南海北部琼东南盆地浅表层沉积物的地球化学特征及对沉积环境的指示*

doi:10.11978/2017091

Abstract

Abstract:

The seabed pockmark is widely found in the Qiongdongnan Basin in the northern slope of the South China Sea. The formation of pockmark is mainly regarded as related to the seabed fluid seepage in recent studies. However, it is still unclear what the geochemical characteristics of the seabed sediment are and what corresponding activities of the pockmark are. Two sediment cores of C14 and C19 from the Southern Qiongdongnan Basin were recovered and investigated in this study. Here, we examine the contents of total sulfur (TS), total carbon (TC) and total organic carbon (TOC) contents as well as chromium reducible sulfur (CRS) content and its stable isotopes (δ³⁴S_CRS) values. Combined with the ratio of total nitrogen to total organic carbon (TN/TOC) and reduced sulfate content in pore water, we analyze the geochemical characteristics. The results suggest that there are two types of sulfate reduction reaction (SR) at the C14 site - the anoxygenous oxidation of organic matter controlled SR (OSR) at the depth of 0~3.91 meters below seafloor (m bsf) and the anaerobic oxidation of methane (AOM) derived SR at the depth of 3.91~7.55 m bsf, and the depth of 3.91 m bsf is the boundary of the two types of reaction of SR. The contents of TS and TC below 3.91 m bsf are higher than those above 3.91 m bsf, and the concentration of sulfate in pore water below 3.91 m bsf decreases linearly with depth, Therefore, the depth of 3.91 m bsf is a significant indicator for geochemical characteristics in sediment core. Moreover, the SR below 3.91 m bsf is controlled by the seepage of methane-bearing fluid. The contents of TS and TC at the C19 site increase with depth. However, the variation of TN/TOC ratios is almost contrary to the patterns of TS and TC contents, that is, because OSR is the main SR type of the whole sediment core at the C19 site and the organic matter is revealed to be deposited in early diagenetic stage. The δ³⁴S_CRS values from cores C14 and C19 are -50.2‰~-46.9‰ and -50.1‰~-42.0‰, respectively. The negative δ³⁴S_CRS values suggest that the major biogeochemical process is the result of sulfate reduction in a relatively open system. Therefore, the methane-bearing fluid for the pockmark in the Qiongdongnan Basin had leaked and the pockmark is now seemingly in a weak seepage stage and/or even to be inactive.

Key words: chromium reducible sulfur (CRS), stable isotopes, anaerobic oxidation of methane (AOM), sulfate reduction (SR), pockmark, Qiongdongnan Basin

CLC Number:

P736.41

Tiantian SUN, Daidai WU, Mengdi PAN, Fei YANG, Nengyou WU, Xuegang CHENG, Lihua LIU. Geochemical characteristics of surface sediments in the Southern Qiongdongnan Basin of the northern South China Sea and its implication for sedimentary environment^*[J].Journal of Tropical Oceanography, 2018, 37(4): 70-80.

Figures/Tables 7

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Tab 1

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站位	柱长/m	纬度	经度	水深/m
C14	6.7	15°54′30″N	110°38′36″E	840
C19	6.9	16°53′53″N	111°09′38″E	1127

送样编号	深度/m	样品性质	测试年龄 /a BP	日历年龄 /cal a BP
C19-46	0.9~0.92	N. dutertrei	18790±80	22865~22465
C19-100	1.98~2.0	N. dutertrei	24310±100	28600~28105
C19-148	2.94~2.96	N. dutertrei	29380±160	33860~33365
C19-292	5.82~5.84	N. dutertrei	32790±260	37695~36185

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Geochemical characteristics of surface sediments in the Southern Qiongdongnan Basin of the northern South China Sea and its implication for sedimentary environment^*

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Geochemical characteristics of surface sediments in the Southern Qiongdongnan Basin of the northern South China Sea and its implication for sedimentary environment*

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Geochemical characteristics of surface sediments in the Southern Qiongdongnan Basin of the northern South China Sea and its implication for sedimentary environment^*