A Kinetic Model for Carbon and Sulfur Fractionation from the Late Ordovician to the Early Silurian in Sichuan Basin,China
[Obective]The significant migration events of carbon and sulfur isotopes(δ13C,δ34S)in the global transi-tion period from the Late Ordovician to the Early Silurian have been widely recognized,but the causes of these iso-topes are still controversial.Quantitative numerical simulation is one of the ways to solve this problem.[Methods]A total of 47 shale samples from the Tianlin and Shaba sections,which were deposited in different regions of the Sichuan Basin during this period(23 samples from the Tianlin section,and 24 samples from the Shaba section),were selected for organic carbon(δ13Corg)and pyrite sulfur isotope(δ34Spy)analysis.The system dynamics software Vensim7.3.5 was used to reconstruct the model and analyze the dynamics.[Results]The main results are as follows:(1)Both δ13Corg and δ34Spy exhibited synchronous positive shifts before the Hirnantian glacial period,reached the peak value in the Hirnantian glacial period,and then decreased.The synchronous positive deviation of C and S isotopes can occur when the burial fluxes of organic carbon were 1.5 to 2.0 times,and pyrite 1.5 to 3.0 times,that of the pre-glacial period.This indicates that the high primary productivity and widely developed anoxic water column of iron were beneficial to the burial of sedimentary organic matter and pyrite.During the Hirnantian glacial period,with the decreased primary productivity and increased oxygen content in water column,the burial flux of organic carbon and pyrite decreased gradually,and the carbon and sulfur isotopes were slightly negative.The intensity of volcanic activi-ty mainly affects the migration of δ13C,and the weakened volcanic activity was beneficial to the positive deviation of δ13C,while the enhanced volcanic activity after the glacial period was the necessary condition for the significant negative deviation of δ13C over a short time(0.5-1.0 Myr).(2)The lower seawater sulfate concentration(approximately 5 mM)in the early stage of the Hirnantian glacial period was the basis of the positive shift of δ34S,with an increase of pyrite burial flux,the ocean sulfate concentration gradually decreased to 3 mM and reached the lowest during the Hirnantian glacial period,while the increase of terrestrial sulfate input flux in the post-glacial period was the main factor leading to the negative deviation of δ34S,making the ocean sulfate concentration gradually return to 5 mM.[Conclusions]The results quantitatively evaluate the main influencing factors of δ13C and δ34S synchronous migra-tion.A new idea and quantitative prediction model are provided for the study of the kinetics of C and S isotope frac-tionation throughout geological history.
Sichuan BasinLate OrdovicianEarly Siluriancarbon isotopessulfur isotopeskinetic model
万方数据
维普
