Age and sedimentary characteristics of beach sediments from Yongle Atoll, South China Sea: implications for sediments supply in a coral reef system

Abstract Low-lying reef islands constructed from unconsolidated sedimentsare of growing interest due to concerns over the future stability of island landforms in response to climatic change and sea-level rise. Sedimentcharacteristics, combined with the constituents and physical processes, are fundamental to understand the links between sedimentproduction and their supply to an island’s beach reservoir. The grain size, constituents, and 230Th age of beach sedimentsfrom five islands at Yongle Atollin the South China Sea were analyzed. Data reveal that coral grains are the dominant constituents of the beach sediments, followed by mollusks, coralline algae, foraminifera, Halimeda, and bryozoans. The mean grain sizes of the beach sedimentwere within a range of 226-4215μm, and the coarsest sands were located near the passages in the atollrim. Transportation by tropical cyclone–generated waves and winnowingby tides are potentially the main factors that influenced the grain-size characteristics. Calibrated 230Th age dates for bulk sand samples date the beach sedimentbetween 424 yr BP and 5953 yr BP. Dating of coral sandsamples from the beach sedimentprovides ages of 686 yr BP to 5780 yr BP; the coral sandsfrom a vertical section (1.2 m depth) located on the beach at Jinqing Island were dated at 1154–3824 yr BP. The age reversal on the vertical section, with similar ages of beach sedimenton any single island and different age ranges on different islands in an atoll, indicates that the coral sedimentsupply processes occurred within a relatively close time period and that different islands had different sedimentsupply rates. The temporal link between the coral-dominated sedimentproduction and the beach reservoir provides an important insight into understanding the sedimentssupply mechanism of atollislands in the northwestern Pacific Ocean.