西沙群岛石岛根管石丛的形成分析*

根管石是高等植物的化石, 它不仅能证实古土壤的存在, 还是古植被重建、古气候反演和沉积环境重建的良好载体。根管石在中国南海西沙群岛石岛东西海岸的潮上带广泛发育, 而在南海其他岛屿鲜有报道。石岛的绝大部分根管石在岩壁及古土壤层中独立分布, 此外还发育了裸露的根管石丛, 形态保存相对完好。本文运用X-射线断层扫描成像技术(Micro-CT)针对石岛根管石丛进行三维可视化分析, 获取其精细的形态特征, 结合植物生态学特性和根管石丛的微观结构, 探讨了西沙群岛石岛根管石丛的形成, 以此为海岸带环境下根管石的形成机制和沉积环境重建提供科学依据。结果表明: X-射线断层扫描得到的石岛根管石丛宏观三维形貌更加直观地展示三层环带状构造。结合岩石薄片, 推测石岛根管石丛内带和中带皆由植物自身交代形成, 而外带则为珊瑚砂固结成岩。植被的形态特征和生态适应性表明石岛根管石丛由茎干形成, 建议使用“根茎石”这一术语更为贴切。同时, 初步推测其原生植被是分布于石岛的优势种草海桐。

Analysis on the formation of rhizolith clumps from Shidao, Xisha Islands

Rhizolith is the fossilized remain of higher plants. It can not only confirm the existence of paleosols, but also provide a lot of information for paleo-vegetation, paleoclimate and sedimentary environment. On the Shidao of Xisha Islands in the South China Sea, rhizoliths are widely distributed in the supratidal zone of the east and west coasts, but they are rarely reported on the other islands in the South China Sea. Most of the rhizoliths are observed as individuals on the rock walls and in the paleosols. In addition, well-preserved rhizolith clumps are also exposed within the supratidal zone. In this study, high-resolution X-ray microtomography (Micro-CT) was used to conduct three-dimensional visualization analysis of the rhizoliths from Shidao to obtain their fine, detailed morphological characteristics. In combination with the phytoecology and microstructure characteristics of the rhizoliths, this paper discusses the formation of rhizolith clumps from Shidao, so as to provide reference information on the formation mechanism and sedimentary environment reconstruction of rhizoliths in coastal environments. The macroscopic three-dimensional morphology, obtained by high-resolution X-ray microtomography (Micro-CT), intuitively demonstrates that the internal structure of the rhizoliths can be divided into three zones. Combined with the results of thin sections, it is inferred that the inner zone and middle zone of Shidao rhizoliths are formed by the metasomatism of plants themselves, while the outer zone is formed by the consolidation of coral sand. The morphological characteristics and ecological adaptability of the vegetation indicate that the rhizolith clumps of Shidao are formed from the stems, rather than the roots. At the same time, it is preliminarily speculated that the native vegetation is Scaevola taccada, a dominant specie on Shidao.