The marine geochemistry of actinium‐227: Evidence for its migration through sediment pore water

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Abstract
227Ac with a half life of 21.8 years has a potential utility as a tracer of deep water circulation and mixing studies on time scales less than 100 years. Here we present the first measurement of 227Ac profile in the pore water of Northwest Pacific deep‐sea sediment and in the ∼ 10,000 m long water column of Izu‐Ogasawara Trench. The results clearly show that 227Ac is supplied from the sediment to the overlying water through migration in the pore water. The model calculation indicates that the molecular diffusion alone through sediment porewater can support only a half of the standing crop of excess 227Ac in the water column and the enhanced supply of 227Ac by particle mixing is necessary to account for the remainder. Thus, bioturbation in the deep sea plays an important role in controlling the flux of some short‐lived radionuclides such as 227Ac and 228Ra across the sediment‐water interface.