【作者】 宋虎跃；

【导师】 童金南； Thomas Algeo；

【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】 古、中生代之交的地质突变期不仅记录了显生宙地球历史上最大的生物灭绝事件,同时也见证了历时最长的生物复苏过程。中生代初迟缓海洋生物复苏和异常海洋环境变化之间的协同演变关系成为当前国内外学者重点关注的研究内容。前人围绕二叠纪-三叠纪之交生物灭绝时期的海洋碳、硫循环和缺氧事件进行了大量研究工作,但对中生代初生物迟缓复苏时期的海洋碳、硫循环和氧化还原条件开展的研究工作相对薄弱。本文选取生物地层研究程度较高的南盘江大贵州滩(打讲、关刀、明塘、边阳)和扬子台地(煤山、大峡口、湖山、平顶山西坡、马家山南坡)早、中三叠世地层,开展高分辨率的碳同位素、硫同位素、铁组分、莓状黄铁矿大小分布等研究工作,尝试探讨该时期异常的海洋碳-硫循环、氧化还原环境与生物迟缓复苏的相互关系。为了探讨华南早三叠世海洋碳同位素水深梯度(△δ13C=δ’3C浅水-δ13C深水)的变化情况及其古海洋意义,选择扬子和南盘江地区代表不同水深的8条剖面,在精细的生物地层学研究基础上进行碳同位素分析和对比、Aδ13C重建及其古海洋意义探讨工作。研究结果表明：(1)浅水剖面δ13Ccarb组成相对偏重,并且记录了早三叠世较为完整的δ13Ccarb标志性漂移事件；深水剖面δ13Ccarb组成相对较轻,“缺失”个别的δ13Ccarb标志性漂移事件。Griesbachian至Spathian早期,δ13Ccarb波动频繁且幅度较大,说明该时期动荡的海洋碳循环；Spathian中晚期,δ13Ccarb的波动频度和幅度均有所减缓,反映该时期海洋碳循环趋于稳定；(2)扬子和南盘江地区均具有较大的A813C(3%0～δ%o),Griesbachian至Smithian晚期Aδ13C较高, Spathian早期Aδ13C发生了显著的变化。通过探讨海洋高生产力和海水停滞分层对Aδ13C贡献,发现海水停滞分层是导致Aδ13C增大的主要原因；(3)早三叠世持续的高pC02和高温背景导致该时期的海水停滞分层、缺氧,温室气候的元凶可能是同时期频繁的火山活动。Griesbachian早期、Dienerian晚期至Smithian晚期较高的Aδ13C指示这两个时期火山活动频繁,海水分层程度严重；(4)Smithian-Spathian之交Aδ13C的转变指示该时期海洋循环发生剧变,海水循环增强,海水分层减缓,与该时期的降温事件相吻合；(5)综合早三叠世δ13Ccarb和A813C的变化情况,对该时期海洋循环,火山活动,生产力变化情况进行了详细划分,为探讨该时期生物与环境的相互关系提供了一定的科学依据。为了恢复中生代初海水硫同位素演化情况和探讨该时期异常的海洋碳-硫循环,对大贵州滩打讲、关刀剖面下、中三叠统进行了高分辨率的碳酸盐晶格硫(CAS)硫同位素研究。以牙形石生物地层为基础对早、中三叠世δ13Ccarb和δ34SCAS进行精细对比,取得以下进展：(1)建立了早、中三叠世高精度完整的δ34SCAS演化曲线。早三叠世δ34SCAS组成(平均值：～26%o)相比晚二叠世(～22%o)和中三叠世(～17‰)偏重,指示早三叠世海洋缺氧严重,硫酸盐还原作用(BSR)较强；(2) Griesbachian至Spathian早期,δ34SCAS波动剧烈,并且与δ13Ccatb具有明显的正相关关系,说明该时期海水硫酸盐浓度很低,导致海水硫酸盐和碳酸盐的滞留时间相当,模拟结果显示该时期硫酸盐浓度为现代海洋的十分之一,小于3mM；(3) Spathian期的δ13Ccarb和δ34SCAS变化率同步减小,并且δ13Ccarb和δ34SCAS之间呈现一定的负相关关系,可能是由于海水循环加强引起的,这与△δ13C的指示意义相吻合：(4)δ13C和δ34SCAS在Griesbachian早期、Dienerian-Sm ithian之交、Smithian-Spathian之交同步正漂,这是由于有机碳、黄铁矿埋藏在缺氧硫化环境中增强造成的,指示这三个时期存在着严重的海洋缺氧、硫化事件；(5)中三叠世,δ34SCAS下降至15‰左右,其变化率也进一步降低,反映该时期海洋以正常的氧化状态为主,硫酸盐浓度进一步升高,碳-硫同位素的正相关关系逐渐消失；(6)中生代初海洋碳-硫循环变化与生物复苏辐射过程相吻合,早三叠世动荡的海洋碳-硫循环及缺氧环境导致生物迟缓复苏,中三叠世正常的海洋碳-硫循环与氧化环境为生物提供适宜的生存环境,促进生物快速复苏、辐射。中生代初的生物复苏过程中,在Smithian-Spathian之交还发生了一次规范不大,但意义深远的次级大灭绝事件。为了探究这一关键时期海洋环境变化与生物灭绝事件的相互关系,我们对地层研究基础较好的巢湖马家山南坡剖面Smithian-Spathian地层进行高精度采样,开展碳、硫同位素、铁组分、莓状黄铁矿大小分布等研究。结果表明：(1)Smithian-Spathian之交δ13Ccarb组成出现高达6‰的正漂,这次正漂事件具有全球可对比性；(2)铁组分、莓状黄铁矿大小分布以及硫同位素组成同时指示Smithian-Spathian之交曾发生严重的海洋缺氧和硫化事件；(3)δ34Spy在硫化环境中明显偏轻,组成相对稳定,根据同时期海水硫酸盐硫同位素组成可推断BSR分馏达到理论最大值。δ34Spy在贫氧-氧化环境中相对较重,分布较为离散,可能受到后期成岩作用的影响；(4)基于Smithian-Spathian之交海洋缺氧与同时期海洋降温、上升流频繁相对应,提出了该时期海洋生物环境事件的发展机理：降温事件引发海洋循环加强,上升流频繁发生并携带大量的深海营养物质促使海洋表层初级生产力升高,δ13Ccarb明显正漂,碳埋藏的加剧和有机质分解引起海洋的缺氧硫化,引发生物灭绝。生产力升高和碳埋藏加强进一步促使大气pC02降低,引发Spathian早期的降温事件。同时,降温事件可能指示该时期火山作用明显减弱；(5)海洋循环的变化与△δ13C变化及碳和硫循环变化情况相吻合,同时说明海洋环境在Spathian早期发生了明显的转变。为深入探索二叠纪-三叠纪之交浅水相区海洋缺氧的演变过程和形成机理,对位于“大贵州滩”台地内部打讲剖面的二叠系-三叠系界线地层中的生物组成和关键地球化学指标进行了系统研究,结果表明：(1)生物大灭绝之前的浅水碳酸盐岩台地表现出低硫(总硫和黄铁矿硫)、低黄铁矿硫/有机碳比值(S,黄铁矿/C有机)、低黄铁矿化系数(DOP)的特征,同时记录了碳同位的负漂和硫化氢气体释放事件,表明该时期浅水台地以氧化环境为主；(2)生物大灭绝之后的各种地球化学指标显示浅水台地开始向贫氧-缺氧环境转变,但整体缺氧程度不高,主要为贫氧-缺氧相；(3)以此为基础,本文提出该时期南盘江盆地缺氧事件的基本演变模式,即大灭绝前频繁的火山活动释放大量有毒气体导致陆地生态系统瓦解,陆地风化加强,从而引起δ13Ccarb负偏；与此同时,陆源物质输入的加强导致海洋贫氧层(OMZ)扩张。OMZ间歇性入侵透光带时,导致H2S气体向浅水台地释放,从而引发黄铁矿硫埋藏脉冲式上升的现象。但该时期浅水台地仍以氧化环境为主。大灭绝之后,陆地风化作用加剧,OMZ急剧扩张,浅水台地开始向贫氧-缺氧相转变；(4)新的地球化学数据也进一步证实了该区微生物岩形成于缺氧环境,“大贵州滩”二叠纪-三叠纪之交氧化还原环境的演变历程也与新近报道的铀同位素研究结果相吻合,从而为进一步探讨该时期海洋缺氧事件与该时期的重大生物突变关系提供了新的认识。综上,本论文建立了中生代初海水δ34SCAS的演化曲线,探明了该时期异常的碳-硫循环状态与氧化还原环境,为探讨该时期生物与环境的相互作用提供了重要的科学依据。更多还原

