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Mg isotope composition (δ26Mg) of seawater may serve as a tool for reconstructingthe global Mg cycle over geologic time scales. Marine carbonate sediments havebeen proposed as a potential archive for seawater δ26Mg values. However, the Mgisotope composition of seawater in the Cenozoic has remained controversial. Here wereport our investigation on the Mg isotope compositions of reef carbonates from thewell "Xike-1" reef core in the Xisha Islands, South China Sea. Magnesium isotopeanalysis was complemented by analyses of Sr-C-O isotopic compositions, majorand trace element concentrations, and mineral compositions. The δ26Mg values inlimestones from the well "Xike-1" reef core range from -5.03‰ to -3.60‰ andthe δ26Mg values in dolostones range from -2.98‰ to -2.72‰Ṁultiple lines ofevidence consistently indicated that biological vital effects are likely the main factorcontrolling the variations of δ26Mg values in limestones, which results in a lagerMg isotopic variation and a lighter δ26Mg value in the limestones. Mg isotopiccompositions of the dolostones from the well "Xike-1" reef core are conservative duringdiagenesis, which is the effective archive of seawater δ26Mg values. The seawaterδ26Mg values are basically invariant during the past 21 Myr. The fractionationfactor of Mg isotope between dolomite and fluids associated with dolomitizationranging from -2.26‰ to -2.02‰Ṁoreover, the Mg isotopic fractionation betweencalcite and fluids should range from -4.20‰ to -2.77‰İn this article, it is likelythat the ocean is at steady state with respect to seawater Mg isotopic compositionin the past 21 Myr. The mass balance calculation of Mg isotopes implies that theMg flux of dolomitization output is about 0.63-0.79 Tmol/yr in the modern ocean,which accounts for about 12%-13% of the Mg flux of riverine input to the ocean. [ABSTRACT FROM AUTHOR]