Redox Evolutions At Earth's Surface From The Cryogenian To Early Cambrian

The Cryogenian to early Cambrian was a crucial period in Earth History,witnessing a major tectonic reorganization,global climate perturbations,a significant oxygenation in the atmosphere and oceans,fluctuations of marine carbon cycle,and the naissance and diversity of macroscopic multicellular animals.These tectonic,climatic,environmental,and ecological events are often considered to be interrelated.Especially,the rise in oceanic oxygen levels may be a trigger for the emergence and radiation of early animals.However,the detailed temporary and spatial variations of this oxygenation event during this period remain unclear and controversial.As a paleo-redox proxy,chromium-isotope provide a new insight that can trace the oxygenation history in the atmosphere.The prerequisite to apply this proxy is the accurate understanding of the Cr cycle in modern Earth's Surface and associated Cr-isotope fractionation.Previous studies have focused on authigenic Cr-isotope compositions of marine euxinic sediments,hypoxic continental margin sediments,oxic siliciclastic sediments,and carbonates.The marine ferromanganese crusts that have widely distributed in oxic environment is a sink for lots of metal elements in seawaters,however,its potential role as a sink of Cr isotopes has not been fully investigated yet.In this study,the Cr-isotope data of modern oceanic Fe-Mn crusts was reported for the first time.The ?53Cr values of the surface scrapings of the Fe-Mn crusts from the central North Pacific seamounts,range from-0.85 to-0.15 ‰,with an average of-0.42 ± 0.34 ‰(2a,n =11),which are clearly lower than those of the Pacific seawaters(0.53-1.43 ‰ with an average of 0.79 ‰).The results reveal preferential removal of light Cr isotopes from seawaters into Fe-Mn crusts,with a fractionation roughly from-2.3 to-0.7 ‰,averaging at-1.2 ‰.It is suggested that such significant negative isotopic fractionation is reached through adsorption of dissolved Cr(VI)anion in the oxic seawater onto the surface of Mn-oxide phases during deposition of the Fe-Mn crusts.However,our modelling analyses imply that even in modern oceanic environment,the deposition of Mn-oxide sediments would make insignificant contribution to the elevation of the global seawater average ?53Cr values.This results lend paleo-environment researchers confidence in applying Cr isotope as paleo-redox proxy.The Yangtze Block in South China is ideally suited for the study of redox fluctuations in Earth's Surface from the Cryogenian to early Cambrian because of widely distributed and continuous banded iron formation,carbonate,and black shale successions deposited through that period.In the basinal setting of the Yangtze Block(Nanhua Basin),an interval of the banded iron formations(BIFs)is preserved at the base of the Fulu Formation,which is thought to be deposited during the waning of the Sturtian glaciation.In this study,Cr-isotope and rare earth elements data of the BIFs from the basal Fulu Formation of the Xiajiajiang section were reported.Strongly positively fractionated authigenic ?53Cr values,?0.71 ‰,were obtained after detrital evaluation,which suggests extensive oxidative Cr cycling occurred at the Earth's Surface and implies that the pO2 was probably higher than 1%PAL during this period.The lack of Ce anomalies(0.90<Ce/Ce*<1.02)suggests that during this period the seawater in which the BIFs were deposited was at a redox state under which the oxidation of Fe2+ to Fe3+ could occur but the oxidation of Ce3+ to Ce4+ could not.A compilation of published ?53Cr data from BIFs,ironstones,and shales deposited from the late Paleoproterozoic to early Cryogenian(1.8-0.7 Ga)suggests that the early increase in atmospheric O2 related to the NOE probably first took place during the late Tonian Period(ca.800-720 Ma).This was temporally coincident with the first appearance of several eukaryotic groups,and the estimated initial divergence of early animals.Chromium-isotope and Ce-anomaly data in carbonates of the Ediacaran Doushantuo and Dengying formations from the Yangtze Gorges area and the Ediacaran-Cambrian Dengying and Zhuajiaqing formations in the Xiaotan section have shown abilities to monitor oxygenation variations in the atmosphere and shallow seawaters of the Yangtze Platform from the Ediacaran to early Cambrian.Slightly positive ?53Crauth values(between 0.18 and 0.43 ‰)and negative shift in Ce/Ce*values(1.04-0.60)from the upper DST-? and DST-? members suggest generally oxidizing conditions in the atmosphere(with pO2 over 1%PAL)and in shallow waters of the Yangtze Platform during the post-Marinoan phase,which may have been related to the diversity of early Ediacaran organisms.Shuram/Wonoka correlative DST-? carbonates are characterized by the fluctuations of?53Crauth values(-0.25-0.97 ‰)and a significant negative ?13C excursion,which may have resulted from partial oxidation of the large deep-water dissolved organic carbon(DOC)reservoir under enhanced oxidative weathering conditions.This process released 13C-depleted carbon into dissolved inorganic carbon pool and isotopically unfractionated Cr(?)into water columns,the latter of which immediately precipitated by adsorption onto the detrital materials.Meanwhile,an increase in local anoxia(and even euxinia)in the intrashelf basin of the Yangtze Platform driven by enhanced microbial sulfate reduction and the resultant H2S,possibly limited regional biological evolution during this phase.The elevated ?53Crauth values in the upper Hamajing(0.40 ‰)and Donglongtan members(0.58-1.18 ‰),and the entire Shibantan Member(0.81-0.96 ‰),together with the decreasing trend of Ce/Ce*values(from 0.98 to 0.33)from the Hamajing Member to Shibantan Member,indicate a more oxygenated environment in the atmosphere and in shallow seawaters of the Yangtze Platform during the late Ediacaran phase,which may have been beneficial to the diversification of late Ediacaran animals.In combination with Ce/Ce*values(0.28-0.67),?53Crauth values from-0.11 to 1.15‰ in the Zhujiaqing Formation indicate that there was a broad oxygenation in the atmosphere and in shallow waters of the Yangtze Platform during the Ed-C transition phase,potentially related to the first appearance of Cambrian animals.This oxygenation event may result in the oxidation of an upwelling DOC pool,leading to the pronounced heterogeneity of ?53Crauth and ?13C values in the Daibu and Zhongyicun members.