RARE EARTH ELEMENTAL GEOCHEMISTRY OF THE SEDIMENTS IN COLD-SEEP AREA IN DONGSHA AREA OF SOUTH CHINA SEA

The Dongsha Area is one of the most important natural gas hydrate zones on the northern slope of the South China Sea. The rare earth elemental geochemical characteristics of surface sediments in this area are controlled not only by their sources but also by the activities of cold seeps. In this paper, a 1 375 cm-long gravity core of 973-4 (21°54.3247′N、118°49.0818′E), which is located on the northern slope of South China Sea at water depth of 1 666 m, is selected as the study subject. The contents of rare earth elements (REE) and some trace and major elements are measured by inductively coupled plasma mass spectrometry (ICP-MS) and X-ray fluorescence spectroscopy (XRF). Combined with the value of total carbon (TC), total organic carbon (TOC) and foraminiferal mass fractions, these data are used to study the influence of cold seeps on the geochemical characteristics of REE and the surrounding diagenetic environment. The results show that the content of rare earth elements in the Core 973-4 sediments reduces significantly in the depth range of 459-619 cm below the seafloor, but the distribution pattern of REE and the REE-related data are consistent to each other. Combined with the study of cold seeps activities, it is found out that the anomalies of the geochemical characteristics of sediments in this area are independent from provenance, but affected by the increase in authigenic carbonate rocks. In addition, by analyzing the variation in δCe value with depth and the distribution pattern of authigenic carbonate and sulfate-methane conversion zone (SMTZ), it is found that the correlation between the three is very high, indicating that the increase of authigenic carbonate came from the anaerobic oxidation of methane(AOM) caused by cold seeps.