最近的一項研究表明,2.5億年前的泛大陸聚合引起了地球多圈層耦合變化以及地球表層環境的惡化,進而導致了古、中生代之交的生物大滅絕事件。
這篇名為「古、中生代之交生物大滅絕與泛大陸聚合」的研究論文發表於《中國科學:地球科學》2013年第10期,從圈層耦合的角度探討了泛大陸聚合與生物大滅絕的關係,由中國地質大學(武漢)生物地質與環境地質國家重點實驗室殷鴻福院士和宋海軍博士撰寫。
二疊紀末至三疊紀最初期(約2.5億年前),全球發生顯生宙最大的生物滅絕事件, 近95%的物種消失。 這次事件破壞了海洋和陸地的生態繫結構, 同時造成了陸地上的煤缺失、海洋中的礁缺失和矽缺失。是什麼原因導致如此規模的生物滅絕事件呢?對其成因與機理的探索已成為國際範圍內該領域研究中的一個熱點。
該研究表明,泛大陸聚合引起的地球多圈層耦合的劇烈全球變化與生物大滅絕關係緊密。泛大陸 (Pangaea) 約於二疊紀初形成,全盛時期是在晚二疊世至早三疊世。泛大陸意味著全球分散的大陸巖石圈匯聚為一,面積約近兩億平方公裡。這樣大的整體大陸巖石圈,其平均厚度顯著大於分散陸塊的平均厚度。根據均衡原理,大陸巖石圈愈厚,則其高出均衡面的部分愈大。與泛大陸相對應,當時全球海水匯聚為泛大洋 (Panthalassa),應遠較現代海洋更深。泛大陸與泛大洋的全盛時期,就是這樣一個高山深盆時期,它必然造成全球性的大海退並影響地球表層系統,特別是氣候。
泛大陸時期火山活動非常活躍,如西伯利亞的通古斯暗色巖、峨眉山玄武巖以及華南二疊-三疊紀之交的火山活動。這種全球規模的火山活動由地幔柱及板塊匯聚引起,可能與泛大陸的聚合有關。峨眉山玄武巖、西伯利亞暗色巖與中二疊世末和二疊紀-三疊紀之交兩次滅絕的時間耦合,它們之間有因果關係。火山爆發對生物滅絕的效應包括: 火山排入大氣中過量的二氧化碳、甲烷、二氧化氮、氰化物等,產生溫室效應,有毒氣體的汙染效應,破壞平流層的臭氧,導致紫外線輻射增強。
CO2 濃度升高導致全球增溫和缺氧事件、C 循環異常;海洋的物理化學異常(海水酸化,表層硫化事件,低硫酸鹽濃度,有機氮同位素異常)使海洋生物或者由於對缺氧和酸化的不適應而選擇性滅絕,或者由於高碳酸血症而死亡;大陸乾旱化及緯度風系取代季風氣候, 導致植被消亡,等等。
綜上,二疊紀-三疊紀之交的全球變化和生物大滅絕是地球各圈層相互作用的結果。地殼活動、大氣圈、水圈、沉積圈和生物圈 (包括各類生物自身演化的因素) 相互作用,從漸變到突變,集中爆發於2.5億年前。
圈層耦合關係是地球系統科學的重大問題,值得深入探討。從地球內部圈層到地球表層系統,再到生物演化,這一過程在時間上有延後性,在因果關係上有許多不確定性,還處於假說階段,需要更多的科學證據。(生物谷Bioon.com)
生物谷推薦英文摘要:
SCIENCE CHINA Earth Sciences DOI:10.1007/s11430-013-4624-3
Mass extinction and Pangea integration during the Paleozoic-Mesozoic transition
YIN HongFu*, SONG HaiJun
The greatest Phanerozoic mass extinction happened at the end-Permian to earliest Triassic. About 95% species, 82% genera, and more than half families became extinct, constituting the sole macro-mass extinction in geological history. This event not only caused the great extinction but also destroyed the 200 Myr-long Paleozoic marine ecosystem, prompted its transition to Mesozoic ecosystem, and induced coal gap on land as well as reef gap and chert gap in ocean. The biotic crisis during the Paleozoic-Mesozoic transition was a long process of co-evolution between geospheres and biosphere. The event sequence at the Permian-Triassic boundary (PTB) reveals two-episodic pattern of rapidly deteriorating global changes and biotic mass extinction and the intimate relationship between them. The severe global changes coupling multiple geospheres may have affected the Pangea integration on the Earth's surface spheres, which include: the Pangea integration→enhanced mountain height and basin depth, changes of wind and ocean current systems; enhanced ocean basin depth→the greatest Phanerozoic regression at PTB, disappearance of epeiric seas and subsequent rapid transgression; the Pangea integration→thermal isolation effect of continental lithosphere and decrease of mid-ocean ridges→development of continental volcanism; two-episode volcanism causing LIPs of the Emeishan Basalt and the Siberian Trap (259-251 Ma)→global warming and mass extinction; continental aridification and replacement of monsoon system by latitudinal wind system→destruction of vegetation; enhanced weathering and CH4 emission→negative excursion of δ13C; mantle plume→crust doming→regression; possible relation between the Illawarra magnetic reversal and the PTB extinction, and so on. Mantle plume produced the Late Permian LIPs and mantle convection may have caused the process of the Pangea integration. Subduction, delamination, and accumulation of the earth's cool lithospheric material at the "D" layer of CMB started mantle plume by heat compensation and disturbed the outer core thermo-convection, and the latter in turn would generate the mid-Permian geomagnetic reversal. These core and mantle perturbations may have caused the Pangea integration and two successive LIPs in the Permian, and probably finally the mass extinction at the PTB.