华南晚泥盆世微生物礁丘对生物事件和环境变化的指示

The purpose of this study is to give insights into the research of F-F mass extinction through: 1)investigating the correlation and synchronism relationships between development of the Late Devonian microbial reef from Liujing, Hengxian, Guangxi and environmental events, such as sea level and nutrient change, and drastic reduction event of stromatoporoids, corals and brachiopods; 2) take the latest knonwn Famennian mud mound from Ganxi, Beichuan, Sichuan as an example, discussing the destruction and restoration history of the Famennian reef ecosystem. This study examines, in detail, conodont stratigraphy of the Liujing microbial reef and nearby stratum to try to find out the temporal relation between development of microbial reef and the F-F biological event. Combining with paleontological and sedimentary research of the Upper Devonian of the Liujing section, restore ecological revolution and environmental change before and after the mass extinction. According to field and indoor observation of its structure, and analysis of microbial assemblages, reconstruct detailed development pattern of the Liujing microbial reef, evaluate the relative controls of calcimicrobes and environmental factors during growth process of the microbial reef. Recognize types of calcified algae, cyanobacteria and stromatoporoids and their distribution in the Ganxi mud mound, compare it with other Famennian and Early Mississippian microbial reefs and mounds all over the world, comprehensively analyze the development and revolution regulation of stromatoporoids and microbial reefs and mounds through the Famennian to Tournaisian interval after the F-F biological event. In addition, a comparative study on reef- and mound-forming organism assemblages, sub-facies, positions in the second order stratigraphic pattern and platform cyclicity of the microbial reef from Liujing and the mud mound from Ganxi will also be conducted in this project.

项目通过考察广西横县六景晚泥盆世菌藻礁的发育与环境事件和生物锐减事件之间的相关性、同步性问题和以四川北川甘溪最年轻的法门期泥丘为例，探讨生物礁生态系破坏和恢复的历史为F-F生物灭绝事件的研究带来新的视角。具体地，通过对六景菌藻礁及附近地层做牙形石生物地层工作，弄清菌藻礁发育与F-F生物事件的时序关系。结合六景剖面上泥盆统古生物学和沉积学方面的研究，恢复大灭绝事件前后的生态演化和环境变迁。依据野外、室内构造观察，微生物类群组合面貌分析，重建六景菌藻礁的详细发育过程，探讨微生物和环境因素对菌藻礁生长过程的相对控制作用。识别甘溪灰泥丘内钙化菌藻类和层孔虫的组成和分布规律，比较该灰泥丘与世界其他法门期、密西西比亚纪早期微生物礁丘的异同点，综合分析F-F生物事件之后层孔虫和微生物礁丘的发展演化规律。比较两地礁和丘中钙质微生物类群和微相的差别，揭示它们在碳酸盐岩层序地层格架中的位置和作用的差异。

项目考察了广西横县六景晚泥盆世菌藻礁的发育与环境事件和生物锐减事件之间的相关性、同步性问题。并以四川北川甘溪法门期泥丘为例，探讨了生物礁生态系破坏和恢复的历史。具体地，从六景剖面融县组菌藻礁及附近地层鉴定出212枚共计28种，2未定种牙形石。通过牙形石重要分子组合将谷闭组顶部至融县组菌藻礁地层划分为早rhenana、晚rhenana、linguiformis和早triangularis四个牙形石带，并判断出六景融县组菌藻礁发育跨时代，其下中部为弗拉期，上部为法门期。识别出六景菌藻礁主体岩性为肾形藻泥晶灰岩、含生物碎屑泥晶藻粒灰岩和生物碎屑泥晶灰岩，其主要组成微生物Izhella，Shuguria和Paraepiphyton的局部含量高达50%。认为融县组下部砾屑灰岩及以上层位层孔虫和珊瑚等造礁后生动物少有痕迹表明其在华南六景地区弗拉晚期就已开始让位菌藻类。并由砾屑灰岩中类型多样的角砾恢复此阶段骨骼后生动物的减少由物理事件引起；该垮塌、甚至大范围的海啸事件发生后，已形成的基底岩石被切割，形成角砾，被同沉积的泥状灰岩粘结，最后由随后繁盛的菌藻类和胶结物形成生物礁。将先前划归法门期的菌藻礁主体归回弗拉末期后，结合类Renalcis可能的异养细菌分类位置继续支持广泛的缺氧是引起F-F生物灭绝的一个重要因素。测试了六景菌藻礁及附近地层7个样品的生物标志化合物，总结出烷烃组分中的正构烷烃、类异戊二烯烃、甾烷和藿烷的含量在菌藻礁之下的砾屑灰岩层段达到最高值，其显示的往上生物量减少，菌藻类相对贡献增加的变化与薄片镜下观察一致。识别出甘溪剖面茅坝组泥丘主要由碳酸盐灰泥组成，其次为Shuguria，此外还含少量层孔虫、床板珊瑚、苔藓虫和毫米级钙质管丝体结壳。综合前人资料判定该泥丘是法门期相对较晚的实例；同时也是法门早中期菌藻礁丘与杜内期微生物泥丘、维宪期海绵-珊瑚-微生物礁的过渡类型。认为茅坝泥丘的发育证明了短暂复苏的层孔虫和与法门早期菌藻礁、丘建造生物类似的菌藻类一起在高能环境固化、粘结沉积物的能力。并提出它们显示相较于法门晚期以前生物礁和丘更大的相似性肯定了F-F生物灭绝之后幸存的和复苏的生物建造细小的、但生物类型丰富的泥丘的能力；而与密西西比亚纪更大的差异性可能源于Hangenberg生物灭绝事件的影响。初步比较了六景菌藻礁和甘溪灰泥丘中钙质微生物类群和微相的差别。