【Abstract】 The Palaeozoic-Mesozoic transition witnessed the largest mass extinction and the longest biotic recovery of Earth life during the Phanerozoic. Recently, an increasing number of geologists focus on the relationship between the delayed marine ecosystem recovery and perturbed ocean conditions. The marine carbon-sulfur cycles and oceanic anoxic events across the Permian-Tirassic boundary have been long concerned, but little has been published on the same extreme events in the aftermath of the Great Dying. To reconstruct marine carbon-sulfur cycles and determine the redox conditions of the Early Mesozoic oceans, several important Lower and Middle Triassic sections (including the Dajiang, Guandao, Mingtang and Bianyang sections in the Nanpanjiang Basin; the Meishan, Daxiakou, Hushan, West Pingdingshan, and South Majiashan sections in the Yangtze platform) in South China have been sampled to study several important gepchemical proxies (including carbon isotope, sulfur isotope, iron species, and distribution of pyrite framboids).In this study, eight Lower Triassic sections from the northern Yangtze area and Nanpanjiang basin, representing several different depositional settings with various water depths, have been sampled to reconstruct δ813C (Δδ13C=δ13Cshallow water-δ13Cdeep water) of the Early Trissic seas. Of these,δ13Ccarb recorded in shallow marine facies sections are enriched in13C. These sections have recorded distinct δ13Ccarb excursions throughout the Early Triassic. However,δ13Ccarb recorded in deep-water facies sections are delepted in13C. These sections also lost some parts of the δ13Ccarb excursions. The δ13Ccarb excursions exbihit three pronounced negative-positive shifting cycles from the early Griesbachian to early Spathian (<2Ma), implying the large perturbation of the contemporaneous marine carbon cycles. There is a positive-negative cycle of δ13Ccarb during middle-late Spatian (-3Ma), indicating the mitigative carbon cycle. Both the Yangtze platform and Nanpanjiang Basin have recorded some large Δδ13C (3‰～8‰) during the Griesbachian to late Smithian. Compared with the high primary productivity, the stratified and stagnant ocean conditions were likely the primary cause of the large Δδ13C. The stratified and anoxic ocean conditions were attributed to the persistent high temperature and high content of pCO2. The strong volcanic activity might have resulted in the high pCO2and high seawater temperature. The large Δδ13C values during the early Griesbachian and late Dienerian to late Smithian, respectively imply the enhanced volcanic activity and seriously stratified ocean conditions during these periods. During the Smithian-Spathian transition, the overturned Δδ13C value indicates the stratified ocean began to circulate and it is agree with the