| Summary: | 碩士 === 國立中山大學 === 海洋科學系研究所 === 106 === In 1842, Darwin proposed that a fringing coral reef surrounding a volcanic island as the island subsided, becoming an atoll, but he did not explain the development of patch reefs. Patch reefs are features of lagoons in atolls. In Dongsha Atoll, there are thousands of patch reefs up to a diameter of about 600 m as viewed by satellite pictures in the lagoon. By using Lidar, the bathymetric details are available, and they can be categorized into flat-top and pinnacle type patch reefs. Two hypotheses are proposed here to explain how patch reefs were formed. Hypothesis 1: Coral reefs grew as more and more reef coral clustered together; Hypothesis 2: Karst topography is a landscape formed from the dissolution of soluble rocks such as limestone. Based on the coral growth hypothesis, can be divided as: the seawater surface change hypothesis, the wave hypothesis. The coverage of the living coral over the top of pinnacles is significantly larger than that of the base coverage. This study draws a 16x16 grid with latitude and longitude lines for a total of 256 points. Lidar recorded various appearances on top of both the flat-top reef and the pinnacle reef near each point. The depth of the flat-top reefs near the back reef is about -1m. Comparison of the depth of the top with the age of the top of the flat-top patch reef. Significant difference, exists between the age of the tops at -1 m~-2 m deep and that at -3m~-6m deep. The core age of one of the oldest flat-top patch reefs, of which the diameter is 100 m and the depth of the top is -5 m, is 3413 years old, and the age of the periphery is 217 years old. The linear regression of the core ages from the center to the periphery is Significant difference. The tree ring pattern denies the Karst Hypothesis. The relative maximum wave heights of the 7 typhoons at each station in 2017. The maximum wave heights taken at the most eastern station near the reefs are relatively smaller. This is supported by the hypothesis of coral growth.
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