Coral-based rare earth element proxy for hydrothermal fluid on the Yongxing Island, South China Sea

Abstract

Coral reefs play an important role in the high-resolution paleoclimate and paleoenvironment records. In particular, the contents, distribution patterns and elemental anomalies of the rare earth elements can reflect the paleoceanic conditions and sedimentary environments. In this study, we focus on the SSZK1 core in the Coral Reef of Yongxing Island, South China Sea (SCS). The sediments of this core can be divided into two stages: P1 (18.00–24.95 m) and P2 (24.95–55.92 m), based on the dating results. The results show that the coral reefs of P1 and P2 stages have slightly different REE contents, but similar REE + Y patterns. In general, the REE + Y patterns of both P1 and P2 stages are similar to those of normal marine carbonates (rocks), showing LREE depletion, negative Ce anomaly and high Y/Ho ratios. This indicates that the REEs of coral preserved their pristine geochemical features although they have experienced the diagenetic alteration to some degree. However, the samples, such as the YX-2300 coral bone formed at 114 ka, have obvious positive Eu anomalies except the similar REE + Y characteristics as the normal marine carbonates (rocks). The positive Eu anomalies indicate that some hydrothermal fluids may be input into the surrounding water at the time of coral bone formation. Based on the model calculations, at least 0.1% of hydrothermal fluids were added to the surrounding seawater during the growth of YX-2300 coral bone. By comparing the data and ages, it is suggested that these hydrothermal fluids could be associated with volcanic activities on Gaojishi Island or Hainan Island. This evidence is important to trace the occurrence of modern worldwide volcanism or deep faults activity in the deep-sea.