Geochemical Characteristics of the Trace and Rare Earth Elements in Reef Carbonates from the Xisha Islands (South China Sea): Implications for Sediment Provenance and Paleoenvironment

Based on the concentrations of the trace elements, rare earth elements (REE), and Sr isotopic compositions in reef carbonates from the well 'Xike-1' reef core of the Xisha Islands, the constraints on sediment provenance and paleoenvironment were defined. Variations of the terrigenous input into the paleoseawater were traced in detail and the paleoenvironment and sediment provenance were further investigated. The results show that the HREE/LREE values in the reef carbonates are negatively associated with their Th and Al concentrations; however, their Al and Th concentrations show positive correlation. The lowest Sr-87/Sr-86 values in the reef carbonates generally coincide with the lowest values of Al, Th concentrations and the highest values of HREE/LREE. These data indicate that the HREE/LREE, Al concentrations, and Th concentrations of the reef carbonates are useful indexes for evaluating the influence of the terrigenous inputs on the seawater composition in the study area. From top to bottom, the changing process of the HREE/LREE values and Al, Th concentrations can be divided into 6 intervals; they are H1 (0-89.30 m, about 0-0.11 Myr), L1 (89.30-198.30 m, about 0.11-2.2Myr), H2 (198.30-374.95 m, about 2.2-5.3Myr), D (374.95-758.40 m, about 5.3-13.6 Myr), L2 (758.40-976.86m, about 13.6-15.5Myr), and H3 (976.86-1200.00m, about 15.5-21.5Myr). Moreover, the changing trend of the HREE/LREE values coincides with that of the seawater delta C-13 values recorded by benthonic foraminiferal skeletons from the drill core of ODP site 1148 in the South China Sea (SCS), but not with that of the seawater delta O-18 values. The high uplifting rates of the Qinghai-Tibet Plateau coincide with the high Th and Al concentrations and the low HREE/LREE values in the reef carbonates. These data indicate that the main factors controlling the changes of terrigenous flux in the SCS are the tectonic activities associated with Qinghai-Tibet Plateau uplifting and the variations of uplifting rates rather than paleoclimatic changes.