Formation Mechanism Of Ministromatolites In The Mesoproterozoic Wumishan Formation In The Border Area Of Hebei And Liaoning And Their Paleooceanographic Significance

The stromatolites are the oldest microbial-related sedimentary formations on Earth,widely distributed in the Precambrian and peaked in the Mesoproterozoic.Although the Mesoproterozoic large-scale stromatolites are diverse in shape and structure,their biogenesis and formation mechanisms are ambiguous due to the widespread dolomitization and recrystallization in carbonates.On the contrary,due to the small size of the individuals,the morphological and structural characteristics of mini stromatolites are more sensitive to changes in the physical and chemical conditions of the depositional environment and become an important window for understanding the paleoenvironment and paleoecology of this era.To further understand the genetic mechanism of mini stromatolites and the paleo-oceanic significance it reflects,this thesis selects the Mesoproterozoic Wumishan Formation sections from Lingyuan,Liaoning,and Kuancheng,Hebei,in the border area of Hebei and Liaoning in North China.Work in sedimentology,microbiology,geometric statistics,mineralogy,and geochemistry,including morphological and 3D morphological reconstruction of mini stromatolites and microbes,geometrical characterization of mini stromatolites and mini stromatolite layers,mineralogical characteristics and cathodoluminescence characteristics of different components in the mini stromatolite layers and in-situ analysis of trace elements by LA-ICP-MS of the carbonate part in ministromatolite.The results show that the ellipsoid-shaped ministromatolites in the Lingyuan section and the microcolumnar stromatolites in the Kuancheng sections used to be located in the intertidal zone and the subtidal zone,respectively.The microbial mats composed of spherical cyanobacteria in the Lingyuan section have changed the distribution of the original aragonite crystals,while in the Kuancheng section filamentous cyanobacteria are merely dwellers of the stromatolite environment.The alternation of the mini stromatolite layer and the flat layer in the Lingyuan section reflects the periodic fluctuation of sea level caused by the tidal control,while the subordinate mini stromatolite layer shares almost the similar thickness with the flat layer,indicating the growth rate of aragonite is much higher than the modern level even though the coccoidal cyanobacteria has limited influence in the growth of aragonite crystals.All of those reflect the seawater conditions with high carbonate saturation in the Wumishan Formation period.Periodic seawater fluctuations also cause periodic exposure,and the disruption of the microbial mat allows atmospheric freshwater to be injected into the mini stromatolites,thereby promoting the transformation of microcrystalline dolomite into fine-grained dolomite.At the same time,the addition of freshwater reduces the local pH value,which is conducive to the occurrence of siliceous precipitation in seawater with a high dissolved silicon content.The simultaneous development of dolomitization and silicification forms the fine laminar structure in the mini stromatolites of the Lingyuan section.In the same harsh environment of mini stromatolites,which are essentially chemically deposited,coccoidal cyanobacteria exhibit stronger adaptability and survival strategies than filamentous cyanobacteria and play a more active role in building the morphology of mini stromatolites.The influence of the coccoidal cyanobacteria,situating at the bottom of the cyanobacterial evolutionary tree,may be due to its nature as the oldest species.