科研之友
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摘要
Coral shingle cays are distributed widely around coral reefs and islands in the South China Sea; however, their formation and evolution are little understood. A Digital Elevation Model (DEM) of three coral shingle cays at the Yongshu, Ximen, and Anda reefs, respectively, in the Nasha Islands had been built after they were investigated in 2017. Based on the modern DEMs of the shingle cays, different DEMs of the shingle cays were reconstructed at the past four intervals of 2011 (2009), 2013, 2015 and 2017 by remote sensing analysis; thus, the geomorphic features and evolution that the cays underwent from 2009-2017 was revealed. During this periods, the shingle cay at the Anda Reef experienced near constant erosion, shrinking continuously and showing losses of 50%, 70%, and 72% in average height, area, and volume, respectively, as part of a general declining trend. The shingle cay at the Yongshu Reef was dominated by horizontal and vertical accretions; its average height, area, and volume increased by approximately 21%, 62% and 103%, respectively, from 2011-2017. The area of the shingle cay at the Ximen Reef showed horizontal accretion, with an increase of approximately 160%, but its average height declined by approximately 31% and changes in its volume were insignificant. At both the Yongshu and Ximen reefs, the shingle cays moved eastward or southeastward into the lagoon or inner reef flat, with the centroid being displaced 41.18 m and 55.71 m, respectively; the average rates of movement were 0.6 m/ month and 0.8 m/month, respectively. In contrast, the shingle cay at the Anda Reef moved northwestward towards the outer reef flat for a larger distance (193.1 m) and at a greater velocity (2.1 m/month) than the other reefs moved. The evolution of three shingle cays was controlled mainly by monsoon- and typhoon-driven waves, and was influenced by differences in hydrodynamics based on their locations and the shapes of the reef flat. As a result, the three shingle cays showed two different modes of evolution: migration and shrinking towards the outer reef flat and migration and extension into the lagoon or inner reef flat. The geomorphic evolution of these coral shingle cays in the Nansha Islands provides an important scientific basis for extensive study of the geomorphic development, and the dynamic mechanism thereof, of coral islands in the South China Sea. Further research will combine long-term remote sensing analysis and regular field observations of the ecology, sedimentology, and hydrodynamics of the coral shingle cays and neighboring reef areas, and focus on the long-term processes of coral shingle cay formation and evolution to reveal the key environmental factors thereof and the mechanisms affecting them.
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