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There are few carbon isotope measurements of acetate in marine sediments, even though acetate could play important roles in biological activity as well as in the carbon cycle in marine sediments. In this study, abundance and carbon isotopic composition of water-soluble acetate are investigated using a surface sediment core (ca. 30cm) from the northwestern Pacific. A large ^<13>C-depletion in acetate by up to 14‰ is observed with increasing depth, as the concentration of acetate decreases. The depletion could reflect a kinetic isotope effect during decarboxylation of pyruvate as the sediment becomes anoxic. On the other hand, there is no difference in ^<13>C between acetate and total organic carbon (ca. -20.6‰) in the upper 13cm of the core, which suggests that the acetate could be produced by fermentative degradation of organic matter and quantitatively consumed. The carbon isotopic composition of acetate can be useful for better understanding the carbon cycle in marine sediments.