Research advance for uranium isotope as a quantitative proxy for paleo-oceans anoxic or oxic environment
Compared with other elements that are sensitive to redox environments(U,Mo,V,S,Fe,Cu,Zn,Ni),uranium isotopes have the advantage to quantitatively reflect long-term and global-scale paleoredox of oceans,due to its uniform composition in the oceans globally,and quantitative relationship with the proportion of the anoxic seafloor areas in ancient oceans.Therefore,uranium isotopes are widely used in the events such as the Late Ediacaran,Early Cambrian,Ordovician/Silurian,Late Devonian,Devonian/Carboniferous,Permian/Triassic,Triassic/Jurassic,and OAE 2(Oceanic Anoxic Event 2).Through a review of previous papers,the author has systematically summarized the principles,methods,and results of using uranium isotopes for quantitative analysis of paleoredox.It has found that the core of current uranium isotopes quantitative method is the mass balance box model of seawater uranium(mass balance box model)and its isotopes and the related variants.By simplifying the uranium mass balance box model based on the proportion and fractionation coefficient of different types of uranium sinks,the redox environment and duration of ancient oceans can be quantitatively reflected.However,there may be ambiguity in the interpretation regarding whether the marine environment contains sulfur or not.In addition,by reviewing previous papers,the author has set up a preliminary coupling relationship among the proportion of anoxic seafloor area(%)reflected from uranium isotopes,atmospheric oxygen concentration,and oceanic anoxic or biological events.It has found that:(1)the uranium isotope analysis results were highly consistent with the occurrence time and extent of various anoxic or biological events,especially being coincident with the events that occurred in the Late Ediacaran,Late Ordovician,Late Devonian Frasnian-Famennian,Devonian/Carboniferous,Permian/Triassic,and Triassic/Jurassic periods,indicating that uranium isotopes are indeed an effective global scale and deep-time scale proxy for quantitative analysis on paleoredox of oceans.(2)The trend of anoxic seafloor expansion is not completely consistent with the trend of the atmospheric oxygen concentration changes.Usually,there is a lag.Only the trend of atmospheric oxygen concentration changes in the Triassic/Jurassic period is similar to the trend of oceanic redox environment changes.Analysis suggests that uranium isotope analysis results indicate the proportion of anoxic seafloor area,that is,the proportion of anoxic bottom seawater area,which is actually the change of redox environment of oceanic bottom water.At first,the change in atmospheric oxygen concentration affects shallow seawater,which first responds to the changes in the atmospheric redox environment,and then gradually affects the biological productivity of the seawater,gradually transmitting the environmental changes caused by redox evolution to the bottom water,and thus leading to a lag reaction of anoxic seafloor's expansion or shrink;or due to climatic changes,sea level rise and fall can cause changes of ocean internal circulation(such as thermohaline ocean circulation),leading to changes in productivity and causing anoxic seafloor area expansion or shrink,which may also result in hysteresis effects.There should be a kind of coupling relationship between the atmospheric redox evolution and ocean redox evolution,it deserves to be paid more attention for further study in future.In addition,through a review and analysis of previous research results,the author believes that there are several problems with the quantitative analysis of global scale paleoredox environments of oceans by using uranium isotopes,such as low accuracy,ambiguity,and the representativeness of samples.It is suggested to improve this method from the following two aspects further:(1)although uranium isotopes have been widely used in many important biological events or oceanic anoxic events since the Ediacaran period,the representativeness of sampling sites for some research results still needs to be improved.Although uranium isotopes are assumed to have global homogeneity theoretically,this assertion has not fully considered the influence of post-diagenesis,weathering and alteration in each sampling site on the uranium isotope composition that reflects the original seawater features.It is necessary to calibrate and verify the analysis results of different samples from different sampling sites of the same period to improve the quantitative analysis accuracy further.(2)The single proxy analysis results of uranium isotopes provides a wide range of anoxic seafloor area proportion,showing insufficient accuracy.It is necessary to carry out comprehensive multi-proxy isotopic quantitative analysis,by adding the results of isotopes like Mo,Tl,and others,which is believed could effectively improve the analysis accuracy for the same event.Finally,the author believes that if the accuracy of uranium isotopes method for quantitative analysis could be improved by comprehensive analysis with combination of other isotopes analysis,it is necessary to find out the coupling relationship between atmospheric redox evolution and marine redox evolution,especially the time effects of marine redox evolution to the atmospheric evolution.Hence,it is also possible to find out the response mechanism of oceanic redox conditions to the atmospheric redox evolution.The coupling relationship and the response mechanism may help the related researchers to predict the impact range and extent of oceanic redox evolution after a certain periods of lagging to the atmospheric redox evolution that is caused by natural and human factors.The results may provide great support for addressing global climate change.
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