Uranium isotopic compositions of the crust and ocean: Age corrections, U budget and global extent of modern anoxia

Abstract

The ^(238)U/^(235)U isotopic composition of uranium in seawater can provide important insights into the modern U budget of the oceans. Using the double spike technique and a new data reduction method, we analyzed an array of seawater samples and 41 geostandards covering a broad range of geological settings relevant to low and high temperature geochemistry. Analyses of 18 seawater samples from geographically diverse sites from the Atlantic and Pacific oceans, Mediterranean Sea, Gulf of Mexico, Persian Gulf, and English Channel, together with literature data (n = 17), yield a δ^(238)U value for modern seawater of −0.392 ± 0.005‰ relative to CRM-112a. Measurements of the uranium isotopic compositions of river water, lake water, evaporites, modern coral, shales, and various igneous rocks (n = 64), together with compilations of literature data (n = 380), allow us to estimate the uranium isotopic compositions of the various reservoirs involved in the modern oceanic uranium budget, as well as the fractionation factors associated with U incorporation into those reservoirs. Because the incorporation of U into anoxic/euxinic sediments is accompanied by large isotopic fractionation (Δ_(Anoxic/Euxinic-SW) = +0.6‰), the size of the anoxic/euxinic sink strongly influences the δ^(238)U value of seawater. Keeping all other fluxes constant, the flux of uranium in the anoxic/euxinic sink is constrained to be 7.0 ± 3.1 Mmol/yr (or 14 ± 3% of the total flux out of the ocean). This translates into an areal extent of anoxia into the modern ocean of 0.21 ± 0.09% of the total seafloor. This agrees with independent estimates and rules out a recent uranium budget estimate by Henderson and Anderson (2003). Using the mass fractions and isotopic compositions of various rock types in Earth's crust, we further calculate an average δ^(238)U isotopic composition for the continental crust of −0.29 ± 0.03‰ corresponding to a ^(238)U/^(235)U isotopic ratio of 137.797 ± 0.005. We discuss the implications of the variability of the ^(238)U/^(235)U ratio on Pb–Pb and U–Pb ages and provide analytical formulas to calculate age corrections as a function of the age and isotopic composition of the sample. The crustal ratio may be used in calculation of Pb–Pb and U–Pb ages of continental crust rocks and minerals when the U isotopic composition is unknown.

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