华北地台西缘早—中寒武世之交的核形石:微组构与生物矿化机制研究

Oncolites from Lower-Middle Cambrian Transition of the Western North China Platform: A Study of Their Ultra-fabrics and Biomineralization

内蒙古乌海地区毛庄组上部灰岩产大量核形石,其内核与包壳含丰富的矿化细菌和有机质残余。显微观察显示:内核以微晶质为主,含少量生屑和陆源碎屑;包壳由亚毫米级亮/暗纹层交互构成。亮层具微亮晶—微晶混合组构,以自养蓝细菌居多;暗层以微晶质为主,富含异养细菌和黄铁矿莓球;这种差异表明两种纹层具不同的生物矿化机制,但都是微生物-化学过程与环境相互作用诱发碳酸盐沉淀并矿化微生物质的结果。超微研究表明:纹层内含大量纳米球粒(40~200 nm),与非晶质胞外聚合物(EPS)密切共生,可能源于EPS的降解。纳米球粒通过粘结-融合形成多面体(0.2~5μm),并进一步集合形成微米级微晶质球粒,后者常被齿状亮晶质环边包围。在寒武纪核形石内部观察到的这种多级微组构的有序组合及其与细菌残余和有机质的规律性配置关系,与近代矿化微生物席和实验研究中观察到的情况相似。本研究认为寒武纪核形石包壳纹层的形成可能与现代微生物席的矿化机制相近,都是微生物与环境相互作用诱发碳酸盐沉淀并矿化有机质的结果。

The lowermost Middle Cambrian carbonate succession at western margin of the North China Platform is rich in oncolite.In order to explore biomineralization mechanisms and microbe-mineral interactions in ancient microbialite,microfabrics of the oncoids at micro-and nano-meter scales were investigated using microscopy and FESEM techniques.The study shows that abundant bacterial and organic relics exist in both nucleus and cortex of the oncoids.Microscopically,the nucleus is mainly of micrite,with few of fine bio-and terrigenous detritus, while the cortex consists of light/dark lamina couplets at sub-millimeter scale.The light lamina consists of mixed microspar and micrite,with filamentous cyanobacteria predominant,and the dark lamina is largely micri-tic,rich in heterotrophs and pyrite grains,suggesting that they may have resulted from different biomineraliza-tion mechanisms.We think that the formation of oncoid cortex in early Middle Cambrian basically coincides with those in present microbial mats,all resulting from carbonate precipitation induced by complex interactions between microbes and geochemistry,and from mineralization of grown mats as well.FESEM observation indi-cates that there exist abundant nanoglobules (40-200 nm in size)associated closely with extracellular polymer-ic substances (EPS),pointing a derivation from anaerobic degradation of EPS.Nanoglobules tend to coalesce into polyhedrons (0.2-5 μm in diameter),which further aggregate into submicron-scaled micritic peloid en-cased with microsparitic rim.Bacterial filaments mainly occur in interstitials between polyhedrons or peloids, while EPS often constitute walls of alveolar structures,or exist in various forms at or near the edges of polyhed-rons and peloids.The oriented assembly of microfabrics at various scales and regular association of bacteria and organic relics observed within oncoid interior are reminiscent of those recognized in present calcified microbial mats and those revealed in culture experiments,possibly implying a general situation for biomineralization of mi-crobialites.We may,therefore,conclude that the formation of oncoid cortex in early Middle Cambrian may ba-sically coincide with those in present microbial mats,all resulting from carbonate precipitation induced by com-plex interactions between various microbes and geochemistry as well as from mineralization of grown mats.