Mechanism And Application Of Chromium Isotopes In Carbonates As A Paleo-redox Proxy

The redox condition of Earth's atmosphere and ocean is widely regarded to have played an important role in the evolution of life.Chromium(Cr)is a redox sensitive element and Cr isotopes have been widely used to trace the evolution of the oxygen level in Earth's atmosphere and ocean.Carbonates are the most widely distributed chemical sedimentary rocks over the geologic history,and have been used as an important archive for paleo-seawater Cr isotopic compositions.However,the behavior of Cr in the deposition environments of marine carbonate is still unclear.Therefore,in this thesis,I investigated the Cr speciation and isotopic composition in carbonates by combining natural sample observations and laboratory simulations,to decipher the geochemical behavior of Cr during carbonate deposition.On the basis of Cr speciation studies,Cr isotopes were applied to investigate the oceanic redox variation across the Permian-Triassic Boundary(PTB),and its relationship to the end-Permian mass extinction and the biotic recovery afterwards.To investigate the speciation of Cr in carbonates,I first determined the oxidation states and isotopic compositions of Cr in sedimentary carbonates ranging from?1.44 Ga to?0.83 Ma in age.X-ray absorption near edge structure(XANES)spectra results show that Cr(?)is the dominant species in all samples regardless of deposition age and Cr isotope composition.This is inconsistent with previous hypothesis that Cr(?)in seawater could directly occupy the position of CO32-in calcite crystal lattice and be preserved in sedimentary carbonates.There are two possible explanations for the absence of Cr(VI)in carbonates.One is that Cr(VI)in seawater was reduced before or after carbonate precipitation,which might cause Cr isotopic fractionation between seawater and carbonates;the other is that marine carbonates preferentially uptake Cr(?)from seawater.As Cr(III)can come from non-redox Cr cycling,which can also cause isotopic fractionation during dissolution and adsorption processes,positively fractionated Cr isotopic values do not necessarily correspond to the rise in atmospheric oxygen level.Although from the above work we know that most of the authigenic Cr in sedimentary carbonates are trivalent,the incorporation behavior of Cr(III)to carbonates is still unclear.Direct investigation of the coordination environment of Cr in natural carbonates is challenging because of the low Cr concentrations and the influence from detrital materials.Therefore,I conducted coprecipitation experiments of Cr(III)with calcium carbonate to constrain the behavior of Cr(?)in carbonate deposition environment.Extended X-ray absorption fine structure(EXAFS)results suggest that Cr(?)is incompatible with calcite crystal lattice.Most Cr in the coprecipitation samples exists as inner-sphere adsorption complex or amorphous Cr(OH)3·xH2O aggregate.A small fraction of Cr could occupy in calcite crystal,but Cr3+could not directly substitute Ca2+,possibly occupy the position of Ca2+ in the form of CrOH2+,leading to a distorted structure,or simply exists as hydroxide inclusions.Besides,it is found that Cr(III)could stimulate polymorph selection of vaterite during calcium carbonate precipitation,probably due to formation of Cr hydroxide at the surface of metastable vaterite crystals,which would retard its transformation to calcite.As Cr(III)is incompatible with carbonate crystal lattice,I suggest that most Cr in natural carbonates might be adsorbed at the crystal surface,and could be acquired after carbonate precipitation.Hence,Cr isotopic composition of carbonates might reflect signal from bottom water or porewater.The investigation of Cr speciation can help to better interpret the Cr isotope data recorded in sedimentary carbonates.Based on this,this study applied Cr isotopes to decipher redox variation in the ocean across the Permian-Triassic Boundary.Multiple observations have shown that widespread oceanic anoxia might have played an important role in the end-Permian mass extinction and the delayed biotic recovery in the Early Triassic.However,it has not been reached a consensus yet on the temporal and spatial changes of the oceanic redox condition.This study presents the Cr isotopic compositions of Upper Permian-Lower Triassic carbonate ramp from the Xiakou section,South China.The ?53Cr values are largely fractionated in the latest Permian(1.15‰ to 2.16‰),which are almost the highest values among Phanerozoic sedimentary carbonates.Such high values are unlikely caused by non-redox Cr cycling,but could be the result of Cr reduction facilitated by the development of reducing environment during the latest Permian,leading to enhanced Cr(?)reduction and sequestration,which could elevate the ?53Cr values of seawater.The high Cr isotope ratios were followed by an abrupt negative excursion of ?53Cr values(from average 1.78‰ to average-0.13‰)across the Permian-Triassic Boundary,which can be best explained by rapid expansion of oceanic anoxia in the eastern Paleotethys.Persistent negative ?53Cr values extending to the Early Triassic indicate that the ocean did not recover from anoxia for an extended period,at least until the Smithian.Chromium isotope results in this study suggest a progressive deterioration of oceanic environments,with moderate oceanic anoxia in the latest Permian,a rapid expansion around the time of mass extinction,and persistent anoxic conditions in the Early Triassic,probably contributing to the profound ecosystem crisis and protracted biotic recovery.