显生宙碳循环异常环境的地球生物学过程研究

颜佳新　王永标　童金南　龚一鸣　宋海军

摘 要：该研究以显生宙碳循环异常环境的地球生物学过程为研究主体，重点研究二叠纪-三叠纪之交和晚泥盆世两大重大地质突变期的地球生物学过程特点和规律，查明碳循环异常的起因及其对生态系统的影响，探索生物与环境的相互作用。二叠纪—三叠纪之交是古生代海洋生态系破坏和中生代型海洋生态系开始重建的转折点，古海洋缺氧对该转折影响深远。通过对华南多条剖面高精度碳同位素、碳-硫形态、碳酸盐晶格硫（CAS）、DOP和Δδ13C等分析测试，该年度研究在认识该时期古海洋缺氧的时限、程度、演化和成因机制等方面取得明显进展。大灭绝前的二叠纪浅水碳酸盐岩台地以氧化环境为主。由于火山活动释放大量的CO2、SO2等气体，导致气温上升、陆地生态系统开始瓦解、陆源输入增加、海洋贫氧层（OMZ）扩张，大灭绝后海洋环境向缺氧环境转变。早三叠世早期δ34SCAS明显比晚二叠世偏重，波动剧烈，且与δ13Ccarb明显正相关（3次同步正漂） ，意味着严重的海洋缺氧、硫化事件，海水硫酸盐浓度很低（<3 mM）。早三叠世早期δ13Ccarb频繁和幅度较大的波动，指示了动荡不稳定的海洋碳循环；之后δ13Ccarb和δ34SCAS变化率同步减小，两者呈现负相关关系，可能是由于海水温度下降、海水循环增强，海洋碳循环趋于稳定。中三叠世δ34SCAS下降，变化率进一步降低，碳-硫同位素的正相关关系逐渐消失，反映该时期海洋硫酸盐浓度进一步升高，以正常的氧化状态为主。大灭绝后动荡的海洋碳-硫循环及缺氧环境导致了生物复苏迟缓。 晚泥盆世弗拉斯-法门（F-F）生物大灭绝事件是显生宙又一重大地质转折时期，集中体现在生物礁生态系中，菌藻类取代后生动物（珊瑚-层孔虫）造礁。对华南地区多个剖面细致的观察和统计分析表明，菌藻类可侵入“活着”的后生动物并抑制后生动物的发育。后生动物骨骼虽然具有自我修复功能，并对菌藻类有反抑制作用，但在环境恶化时，这种平衡会被打破，导致后生动物个体无法生长而死亡，直至后生动物生物礁被菌藻类生物礁取代。中-晚泥盆世植物登陆，陆生植物粗大根系的发育，导致陆地生物-化学风化急剧增加，近岸海域海水营养水平激增，可能是晚泥盆世菌藻类“大举入侵”珊瑚-层孔虫礁生态系最终取而代之的直接推手。

关键词：碳循环 地球生物学 古氧相 二叠纪-三叠纪之交 弗拉斯-法门 地微生物

Abstract： This research was designed to focus on records of Permo -Trassic and Frasian - Famenian transitions in the South China， to decipher the origin and evolution of the abnormal carbon cycle and their influences on the coeval marine ecosystem， as well as the interactions between the organism and environments. The Permo-Trassic transition is the turn-point of the Palaeozoic ecosystem to Mesozoic ecosystem， with recurrent lethal marine oxygen deficient pulses （ODP）. Present works， mainly based on isotopic analyses of δ13Ccarb， δ34SCAS， Δδ13C， and chemical analysis of DOP， delimits the extension， temporal pattern of these marine ODP. Oxic condition prevails on carbonate platform before the end-Permian mass extinction. Because of CO2 and SO2 released by volcanic activity， the resultant rise of the temperature and deteriorating vegetation would increase the terrestrial input to marine ecosystem and result in the expansion of the oxygen minimum zone. This scenario has been detailed by present investigations. The δ34SCAS values strongly fluctuate， and correlate positively with δ13Ccarb in period from the Griesbachian to late Smithian stages， indicating an oxygen deficient condition with low sulfate content， and an unstable carbon cycle. Then a synchronic decrease of the negatively correlating δ13Ccarb and δ34SCAS values coincides with the drop of the sea surface temperature， implying an improvement of the marine circulation and carbon cycle. From the middle Triassic on， the δ34SCAS value decrease and its variation declines. And the synchronic variation vanishes， indicating an oxic condition companying with a rise of the sulfate content. The unstable carbon and sulfur cycle with lethal ODPs delays the organism recovery after the big extinction. The Frasinian Famenian （F-F） extinction is exemplified by the replacement of the coral- stromatoporoids reef by bacterial reef. Detailed observation and statistic analyses on F-F sections in the South China indicate that bacteria might intrude and inhibit live metazoan. Although of healing and anti-inhabitation， individual metazoan organism might stop growth， even die and be replaced， while it was attacked by bacterial intrusion in exacerbated environments. Accelerating biochemical weathering on land， due to plant landing， would increase nutrition input to littoral environments， where reef habitat commonly located， and trigger the replacement.

Key Words： Carbon cycle； Geobiology； Paleo-redox facies； Permo-triassic； Frasinian-famenian； Geomicrobe

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