A Comparative Study Of Microbialites In Early-Middle Cambrian And Mesoproterozoic From North China Platform: Interactions Between Microbial Communities And Environment

Ancient microbialites reflect interactions between microbial communities and environmental conditions. Microbialites provide a record of the earliest ecosystems on Earth when they formed a significant component of Earth’s platform and shelf carbonates. Their dominance in shallow marine environments throughout early Earth history makes them the best records of microbial community interactions. As biosedimentary structures, microbialites also reflect ancient environmental conditions through interactions between microbial communities and their environment. 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 mirite, 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 micritic, rich in heterotrophs and pyrite grains, suggesting that they may have resulted from different biomineralization mechanisms. We think that the formation of oncoid cortex in early Middle Cambrain 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 indicates that there exist abundant nanoglobules (40-200nm in size) associated closely with extracellular polymeric substances (EPS), pointing a derivation from anaerobic degradation of EPS. Nanoglobules tend to coalesce into polyhedrons (0.2～5μm in diameter), which further aggregate into submicron-scaled micritic peloid encased 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 polyhedrons and peloids. In addition, the wide distribution of oncoids at Early-Middle Cambrian transition in different continents seems to link temporally to a dysaerobic paleoceanography caused by significant volcanic eruptions such as Kalcarindji LIPs and consequently rapid transgression at that time, thus implying a high stressed environment in which metazoan benthos had been largely suppressed.Thrombolites are an important type of microbialites, which were mainly found after Neoproterozoic, especially during Phanerozoic with high stress environment, but rare in Mesoproterozoic. Researchers hold different opinions on their origin:some propose that they may originally be stromatolite but disturbed by metazoan. Through further study on mesoclots, the key structure of thrombolites, we infer that they are of biogenic origin, and related to specific ocean chemistry. This result provides important evidence for further understanding of Mesoproterozoic marine environment conditions, and indicates that the difference of various microbialites maybe mainly related to environmental conditions and biological mineralization process.The evolution of chemical conditions of Proterozoic oceans has been a hot issue. Through systematic study on geochemical redox indicators of Gaoyuzhuang Formation, we found that the redox-sensitive elements (U, Mo and V) are only concentrated in the bottom and top parts of third member. In addition, stable carbon isotopes (organic and inorganic carbon isotopes) also have undergone a similar variation. So we think the two layers represent two periods of ocean euxinic. Carbon isotopic variations may be related to stratified water chemistry conditions. In euxinic conditions, heterotrophic bacteria are dominated microbial communities,δ13Corg would be obviously biased negative; while in the non-euxinic environment, autotrophic cyanobacteria are dominated, δ13Corg would be biased positive. This distinct vertical 813Corg gradient presented here indicates that stratified conditions prevailed on Mesoproterozoic continental shelf, and provides new information and evidence for further studying in the evolution of Mesoproterozoic ocean chemistry.