cooling event in that time. An oceanographic model throughout the Early Triassic has been developed by integrating the δ13Ccarb, Δδ13C and chemical weathering results.The Lower and Middle Triassic strata of the Dajiang, Upper Guandao and Lower Guandao sections, located at the Great Bank of Guizhou, have been sampled to undertake the δ34SCAS study during the Early Mesozoic. The δ34SCAS compositions of the Early Triassic (average:～26‰) are more positive than the Late Permian (～22‰) and Middle Triassic (～17‰) counterparts and reflect the enhanced ocean anoxia and bacteria sulfate reduction during the Early Triassic. Based on the conodont stratigraphy and radiometric dating, the relationship between the δ13Ccarb and δ34SCAS has been established. The δ34SCAS compositions show great change rates and strong positive relationship with the δ13Ccarb values through the Griesbachian-early Spathian. These phenomenona indicate that the sulfate concentrations of the contemporaneous seawater were very low and the residence time of the sulfate was equal to the carbonate. Simple modeling shows that the sulfate concentrations during the Early Triassic decreased to one tenth of the modern ocean (<3Ma). During the Spathian, the change rates of δ13Ccarb and δ34SCAS decreased abruptly, and there is a negative relationship between them. These changes may be attributed to the enhanced ocean circulation. Both δ13Ccarb and δ34SCAS exhibit distinct positive excursion during the early Griesbachian, Dienerian-Smithian boundary and Smithian-Spathian boundary, respectively. Both enhanced organic carbon and pyrite in anoxic (euxinic) conditions resulted in the positive excursion events. Thus, the δ13Ccarb and δ34SCAS excursions indicate three super ocean anoxic (euxinic) events during these periods. The δ34SCAS values decreased to15‰in the Middle Triassic, indicate that the oxic conditions dominated the ocean. The increased sulfate concentration resulted in the small change rate of the δ34SCAS.The marine sulfur-carbon cycles coincide with the delayed biotic recovery. The large perturbed sulfur-carbon cycles during the Early Triassic represent the periodic ocean anoxic (euxinic) events that resulted in the mass extinctions and delayed recovery. The normal sulfur-carbon cycles in the Middle Triassic indicate the hospitable ocean conditions for the biotic radiation.The Smithian-Spathian boundary coincides with a second-order mass extinction, global cooling, and large positive C-isotope excursions. However, the causes of these biotic and environmental changes have not been resolved. In order to evaluate the changes in oceanic redox and environmental conditions, petrographic and geochemical analyses were undertaken in this study, including the distribution of pyrite framboids and carbon and sulfur isotopes, and iron species across the Smithian-Spathian boundary at the southern Majiashan section. An～6‰positive shift in δ13Ccarb across the Smith-Spathian boundary indicates a global event. The iron speciation, sulfur isotopes and pyrite