Sulfur Isotope Compositions Of Different Sulfur Componds In Mesoproterozoic Samples From North China

Mesoproterozoic is an important period in which ocean environment evolved from anoxic-ferruginous condition to oxic condition.As a result of the old age of Mesoproterozoic strata and the lack of evaporates,sulfur isotope data remains scare and the significance and variations in sulfur cycle are still unclear in Mesoproterozoic.Based on sulfur isotope analyses of carbonate-associated sulfate and pyrite in Mesoproterozoic sedimentary samples from Jixian section and three drill-cores at Kuancheng,this paper tempts to study Mesoproterozoic oceanic sulfur cycle in time and space,and tries to discuss the impact of contemporaneous environmental changes on organisms.Sulfur isotope compositions show that the Mesoproterozoic sulfur cycle is a seawater sulfate concentrations gradually increase at the interval from the bottom to the top of the Changcheng Group.?³⁴S_CAS values appear large positive and negative excursion at the age of Gaoyuzhuang Formation and Wumishan Formation.?³⁴S values vary from-7.2‰?+26.2‰indicate that the concentration of seawater sulfate had reach the threshold of bacterial sulfate reduction,and the atmospheric oxygen level had been further improved.?³⁴S_CASvalues appear different degrees of negative excursion in some regions in Gaoyuzhuang Formation,indicating that seawater sulfate concentrations are not uniform in space,and the oxidation of seawater at different depths is lagging behind in space.The oxidation of deep seawater may promote the emergence of multicellular eukaryotes and the improvement of biodiversity.?³⁴S values vary from+30?+40‰shows that euxinia environment is developing at the interval of the deposition of Hongshuizhuang Formation.It indicates that the increase of atmospheric oxygen level in this period leads to the increase of terrestrial sulfate input and the bacterial sulfate reduction is further enhanced.As a result of continuous sulfate consumption,Tieling and Xiamaling Formation show low concentrations of sulfates.So it is difficult to form the rebound of euxinia environment to oxic environment,which is conductive to the survival of organisms.In conclusion,variations in sulfur isotope compositions in Mesoproterozoic strata indicates significant changes in marine redox conditions,atmospheric oxygen level and other environmental conditions,and those environmental changes may have contributed to the biological evolution in the Mesoproterozoic.