Geochemical evidence for the development of coral island ecosystem in the Xisha Archipelago of South China Sea from four ornithogenic sediment profiles

Geochemical characteristics of four ornithogenic sediment profiles (GQ, GJ3, JQ and JY2) from Ganquan (GQ), Guangjin (GJ3), Jinqing (JQ) and Jinyin (JY2) coral islands in the South China Sea were analyzed. We noted clear differences in the distribution of organic matter (OM) and elements above and below the respective critical depths of 11cm (GQ), 16cm (GJ3), 9cm (JQ) and 12cm (JY2) in the four profiles, revealing an apparent change in sediment source materials. For all the sediment profiles, there were statistically significant positive correlations between TOC and P in the sediments below these critical depths, indicating that the composition of organic matter was predominantly derived from guano. However, these positive correlations were significantly weakened above the critical depths, probably due to the increased contribution of plant humus to the organic matter. Our results suggest that the source materials of ornithogenic sediments have changed gradually from a two-component (coral sand, guano) mixture to a three-component (coral sand, guano and humus) mixture, likely indicating the slow development of vegetation following seabird occupation. Cu, Cd, Zn, P, As, Se and Ba, whose controlling factors are guano input, were identified as a group of avian bio-elements in the Xisha Archipelago. The distributions of other mineral elements, such as Fe, Al, Ti, Mn and K, were mainly controlled by organic matter. Based on this integrated analysis of the geochemical characteristics of inorganic elements, we inferred that the compositional materials (coral sands, guano and plant residues) were the key factors controlling the geochemical behavior of elements in the ornithogenic sediment profiles on the coral islands of the South China Sea. Seabird activities and vegetation development have significantly changed the original sediment structure and have exerted a positive impact on terrestrial production and soil development on the islands.