近日,国际期刊Chemical Geology在线刊登了中科院地质与地球物理研究所 研究人员的最新研究成果“Evolution from an anoxic to oxic deep ocean during the Ediacaran-Cambrian transition and implications for bioradiation,”,文章中,研究者发现深海氧化与寒武纪“生物大爆发”的关系。
在埃迪卡拉纪-寒武纪(E-C)转折期,包括构造格局、古海洋、古气候以及生物都同时发生了重大的改变,这其中最为显著的就是新元古代末期埃迪卡拉动物群的消亡以及早寒武世宏体后生动物门类的爆发性分异、辐射和生态扩张(即“寒武纪生命大爆发”)。目前,关于这一关键时期生物快速辐射和生态扩张的驱动机制是国际学术界非常关注的热点问题,但也存在着较大分歧。虽然部分研究者认为,该时期大气氧浓度的增加并伴随深海的氧化是这一生物事件的环境触发因素之一,但由于对这一重要转折期深海的演化缺少详细的研究,因此其氧化还原状态转变过程还不是很清楚。另外,由于缺少地层对比上的约束,海洋的氧化过程与生物辐射的关系也并没有得到很好地阐述。
中科院地质与地球物理研究所油气资源室汪建国博士后在合作导师陈代钊研究员指导下,选择湖南西部跨越E-C界线附近的深水剖面为研究对象,以δ13Corg、δ34Spy以及铁组份为分析手段,重建了E-C转折期深海的演化过程。结果表明:E-C海洋的演化可分成两个阶段(阶段1和阶段2)。阶段1(Phase 1):具有高的FeHR/FeT比值(所有样品都大于0.38)以及低的Fep/FeHR比值(绝大部分样品都小于0.8),显示该时期深海为富铁缺氧,但在牛蹄塘组下部有两个样品的Fep/FeHR比值大于0.8,说明在早寒武世期间这种富铁缺氧的海水曾短暂的被硫化缺氧水体所取代,同时该时期δ13Corg、δ34Spy值具有一定的耦合关系。阶段2(Phase 2):与阶段1存在明显的区别,FeHR/FeT比值快速降低(总体低于0.38),Fep/FeHR比值向上有上升的趋势,但没超过0.5,δ34Spy从30‰快速下降到-10‰,而且与δ13Corg变化明显脱耦,说明该时期深海快速从缺氧转变为富氧环境,这与大量海绵个体以及海绵骨针的出现相一致。
在结合地层以及生物资料的基础上,通过碳同位素地层对比,可以看出深海从还原到氧化状态的快速转变与Atadabanian早期澄江动物群出现在时间是大致耦合的,这说明大气-海洋的快速增氧(如深海的氧化)是早寒武世生物门类、生态快速分异、辐射和扩张的重要环境驱动力。深海氧化表明当时大气-海洋氧浓度已经升高到一个临界值,这为依赖于扩散(diffusion)有氧代谢的早期生命提供了一个前所未有的环境和发育(基因)创新机会,触发了生物门类、形态的快速分异、形体设计复杂化和大型化;同时深海氧化提高了资源可利用率、拓展了适合生物生存的生态空间(或范围),进一步激发了宏体生物快速的生态开发和辐射。(生物谷Bioon.com)
doi:10.1016/j.chemgeo.2012.03.005
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Evolution from an anoxic to oxicdeepocean during the Ediacaran–Cambriantransition and implications for bioradiation
Jianguo Wanga, Daizhao Chena, , , Detian Yanb, Hengye Weia, Lei Xianga
The Ediacaran–Cambriantransition, one of the most critical intervals in Earth's history, is marked by dramatic biological, oceanic and geochemical turnovers. Here high-resolution carbon and sulfur isotopic data respectively for organic carbon and pyrite, and iron speciation data are presented from the deep-water Liuchapo and Niutitang Formations on the Yangtze block, South China. The carbon isotopic data, together with biostratigraphic and radiometric dating, offer the compelling evidence for the placement of Ediacaran–Cambrian boundary within the Liuchapo Formation (chert succession), and for its correlation with shallow-water equivalents elsewhere. In this context, iron speciation and sulfur isotopic data further suggest a predominant anoxic and ferruginous deepocean over the transitional time until the middle Early Cambrian (Atdabanian or Stage 3) when the deepocean was rapidly oxygenated. Coincidently, during this interval, large-body metazoans (i.e., sponges) abruptly appeared in the deepocean, which was temporally associated with the highly diversified large-body skeletonized animals (i.e., Chengjiang Biota) which colonized in shallow-water niches particularly in southwestern China. This scenario suggests a causal link between deep oceanic oxygenation and the explosive diversification of large-body skeletonized organisms in the Early Cambrian.