南海东沙海域冷泉渗漏区沉积物稀土元素地球化学特征

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

  • 摘要: 东沙海域是我国典型的冷泉活动区,该区域表层沉积物的稀土元素地球化学特征不仅受其源区控制,同时也可能会受到冷泉渗漏活动的影响。本文选取位于南海北部陆坡973-4站位的柱状样为研究对象,该站位位于21°54.3247’N、118°49.0818’E,水深为1666m,柱状样总长1375cm,采用电感耦合等离子质谱(ICP-MS)、X射线荧光光谱(XRF)等分析测试方法,测得样品的稀土元素以及部分微量和主量元素数据,并结合总碳(TC)、总有机碳(TOC)以及有孔虫质量分数等数据,探讨了冷泉泄漏对周围成岩环境及沉积物稀土元素地球化学特征的影响。结果发现,973-4站位柱状样在海底以下459~619cm深度范围内稀土元素含量整体异常减少,但所有样品稀土元素分布模式及相关稀土元素参数均十分一致。结合冷泉活动研究,表明该区域沉积物元素地球化学特征的异常与物源无关,是受自生碳酸盐岩的增加所影响。此外,通过分析δCe值随深度的变化以及自生碳酸盐岩和硫酸盐-甲烷转换带(SMTZ)的分布情况发现三者相关度很高,表明自生碳酸盐岩的增加很可能是该区域发生的冷泉渗漏导致的甲烷厌氧氧化作用(AOM)所产生的。

     

    Abstract: 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.

     

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