光合作用生物膜诱发的放射鲕粒:以江苏徐州贾旺剖面寒武系苗岭统张夏组为例*

几年来针对巴哈马现代文石鲕粒的持续性研究表明,微生物和细胞外聚合物质(EPS)在鲕粒的形成和发育中起着关键而重要的作用,从而产生了一个重要的认识,即: 鲕粒可以看作是“纹层状的有机沉积构造”并遵循着微生物岩体系的一些形成特征。但是,鲕粒30亿年的发育历史、多样化的产出环境、特征性的矿物构成和各种各样的沉积组构,确实赋予了鲕粒生长和形成机理的复杂性和神秘性,因为鲕粒在何处而且如何形成、以及鲕粒究竟记录着何种生物与非生物过程的许多问题还存在剧烈争论。来自于江苏徐州贾旺剖面苗岭统张夏组上部鲕粒滩相灰岩,由较为典型的方解石放射鲕粒所组成,表现出放射状、放射—同心状和泥晶质的沉积组构,而且在鲕粒核心、鲕粒皮层以及在鲕粒间的不规则团块或凝块的暗色泥晶质构成中高密度地保存着精美的葛万菌(Girvanella)化石,进一步表明了这些暗色泥晶构成代表着较为特征的光合作用生物膜,从而提供了一个苗岭世方解石海中放射鲕粒形成较为直接的微生物证据,以及与光合作用生物膜之间复杂的成因联系,因为葛万菌是相对较为肯定地类比于近代钙化织线菌(Plectonema)的丝状蓝细菌化石,尽管还可类比于现代的伪枝菌(Scytonema)。虽然形成放射状鲕粒皮层的放射纤维状方解石的沉淀作用确实不能解释为直接的微生物沉淀作用的结果,但是,这些放射鲕粒确实表现出光合作用生物膜诱发、滋养并促进了放射纤维状方解石皮层增生作用的重要证据,为拓宽“鲕粒谜”的阐释提供了一个较为重要的典型实例,而且还成为寒武纪苗岭世方解石海与后生动物辐射相耦合的蓝细菌繁荣的重要证据。

Radial ooids induced by photosynthetic biofilms: an example from the Cambrian Miaolingian Zhangxia Formation at Jiawang section in Xuzhou city of Jiangsu Province,North-China Platform

Recent continuous studies on the modern marine aragonite ooids in Bahama demonstrate that microbes and extracelluar polymeric substances(EPS)are very important in the formation of ooids,which results in an important scientific understanding,i.e. these ooids can be regarded as “laminated organic sedimentary structures”that own the formation characteristics of microbiolith systems. Nevertheless,the formation of ooids presents many mysteries and complexities caused by many reasons: (1)3.0 billion years’ development history;(2)diversified formation environments;(3)characteristic composition of minerals;(4)a variety of sedimentary structures. There are still many strong debates on where and how do these ooids form,and what kinds of biological and abiotic processes they record. The oolitic beach facies limestone found at the upper part of the Miaolingian Zhangxia Formation at the Jiawang section in Xuzhou city of Jiangsu Province are made of calcite,radiative oolites,which show radial,radial-concentric,and micritic sedimentary structures. Plenty of excellent Girvanella fossils are preserved in the core and cortex of these ooids,as well as the clump or clot among ooids,and the dark micrite that are composed of irregular agglomerate and clots. These dark micrite represent the characteristic photosynthetic biofilm,which provides the direct microbial evidence for the formation of radiate oolites in the Miaolingian calcite sea. Besides,the complex genetic relationships with the photosynthetic biofilm have been established because the Girvanella are the fossils of filamentous cyanobacteria that is similar with the recent calcified Plectonema and the modern Scytonema. Although the deposition of radial-fibrous calcite formed at the cortex of radiate oolites cannot be interpreted as the results of direct microbial precipitation,these radiate oolites present the important evidences that are possibly nourished by the photosynthetic biofilm which promotes the accretion of the cortex made up of radial and fibrous calcite in the Zhangxia Formation. Ultimately,our studies provide an important example for the further understanding of the “ooid dilemma”,which also provides an important evidence for the cayanobacterial bloom in the Cambrian Miaolingian calcite sea that is coincided with metazoan radiation.