Behaviors of ²¹⁰Pb and ²¹⁰Po around hydrothermal vents in the Okinawa Trough

Vertical profiles for the uranium-series radioisotopes ²¹⁰Pb and ²¹⁰Po were obtained at the two hydrothermal vent sites,the Iheya Ridge and the Minami-Ensei Knoll,in the Mid-Okinawa Trough in 1993 and 1994,respectively.In 1995,both radioisotopes were measured at the Minami-Ensei Knoll again.At the Iheya Ridge,where the hydrothermal activity is not active as reflected by the CH₄,and ²²²Rn data,both the total ²¹⁰Pb and ²¹⁰Po activities show deficiency relative to their parents,and the mean residence time of ²¹⁰Pb and ²¹⁰Po is approximately equal to 20 and 2-5 a,respectively.At the Minami-Ensei Knoll,which is characterized by black smokers,the total ²¹⁰Pb(0.167×10^-3-2.5×10^-3 Bq/kg) around the plumes is deficient relative to ²²⁶Ra but the total ²¹⁰Po activities (1.83×10 ^-3-2.83×10^-3 Bq/kg) are in excess relative to ²¹⁰Pb.The ²¹⁰Po activities are higher than those in the East China Sea and the Okinawa Trough and excess ²¹⁰Po has been found.The ²¹⁰Pb/²²⁶Ra and ²¹⁰Po/²¹⁰Pb activity ratios are 0.1-0.4 and 1.1-7.8,respectively.²¹⁰Pb is preferentially scavend in the hydrothemial plumes.The possible mechanisms governing the concentration of ²¹⁰Pb in the Minami-Ensei Knoll involve a lateral transport of ²¹⁰Pb via diffusion.The high ²¹⁰Po/²¹⁰Pb activity ratios in the hydrothermal plumes suggest a depletion of ²¹⁰Pb and addition of ²¹⁰Po in the hydrothermal vent area.