三沙永乐龙洞洞内侧壁礁体矿物和元素组成及其晚更新世以来的形成演化

三沙永乐龙洞深度在300m左右,是目前全球已知最深的海洋蓝洞,但是对其性质和成因了解极其有限。本研究使用X射线粉晶衍射仪、X射线荧光光谱仪、AMS14C测年等分析测试方法对采自不同深度的洞壁礁体的矿物物相、元素含量和形成年代进行了测定,探讨了洞壁性质和龙洞的演化。研究结果表明:三沙永乐龙洞洞内侧壁矿物均属于碳酸盐矿物,包括文石、高镁方解石和低镁方解石,平均含量分别为49.0%、46.6%、14.3%,它们主要来自钙质生物碎屑;洞内侧壁的主要元素是Ca、Mg、Sr,平均含量分别为36.8%、0.98%、0.48%。三沙永乐龙洞是一个复合体,17m以上岩石年龄晚于7.5cal kaBP,是全新世海平面上升时期形成的现代珊瑚礁体,没有经历过海平面下降引起的成岩作用; 17—35m岩石形成时代早于25cal ka BP,是经历了大气淡水成岩作用的晚更新世喀斯特溶洞,且在高海平面时期于17—23m以浅的空间内广泛发育洞内珊瑚礁。

MINERAL AND ELEMENTAL COMPOSITION, FOMATION AND EVOLUTION OF THE REEFS IN THE INNER WALL OF SANSHA YONGLE BLUE HOLE, SOUTH CHINA SEA

The Sansha Yongle Blue Hole is located in the Yongle coral atoll among Xisha Islands, Jinle Island, and Shiyu Isles. Being deeper than 300 m, the blue hole is the deepest marine blue hole in the world. The mineral composition, element content, and age of the cave reefs situated on the wall in different depths of the blue hole were analyzed with methods of X-ray diffraction, X-ray fluorescence, and AMS¹⁴C dating. The results show that the mineral composition of the cave reefs in the Sansha Yongle Blue Hole is quite simple, mainly carbonate species, including aragonite, high-magnesium calcite, and low-magnesium calcite, being derived mainly from calcareous bioclastics on average of 49.0%, 46.6%, 14.3%, respectively. The main elements are Ca, Mg, and Sr, on average of 36.8%, 0.98%, and 0.48%, respectively. Sansha Yongle Blue Hole is a complex cave. The reef rock above 17m was formed later than 7.5cal ka BP, which is a modern coral reef formed during the Holocene sea level rise, and did not experience diagenesis caused by sea level decline. However, the rocks between 17m-35m were formed earlier than 25cal ka BP, corresponding to the late Pleistocene karst event undergone atmospheric freshwater diagenesis, during which extensive coral reefs developed in shallow depths of 17-23 m in high sea level regime.