Spatiotemporal Variations And Formation Mechanism Of The Marine Redox Conditions In The Southeastern Deep-water Area Of The Nanhua Basin (South China) In The Early Cambrian

The early Cambrian?⁵41-509 Ma?witnessed the first explosive radiation of metazoan body plans,known as“Cambrian Explosion”,which is characterized by the occurrences of fossils of almost all animal phyla in a relatively short time.This remarkable biological event may have strongly linked to the changes in coeval atmosphere,ocean and biogeochemical cycle of key elements.The marine redox evolution of this interval is one of the most important keys to understand the relationship of the co-evolution of life and environments.The Nanhua Basin,which is located between the Yangtze Block at the northwest direction and Cathaysia Block at the southeast direction,was a passive continental-margin basin during the Ediacaran and Cambrian that was probably well connected to the open ocean at both its northeastern and southweatern ends.The lower Cambrian sedimentary succession in the Yangtze Block side of the Nanhua Basin are not only relatively continuous and well preserved,but also yielded the richest fossil record of this crucial time period,which has been a research focus resulting many important findings for the early Cambrian oceanic redox evolution in past decades.However,little attention has been paid to the deep basin area of the southeast edge of Nanhua Basin next to the Cathysia Block because of the lack of fossil record and widespread metamorphic and igneous rocks in this area,resulting little known about marine redox evolution in this area,which prevents our understanding of marine redox evolution at a basin scale and its relationship with coeval life evolution.In this study,we focus on two fresh early Cambrian sections at Shimen and Silikou in Sanjiang county,Guangxi province,South China.These two sections contain Ediacaran cherts of the upper Laobao Formation and early Cambrian black shales of Qingxi Formation,and are characterized by continuous deposites and weak metamorphism.Sanjiang area was in the deep basin area of the Nanhua Basin and geographically close to the Cathyasia Block in early Cambrian,thus provide a valuable opportunity for studying marine redox evolution in the Cathaysia Block side of the Nanhua Basin.Based on fundamental geology of study area,a comprehensive investigation was conducted involving stratigraphy,sedimentology,chronology and geochemistry in order to understand the spatiotemporal evolution of redox conditions in study area and mechanism behind.Furthermore,a comparison with adjancent deep-water basin facies on the Yangtze Block was also conducted in order to understand the redox evolution and mechanism at basin scale.Main conclusions and findings are given as below:?1?We reported a SIMS zircon U-Pb age of a tuff layer firstly found in the topmost Laobao Faormation at the Shimen section.A weighted mean age of 540.5±4.6 Ma is calculated based on thirteen concordant SIMS measurements,suggesting that the Precambrian-Cambrian?Prc-C?boundary is at the topmost of the Laobao Formation in Sanjiang area.Regional correlation confirms the diachronous development of Laobao Formtion and the location of Prc-C boundary within the Laobao/Liuchapo Formation varies in the basin,which was controlled by the syndepositional fault/fracture and the silicic different sources.The SIMS U-Pb age reported here,in combination with the occurance of the enrichment layers of organic carbon and redox-sensitive trace elements?RSTEs?indicates that the organic-rich black shales in the basal part of Qingxi Formation are in age of the early Cambrian Fortunian Stage to Stage 2.?2?We analyzed iron species,RSTEs,and S-N-C isotopes of deep-water black shales in the Silikou section,which represent extensive clastic input mainly from the Cathaysia Block during the early Cambrian?⁵41-509 Ma?.Integrated data reveal a continuous shift in bottom-water redox conditions at Silikou from strongly euxinic in the lowermost black-shale interval?BS1,0-37.4 m?to fully oxic in the uppermost black-shale interval?BS4,296-372.9 m?accompanying a progressive movement of the O₂/H₂S redoxcline into the sediment.In between,most importantly,the BS2 interval?91-154 m?accumulated under manganous-ferruginous conditions?i.e.,Fe-Mn reduction zone?characterized by an active Fe-Mn particulate shuttle,and the BS3interval?204-260 m?under nitrogenous conditions?i.e.,nitrate reduction zone?characterized by strong denitrify ication processes,suggesting highly complex redox conditions in early Cambrian oceans.The observed sequence of redox conditions supports a strong depth-related redox stratification in the early Cambrian ocean,including?from surface to deep and from nearshore to offshore?oxic,nitrate reduction,Fe-Mn reduction,and euxinic zones,which were sequentially recorded at Silikou likely due to secular expansion of the oxic surface layer and/or long-term relative sea-level fall.Our study highlights the complexity of paleoredox conditions and the need for continued paleo-redox studies to explore the redox heterogeneity of early Cambrian oceans.?3?The integrated data of iron species?RSREs and C-S-N isotope at Shimen section indicate that bottom-water redox conditions in the lower interval of Shimen section was anoxic ferruginous and even euxinic conditions,but changed to dyoxia-dominated,while oxic?ferruginous and euxinic conditions dynamically occurred in the upper interval.This finding is consistent with the redox evolution trend of increasing oceanic oxygenation revealed by redox-related parameters in the Silikou section.?¹⁵N_bulkulk value?-2.3 to+2.3‰?obtained from Qingxi Formation is significantly lower than those?+5‰?from shallow-water sections in the Nanhua Basin.The?¹⁵N_bulkulk depth-gradient in the basin is interpreted as the consequence of improved N₂-fixation under widespread anoxic conditions in the basinal area.The?³⁴S_py of Qingxi Formation shales at Shimen section is controlled by the sulfate concentration provided by riverine input from Cathaysia Block and the availability of organic matter,while the?³⁴S_py of deep basin at central Yangtze Block is manipulated by low sulfate content only.?4?The redox reconstruction of Qingxi Formation at both Shimen and Silikou section?southeast deep-water facies close to Cathaysia Block?suggest continuous euxinic conditions during the Cambrian Fortunian Stage to Stage 2,meanwhile,anoxic ferrugineous without euxinic conditions prevailed the deep-water facies on the central Yangtze Block based on previously published data?for example:Songtao in northeast Guizhou and Longbizui section in north Hunan?.The redox differences indicate a highly heterogeneous redox state existed at deep-water facies of the Nanhua Basin.The euxinic conditions for the basal black shales at Shimen and Silikou section imply that“euxinic wedge”reported for the Yangtze Block side also developed along the Cathaysia Block side,and the euxinic waters expanded toward to southeast deep-water region of Yangtze Block.In contrast to cool-dry climate and weak chemical weathering of source rocks for the Yangtze Block reported in previous studies,the calculated chemical index of alteration(CIA_corr)value in Qingxi Formation at both Shimen and Silikou sections range persistently between 76-84,indicating that their source rocks experienced a middle-to-intense chemical weathering and were likely in a warm and humid climate.Intense chemical weathering of source rocks may have caused high sulfate and nutrient fluxes into the basin,and further fueled the primary productivity and bacterial sulfate reduction?BSR?,eventually resulted in developmet of euxinic waters in Sanjiang area and following expansion into deep-water facies of Yangtze Block.Major and trace element patterns of Qingxi Formation at study sections suggest that major source rocks of the studied shales were granite with only minor granodiorite,which were distinctly different from the uniform granodiorite as source rocks for coeval black shales deposited in the central Yangtze Block.Therefore,we conclude the difference continental weathering degree determined by different source rock type and climatic conditions may have caused the marine redox heterogeneity between the deep-water facies in Sanjiang area and in the central Yangtze Block area.Broadly,our results imply that continental weathering and the related terrestrial fluxes likely have played an important role in oceanic redox spatiotemporal evolution in the early Cambrian.