Response Of Cretaceous Sedimentary Organic Matter To Oceanic Anoxic Event 1b In The Gulf Of Mexcio

The Cretaceous Oceanic Anoxic Event 1b(OAE1b)is considered to be a non-global Oceanic Anoxic Event,and has different depositional mechanism in different places.We collected samples from the Lower Cretaceous Aptian to Albian age in the Gulf of Mexico in the United States,which preserved complete OAE1b sedimentary sequence.Sedimentary organic matter biomarkers,major and trace elements,Fe-speciation,and pyrite sulfur isotope are systematically analyzed to disscuss the dynamic process and controling factors of sedimentary organic matter during OAE1b,and primary understandings are follows.(1)The main sources of sedimentary organic matter deposited in Lower Cretaceous Aptian-Albian age in the Gulf of Mexico are benthic macroalgae,planktonic algae,and bacteria.The increase of C₁₇ n-alkane and 4-methyl steranes in Bexar member indicates the phanktonic alage flourished during this period.At the same time,the increase of P,Ni,Zn which related to productivity reveals the significant increase of productivity during this period.The driving force of productivity increase and planktonic alage blooming may be the release of large amounts nutrients from the eruptions of LIPs and the transgression of Cretaceous.(2)Glen Rose member is charactered by the abnormal distribution of 17-??hopanes,which is expressed in the increase of C₃₁(S+R)/2/C₃₀ and C₃₂(S+R)/2/C₃₀values,which can reach 1.13 and 1.12.Combing wih the characteristics of low n-C₁₇/n-C₂₁ ratio,C₂₉???regular sterane advantage,low TOC content,heavy carbon isotope composition,high Pr/Ph and Ts/Tm ratio,samples from Glen Rose member are mainly deposited in a shallower water environments,macrobenthic alage are main contributors of organic matter.(3)The sedimentary environment of Cretaecous Aptian-Albian stage in the Gulf of Mexico is dysoxia environment.The results of Mo_xs,U_XS,Pr/Ph,Pr/n-C₁₇ and Ph/n-C₁₈ indicate,with the increase of productivity and the degradation of organic matter,the bottom water reduction environment is strengthened in Bexar member,but due to the shallow water environment,it is diffcult to form stratification,and has not reach euxinic environment,the Fe _HR/Fe _T and Fe_py/Fe _HRindex fall in the range of dysoxia.The EF_Mo-EF_Ucovariation index falls in the range of 3×SW,indicating the geographic background of the semi-restricted basin in the Gulf of Mexico has an important influence on this Oceanic Anoxic Event.The change of redox environment is the main reason for the change of pyrite sulfur isotope.