2018 Volume 20 Issue 1 Pages 1-20
Beach sediments in the Ryukyu Archipelago are mainly composed of skeletal fragments and shells produced by calcifying organisms. However, the production age, transport time and depositional age of beach sediments have not yet been fully understood. Here we show the age, transport and depositional process of beach sediments around Sesoko Island (Okinawa, Japan), based on the abrasion grades and radiocarbon (14C) ages of empty tests of Baculogypsina sphaerulata (Foraminifera, known as star sand). Results of abrasion-grade analysis showed that well-preserved tests of B. sphaerulata (a pristine test with most spines remained) became fewer from a reef flat to a beach, where the abraded tests with no spines were found abundantly. Results of 14C dating showed that all B. sphaerulata test ages were after ca. 700cal yr BP (calibrated year before 1950 AD). B. sphaerulata test ages generally became older from a reef flat toward a beach, although relatively old abraded tests were found in reef sediments as well. B. sphaerulata test ages in beach sediments were generally younger in the north side than the central and southern parts. Most 14C ages of coral fragments and mollusk shells in beach sediments were younger (Modern: after 1950 yr AD) than those of B. sphaerulata tests. These results suggest that the production of B. sphaerulata tests increased after ca. 700cal yr BP. This is likely caused by the development of a reef flat and the increase of B. sphaerulata habitats associated with a relative sea-level fall at late Holocene. After being transported from a reef flat to a beach by wave-induced currents within tens to hundreds of years, B. sphaerulata tests were gradually accumulated into the central part of a beach mainly from the north side by nearshore tidal currents. The presence of modern coral fragments brought by strong waves and typhoons suggest that beach progradation continues to the present.
