NATIONAL ACADEMY OF SCIENCES-NATIONAL RESEARCH COUNCIL WASHINGTON DC PACIFIC SCIENCE BOARD
Onotoa is a dry atoll just south of the equator and west of the international date line. Its yearly rainfall averages only about 40 inches, droughts occur periodically, and ground cover vegetation is sparse. Island deposits are almost exclusively unconsolidated calcium carbonate gravel and sand, the gravel mainly toward the sea and the sand mainly lagoonward. The shape of the lagoon bottom is derived from echo sounding and direct observation. It comprises three shallow basins maximum depth 8 fathoms that are separated from one another and from the sea beyond by still shallower water, the whole with numerous small patch reefs that rise to or near the surface. The near-surface framework of the Onotoa reefs consists primarily of the blue alcyonarian Heliopora, a genus that is not extensively developed there among now living corals. Fish are shown to be important in the production of lagoonal sediments. The sediments, soils, and surface waters of the island areas of Onotoa, and the ecologic zones and deposits of its shallow marine waters, are provisionally described and classified. Preliminary identifications of coral collections indicate them to include about 26 genera and 50 to 60 species. Limited observations on the chemistry and movement of some of the shallow marine waters show a diurnal variation in pH and an out-flowing gravity current across the windward reef flat and upper benched reef slope.
Report on Scientific Investigations in Micronesia.