A Study On The Carbon Cycle Of Nantuo Ice Age In Eastern Guizhou Province,China

According to the Neoproterozoic“Snowball Earth”hypothesis,during the Marinoan glacial period,the Earth surface?including continents and oceans?was completely frozen,and the surface was covered with ice and snow,which caused the chemical weathering and water circulation of the surface to stop,resulting in a sharp decrease in biological productivity.However,some scholars dispute whether the surface of the Earth was entirely covered by ice and snow.Several lines of sedimentary,paleonto-logical,and geochemical evidence suggested that open waters and active biological productivity existing during the Marinoan Period.The Neoproterozoic“Snowball Earth”event represents the extreme climatic conditions experienced by the Earth and has a profound impact on the subsequent atmospheric and marine oxidation,biogeochemical cycle,and eukaryote evolution.Despite the prolonged evolution stasis between the origin of eukaryotes and their rise to dominance in marine planktonic ecosystems for eukaryotes before 720 Ma,the Cryogenian witnessed the transition from dominant bacterial to eukaryotic marine primary productivity,one of the most profound ecological revolutions in the Earth's history.The transition also reorganizes the biogeochemical cycling of carbon and phosphorus in the water column and paving the way for subsequent eukaryotes diversification and animal evolution.Instead of being the evolution bottleneck,the extreme Neoproterozoic global glaciation could be the evolution prompter for the eukaryotes.Nevertheless,the ocean redox environments of this transition,as well as possible links with rising atmospheric oxygen levels and the evolution of animals,remain unknown.The carbon cycling plays the crucial role of regulating the redox state of the hydrosphere and the atmosphere,and it could be recorded in stable carbon isotopic compositions from the carbonate rocks from which they precipitated.However,the lack of chemical sedimentary rocks in the same ice age severely restricted the understanding of the marine environment in the critical transition.In this paper,syn-glacial dolostones samples were collected from the Nantuo Formation of two drilling cores in Songtao County,Guizhou Province,which provides an ideal material for the study of the marine environment of Nantuo glacial period.Dolostone layer in core ZK505 and ZK309 is located in the lower part of Nantuo formation,sandwiched between two sets of conglomerate and silty mudstone.The polarized light microscope and scanning electron microscope show that the lithology is microcrystalline dolostone.Delta V Advantage stable isotope mass spectrometry was conducted to analyze the carbon and oxygen isotopic compositions for the dolostone samples from the Nantuo formation.The results show that?¹⁸O_V-PDBof core ZK309 is between-3.98‰and 1.76‰,?¹³C_V-PDBis between-8.57‰and-6.67‰,?¹⁸O_V-PDBof core ZK505 is between-16.97‰and-8.37‰,?¹³C_V-PDBis between-9.68‰and-8.42‰,and before and after glaciation The results show that?¹³C_V-PDBhas a significantly low negative value.The results show that the contents of iron and manganese in the dolostone samples from ZK505 and ZK309 are very high,among which the average value of iron and manganese in ZK309 is about 7553.16×10^-6and 6850.85×10^-6respectively;the ratio of Mn/Sr is relatively large,the average value is about 40.94;the average value of iron and manganese in ZK505 is about 92867×10^-6and 92867×10^-6respectively The average value of Mn/Sr ratio is about 26.89.According to the correlation of carbon and oxygen isotopes and the characteristics of trace elements,the carbon isotopic composition of the dolostone samples in the Nantuo formation represents the original sedimentary record.We conclude that:?1?the carbon isotopic composition of Nantuo syn-glacial dolostones provides a test basis to decipher the formation cap carbonates in the meltwater environments and the hypothesis of methane seepage.?2?Based on the study of carbon isotopic compositions of dolostones of Nantuo formation in the east of Guizhou Province,it is inferred that the biological productivity was low during the Nantuo glacial period.?3?The discovery of synglacial dolostones in the Nantuo Formation of Guizhou province indicates that there were open water areas in the South China area during the Neoproterozoic Nantuo glacial period,providing habitats for the eukaryotes during the Snowball Earth period.?4?The study of carbon isotopes and trace elements of dolostones from the Nantuo Formation in Guizhou Province shows that the ocean of Nantuo glacial period was predominantly anoxic and iron-rich environments.?5?The low negative value of carbon isotope of dolostones in the Nantuo Formation probably was caused by anaerobic oxidation of methane.?6?It was suggested that the organic carbon has a high burial ratio during“Snowball Earth”,which provides a geochemical basis for the accumulation of oxygen in the atmosphere during the glacial period.