Geochemical Characteristics Of The Black Shales From The Shuijingtuo Formation In The Shennongjia Area And Their Paleoenvironmental Implications

In order to deduce the redox state of the early Cambrian paleo-ocean,we present new whole-rock geochemistry and other redox state indexes for the Lower Cambrian Shuijingtuo Formation from the Shennongjia area in the north Hubei province,such as major and trace element contents,total organic carbon contents(TOC),organic carbon isotopes(?¹³C_org),inorganic carbon-oxygen isotopes(?¹³C_carb-?¹⁸O),Re-Os isotopes and molybdenum isotopes(?^98/95Mo).Moreover,we have also improved the traditional Re-Os isotopic analysis method of organic-rich sedimentary rocks.The traditional Re-Os isotopic analysis method cannot preferentially liberate hydrogenous Re and Os in organic-rich sedimentary rocks.With the aim of overcoming this disadvantage,we use pure nitric acid(HNO₃)as a new digestion medium for Re-Os isotope analysis to replace the traditional medium-inverse aqua regia.After a series of Re-Os experiments on international reference material SGR-1b(oil shale)and BIR-1a(basalt),we conclude that the temperature and digestion time take control of the spike–sample equilibrium and the amount of detrital Re and Os incorporated into the system.The optimum method is the digestion condition of 230?and 24 h to guarantee the isotopic equilibration of spike–sample,and to minimize the dissolution of detrital Re and Os in the system.We have analyzed the Re-Os isotopes of the black shales in the Lower Cambrian Shuijingtuo Formation to constrain the depositional age of this stratum.By comparing the different Re-Os isotopic data analyzed by the HNO₃ solution and the traditional inverse aqua regia solution,we found that sample collection is a prerequisite to obtain an accurate Re-Os isochron age for black shales or other organic-rich sedimentary rocks.During sample collection,samples in stable sedimentary environment with appropriate sampling interval are preferential.The initial¹⁸⁷Os/¹⁸⁸Os ratios of the black shale samples from the Shuijingtuo Formation vary from 0.627to 1.799(T_MA=520 Ma)and these samples have heterogeneous the initial Os isotopes,leading to invalid Re-Os isochron ages.Based on the comparison of inorganic carbon isotopic variations(?¹³C_carb)and the summary of the previous zircon U-Pb dating results of tuff interlayers,we suggest that the depositional age of the Shuijingtuo black shales should be515-526 Ma.The inorganic carbon isotopic data of the Lower Cambrian Shuijingtuo Formation display two positive deviations and two negative deviations.By comparing the carbon isotopic variations between the Shuijingtuo samples and the representative early Cambrian sedimentary profile,we consider that the last positive carbon isotope deviation is consistent with the positive carbon isotope deviation(CARE),corresponding to the stage of Cambrian explosion.Based on the redox indexes(e.g.organic carbon isotope composition and TOC contents,the redox-sensitive elements(RSE)contents and the RSE element ratio),the sedimentary profile is dived into two intervals(lower part as interval I and upper part as interval?).The samples of interval I have low?¹³C_org(-31.80‰?-29.70‰),relatively high TOC contents(2.9±1.0 wt.%),high RSE contents(U 18.6±5.4 ppm;V 1023.6±344.2 ppm;Mo 43.6±18.6 ppm;Re 40.4±30.9 ppb),and high V/Cr(15.78±3.50),Ni/Co(15.60±4.40),Mo _EF/U_EF(Mo _EF and U_EF:enrichment factors;5.1±1.6)ratios,indicating an euxinic depositional environment.In contrast,the samples of interval?have higher?¹³C_org(-28.70‰?-25.90‰),lower total organic carbon(TOC)contents(mean 1.4±0.6 wt.%),lower RSE contents(U 4.8±1.5 ppm;V 135.0±47.8 ppm;Mo 12.8±6.5 ppm;Re 10.0±5.4 ppb),and lower V/Cr(2.08±0.48),Ni/Co(5.74±1.75),Mo _EF/U_EF(4.7±1.6)ratios,indicating an oxic depositional environment.The Mo _EF-U_EF data of this stratum show that the depositional environment of interval I to interval II changes from enuxinic to suboxic.Relatively high Mo/TOC ratios(mean 16.21±8.11 ppm/wt.%)of interval I imply that this interval was deposited at an open to semi-open continental margin basin.In interval I of the Lower Cambrian Shuijingtuo Formation,two positive deviations of Mo isotopic data indicate that the water column was euxinic and had underwent two limited pulsed oxygenations.The totally slow increasing of?⁹⁸Mo values in interval I suggest a decreasing of water column reduction.