Chromium Isotopic Fractionations And Its Applications In Surface Environments

With the development of multi-collector mass spectrometer,stable chromium(Cr)isotope system has been widely applied to understand various geochemical processes.In surface environments,Cr exists as either Cr(?)or Cr(?).Cr(?)in igneous rocks can be oxidized to soluble Cr(?)by manganese oxide,and Cr(?)can be reduced to insoluble Cr(?)by organic matter and Fe(?)in the environment.The valence change between Cr(?)and Cr(?)is accompanied by isotopic fractionation.Thus,Cr isotope system is a potential proxy to trace the Cr cycling in surface environments.In this thesis,we conducted a series of work to study the Cr isotopic fractionation behavior during surficial processes and its applications in natural environments.Firstly,clarifying the transformation mechanism of Cr in different components and the accompanied Cr isotopic fractionation in soil during weathering process is of great significance for understanding the surface Cr cycling.Red soil is an important soil type in China,and paddy soil can be intensively modified by farming.With intense weathering and influence from human activities,the elemental cycling in paddy fields from the Red Soil Critical Zone Observatory(RSCZO)in southern China is very complex.We measured the Cr isotopic compositions of two paddy field profiles and two drillcores.The young paddy fields were converted from uplands about 20 years ago,and the ?53Cr values of the profile range from-0.34‰ to-0.22‰,while the old paddy fields were converted about 100 years ago,and the ?53Cr values of the profile range from-0.20‰ to-0.06‰.The ?53Cr values for the old paddy fields are generally more positive than that for the young paddy fields,indicating that the Cr isotopic compositions of the red soils have been changed due to the long-time rice cultivation.The BCR three-step sequential extraction method divides the Cr in soils into four forms:exchangeable(extraction Step 1),reducible-iron/manganese oxides(extraction Step 2),oxidizable-organic matter and sulfides(extraction Step 3),and residual Cr.The leaching results showed that Cr adsorbed by Fe-Mn oxides is rich in heavy Cr isotopes.The organic matter prefers to adsorb or reduce light Cr isotopes,leaving the remaining Cr(?)in the soil pore water enriched in heavy isotopes.The results can help to a better understanding of the mechanism of Cr isotopic fractionation during oxidative weathering.The fractionation during oxidation of Cr(?)to Cr(?)is relatively small,and the negative ?53Cr values of the weathering residues may result from the reduction of Cr(?).In addition,the results from the two drillcores are similar to that from the profile samples.The weathering products migrate to rivers and eventually to the ocean.The river system plays as a link between the land and the ocean.In this thesis,Cr isotopic compositions of the sediments from upstream to estuary along the Xiaoqing River in Shandong Province were measured.In addition,the Xiaoqing River is one of the most polluted rivers flowing into Laizhou Bay,and the Cr isotopic compositions of river sediments may indicate the contributions of Cr from different sources.Because bulk samples contain a large amount of detrital Cr,we need to subtract it to obtain information on authigenic Cr.The authigenic Cr isotopic compositions(?53Crauth)obtained from both the leaching experiments and the calculation using the detrital index(Cr/Ti)are essentially identical.The results showed that the positive ?53Crauth values in the upstream samples indicate a Cr source from the anthropogenic activity.The slightly positive ?53Cr values in the midstream samples probably result from natural oxidative weathering.The ?53Cr values in the downstream,estuary and Laizhou Bay are within the range of igneous reservoirs,suggestive of a source of detritus.Last,on the basis of understanding the Cr cycling and the Cr isotopic fractionation mechanism in modern surface environments,we applied Cr isotope system to understand the redox change in paleoenvironment.The Ediacaran Period(635 Ma?541 Ma)is a crucial period in the biological evolution history.During this period,macroscopic multicellular animals first arised,which is widely thought to be related to the rise of the atmospheric oxygen level.However,the redox states of atmosphere and ocean during the Ediacaran Period still remain controversial.We measured the Cr isotopic compositions of two different Ediacaran successions,deposited in the inner shelf and the slope/basin environment of Yangtze platform,respectively.The distinct?53Cr values of shales between the shallow water succession(the Wangji succession,up to 1.11‰)and the deep water succession(the Lantian succession,the ?53Cr values are within the range of igneous reservoirs)suggest redox heterogeneity of the Ediacaran ocean.The deep water was probably still anoxic at that time,and the Cr(VI)enriched in heavy Cr isotopes from oxidative weathering had been reduced before it could be transported to the Lantian area.In the carbonates of the upper Doushantuo Formation,the positive excursion of ?53Cr values correlates with the ?13C positive excursion,probably indicating the enhanced biological productivity at that time.In conclusion,Cr isotope system is a reliable proxy to trace the Cr cycling in soil,to identify the Cr sources in river sediments,which can serve as a guide for environmental remediation,and to reconstruct the redox evolution in the geological history.