南科1井第四系暴露面特征及其与海平面变化的关系*

珊瑚礁地层中的暴露面是海平面变化的忠实记录, 对地层层序的划分和研究珊瑚礁体的生长发育过程具有重要意义。本文基于手标本观察与薄片鉴定、碳氧同位素和矿物组成分析, 识别了南沙美济岛南科1井第四纪生物礁碳酸盐岩地层中的典型暴露面, 剖析了主要暴露面与海平面变化及珊瑚礁发育演化的关系。在南科1井(NK-1)第四纪地层中典型暴露面附近以出现大量溶蚀孔穴和红褐色或锈黄色钙质结核为特征, 其碳氧同位素偏负, 并富集Al、Th、Fe和稀土元素, 具有典型碳酸盐岩风化壳的特征。南科1井全新世礁体发育在晚更新世暴露面之上, AMS¹⁴C和U-Th年龄数据证实美济岛全新世珊瑚礁生长时段与南海其他珊瑚岛礁一致, 主要发育在8.2~4.7ka时期, 该时段海平面的缓慢上升为珊瑚礁连续垂向生长提供了有利环境。Sr同位素和古地磁年龄标定的南科1井更新世地层中的主要暴露面时代与南沙永暑礁和西沙群岛珊瑚礁地层中的暴露面时代基本一致, 主要暴露面对应于全球第四纪低海平面时期。

Characteristics of Quaternary exposure surfaces in Well Nanke-1 and their relationship with sea-level changes

Exposure surfaces in shallow-water carbonates are considered to be the faithful archive of paleo sea-level fluctuations, which are important for studying carbonate sequences and development of coral islands. Based on core observations, thin section identifications, and analyses of stable isotopes and mineral compositions, we identified nine main exposure surfaces in the Quaternary reef carbonates from a deep borehole “Well Nanke-1” (NK-1) drilled on Meiji Island, and we discussed their relationship with sea-level change and coral reef evolution in the South China Sea. Exposure surfaces in the Quaternary reef carbonates in Well NK-1 are characterized by heavily dissolved cavities and rich calcareous nodules with reddish-brown or yellowish colors. Weathered carbonates around most exposure surfaces have negative carbon and oxygen isotopes, and higher contents of Al, Th, Fe, and rare earth elements. The AMS¹⁴C and U-Th age data confirm that the Holocene coral reefs have developed during 8.2~4.7 ka on Meiji Island; and its growth period is consistent with that of other coral reefs in the South China Sea. The slow rise in sea level provided a favorable environment for the vertical growth of most coral reefs. Most exposure surfaces within Pleistocene stratum of Well NK-1 correspond to low sea-level stages, and the dated ages of these exposure surfaces by Sr isotopes and paleomagnetic studies are consistent with the records from the Xisha Islands and Yongshu Island.