Redox Condition Changes In Early Silurian Ocean And Their Influence On The Late Aeronian Sedgwickii Event

The early Silurian(-443-433 Ma)was a special interval of environmental improvement and biological recovery following the Late Ordovician mass extinction.The atmosphere-ocean-ecology system was ever considered relatively stable,but remarkable positive carbon isotope excursions in the early Silurian(late Aeronian stage,middle Llandovery)suggest significant perturbations of the carbon cycle.Coeval glaciations also indicate climate instabilities.Furthermore,a rapid turnover of graptolite diversity(called as graptolite sedgwickii event)has been documented in sedgwickii biozone,while the underlying mechanisms that caused this crisis remain poorly understood.In order to explore the marine redox conditions and the links to extinction event,we present high resolution organic carbon isotopes,total organic carbon(TOC)contents,iron speciation,trace metal concentrations(including Ba,Mo,U,V,Ni,Al and rare earth elements)and nitrogen isotopes data for early Silurian(Longmaxi Formation)black shales from Bao201 drill core section in Chongqing Municipality,Southwest China.The ?13Corg curve shows a slightly positive excursion(from-31.1‰ to-29.4‰),possibly indicating increased organic carbon burial under anoxic conditions,which is consistent with coeval high TOC contents.Along with more organic carbon burial was the decrease of atmospheric pCO2.When the temperature dropped below the threshold,it would lead to the formation of glaciers.The exposure and weathering of 13C-enriched carbonate platform,caused by sea-level fall,would lead to rapid and significant positive carbon isotope excursions in the upper Longmaxi Formation.Therefore,the organic carbon isotope data in Bao201 drill core section suggest anoxic conditions during the sedimentation of lower Longmaxi Formation.With the increased oxygen solubility due to lowering sea-surface temperature,the anoxic condition might be weakend during the sedimentation of upper Longmaxi Formation.The iron speciation,V and Ni concentrations data in Bao201 drill core section indicate anoxic and ferruginous conditions during sedimentation of Longamxi Formation.However,Ce anomalies decrease in the upper Longmaxi Formation,suggesting weakened water column anoxia,which is consistent with coeval Mo-U covariation patterns.Benthic redox variations play a critical role for Mo-U enrichment pattern in Longmaxi Formation shales.The lower Longmaxi Formation was suggested to be deposited under anoxic and ferruginous-sulfidic conditions.The middle Longmaxi Formation was deposited under anoxic and ferruginous conditions,which were intermittently weakened during sedimentation of the upper Longmaxi Formation.Two obvious ?15N excursions observed in the Longmaxi Formation indicate significant perturbations of the marine nitrogen cycle.The relatively low ?15N values(from-1.6‰,to+0.1‰)in lower Longmaxi shales reflect that NO3-/NO2-/NH4+were removed from the bioavailable nitrogen reservoir through denitrification and anammox processes,and the N2 fixation processes played the critical role in coeval nitrogen cycle.It suggests that the lower Longmaxi Formation were deposited under anoxic conditions.The ?15N values in lower to middle Longmaxi Formation gradually decrease to the minimum value of-3.6‰,reflecting partial NH4+ assimilation in NH4+ replete environment,which indicates an expansion of oceanic anoxia with redox transition zone moving upward to the photic zone.The positive excursion of ?15N values in upper Longmaxi Formation reflects that the partial NH4+ assimilation processes were inhibited,indicating weakened anoxia along with redox transition zone moving downward.The multi-proxy analyses for Bao201 drill core section suggest that the Longmaxi Formation were mainly deposited under anoxic conditions.The organic carbon isotopes,nitrogen isotopes,Ce anomalies and Mo-U covariation results indicate that the anoxic conditions were weakened during the sedimentation of upper Longmaxi Formation,which is consistent with the sedgwickii graptolite extinction event.Therefore,we proposed that there would be potential links between the biotic crisis and coeval marine redox evolutions.Due to the discrepancy of foraging behavior and habitat preferences,the graptolites in Paleozoic ocean inhabited in epipelagic zone and the upper boundary of OMZs(oxygen minimum zones)respectively.Compared to epipelagic communities,the deep water graptolite communities were more sensitive to the changes in marine environments,because their nutrients and food source were both supplied from the nitrification and denitrification processes in the OMZs.The contraction of OMZs would lead to the decrease of nutrients and food supplies,leading to survival pressure to some graptolite species,which is possibly related to the reduction of graptolite diversity in sedgwickii biozone.