪(P—T)之交发生了显生宙以来最大的一次生物绝灭事件,生物灾变事件和环境剧变伴随着全球碳循环的动荡。尽管二叠纪—三叠纪之交碳同位素的研究已经十分深入,但是还存在很多分歧和不足。华南乃至世界上浅水碳酸盐台地剖面碳同位素的研究日趋成熟,但是深水相区仍需完善。浅水碳酸盐台地的二叠系—三叠系界线剖面往往由于地层高度凝缩或/和受地层层序的局限而造成许多地质信息缺失,或不易被识别。黔中地区发育深水相剖面,沉积连续,剖面完整,岩石特征和生物特征明显,保留了许多较浅海相地层而言更为精细的地质信息。对较深水地层中的碳同位素变化规律的研究,是更加全面地认识P—T转折时期生物—环境地质事件的演化不可或缺的一部分。本文研究了贵州新民二叠系—三叠系界线(PTB)剖面碳同位素(δ13Corg和δ13Corg)组成和疑源类、菌藻类丰度的变化特征。在牙形石生物地层学的基础上,结合野外沉积学特征,一方面研究碳同位素变化规律,并与“金钉子”(GSSP)煤山剖面等进行对比,以完善深水相地层碳同位素的研究资料,试图从碳循环的角度解译该转折时期生物—环境事件的重大改变；另一方面,综合利用δ13Ccarb和δ13Corg的变化规律,利用已有的生产力模型初步评估深水相地层生产力和有机碳埋藏状况。(1)新民剖面与煤山剖面可以进行等时框架对比。δ13Ccarb在晚二叠世绝灭界线层(MEB)以下发生缓慢负偏,暗示碳同位素异常早于生物绝灭事件。δ13Corg在二叠纪末期经历了缓慢降低(Clarkina yini带)和陡然降低(Clarkina meishanensis带)两个阶段,表明海洋环境的变化经历了由渐变到突变的过程。(2)新民剖面PTB附近的粘土岩是火山成因的,其原岩为长英质流纹岩,来源于板内火山喷发。在MEB层以下,单期次的火山活动在一段相对短暂的时间内,可以使δ13Corg值发生明显变化,但其影响幅度不足以使得海洋生态环境长期发生巨变；而在大绝灭之后的一段时间内,由于海洋生态系发生了翻天覆地的变化,δ13Corg变化已不是直接明显地对火山活动有较好的响应。暗示在生物大绝灭层(MEB)上下,海洋碳循环体系处于不同的碳同位素背景。(3)在MEB层内,δ13Corg在经历短暂高值后陡然降低。δ13Corg短暂高值的层位与绿硫细菌出现的层位相对应,且该层中产出较多的细粒状和草莓状黄铁矿,暗示绿硫细菌可能是导致这一异常高值的主要原因：海洋深部的底层水上涌可能造成新民剖面界线粘土层指示的海水透光带被H2S酸化缺氧的环境(PZE),以绿硫细菌占据主导地位的海洋菌藻类或疑源类的繁盛,致使δ13Corg在绝灭期内出现高异常值。对比世界著名的PTB剖面δ13Corg变化规律,发现MEB层内短暂δ13Corg高值在特提斯域比较明显,而在远洋剖面则相对缺失,暗示在只有相对局限的海域中PZE现象比较强烈。不同类型的海域上升流强弱差异及离陆远近引起陆源输入不同,可能是造成不同海域PZE及菌藻繁盛差异的重要原因。δ13Corg在MEB层内的陡然降低具有全球一致性,火山活动和海洋生态系崩溃以及可能存在甲烷水合物释放是绝灭期出现δ13Corg陡然降低的主要驱动因素。(4)残余TOC由初级生产力、有机碳埋藏分数forg和埋藏环境条件共同决定,但新民剖面的数据表明在晚二叠世初级生产力对残余TOC的贡献更大。这一结论还需进一步论证：我们在运用Kump的模型时,发现forg主要受控于δ13Ccarb。该模型认为火山活动对δ13Ccarb的影响甚微,并只把火山作用和风化作用的碳同位素值设定为δw=-5%o,即仅考虑火山作用在碳同位素分馏过程中的直接效应,而忽略了火山作用造成的一系列次级的环境效应,如“火山冬天”、海水酸化、有毒物质的喷出等对海洋环境造成的影响。然而晚二叠世大规模的火山作用造成的间接影对使海洋碳循环产生较大的异常。因此在定量—半定量计算特殊地质历史时期有机碳埋藏分数时,该模型还得进一步改进。更多还原

【Abstract】 Stable carbon isotope(813C) is widely used in the study of stratigraphic division and comparison, paleo-environment, and geological events. The variation ofδ13C accompanied with environmental changes and life activities, and the fluctuation of carbon isotope composition in stratigraphic units can reflect the perturbation in the global carbon cycle directly or indirectly. The abnormalδ13C is usually related to geological events that had great influence on life evolution such as sea level change, volcanism, ocean anoxia, asteroid impacts and so on, which is often consistent globally or in a wide region.The Permo—Triassic mass extinction was the greatest mass extinction of the Phanerozoic, associated with global carbon cycle disturbance. Although the study of carbon isotope during Permo—Triassic transition has been very thorough, there are still many differences and inadequacies. Those researches are relatively abundant on shallow-water carbonate platform sections in the South China even in the world, but still need more improvement in deep water areas. Some records usually missed or can not be easily identified in shallow water sections due to high degree of stratigraphic condensation and/or subject to limitations of the stratigraphic sequences. The deep water marine PTB sections were well developed in the middle of Guizhou province. The Xinmin section may be one of the best sections for studying the curious geological events during this transition because of its well exposure, uninterrupted sediments, light change of the lithology, well development of claystone and fauna, and clear lithological boundary, all of which may have record much more information than shallow-water sections. Therefore, the study of carbon isotope variations on deep-water section is an integral part to comprehensivly understand the evolution of environmental—biological events in the P—T transitional period.Bulk organic and inorganic isotope compositions and the abundance of acritachs, algae and microbes were examined from the Permo—Triassic boundary at Xinmin section, Guizhou. Based on