Study On The Coevolution Between Early Siliceous Organisms And Oceanic Environment During Late Ediacaran-early Cambrian

Biosilicification has been reported from all the extant eukaryotic supergroup.The earliest siliceous organisms that use dissolved silicon to build siliceous skeleton are known as siliceous sponges and radiolarians,which were reported from Ediacaran-Cambrian transitional period as an important part of Cambrian explosion.However,molecular phylogenetic studies and biomarkers indicate a Neoproterozoic origin of sponges and radiolarians,which is earlier than fossil records.So,more research on the origin of siliceous organisms is needed to solve the gap among different research methods.Due to lack of siliceous organisms,Precambrian silicon output is controlled by inorganic precipitation,while in modern ocean occupied by flourishing siliceous organisms and silicon is usually precipitated via biological activities.Therefore,the appearance of siliceous organisms plays an important role in the transformation of oceanic silicon cycle.Silicon cycle is also closely related to carbon cycle through weathering process,which consumes CO₂and release dissolved silicon to ocean.Therefore,research on siliceous organisms will shed light on exploring both the evolution of organisms and the carbon-silicon cycles.A series of widespread and continuous cherts were deposited on South China during late Ediacaran-early Cambrian,where the earliest siliceous sponge spicules and radiolarians were discovered,thus opening an excellent window to study the evolution of early siliceous organisms and oceanic environment,as well as their relationships.This study focuses on three sections:Gunshi’ao section in Zigui,Hubei Province,Yangtou section in Tongren,Guizhou Province and Yanziqiao section in Yiyang,Hunan Province.The Yanjiahe Formation of Gunshi’ao section is deposited in shallow intraplatform basin,while the Liuchapo Formation of Yangtou section and Yanziqiao section are deposited in slope-basin environment.The thesis structure can be divided as:（1）summarizing the organismal facies at the studied sections through observations of thin sections and microfossils;（2）exploring the origin and forming mechanisms of Ediacaran-Cambrian cherts based on geochemical proxies such as Al-Fe-Mn,Ge/Si and rare earth elements;（3）analyzing the oceanic redox conditions according to the features of Mo,U and V/Cr,and discussing the relationship between redox conditions and organismal transitions;（4）using bulk silicon isotopes and box model to investigate the silicon cycle during Ediacaran-Cambrian transition.Major conclusions include:（1）In late Ediacaran,Gunshi’ao section and Yanziqiao section only contain some irregular organic mass without any clear biological structure,while Yangtou section is dominated by Horodyskia minor and Palaeopascichnus jiumenensis.Abundant fossils,including sponge spicules,multicellular algae,small shelly fossils,sheet-like fossils and radiolarians,flourish in early Cambrian.（2）The forming process of Ediacaran-Cambrian cherts can be classified into three mechanisms:direct precipitation from hydrothermal vents,which exists as regional phenomenon;cherts on shallow platform are formed by the replacement of carbonate and sponge uptake;cherts in slope-basin environments are controlled by organic adsorption and sponge uptake.（3）According to Mo-U enrichment factors and V/Cr values,late Ediacaran is featured by suboxic environment while early Cambrian is dominated by more anoxic condition.So,at least the first stage of Cambrian explosion is not caused by oxygenation,instead,anoxia may have promoted the revolution of biological functions or ecological systems.Also,we can speculate that early animals such as sponges have low oxygen requirements.（4）New and compiled silicon isotope data from seven sections spanning the E-C boundary show an increase in divergence ofδ³⁰Si values at ca.533Ma.Using mass-balance arguments,we interpret this as evidence that the radiation of siliceous sponges and radiolarians contributed to a substantial（ca.80%）decrease in oceanic dissolved silicon concentrations.Before the silicon decline,the ocean is characterized by large silicon inventory,which can buffer short-term or regional silicon fluctuations,thus maintaining a homogenous ocean in respect of silicon.After the silicon decline,the ocean is characterized by small silicon inventory,thus becoming sensitive to any silicon fluctuations and recording diverged silicon signals.（5）Sponge animals may have existed since Precambrian and they may also have evolved the genes for biosilicification.However,the phenomenon of biosilicification might be absent until early Cambrian,indicating an important role of external factors.The removal of harmful high silicon concentration by flourishing siliceous organisms may benefit the main stage of Cambrian explosion.