ABSTRACT

The excess ²¹⁰Pb dating method was applied to two young straw stalactites collected from a cave in southern Okinawa Island, Japan. The ²¹⁰Pb profiles only in the matrix of the straws exhibited an exponential decrease with distance from the tip. The ²¹⁰Pb profiles of the two straw samples gave the values of 2.2 and 5.9 mm/y for longitudinal growth rates. The excess ²¹⁰Pb is distributed uniformly over the outer and inner surfaces of the straws. Such an excess of ²¹⁰Pb is presumably produced from the airborne ²²²Rn which exists with high concentrations in the cave air and the ground water running through the central channel of the straws. During growing process of the straws, a part of the ²¹⁰Pb will be adsorbed on the tip of straw, and they are supposed to be incorporated into matrix part during reprecipitation of CaCO₃.