framboid data provide unambiguous evidence for an intense but transient episode of euxinia around the Smithian-Spathian boundary. In the euxinic interval, the pyrite sulfur isotopes (δ34Spy) are negative and agminated. The difference between the δ34SCAS and δ34Spy implies the largest of the fractionation in the bacterial sulfur reduction. The δ34Spy in suboxic and oxic conditions are positive and scattered. The scattered compositions are attribute to the subsequent diagenesis. Based on the ocean anoxia, abrupt cooling and upwelling events during the Smithian-Spathian boundary, this study infers that these events were due to the reinvigorated oceanic thermohaline circulation and a resultant increase in marine primary productivity. Enhanced organic carbon burial and drawdown of atmospheric pCO2resulted in global cooling and concomitant changes in marine and terrestrial ecosystems. This hypothesis agrees with the Δδ13C and carbon-sulfur cycles.Mutil-geochemical proxies, including the total sulfur (TS), pyrite sulfur concentrations, Spyirte/Corg ratios, the degree of pyritization (DOP), and carbon isotopes through the Permian-Triassic boundary strata at the Dajiang section have been analysed. The results show that the TS, pyrite sulfur concentration, Spyirte/Corg ratio and DOP are in low values below the Late Permian mss extinction (LPME) horizon, indicating oxic condition dominated the shallow water facies. Besides, the geochemical data presented a negative excursion of carbon isotope and a hydrogen sulfide release event during this period. The increase of sulfur concentration, Spyirte/Corg ratio and DOP following the LPME demonstrates the dyoxic-anoxic condition in the shallow water facies. Consequently, the ocean anoxia developed in the Nanpanjiang Basin during Permian-Triassic transition. During the Late Permian, the enhanced volcanic activity released huge CO2and SO2, destroyed the terrestrial ecosystem, and resulted in the increased terrestrial weathering. Strong terrestrial weathering brought a large amount of organic carbon into the ocean and lead to the carbon isotopic negative excursion and the expansion of the oxygen minimum zone (OMZ). The H2S would diffuse into the shallow water periodically when the OMZ expanded into the euphotic zone, and further resulted in the pulse of enhanced pyrite burial. Nevertheless, the shallow water was still in oxic condition during this period. During the LPME and its aftermath, enhanced terrestrial weathering caused the further expansion of the OMZ and resulted in the dyoxic-anoxic condition of the shallow water. In addition, the microbialites that occurred in the immediate aftermath of the LPME were formed in anoxic condition.In conclusion, this study has established a high resolution δ34SCAS curve in the early Mesozoic, and clarified the unusual carbon-sulfur cycles and marine redox conditions. This provide important evidence for further understanding of the biotic and environmental events in the aftermath of the great end-Permian mass extinction.更多还原