Mesoproterozoic Microbialites From North China Platform:Microfabrics, Organomineralization Processes And Their Palaeoenvironmental Distribution

Mesoproterozoic spans a time interval from1.6Ga to1.0Ga and represents a transitional stage between the two Great Oxidation Events in Earth’s history. During this stage, due to the significant raises in atmospheric oxygen level, sulfate influxes and therefore active bacterial sulfate reduction, the ocean became strongly stratified with a wedge-shaped euxinic layer presenting between the upper oxygenated layer and lower ferrous layer. Microbialites and microbially induced sedimentary structures (MISS), as the products of microbe and environment interactions, are widespread in the Mesoproterozoic epeiric sediments of the North China platform, providing a nice opportunity to explore the ancient microbial communities, their living environments, and the interactions between the two. In order to study these characteristics, microbialites and MISS preserved in the Mesoproterozoic sediments were chosen as the major research objects.The extensive study on various microbialites reveals that domal thrombolites mainly develop in lower deep subtidal setting, tabular thrombolites widely distribute in the environments of middle subtidal to upper subtidal, while MDS and domal stromatolites are largely restricted to the areas from upper subtidal to lower intertidal, and microbial mat-rich dolomites are commonly seen in the intertidal zone. Microbial mat destruction structures (a type of MISS) are often concentrated in supratidal, and dark biolaminites in subtidal of semi-closed lagoon environments. These features clearly indicate that environment indeed have obvious controls on the morphogenesis and distribution of microbialites. Though analyzing the redox sensitive element contents and the abundances of carbonate seafloor precipitates in the microbialites deposited in deep subtidal to supratidal environments, we have confirmed that the Mesoproterozoic ocean was indeed stratified and had a shallow chemocline, likely around a depth of～25m, and reveal that thrombolite and dark biolaminite deposited in anoxic to dysoxic water, while the other microbialites are mainly formed in dysoxic to oxic water.The study on the MISS from siliciclastics shows that the morphologic variation and association of various MISS are mainly influenced by hydrodynamics, substrate exposure duration and water supplement, which are largely regulated by topography. The supratidal is rich in various MISS, especially sand cracks; the upper intertidal is featured by mat protected ripple marks and chips; the lower intertidal to subtidal zone is lack of in situ MISS but has some redeposited mat chips. From the subtidal to supratidal, four MISS zones have been recognized, each of which has its own distinctiveness in MISS morphological association, and can be used as a potential indicator for palaeoenvironmental reconstruction.Study on the micro-to ultra-structures of various microbialites, including thrombolites, MDS and dark biolaminites, shows that, though variable in morphology, different types of microbialites may share a common mechanism in their organomineralization:(1) synthesizing nanoglobules by replacing organics,(2) aggregation of nanoglobules and organic relics to form polyhedrons,(3) further clumping of polyhedrons and organic relics to form micropeloids or fibers (such as the vertical fibers in MDS),(4) orderly assembly of micropeloids and fibers with inorganic precipitates to form various microbialites. The ordered assembly of organominerals from the process (1) to (3) seems to be common in different microbialites, and may potentially be used as biosignatures for the recognition of biogenic carbonates; whereas the process (4) varies in different types of microbialites, possibly depending on particular environment and specific microbial process. This study also shows that there exist possibly two basic patterns in the mineralization of organic materials:(1) direct mineralization, where the nanoglobules replace organic remains, such as EPS and bacteria relics;(2) indirect mineralization, where the carbonate precipitates are induced by increasing carbonate alkalinity in the micro-environment due to organic matter decomposition.The study on the possible interactions between microbe and environment recorded by Wumishan dark biolaminites revels that they are composed of alternating fibrous dark and micritic light laminae. Evidence from their microfabrics and isotopes indicate that dark biolaminites are similar to Holocene tufas in having clear annual laminations. The light laminae, possibly formed in late autumn to early spring, contain minor amount of randomly dispersed filaments and dominated by micrites that likely derived from the calcification of strongly degraded microbes. The dark laminae, formed in late spring to early autumn, contain abundant putative bacteria filament relics, and vertically aligned fibers that possibly resulted from rapid calcification of microbial filaments through sheath encrustation. Time series and wavelet transform analyses on the thicknesses of laminar couplets and the concentrations of Ca, Fe, Co/Ti, Cr/Ti and Br elements in the laminae revealed a persistent-11-yr period, corresponding to sunspot cycles. This result may indicate that the solar irradiance fluctuations in Mesoproterozoic, in a similar way to the modern solar irradiance variation, have influenced the lamina couplet thicknesses probably by influencing microbial growth and carbonate saturation in the micro-environments. They may have modulated the Ca, Fe and Cr/Ti concentrations in the laminae through solar-forcing Earth’s climate change, and the Co/Ti and Br contents through solar-forcing microbial activity.