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

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 interactionsnbetween 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.