华北地台西缘早—中寒武世过渡期核形石:微生物群落对浅海缺氧环境的响应

Mass-occurrence of oncoids in the Early-Middle Cambrian transition at western margin of North China Platform: A response of microbial community to shallow marine anoxia

地质记录表明在早—中寒武世过渡期发生了一次重要的生物灭绝事件,但对其成因的认识仍存在分歧。作者试图通过对华北地台该时段广泛发育的核形石微组构和矿化过程研究,揭示微生物群落对灾变事件和海洋环境变化的响应。研究发现核形石内富含细菌化石;包壳的微组构和有机矿化特征表明其形成于细菌硫酸盐还原(BSR)作用活跃的高碱度海水条件,密集的莓状黄铁矿微粒和异养细菌残余指示为缺氧环境;而沉积相分析显示核形石发育于浅海陆棚背景。研究认为,同期的核形石及其他可对比微生物岩不仅在华北广泛分布,在其他大陆和板块上也有良好记录,表明在这个生物灾变期浅海环境有广泛的微生物群爆发,并可能与大火成岩省喷发、全球气温升高以及深部缺氧海水向陆棚侵进引起的浅海广泛缺氧相关。早—中寒武世过渡期广泛发育的微生物岩可能记录了底栖动物大量灭绝后,微生物群在缺氧水体中的快速繁盛与生态扩张过程。

Geological records showed that an important biological extinction event happened during the Early-Middle Cambrian transition period, but there are controversial views about its orign. In order to reveal the response of microbial community to the biotic crisis and the marine environment change, microfabrics and organo-minerals in the oncoid cortices formed in that period at the North China Platform were studied. The study revealed that abundant microbial fossils exist in the oneoids. Study of microfabrics and organomineralization patterns in the oncoid cortices suggested that the oncoids were likely formed in seawater with high alkalinity and active bacterial sulfate reduction （BSR）. Rich pyrite framboids and purported heterotrophic remains in the cortices represented an anoxic bottom-water condition, and facies and depositional sequence analyses suggested a shallow shelf environment. A wide correlation indicated that the timeequivalent oncoids or related microbialites were widespread in the North China Platform, and also well recorded in some other continents or plates. This may imply an extensive bloom of microbial communities in the aftermath of terminal Early Cambrian biotic crisis, which was likely related to a pervasive anoxia in shallow marine environments caused by LIP （Large igneous province） eruptions, and subsequently global warming and invasion of anoxic deep seawater onto the shelves at that time. The broad distribution of oncoids and other related microbialites at the Early-Middle Cambrian transition may have reflected an ecological response and rapid expansion of microbial community to the high-stressed environments where benthic metazoans had been largely suppressed during the biotic crises.