Geochemical Records Arcoss The Late Ordovician-Early Silurian Transition And The Implications To The Formation Mechanism Of Black Shales In South China

The Ordovician-Silurian transition was an interval which occurred major changes in the Earth’s biotic,climatic,and environmental systems.The organic-rich shales in the Late Ordovician Wufeng Formation and Early Silurian Longmaxi Formation are the important source rocks in South China,however,the deposition mechanism of these shales is still under debate.In order to reconstruct the climatic and oceanic environments during the Late Ordovician and Early Silurian transition,and investigate the influence of significantly geological events on the deposition of organic-rich shales in the Wufeng and Longmaxi formations,this study presents the sedimentary characteristics,geochemical and isotope geochemical compositions of the Upper Ordovician-lower Silurian succession of the Yangtze Platform.The Wufeng and Longmaxi formations are mainly composed of siliceous shale,argillaceous shale and calcareous shale,based on the mineral compositions.Two 3th order sequence framework was established based on the drilling data,core photo,and seismic profiles of the Sichuan Basin.The Wufeng Formation and Guanyinqiao Member are a 3th order sequence（SQ1）,the Wufeng Formation was interpreted as transgressive systems tract（TST）,and the Guanyinqiao Bed was interpreted as highstand systems tract（HST）.The Longmaxi Formation consists of two members,including Long-1 Member and Long-2 Member.The Long-1 Member was interpreted to be a 3th order sequence（SQ2）,which is divided into transgressive systems tract（TST）,highstand systems tract（EHST）and a late highstand systems tract（LHST）.The Wufeng Formation and lower Longmaxi Formation are mainly composed of organic-rich siliceous shales,which are high quality shale in this interval.Lithium isotopes from carbonate sections are reported and continental weathering is reconstructed.Highδ⁷Li values occur in the late Katian,early Hirnantian and late Hirnantian,which reflect lower denudation rates and riverine Li fluxes.However,multiple negative excursions ofδ⁷Li_carb are recorded during the latest Katian,and mid-Hirnantian interglaciation and latest Hirnantian periods,which can be interpreted as episodes of enhanced denudation rates and riverine Li fluxes due to warm and humid climates triggered by intense volcanic activity.Continental weathering appears capable of eventually stabilizing climate when the climatic system is out of balance.In addition,higher weathering and erosion increases the availability of dissolved nutrients and sulfate influx,which enhances oceanic primary productivity,which then leads to the expansion of euxinic seawater and triggered the Late Ordovician Mass Extinctions.Theδ¹³C profiles from the Yangtze Platform are consistent with earlier reports of Hirnantian shelf gradients in sedimentaryδ¹³C_DIC values.In addition to intense carbonate weathering,the high level of photosynthesis in the inner shelf caused by abundant nitrate and nutrients may have enhanced the dissolved inorganic carbon gradient on shelves globally,driving sedimentaryδ¹³C values even higher than those of sediments deposited in the outer shelf where carbon exchange with the surface ocean was less restricted.The organic-rich Wufeng and lower Longmaxi shales are characterized by isotopically negative carbon shifts;however,obviously highδ¹³C_org values are shown in the organic-poor shales of the upper Longmaxi Formation.The negativeδ¹³C_org excursion is attributed to ¹²C-depleted CO₂ sourced from volcanic activity.Furthermore,the positiveδ¹³C_orgexcursion in the upper Longmaxi Formation occurs because a large amount of ¹²C was removed from the ocean-atmosphere system through continuous burial of organic carbon.The variation inδ¹⁵N_bulk,which maintains persistently high values in Wukemuchang section,which can be interpreted as aerobic nitrogen cycling in the inner shelf.The highδ¹⁵N_bulk values in the inner shelf section are likely a consequence of the partial denitrification in the water column,leaving the residual nitrate pool enriched in ¹⁵N.In contrast,anaerobic nitrogen cycling with more rapid denitrification is attributed to the low levels of dissolved oxygen in the outer shelf.Volcanic activity and its influence on continental weathering are important reasons for controlling the deposition of organic-rich black shale in the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation.First of all,the strong volcanic activity in Late Ordovician and Early Silurian can release a large amount of nutrients directly into the sea.Second,volcanic activity could release large amounts of greenhouse gases caused a warm and humid climate conditions,resulting in strong continental weathering,this process will also release a lot of nutrients into the ocean.Therefore,both the volcanic activity and intense continental weathering promote the ocean surface primary productivity,leading to the redox stratification in the Yangtze sea.Finally,organic-rich black shales were deposited in the deep water areas.