南海西沙永乐龙洞沉积物组成、来源及其沉积作用

南海永乐龙洞发育于永乐珊瑚礁台地,龙洞深度达300m,为世界之最。沉积物堆积在龙洞的洞壁斜坡、龙洞中部的转折平台以及洞底等部位。使用激光粒度仪、X射线粉晶衍射仪、X射线荧光光谱仪等对采自不同深度的沉积物进行了粒级、矿物物相、元素含量的研究。研究结果表明:龙洞沉积物绝大部分为钙质生物碎屑,以砂粒级碎屑为主,含砾石碎屑、粉砂碎屑,分选和磨圆差;沉积物矿物组成以文石、高镁方解石为主,含少量低镁方解石,其平均含量分别为69%、28%、3%;化学组成以Ca、Mg、Sr为主,平均含量分别为35.5%、0.9%、0.5%,含少量Si、Al、Ti、P、S等元素。该区沉积物来源包括礁坪生物碎屑和东亚季风风尘陆源物质两个方面,以礁坪来源的生物碎屑为主;龙洞沉积作用包括机械捕获作用和垂直沉降作用两种方式,而以机械捕获作用为主。

SEDIMENTS OF THE YONGLE BLUE HOLE IN XISHA ISLANDS, SOUTH CHINA SEA: ORIGINATION, COMPOSITION, AND SEDIMENTATION

The Yongle Blue Hole is located in the Xisha Islands, and it is the deepest one (over 300m) in the world. Sediments are distributed and sampled on the wall, the middle platform, and the bottom of the Blue Hole in different depths. The grain size, and the mineral and geochemical components of the sediments were analyzed by a laser particle analyzer, and X-ray diffraction and fluorescence instruments. The results show that the sediments are composed of mainly biogenic debris. Dominant grain size of the sediments are sand, some occasional breccia, and a few silt and clay. The debris is poorly sorted and rounded. Three minerals, i.e., aragonite, high-magnesia calcite, and low-magnesia calcite were found in the sediments, taking 69.1%, 28.1%, and 2.8%, respectively. Elements Ca, Mg, Sr are major components of the sediments, accounting for 35.5%, 0.9%, and 0.5%, respectively, plus rare Si, Al, Ti, P, and S. The results suggest that sediments of the Blue Hole are mainly originated from adjacent reef flat, and a few are terrigenous that transported by winter East Asia Monsoon. Sedimentation of the sediments in the Blue Hole is mainly by mechanical capture, and few via particle settling.