Transition Metal Isotope Fractionation At Low Temperatures, And Its Applications In Paleoceanography

Studies of non-traditional stable isotopes have being developed quickly in recent years thanks to the introduction of multi-collector plasma-source mass-spectrometry(MC-ICP-MS).As new stable isotope systems,the premise and key of using them to constrain important issues is to understand its fractionation mechanism and process adequately.As the extremely low growth rate and stable forming environment,Fe-Mn crust is the ideal fossil of Paleoceanography and global change.The work of this thesis is consisted mainly of two-parts:the experimental study of mass fractionation during the process of crystallization, reduction and hydrolysis;investigation the isotope variations of Fe,Cu and Zn isotope in Fe-Mn. In addition,technical improvements for Cu and Zn isotope measurements have been madeThe main achievements of the thesis are as following:1.Improvements of instrumental stability and precision through using the sampling medium..2.The process of CuSO₄·SH₂O crystallization can induce Cu isotope fractionation,where the cystal shows enrichment of heavy isotope,and equilibrium mass fractionation factor in 65Cu/63Cu isα=1.00017 at room temperature.The process of reduction of Cu²⁺ in to native copper can induce fractionation,where the productant copper shows light isotopeenrichment.As this is a unidirection reaction,it is a kinetic process.3.The heavy isotope of Fe is depleted in the poductant during Fe(â…¢) hydrolysis.The instantaneous equilibrium fractionation factor is decreasing with the temperature increasing.The relationship between the instantaneous equilibrium fractionation factor and temperature is:10³lnα=-0.17×10⁶/T²+0.91.4.Variations in isotope compositions of transition metals in paleocean over the last 80Ma have been obtained through investigation of three Pacific Fe-Mn crusts,which adding our knowledge on the isotope evolution of transition metals in ocean back to late Cretaceous.5.Through combining the data obtained in this study with previous ones,it has been found that:(1) the Fe isotope compositions of surface layers of Fe-Mn crusts are rather heterogeneous even within a same oceanic basin;(2) The Fe isotope compositions of Fe-Mn crusts in different oceans are largely overlapped,but with a general trend of heavy isotope depletion from Atlantic, through Indian,to Pacific Ocean.6.The correlation between Fe isotope and Pb isotope is existed widely in Fe-Mn crusts, which shows the effect of source input on Fe isotope compositions of Fe-Mn crusts to some degree.7.The ranges in isotope variartions of Cu and Zn are small relative to that of Fe isotopes. This result most likely from the differences of theresident times of these elements in oceans,as the residence time of Fe is much shorter than that of ocean circle,implying that the isotope composition cannot be homogenized thus sensitive to local input.In contrast,the resident times of Cu and Zn are much longer than that of ocean circulation,and their isotopes have been rather homogenized,implying the effect of local input has been largely cancelled out8.The patterns of isotope evolution obtained from different Fe-Mn crusts are rather similar at long-term scale,even their isotope compositions are different.This means that the long-term evolution of Fe isotopes recorded in these crust are controlled by factors with regional or even global effects.Through comparing the paleocean environments and Fe isotopes,this is proposed that the Fe isotopes maybe used as a proxy of ocean productivity or environment change.