Abstract
Minami-Daito and Kita-Daito Islands are raised atoll reefs consisted of marginal hills (height; about 50 m above sea level) and central lowland (height; about 1 or 2 m a.s.l.). The islands would be more raised in the last ice-age than at present, and many ponds in the central lowland are considered to be originated from dolines or uvalas formed in that age (Fig. 2).
Lake sediment is mainly composed of humus and a clay layer is found in the lowest part. After NAKAO (1977), result of carbon dating of wood pieces taken just above the clay layer was 8, 560 Y. B. P, for Amida-ike in Minami-Daito Island (12.6 m under the lake bottom and 20.1 m under the water surface) and 7, 550 Y. B. P. for Aka-ike in Kita-Daito Island (17.8 m under the bottom and 19.8 m under the water surface). This means that dolines and uvalas formed in the last ice-age had submerged after it and created the ponds.
In Minami-Daito Island, flat and marshy area (L4) is found in the south-west part of the central lowland, and isolated hills and dolines (Fig. 3) are found in the north-east part of the lowland (L3). L3 surface may be the original landform during the ice-age, and in the south-west part, it would be submerged under L4 surface which consists of recent deposit (Fig. 4). Most of ponds distribute on L4 surface, but several lakes in Minami-Daito Island situate on L3 surface. In Kita-Daito Island, L4 surface is narrow and ponds are situated on L3 surface.
Distribution of electric conductivity of lake water and shallow groundwater is shown in Fig. 5, where the values for lake water is low as compared with that for groundwater. This means that sea-salt intrusion into the central lowland, especially into lakes, may be small and intermittently.
