海南岛珊瑚岸礁的特征

本文分析了海南岛珊瑚岸礁的成礁环境,论述了本岛岸礁的结构特征,进而划分了岸礁类型,最后探讨岸礁发育与海平面变化的关系。

THE CHARACTERISTICS OF FRINGING REEFS OF HAINAN ISLAND

The fringing reefs of Hainan Island are one of the most developing fringing reefs in China. There are many coral reefs discontinuously distributed around the island. In the eastern and southern coasts, the fringing reefs develop on a large scale. In the western and northwestern coasts, the fringing reefs have some different characteristics. The bases of the fringing reefs are different from place to place, such as basaltic, granitic, limestones of ordo-vician system quartziferous conglomerate of Devonian system or loosing slope wash and alluvium etc.The climate of Hainan Island belongs to the tropical monsoon island climate. The mean annual temperture of air is 25.4℃,and the mean annual tem-perture of sea-water 26.7℃ in the southern coast, whereas the air temperture is 23.4℃ and the temperture of the sea-water 24.2℃ in the northern coast. The mean salinity of the surface sea-water is about 32-33‰ which is slightly lower than the mean value of the South China Sea (34‰). Generally speaking, the quantity of evaporation is greater than that of rainfall. The whole area is subjected to NE monsoon and SE wind. As usual, The NE monsoon is the prevailing wind in winter and the SE wind prevails in summer. Spring and autumn are the alternative seasons of the wind. In a word, the coastal climatic and hydrographic coditions are suitable for the growth and development of coral reefs, but the seasonal change moderates the growth of the coral reefs.The fringing reefs of Hainan Island can be divided into the following five zones:1. The protogenous reef zone. This zone is situated in the lower tide zone. It forms a slope, on which many kinds of corals, algae and reef-like creatures are developed. This zone can also be subdivided into upper zone and lower zone according to the combination of the hydrodynamic conditions and the coral species. The depth of these zones is approximately 5m. The lower limit for the living corals is about 10m deep.2. The reef flat zone. This zone is situated in the tidal zone. It has a broad and flat relief, on which the turdidites are accumulated and there live a few living corals. This zone can also be subdivided into marginal shoal, inner and outer subzones.3. The beach. This zone is situated in the tidal zone.The beach is a slope. Upward it becomes the berm of the upper-tide zone.4. The coastal biogenic berm. This zone is situated in the upper-tide zone and is the highest place of the fringing reef. The berm is composed of the biogenic shingly sand accumulation.5. The lagoon and swampy zone. This zone is situated in the upper-tide zoneand distributed in the side of the coastal biogenic berm or between twoberms. It is composed of biogenic sand and shingle.In terms of the coast characteristic, the environment of reef formation and the conditions of the development of coral reefs, the reefs of Hainan Island can be divided into the following four types. 1. Straight fringing reefs, 2. gulf fringing reefs, 3. lagoon fringing reefs, and 4. offshore-isle fringing reefs.Because of the different environments of the reef formation, the scale of fringing reefs, coral species and contents of terrigenous materials differ between southeast and northwest coasts of Hainan Insland.By the determination of the isotope C, there were several periods of higher sea-level around Hainan Island in the last 8000 years, and in the period of 5000 years B.P. the sea level was the highest (about 2-4m. above the present sea level).