Redox State Evolution Of The Late Neoproterozoic To Early Cambrian Ocean

The Earth system experienced tectonic,climatic,ecological and biological upheavals between late Neoproterozoic to early Cambrian.One of the significant outcome was the Earth's second major oxygenation event(ca.720-520 Ma),when the oxygen level in the atmosphere and ocean approached the modern level.The redox condition on Earth's surface is closely related with major biogeochemical cycles,so that it closely in-teracts with the evolution of life.Especially,as one of the major chemical components in determining the redox condition,oxygen(O2)probably had facilitated the origin and diversification of multi-cellular life.This thesis mainly studies the redox-related geochemical indices and reviews the bi-ological evolution,and discusses both of them in an integrated stratigraphic framework in order to further study the coevolution between life and the redox condition during the late Neoproterozoic-early Cambrian interval.The basal Datangpo Formation was deposited during the interglacial period be-tween the late Neoproterozoic Sturtian and Marinoan snowball Earth glaciation.It is rich in highly-reactive iron and acid volatile sulphide,and has relatively low content of pyrite,all suggest that the local sedimentary environment was lack of oxygen and sul-phide,but rich in ferrous iron.Moreover,the ?34S value of pyrites in the basal Datangpo Formation and many other equivalent units in Namibia,Canada and Australia,reached an unprecedentedly high level in Earth's history,which indicates scarcity of sulphate in global ocean.This may have been due to low input of sulphate from weathering of con-tinents,and high output of oceanic sulphur in the form of pyrite.And both of them may have linked to the ocean stratification during the global glaciations and the secure anoxic state of Earth's surface.The lower than mantle values of ?13Ccarb and the anomaly in the linear relationship between ?13Ccarb and Acarb-OM suggest there may have been a large dissolved organic carbon(DOC)reservoir in the ocean.Microbial carbon pump may have played vital role in the accumulation of this large DOC reservoir,and the modern-style carbon pump may have been not in function due to lack of eukaryotic phytoplankton and macroscopic animals.The arising difference of sulphate isotopic compositions between pyrite and sul-phate from the bottom to the top of Ediacaran units suggests an increase in the oceanic sulphate reservoir.Oxygenation events may have followed the Marinoan and Gaskiers glaciation,as indicated by the moderately high Mo concentrations and ?98/95Mo values.The ocean redox state may have been spatially heterogeneous during most of the Edi-acaran Period,with considerable oxygenation in the surface and ferruginous condition in the deep.Moreover,mid-water sulphidic iron-minimum zone(S-IMZ)may have de-veloped on the continental margins.We use both local and global redox indices to reveal a possible expansion of such S-IMZ during an oxygenation event in late Ediacaran.This event may have hampered the healthy development of eukaryotes by poisoning the aer-obic respiration.The shallow ocean during the latest Ediacaran may have experienced gradually oxygenation as indicated by decreasing Ce/Ce*value in Dengying Formation,which probably related with the diversification of macroscopic bilaterians preserved as more complicated trace fossils and skeletons.The primary producers in the ocean may have still been prokaryotes.Although animals began to diversify,majorities of them were soft-bodied sessile epibenthic osmotrophs(sponges and Ediacara-type fauna etc.)who had low oxygen level requirements.However,they may have contributed to the con-sumption of the large dissolved organic carbon reservoir in the ocean,which facilitated the further oxygenation of the ocean.The maximum ?98/95Mo values of carbonaceous rocks in South China increased steadily in early Cambrian,and reached modern open ocean value at about 521 Ma,which indicates that modern-like oxygen levels characterised the ocean in early Cam-brian for the first time in Earth history.The expansion of strongly and moderately oxic seafloor may have provided a significant increase in the size of stably habitable space for animals on the continental margins and liberate animals that possibly suffered from fluc-tuating redox conditions during Ediacaran time.It is further demonstrated that the areal extent of oxygenated bottom waters increased in step with the early Cambrian bioradi-ation of animals and eukaryotic phytoplankton.Animals developed many ecologically important characters during this biotic event,such as increased animal body size,ac-tive locomotion,bioturbation,carnivory and cropping.Eukaryotic phytoplankton may have begun to contribute a major proportion to the primary production,and provided food sources for mesozooplankton.The marine ecosystem with extended trophic levels and size hierarchy may have become more like the modern ones.Most of these newly-evolved life forms and their characters are facilitated by the early Cambrian oxygena-tion of the ocean,while some of them affected both carbon and nutrient cycling,first facilitating and then stabilizing more widespread oxygenation in the world's oceans.