最近的一项研究表明,2.5亿年前的泛大陆聚合引起了地球多圈层耦合变化以及地球表层环境的恶化,进而导致了古、中生代之交的生物大灭绝事件。
这篇名为“古、中生代之交生物大灭绝与泛大陆聚合”的研究论文发表于《中国科学:地球科学》2013年第10期,从圈层耦合的角度探讨了泛大陆聚合与生物大灭绝的关系,由中国地质大学(武汉)生物地质与环境地质国家重点实验室殷鸿福院士和宋海军博士撰写。
二叠纪末至三叠纪最初期(约2.5亿年前),全球发生显生宙最大的生物灭绝事件, 近95%的物种消失。 这次事件破坏了海洋和陆地的生态系结构, 同时造成了陆地上的煤缺失、海洋中的礁缺失和硅缺失。是什么原因导致如此规模的生物灭绝事件呢?对其成因与机理的探索已成为国际范围内该领域研究中的一个热点。
该研究表明,泛大陆聚合引起的地球多圈层耦合的剧烈全球变化与生物大灭绝关系紧密。泛大陆 (Pangaea) 约于二叠纪初形成,全盛时期是在晚二叠世至早三叠世。泛大陆意味着全球分散的大陆岩石圈汇聚为一,面积约近两亿平方公里。这样大的整体大陆岩石圈,其平均厚度显著大于分散陆块的平均厚度。根据均衡原理,大陆岩石圈愈厚,则其高出均衡面的部分愈大。与泛大陆相对应,当时全球海水汇聚为泛大洋 (Panthalassa),应远较现代海洋更深。泛大陆与泛大洋的全盛时期,就是这样一个高山深盆时期,它必然造成全球性的大海退并影响地球表层系统,特别是气候。
泛大陆时期火山活动非常活跃,如西伯利亚的通古斯暗色岩、峨眉山玄武岩以及华南二叠-三叠纪之交的火山活动。这种全球规模的火山活动由地幔柱及板块汇聚引起,可能与泛大陆的聚合有关。峨眉山玄武岩、西伯利亚暗色岩与中二叠世末和二叠纪-三叠纪之交两次灭绝的时间耦合,它们之间有因果关系。火山爆发对生物灭绝的效应包括: 火山排入大气中过量的二氧化碳、甲烷、二氧化氮、氰化物等,产生温室效应,有毒气体的污染效应,破坏平流层的臭氧,导致紫外线辐射增强。
CO2 浓度升高导致全球增温和缺氧事件、C 循环异常;海洋的物理化学异常(海水酸化,表层硫化事件,低硫酸盐浓度,有机氮同位素异常)使海洋生物或者由于对缺氧和酸化的不适应而选择性灭绝,或者由于高碳酸血症而死亡;大陆干旱化及纬度风系取代季风气候, 导致植被消亡,等等。
综上,二叠纪-三叠纪之交的全球变化和生物大灭绝是地球各圈层相互作用的结果。地壳活动、大气圈、水圈、沉积圈和生物圈 (包括各类生物自身演化的因素) 相互作用,从渐变到突变,集中爆发于2.5亿年前。
圈层耦合关系是地球系统科学的重大问题,值得深入探讨。从地球内部圈层到地球表层系统,再到生物演化,这一过程在时间上有延后性,在因果关系上有许多不确定性,还处于假说阶段,需要更多的科学证据。(生物谷Bioon.com)
生物谷推荐英文摘要:
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.