Redox Characteristics Of Sedimentary Water Column In Marine Shale And Its Control Mechanism On Organic Carbon

During the Late Ordovician to Early Silurian（OST）,a set of black shale rich in organic matter was widely developed worldwide,which was an effective carrier for studying environmental changes and their relationship with biological evolution.Based on the geochemical test of marine shale samples from typical sections in the Sichuan Basin during the OST period,the evolution of redox conditions,dynamic changes of carbon and sulfur in sedimentary water,microbial reduction of iron-sulfur coexistence system,and transformation of anoxic water in ferruginous-euxinic coexistence system were simulated.The redox characteristics of water in four sedimentary sections of Tianlin,Changhebian,Wc-01 well,and Shaba were analyzed.Combined with the published data of other sections in the Sichuan Basin during this period,the temporal and spatial evolution of redox conditions of sedimentary water was discussed,the evolution characteristics of C,S cycles and their isotopes were revealed,and the influencing factors of spatio-temporal heterogeneity distribution of anoxic water,especially euxinic condition were analyzed.Lastly,the controlling mechanism of C-S-Fe interaction on organic carbon under anoxic conditions were summarized.The main results as follows:1)There are significant temporal and spatial differences in the redox conditions of sedimentary water columns in the Sichuan Basin during the OST period.Vertically,it shows the evolution characteristics from oxic-suboxic conditions to anoxic conditions,then to oxic-suboxic conditions,and finally to anoxic conditions.Horizontally,the development range and duration of anoxic water in southern Sichuan sedimentary area are lower than those in eastern Sichuan sedimentary area.2)The change of sulfate concentration on seawater is the main controlling factor of the development degree of euxinia.The concentration of SO₄^2-in the sedimentary water during this period was about 1/6 of that in the modern seawater.The distribution area of the euxinic water column was the smallest in the Hirnantian glacial period and slightly expanded in the non-glacial period.3)The increase in the burial flux of pyrite and organic carbon drove the synchronous positive excursion of C and S isotopes.The changes in microbial sulfate reduction and volcanic activity intensity play a certain role in controlling the cycle disturbance and isotope excursions of C,S cycles,and isotope evolution.4)The non-linear relationship between Fe and S microbial reduction affects the stable state of anoxic water by controlling the output flux of Fe S,organic sulfur and other componts,and then controls the burial and preservation of organic carbon.The results of this thesis clarify the complex coupling relationship between C-S-Fe in marine shale during the OST period and its response to the evolution of redox conditions in sedimentary environment.The control mechanism of“preservation mode”on organic matter enrichment is revealed,which is instructive to the study of organic carbon prevervation under abrupt environment changes in geological history.