Phosphorites' Sedimentary Event In Sinian Doushantuo Period, West Yangtze Area

Sedimentary of marine phosphorites between the late Neoproterozoic and Cambrian was the first large-scale global phosphogenesis event.Phosphogenesis also emerged on the Yangtze platform in Sinian Doushantuo period. The broad transgressive activity in Doushantuo period formed carbonatite -phosphorite- black shale deposition fabric widely distributed on the Yangtze platform.The problem of marine phosphorites'deposition is one of the difficulties in the geological research field.There are many controversies. The phosphogenesis and biological evolution events were associated with changes of global environment. For example,the disassembly of the Rodinia supercontinent, the late Neoproterozoic glacial events' closing and the reconstruction of new ecosystem in ancient marine,which influenced the phosphogenesis and biological evolution and innovation of life.Biological congregation of phosphorus is an important way to phosphorus enrichment.Its large-scale burial records could reflect the important information about changes in the ancient ocean.At the same time, biological metabolism, photosynthesis and other activities of life are inseparable from the participation of phosphorus, phosphorus plays an important role on the evolution of life.More and more research shows that marine phosphorites exist in close contact with the biological effect, which also reflects the co-evolution of the relationship between environment and biology.The subject is supported by Natural Science Foundation(N0. 40743016) and Ministry of Education university special scientific research fund for doctor(N0.20070616014).Based on the advances in the study of genesis of phosphorite and biota of Doushantuo period, analyses of petrography, paleontology, sedimentology,elemental-chemistry, isotopic geochemistry and organic geochemistry are conducted on representative phosphorite of Doushantuo Formation-Weng'an phosphorite in Guizhou and Yichang phosphorite in Hubei. The purpose of this study is to explore Doushantuo phosphorite's formation in the upper Yangtze region and its biomineralization and the relationship between phosphogenesis and biological evolution.There is less contrast material fabric-sedimentary environment-genesis of phosphorite between the carbonatite phosphatic series in Weng'an and the shale-carbonatite phosphatic series in Yichang.The study of organic geochemistry is deficient too.At the same time, Weng'an biota and Miaohe biota were discovered in Guizhou and Hubei.Therefore, we probed into the relationship between Doushantuo phosphorite's formation and sea-level changes and the bio-mineralization in the upper Yangtze region, yielded the following results:(1)An organic geochemical analysis of the phosphorite samples coming from the stratigraphic position of Weng'an biota has been carried out, and the hydrocarbons fractions contain n-alkanes,terpanes, steranes and isoprenoids. The results show that the distribution pattern of n-alkanes has a wide range of carbon atom numbers.The OEP near 1.The Pr/Ph ratio indicating phytane dominance.The sterane content of C27>C29>C28 shows asymmetrical"V"shape.The dominance of C27 may be caused by acritarchs. The curve of the carbon isotope compositions of individual n-alkanes of the phosphorite has a narrow variation range.Carbon isotope compositions of individual n-alkanes show that the organic material of the phosphorite from the Doushantuo Formation originates is singleness.The biomarkers characteristics indicate that the main sources of the organic matters are algaes and bacteria.(2)The Doushantuo Formation of Baiguoyuan could be divided into some third-order sequences. Compared Yichang and Weng'an phosphorite's depositional environment, we could conclude that subtidal zone was favorable area for phosphogenesis,which had high energy and abundant algaes.The physical weathering during glacial epoch and chemical weathering during the Doushantuo period produced the phosphoric material,which was input into ocean,with high biological productivity,providing opportunity for biomineralization.Phosphogenesis and transgression have the closed relationship.Intermittent fluctuations of sea level provided the source and power conditions for phosphorite's deposition. The transgression was very favour of phosphorite'sformation,high grade phosphoria formation corresponds to the maximum transgression.(3)After the great glaciation period,the biological productivity recovered rapidly, providing condition for biomineralization. The Yichang phosphorite's organic carbon content is between basin phosphorite and platform phosphorite's content, intergrade is characteristic of its.Because the environment was retention basin of semi-limitations, the phosphorites of Baiguoyuan were resulted from weak reduction-weak oxidation environment.Compared with platform phosphorite of Weng'an,platform phosphorite has the characteristic of low organic carbon content and high P2O5 content as a result of more oxidative environment.Carbon content and high P2O5 content indicated that organic material played a significant role in phosphorus element's enrichment. (4)The source of Baiguoyuan phosphatic series' was discussed. The deep phosphorus' input caused by volcanic eruptions was unlikely.The physical weathering during glacial epoch and chemical weathering during the Doushantuo Period produced the phosphoric material,which was input into ocean. The Baiguoyuan lower phosphate rock Ceanom indicators reflect a relatively reducing environment,after the Nantuo ice age,the conditions of ancient sea was a still relatively reducing environment.Ancient ocean water was not completely oxidized.(5)The isotope data indicate that the biological productivity was gradually increased in the early stages of the Doushantuo period.It implied that upwelling of phosphorus-rich bottom water brought the phosphorus source.And then,biological productivity's increase causedδ13C value's changes. The migration of phosphorus water provided nutrients for the Doushantuo biota's flourished, and biota's flourished provided the condition for phosphorus's further aggregation and precipitation. Began the analysis from Weng'an Biota's occurrence layer, the value ofδ13Corg gradually diminished,and the sea-level changes unanimously,showing the obvious characteristics of fluctuating cycles.With theδ13C value's shocking, the impact of various conditions caused the large fluctuations of biological productivity.(6)With the analysising of Weng'an phosphate rock's characteristic ratios of trace elements,the evidence did not support the phosphate rock's hydrothermal sedimentation. Datang and Chuanyandong's lower seam ofΣREE was much higher than the upper seam;The LREE/HREE of Weng'an phosphorite was less than the western of Hubei.