ABSTRACT

The growth rate of terrestrial ferro-manganese nodules formed in limestone areas of the Ryukyu Islands has been determined by a radiochemical method. The ionium-thorium ratios exponentially decrease with depth from the surface of the manganese nodule, and the growth rate has been evaluated as (61 ± 9)mm per 10⁶ years for samples collected from a reddish soil accumulated on the stratum of Naha Limestone at Nakijin-son, the northern part of Okinawa-jima. The growth rate will lead to the age of (16 ± 2) × 10⁴ years for a manganese nodule about 2.0cm in diameter, which is of the largest size in the area. This may give an estimation of the upper limit age for the Riukiu (Naha) Limestone formation.