The Applications Of Cu,Zn And Sr Isotopes In Oceanic Environments During The Cambrian Explosion And Late Devonian Mass Extinction

This doctoral dissertation focuses on the evolution of life during the two critical periods of the Earth's history,namely,the life explosion and the mass extinction.The“Cambrian Explosion”is one of the most important biotic events in geological history,and it has been widely attributed to increasing oxygen levels in the atmosphere-ocean system.However,there are considerable controversies on the redox conditions of the early Cambrian ocean.Some studies suggested that the ocean was widely oxygenated prior to“Cambrian Explosion”,while other studies inferred that the early Cambrian ocean still remained anoxic or even euxinic conditions.The Frasnian-Famennian?F-F?mass extinction of the Late Devonian is one of the“Big Five”biotic crises during the Phanerozoic.The cause of this catastrophic event still remains controversial,and hydrothermal activities/volcanisms have been proposed to explain this extinction event,but geochemical evidence in support of the model is scarce.Copper?Cu?and zinc?Zn?isotopes can be significantly affected by oceanic sulfidic environments,and Zn and strontium?Sr?isotopes can be utilized as indicators of hydrothermal activities/volcanisms.In order to explore the oceanic environments during the“Cambrian Explosion”and F-F mass extinction,in addition to other traditional geochemical proxies,we analyzed the Cu and Zn isotopes of Lower Cambrian black shales from the Meishucun section in Yunnan,and Zn and Sr isotopes of carbonate rocks across the F-F boundary from the Fuhe section in Guangxi.Three stages among black shales in Meishucun section show the decreases of?⁶⁵Cu and?⁶⁶Zn,and enrichments of Mo,U and V,indicating the developments of three phases anoxic?even euxinic?water during the deposition of black shales.The appearance of anoxic?even euxinic?water during the Qiongzhusi Age probably hindered the evolution of small shelly faunas,and probably also postponed the major phase of the“Cambrian Explosion”.Mo/TOC ratios of black shales deposited in anoxic?even euxinic?water settings continuously increase,and?⁶⁵Cu_authuth and?⁶⁶Zn_authuth of black shales deposited in oxic-suboxic water settings gradually become heavier.?¹³C_org also increases along the section.It indicates a progressive oxygenation of the atmosphere-ocean system between the two phases of the“Cambrian Explosion”,which results in increase of the oceanic primary productivity and promotes the evolution of early animals.The ?⁶⁶Zn and ⁸⁷Sr/⁸⁶Sr ratios do not show coupled negative shifts in or before the two phases Kellwasser Event Horizon,indicating that hydrothermal activities or volcanisms might be limited in affecting the F-F marine ecosystems in South China.On the contrary,a coupling of elevated?⁶⁶Zn and ⁸⁷Sr/⁸⁶Sr values is observed in the LKWH,UKWH and Palmatolepis delicatula platys to P.minuta minuta conodont zones.The positive?⁶⁶Zn and ⁸⁷Sr/⁸⁶Sr shifts are exactly located in the same horizons characterized by low?¹³C values and sea-level fall stages,and are likely induced by enhanced carbonate weathering as a result of climate cooling.A simple numerical modeling of the relationship between enhanced carbonate weathering and change of Zn-Sr isotopes in seawater supports this explanation.We suggest that climate cooling might have caused wide-spread demise of shallow-water tropical stenothermal faunas,and the resulting sea-level falls could have reduced the habitats of marine benthic organisms and finally caused the F-F mass extinction.