conodont biostratigraphy and sendimentary characteristics, we study the variations of carbon isotope at Xinmin section, and compare it with that on Global Stratotype Section and Point Meishan section, supplying the carbon isotope data on deep—water sections. We will focus on the aspects that affect the carbon isotope fractionation in order to discuss the great changes of biological—environmental events during the P—T transitional period. And with the variations ofδ13Ccarb andδ13Corg, we will utilize the productivity model to assess the paleo-productivity and organic matter burial ground conditions of deep-water section.(1) Xinmin section and Meishan section can be compared in the same time scale. The beginning ofδ13Ccarb negative excursion below the mass extinction boundary(MEB) in the Late Permian implied that carbon cycle perturbation preceded the mass extinction event itself.813Corg data highlight environment changes in the latest Permian and show the following features:a gradual decreasing phase begins in Clarkina yini zone, and a sharp decreasing phase occurs during Clarkina meishanensis zone, indicating that the marine environment changed from the gradual transformation to mutation process.(2) The clay rocks near PTB in Xinmin section are formed from vocanic felsic rhyolite, generated in within-plate. The volcanic activity in one phase may make significant change ofδ13Corg value in a relatively short time, but no dramatic impact on marine environment for longer time scale. During the period following the mass extinction, the magnitude fluctuation ofδ13Corg was not directly to the volcanic layers. It implied that the ocean carbon cycle system changed greatly after the mass extinction.(3)δ13Corg exhibit a positive peak before the sharp decrease above the event horizon, and the flourishing of green sulfur bacteria(GSB) may result in this positive peak:because we found large amount of frambiodal and granule pyrite. The deep anoxic bottom water upward excursion may cause photic zone euxinia(PZE) which indicate the photic zone layer of the ocean is anoxic and riched in H2S. Then some microbes such as green sulfur bacteria may become dominant in the photic zone through this time, causing this positive peak ofδ13Corg in MEB layer. The temporary peak of 813Corg value exist in Tethyan PTB section, but relatively scare in the pelagic ocean PTB sections, suggesting PZE developed strongly only in the limited sea such as Tethys. However,δ13Corg decreased sharply near the MEB layer in Xinmin section with global consistency, which could be caused by the combination of volcanic activity, marine biological breakdown and a sudden release of methane hydrate.(4) Residual TOC is determined by the combinations of the primary productivity, organic carbon burial fraction(forg) and burial redox conditions. But the data in Xinmin section show that the paleo-productivity plays a dominant role on the value of residual TOC. This conclusion still needs further proof:we found that forg is mainly controlled byδ13Ccarb by Kump’s model, in which it concluded increased volcanism leads to only small changes inδ13Ccarb, and set the carbon isotope value of volcanic activity and weathering asδw=-5‰. That is to say, a series of secondary environmental effects caused by volcanism were ignored, such as "volcanic winter", ocean acidification, smoke and other toxic substances and so on, all of which had great impact on the marine environment. However, large-sized volcanism in the Late Permian could result in serious perturbation to marine carbon cycle. Therefore, the model still need to be further improved on calculating organic carbon burial fraction quantitatively or semi-quantitatively during this special geological period.更多还原