Redox Condition Of Deepwater In The Nanhua Basin At The Ediacaran-Cambrian Transition

The Ediacaran-Cambrian（E-C）transition（ca.635～520 Ma）is a significant evolutionary period in the earth history,during which“Ediacaran Mass Extinction”and“Cambrian Explosion”and other life evolution events occurred successively.The exploration of these events has important scientific significance and research value.However,there are divers views on the redox condition of deepwater or the existence of euxinic deepwater near the E-C boundary.This paper focuses on the change of deepwater environment of the Nanhua Basin during the E-C transition,and discusses the influence of the change of seawater redox on the Ediacaran life.The divergences could be caused by the following physical factors:ⅰ)the position of E-C boundary is uncertain,which makes inaccurate time,which causes the contradictory of lateral correlation.ⅱ)the rock samples were not collected due to the mulched strata near the E-C boundary,which leads to the failure of geological information extraction.ⅲ)the large sampling spacing results in the omission of geological information,due to the extreme condensing strata in the deepwater facies sections of Ediacaran.Therefore,in order to avoid the incomplete geological information caused by these factors,the working methods and research ideas were further modified in this study.High-resolution rock sampling was adopted for the cherts from the topmost Liuchapo Formation in the Pingyin section in Jiangkou,Guizhou,and then geochemical analysis of rock samples was conducted and the E-C boundary will be constrained by isotopic chronology result.This paper aims to extract the seawater environmental information contained in rock samples near the E-C boundary,and reconstructs the detailed change of paleo-redox condition in the Nanhua Basin at the E-C transition.The Neoproterozoic to Cambrian strata from the Yangtze Platform in South China is complete and continuous,and the Liuchapo Formation of the Pingyin section that contains two layers of K-bentonite belongs to a set of deepwater-facies deposits,which is an ideal place to unravel the evolution information of marine environment during the E-C transition.Work in this study will be carried out in the following three aspects:1.We reported a zircon SHRIMP U-Pb age of K-bentonite found in the topmost Liuchapo Formation in the Pingyin section through the isotopic chronology,constrained the E-C boundary and obtained an accurate time.2.According to the investigation of the geochemical characteristics of major-and trace-elements concentrations in the rock samples from the topmost Liuchapo Formation in the Pingyin section,the genesis and sedimentary environment of the rock samples are discussed,which evaluate whether this set of cherts has the criteria to unravel the information of paleo-seawater environment.3.Extracted the paleo-seawater information recorded in the cherts by using the corresponding redox proxies,get finely insight to the redox condition of deepwater in the Nanhua Basin at the E-C transition,and analyze preliminarily the potential link between the seawater redox and biological evolution.Finally,the following conclusions were obtained:1.A zircon SHRIMP U-Pb geochronology of K-bentonite from the topmost Liuchapo Formation in the Pingyin section,Guizhou yielded an age of 540±3 Ma（2σ,n=26,MSWD=0.94）.Due to the increased number of zircons,the new age was recalculated based on previous work,the data is more statistically significant and more objective,and the accuracy of the dating data has been further improved.The age in this study also supports the scheme on the constraints of the E-C boundary in deep-water realm in South China,which suggested that the E-C boundary should be placed at the horizon that records a significant negative organic carbon isotope excursion.In addition,according to the comparison of the geochemical characteristics the widely distributed K-bentonite from the equivalent sequence in the Yangtze Platform of South China,it shows that the K-bentonite should record the large-scale volcanic activity in this period.2.The cherts from the topmost Liuchapo Formation in the Pingyin section,Guizhou are normal seawater origin,which can be used as a carrier to reflect the redox condition of deepwater in the Nanhua Basin during the E-C transition.This set of cherts has the following geochemical characteristics:1)Major elements:the average contents of SiO₂,Al₂O₃,Fe₂O₃ and MnO are 93.3 wt%,1.89 wt%,0.53 wt%and0.006 wt%,respectively;2)Trace elements:the mean value ofΣREE（Rare Earth Elements）is 34.7 ppm;the distribution pattern of REE under the anoxic environment was observed;the mean values of La _N/Ce _Nand La _N/Yb _N are 1.49 and 0.41,respectively;the mean values of Ce/Ce^* and Eu/Eu^* are 0.64 and 1.08,respectively,the cherts have moderate and weak negative Ce anomalies and no obvious Eu anomalies;the RSE（U,V,Mo）showed extreme weak correlation with Al₂O₃,while there was no positive correlation between P₂O₅ and Y content.The above geochemical characteristics and corresponding proxies indicate synthetically that the rock samples from the topmost Liuchapo Formation in the Pingyin section belong to a set of siliceous rocks with high silicon purity,they do not have the characteristics of typical hydrothermal and biogenic origin and should be normal seawater origin.The RSE of the cherts concentrated through authigenic enrichment rather than detrital input.The combination of geochemical data and paleomagnetism results proves that the paleogeographic location of Pingyin section should be the transitional zone from the lower slope of continental margin to the basin,and should be connected with the open ocean.3.Cherts near the E-C boundary in the topmost Liuchapo Formation in the Pingyin section,Guizhou recorded a prominent change of the deepwater redox in the Nanhua Basin at the E-C transition.The various proxies for the deepwater redox display a remarkable shift.The concentrations and enrichment factors of U,V,Mo in the cherts within the underlying strata are much lower than those above the E-C boundary.Th/U ratios increased progressively from 0.69 to 2.29 below the boundary,and then dropped sharply to below 0.29 near the boundary.V/（V+Ni）ratios increase to>0.84 near the boundary.And almost the chert samples have Fe _HR/Fe _T ratios of>0.38,and their Fe _PY/Fe _HR ratios below the boundary are lower than 0.80,whereas those of the samples around the boundary are higher than 0.80.The pyrite sulfur isotope compositions(δ³⁴S_py)and the whole rock nitrogen isotope compositions(δ¹⁵N)show respectively a negative excursion and a positive excursion at the boundary.Besides,the enrichment factors of RSE and isotopic compositions(δ¹³C_org,δ³⁴S_py and δ¹⁵N)show a relatively synchronous trend.The above evidences demonstrate that a rapid shift of deepwater redox from anoxia to euxinia occurred at the paleogeographic location of Pingyin section during the E-C transition.Further,integrated with existing data,we thus found that an expanded deepwater euxinia occurred in the Nanhua Basin during the E-C transition,and it may be global.The negative excursion of organic carbon isotope near the E-C boundary was coupled with this expanded euxinic deepwater,since organic carbon isotope compositions can be used as an expression of marine primary productivity,it hints that the expanded euxinic seawater restricted the development of marine organisms,and even is a major cause of Ediacaran faunal extinction.