Geochemical Characteristics Of Black Shale And Paleoceanic Implications Of Niutitang Formation Of Early Cambrian, Guizhou And Hunan Province

Following the the breakup of Rodiania and demise of the most severe ice age,the Ediacaran-early Cambrian witnesses the rapid develoment of metazoans,culminating in'Cambrian Explosion'.This biological innovation has been widely attributed to shift in ocean redox conditons from dominately anoxic to more oxic,yet,the redox evolution of this critial period remains highly controversial.To further address this question,this paper carried out systematic petrological observation and geochemical analyses for the Early Cambrian Niutitang black shales in the Hejiapu section of northeastern Guizhou and the Siduping section of western Hunan.Paleogeographically,both sections were situated in the transitional belt between the shallow platform and deep basin facies.Our research,in combination with the data published previously from other sections of the same period from south China,provides better constrain for the redox conditions of the paleo-ocean during Early Cambrian.The Niutitang black shales from both sections have lower Al,Th compared to the Past-Archean Australian Shale?PAAS?value,indicating little affection of terrigenous detritals.All the analyzed samples display neither significant shale-normalized negative Ce anomalies,nor positive Eu anomalies,with only low rare earth elements?REE?,suggestive of none or little hydrothermal origin.Instead,the enrichments of light rare earth elements?LREE?and the light negative Ce anomalies in the samples more likely imply an anoxic or euxinic condition in the water column.Based on the analyses of RSE contents and the relationship between Mo and total organic carbon?TOC?and Mo/TOC ratios,a weakly restricted to open basin conditions are suggested for the Hejiapu section;while a weakly to strongly restricted basin conditions were proposed for middle part of the Siduping section.The upper part of this section shows a clear tendency of waning basin restriction.Overall,the Siduping section shows a property more close to the platform than that of the Hejiapu section.Framboidal pyrites are present in the black shale of middle Hejiapu section,while many framboidal pyrite molds occur in upper part of the section,with an average framboid diameter less than 10?m.Redox sensitive elements?RSE?are extremely enriched in the black shales at the bottom and middle section,but show a clear decrease in the top section.Other proxies,such as V/Ni,Ni/Cr,V/?V+Ni?,Mo_EF-U_EF,DOP-T,and Fe/Al ratios all displaythe same pattern.In conclusion,black shales in the bottom and middle parts of the Hejiapu section were deposited under a persistent euxinic condition,while those in top part of the section were formed in a lesseuxinic condition,suggesting a change in sea-water redox condition.In the Siduping section,framboidal pyrites are observed in the black shales both from lower and upper parts,with an average diameter less than 10?m.In the middle part of the section,however,pyriye framboids are less common.The enrichments of RSE in the middle part of the section is lower than those in the Hjiapu section,but in the top part of the section RSE are highly enriched.Other proxies in this section also show the same pattern.Overall,this section shows a pattern of decreasingeuxinic condition upwards.Our data further confirm that in the earlier period of Early Cambrian,the paleo-ocean of South China most likely remained stratified,with a euxinic mid-depth waters,and both sections studied in this paper were situated in the“euxinic wedge”.Both sections showed a decresing trend in euxinic conditions upwards,likely suggesting a gradual increase of oxygenation in the water column.The consequent increases of Fe and Mo concentrations as important micro-nutrients in the ocean likely paved the way for the onset and rapid diversification metazoans in the early Cambrian.