The Geochemical Research On Gaoyuzhuang Formation,Jixian Section And Its Implication On The Redox State Of The Early Mesoproterozoic Ocean

The Mesoproterozoic era has long been considered a time of relatively environmental and biological stasis,which is termed as the‘boring billion'.However,emerging research works,like the discovery of decimeter-size multicellular eukaryotic macrofossil and the report of oxygenation event,suggest that this period may have been more dynamic than previously considered.Nevertheless,the environmental evolution of this immense period is poorly constrained.The Jixian system as a typical Mesoproterozoic stratotype in China,preserves one of the most complete and continuous Mesoproterozoic marine sedimentary sequences in the world.This sequence only experienced very weak metamorphism and well kept its primary geochemical record during the deposition.Therefore,for discussing more about detailed redox state of the early Mesoproterozoic ocean and providing new constraints on its oxygenation degree,this research works on Gaoyuzhuang Formation,Jixian group(?1560 Ma)by using multiple geochemical redox proxies,including major and trace element contents,total organic carbon contents,iron-speciation analysis,inorganic carbon-oxygen isotope(?¹³C_carb-?¹⁸O),and molybdenum isotope(?^98/95Mo).The main conclusions of this research are as follows:(1)Three times of swift?¹³C_carb negative excursions are recorded in the Gaoyuzhuang profile,accompanying with extensive enrichment of Fe and Mn.The enrichments of these elements suggest the increase of oxygen in the water column during the three intervals,therefore resulted in soluble Fe(II)and Mn(II)in seawater was oxygenated to insoluble Fe(III)and Mn(IV)in association with Fe and Mn(oxyhydr)oxides drawndown.Each of the three?¹³C_carb negative excursions coincides with increasing of water depth from intertidal,subtidal,to deep subtidal(even under storm wave base).Therefore,the three times of pulsed oxygenation together with deepening sedimentary water depth indicate the deepening of the oxycline of the ocean,which might lead to a progressive oxygenated ocean during the early Mesoproterozoic era.(2)All chosen samples for Fe-speciation analysis in the lower Gaoyuzhuang Formation contain high Fe_HR(high reactive iron)values with Fe_HR/Fe_T>0.38,and extremely low Fe_py(pyrite)with Fe_py/Fe_HR<0.7,which indicate a non-euxinic/ferruginous setting of the bottom water and agree with the previous model of stratified ocean in Mesoproterozoic era.(3)The molybdenum isotopic compositions of the carbonates are easily affected by the adsorption of Fe-Mn(oxyhydr)oxides.Therefore,for evaluating the molybdenum isotopic composition of global seawater,we use samples from Gaoyuzhuang member IV with low Fe,Mn contents to reflect coeval?⁹⁸Mo_sw.Besides,they are characterized by low TOC contents,Mn/Sr ratios and have?¹⁸O>-10‰,in addition to display REE+Y patterns with weakly oxic seawater features.Thus,these samples may more faithfully record the Mo isotopic composition of coeval seawater.According to their?⁹⁸Mo values,we limit that the global seawater?⁹⁸Mo_swis 0.85-1.41‰during the early Mesoproterozoic era.Moreover,we use an updated database of published Mo concentration data in euxinic shales and estimate the Mesoproterozoic seawater[Mo]concentration is 6-34 ppm.On the basis of the principle of seawater Mo isotope mass-balance,we have built a single reservoir Mo cycling model to further constrain the oxic area of the seafloor.The modeling result suggests a minimum oxic seafloor area of?30%of the total seafloor area.As for the reducing seafloor area,the euxic area only account for 1-10%,therefore the seafloor was dominantly bathed beneath the ferruginous water column during the early Mesoproterozoic era.