南沙群岛珊瑚礁区仙掌藻的现代沉积特征

仙掌藻为温暖水体中钙化的绿藻，是南沙群岛珊瑚礁区重要的钙质沉积物源。对南沙群岛８座环礁现代沉积物样品的分析表明，仙掌藻碎屑在环礁各地貌沉积带沉积物中的含量，以泻湖盆底最高，平均为32.66％，最高可达75％，泻湖坡次之，平均9.22％，礁坪含量低，平均6.06％。南沙群岛仙掌藻以砂质基底上生长的直立类型为主，能生长于各个地貌沉积带，最适生态环境为封闭性好、泻湖面积大、水深较大（10～25m）、水动力弱的砂质泻湖盆底。仙掌藻的现代沉积特征反映了其生态特征，可作为中新世以来珊瑚礁沉积相划分的依据。

Modern Sedimentary Characteristic of Halimeda on Coral Reefs of Nansha Islands

Halimeda , a genus of calcified green alga (Chlorophyta) of warm seas, is an important producer of carbonate sediment on coral reef areas of Nansha Islands, South China Sea. This study analysed the modern sedimentary characteristic of Halimeda in 52 clastic samples from the 4 geomorphic sedimentary zones of 8 atolls. Among the 4 zones, i.e.,reef front slope, reef flat, lagoon slope and logoon basin floor, Halimeda is the richest in lagoon basin floor and constitutes averagely 32.66% of the sediment, and it ranges from 16% to 75%. Observation under microscope indicates that in the sediment of lagoon basin floor there are most well preserved Halimeda segments and they are deposited in situ. In the sediment of lagoon slope, Halimeda ranges from 8% to 16%, with an average of 9.22%. In reef flat, Halimeda composition is low, and ranges from 1% to 8%, with an average of 6.06%. From the present data, the contribution of Halimeda to the reef front carbonate production is small. The analysed results are consistent with the field investigation. Halimeda of Nansha Islands can grow on any zone of coral reefs either in sand or on solid substrata, the optimum habitat is well closed, big lagoon area, deep water, weak dynamic, sandy lagoon basin floor, where it develops well in erect. This paper concludes that the sedimentary characteristic of Halimeda on coral reefs of Nansha Islands are in line with its ecologic distribution. Coral reef drilling research shows that the paleoenvironment setting of the Cenozoic is comparable with that of recent Halimeda , so the characteristic can be the bases of analysing sedimentary facies of ancient and modern coral reef sediment.