顯生宙碳循環(huán)異常環(huán)境的地球生物學過程研究

顏佳新　王永標　童金南　龔一鳴　宋海軍

摘 要：該研究以顯生宙碳循環(huán)異常環(huán)境的地球生物學過程為研究主體，重點研究二疊紀-三疊紀之交和晚泥盆世兩大重大地質突變期的地球生物學過程特點和規(guī)律，查明碳循環(huán)異常的起因及其對生態(tài)系統(tǒng)的影響，探索生物與環(huán)境的相互作用。二疊紀—三疊紀之交是古生代海洋生態(tài)系破壞和中生代型海洋生態(tài)系開始重建的轉折點，古海洋缺氧對該轉折影響深遠。通過對華南多條剖面高精度碳同位素、碳-硫形態(tài)、碳酸鹽晶格硫（CAS）、DOP和Δδ13C等分析測試，該年度研究在認識該時期古海洋缺氧的時限、程度、演化和成因機制等方面取得明顯進展。大滅絕前的二疊紀淺水碳酸鹽巖臺地以氧化環(huán)境為主。由于火山活動釋放大量的CO2、SO2等氣體，導致氣溫上升、陸地生態(tài)系統(tǒng)開始瓦解、陸源輸入增加、海洋貧氧層（OMZ）擴張，大滅絕后海洋環(huán)境向缺氧環(huán)境轉變。早三疊世早期δ34SCAS明顯比晚二疊世偏重，波動劇烈，且與δ13Ccarb明顯正相關（3次同步正漂） ，意味著嚴重的海洋缺氧、硫化事件，海水硫酸鹽濃度很低（<3 mM）。早三疊世早期δ13Ccarb頻繁和幅度較大的波動，指示了動蕩不穩(wěn)定的海洋碳循環(huán)；之后δ13Ccarb和δ34SCAS變化率同步減小，兩者呈現(xiàn)負相關關系，可能是由于海水溫度下降、海水循環(huán)增強，海洋碳循環(huán)趨于穩(wěn)定。中三疊世δ34SCAS下降，變化率進一步降低，碳-硫同位素的正相關關系逐漸消失，反映該時期海洋硫酸鹽濃度進一步升高，以正常的氧化狀態(tài)為主。大滅絕后動蕩的海洋碳-硫循環(huán)及缺氧環(huán)境導致了生物復蘇遲緩。 晚泥盆世弗拉斯-法門（F-F）生物大滅絕事件是顯生宙又一重大地質轉折時期，集中體現(xiàn)在生物礁生態(tài)系中，菌藻類取代后生動物（珊瑚-層孔蟲）造礁。對華南地區(qū)多個剖面細致的觀察和統(tǒng)計分析表明，菌藻類可侵入“活著”的后生動物并抑制后生動物的發(fā)育。后生動物骨骼雖然具有自我修復功能，并對菌藻類有反抑制作用，但在環(huán)境惡化時，這種平衡會被打破，導致后生動物個體無法生長而死亡，直至后生動物生物礁被菌藻類生物礁取代。中-晚泥盆世植物登陸，陸生植物粗大根系的發(fā)育，導致陸地生物-化學風化急劇增加，近岸海域海水營養(yǎng)水平激增，可能是晚泥盆世菌藻類“大舉入侵”珊瑚-層孔蟲礁生態(tài)系最終取而代之的直接推手。

關鍵詞：碳循環(huán) 地球生物學 古氧相 二疊紀-三疊紀之交 弗拉斯-法門 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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