Geochemical zoning of surface sediments and its environmental implications in the Qizhou Island sea area, northwest South China Sea.

Abstract

The Qizhou Island sea area, situated in the northwest region of the South China Sea, is a significant area influenced by the influx of materials from surrounding rivers and a diverse range of marine environments. This study aims to examine the geochemical characteristics of surface sediments collected from 80 stations, employing correlation analysis, factor analysis, and hierarchical cluster analysis. The spatial distribution of major and trace elements reveals distinct geochemical zoning, carrying important environmental implications. Correlation analysis reveals a strong positive correlation between Al₂O₃ and TiO₂. Additionally, CaO and Sr exhibit significant positive correlations with water depth, primarily driven by the combined effects of nearshore coral reef enrichment and carbonate-rich late Pleistocene relict sediments on the outer shelf. Factor analysis identifies two main factors: F1, which represents terrigenous fine-grained sediments, and F2, which reflects biogenic carbonate deposition. Hierarchical cluster analysis categorizes the sediments into two primary zones: Zone I, located at depths shallower than 70 m, is influenced by contemporary hydrodynamic processes, with sediments primarily derived from nearby rivers, including the Pearl River and eastern Hainan Island; Zone II, situated below 70 m, is characterized by calcareous bioclasts, likely indicative of relict littoral sediments from the late Pleistocene low sea level period. The spatial distribution of geochemical elements in this region has been significantly shaped by the rapid rise in sea level since the Last Glacial Maximum and the formation of modern ocean currents, highlighting the environmental significance of sedimentation processes in the Qizhou Island sea area.