Spatiotemporal Variations Of Ocean Redox Conditions And Its Co-evolution With Early Animals During The Early Cambrian,South China

The early Cambrian is a key episode of Earth's life evolution and environmental change.The surface biosphere on earth witnessed a significant shift from a microbial world in the Precambrian to the microalgae and metazoan-dominated world during the Cambrian.Furthermore,the biosphere during the early Cambrian are characterized by a radiation of bilaterians and biomineralization as represented by small shelly fauna during the Cambrian Fortunian Age and Age 2,the peak in morphospace and ecospace during the Cambeian Age 3,the enhanced bioturbation,a rise of the animal body size,the expansion of the complex biota as represented by arthropods from shallow to deep waters.Eukaryotes particularly in animals require suitable environment such as enough oxygen to sustain their metabonisms.However,the advent of eukaryotes and their activities could result in the changes in the surrounding even global environments such as an increase or decline in the atmospheric-oceanic oxygen levels as well as a rise of seawater sulfate concentrations.Thus,it is very important to explore the co-evolution of ocean chemistry and life evolution during the early Cambrian.At present,scholars have done many related works and a lot of propresses have been made.However,there are absence of systematic and deepgoing researches on the co-evolution of ocean chemistry and life during the early Cambrian.The lower Cambrian strata in South China are relatively continuous and well preserved.Importantly,there are abundant data for the stratiraphy and paleontology,which provides solid basis for the stratigraphic division and correlation.In order to explore the relationship between the early Cambrian ocean chemistry and life evolution,we conduct this work from four following angles:(1)the general co-evolution of spatial variability of ocean chemistry and early animals through the reconstruction of spatial structure of ocean chemistry in South China;(2)the influence of spatial variability of water redox conditions on animals and its activities through the investigation of the water redox reconstruction on the beds with different complex fossil assemblages;(3)the impact of bottom-water and photic euxinic conditions on the benthic and planktonic animals;(4)that whether the ocean oxygenation event occurred in the Cambrian and its relationship with animal evolution through the investigation of oxic waters in South China and oxic process globally.Thus,we conducted analyses of iron speciation,redox-sensitive trace metal,sulfur isotopic composition of pyrite,and nutrient-related proxies such as total organic carbon(TOC)and excess barium for seven sections with different paleo-water depths during the early Cambrian in South China.These new results in combination with the published data globally provide a unique opportunity to explore the fluctuations of early Cambrian ocean chemistry.Ultimately,comparison of the spatiotemporal variability of ocean chemistry to the fossil distribution in South China even global elsewhere can provide a new insight to explore the co-evolution of ocean chemistry and life evolution.The main results related to this study as given as follows.1.The iron speciation and redox-sensitive trace metal data from the lower Cambrian strata located from inner-shelf to basinal settings in South China suggest that the mid-depth euxinic waters were co-existed with oxic surface waters and deep ferruginous waters.In combination with ocean redox conditions in global elsewhere including Oman,Canada,and Italy,we infer that the global ocean exhibited a high degree of redox heterogeneity during the early Cambrian,.Our results further show that bottom-water euxinic conditions were observed in mid-depth and some basinal environment,whereas phothic euxinic conditions were found in some basinal settings.This spatial heterogeneity of bottom and phothic waters might result from a stepwise decline in upwelling.These findings are consistent with low atmospheric oxygen levels(~10-40% of present atmospheric level,or PAL)inferred recently but inconsistent with conventional view of a full oxygenation of Cambrian atmosphere and ocean.2.The reconstructed redox conditions in South China point to an expansion of oxic waters from shallow to deep waters.Interestingly,the compilation of our new iron speciation data together with pulished ones globally from Ediacaran to Cambrian suggest an oxygenation event during the Cambrian Age 2 to 4.Our findings disagree with the stable or decreasing marine oxygen levels during the early–middle Cambrian suggested in previous studies.3.A large spatial gradient in pyrite sulfur isotopic compositions(?34Spy),which vary from an average of-12.0‰ in nearshore to +22.5‰ in distal deepwater areas.This finding suggests low concentrations and spatial heterogeneity of water sulfate,consistent with a limited oceanic sulfate reservoir globally rather than the conventional view of high oceanic sulfate in Cambrian ocean.4.We conduct an investigation of the relationship between TOC and the ratio of pyrite iron to highly reactive iron(FePy/FeHR)from sections located in different sedimentary facies.These results show a good relationship(R2=0.66)in the outer-shelf Wangjiaping section,but weak relationship(R2?0.22)for deeper-water areas including the outer-shelf Yangjiaping,slope Songtao and Longbizui sections.Furthermore,both ?34SPy(-2.1±5.3‰)and TOC(2.4±1.1%)values at Wangjiaping are lower than ones at Wangjiaping(?34SPy: +8.3±4.7‰;TOC: 10.7±2.6%),Songtao(?34SPy: +8.6±7.7‰;TOC: 8.3±4.0%),and Longbizui(?34SPy: +21.0±6.8‰;TOC: 6.2±1.7%).These findings suggest that the developments of euxinic conditions at Wanjiaping and other deeper-water areas are up to the organic matter and sulfate availability,respectively.This interpretation is in agreement with a gradient of seawater sulfate concentrations.5.Coupling our redox proxy work with the extensive paleontological record in South China,we find that the stepwise ocean oxygenation is well coupled with a series of important biotic events including an increase in biodiversity,the expansion of complex fauna as represented by arthropods from shallow to deep waters,the complex fauna instead of simple one as represented by small shelly fauna or sponge.This finding suggests that the oceanic oxygen levels play a key role in life evolution during this period.Further,our results show that many benthic trilobites and bioturbation were found in the oxic conditions of inner-shelf areas,whereas only the appearance of planktonic trilobites without bioturbation were exhibited in the anoxic even euxinic conditions of outer-shelf to basinal areas.These findings indicate the oceanic redox conditions substantially controlled the spatial distribution of trilobite and bioturbation.Amazingly,the benthic trilobites are only found in the stable and persistent oxic waters,whereas the benthic sponges are discovered in the dynamic environment dominated by suboxic conditions.This observation might suggest high requirement of oxygen levels for skeletal animals.However,deep waters remained anoxic and ferruginous even as the appearance of macrozooplankton and suspension-feeding mesozooplankton,which is inconsistent with a prevous view that the onset of these animals resulted in oxygenation of deep oceans.Further,although high seawater sulfate are inferred in some local areas,it remained low oceanic sulfate reservoir as the enhanced bioturbation,which is inconsistent with a previous view that a rise of bioturbation caused high oceanic sulfate reservoir.In summary,the influence of early Cambrian animals on contemporaneous ocean chemistry,as proposed in previous studies,may be overly exaggerated.We also find that no planktonic and/or benthic fossils were observed in anoxic and suboxic conditions(e.g.,manganous;at least ~10 ?M of dissolved oxygen level),suggesting other factors on the life evolution such as ecology.