Journal of Tropical Oceanography >
Geochemical characteristics of surface sediments in the Southern Qiongdongnan Basin of the northern South China Sea and its implication for sedimentary environment*
Received date: 2017-08-29
Request revised date: 2017-10-10
Online published: 2018-07-16
Supported by
National Natural Science Foundation of China (41273022);PetroChina-Chinese Academy of Sciences Science and Technology Cooperation Project (2015A-4813);Chinese Academy of Sciences Youth Innovation Association (2014321);Open Fund of the Qingdao National Laboratory for Marine Science and Technology (KC201703)
Copyright
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 (δ34SCRS) 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 δ34SCRS values from cores C14 and C19 are -50.2‰~-46.9‰ and -50.1‰~-42.0‰, respectively. The negative δ34SCRS 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.
SUN Tiantian , WU Daidai , PAN Mengdi , YANG Fei , WU Nengyou , CHENG Xuegang , LIU Lihua . 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 . DOI: 10.11978/2017091
Fig. 1 Map showing the locations of the sediment cores used in this study. Modified after Zhang et al (2014)图1 研究区取样站位地理位置[据张劼等(2014)修改] |
Tab 1 Information on the two cores used in this study表1 重力活塞沉积柱基本信息 |
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 |
Fig. 2 Profiles of geochemical parameters at C14 (a) and C19 (b). The sulfate data are from Luo et al (2015)图2 C14 (a)和C19 (b)两个站位地球化学参数剖面 |
Fig. 3 Contents of chromium reducible sulfur (CRS) and δ34SCRS values of sediments at C14 (a, b) and C19 (c, d)图3 C14 (a, b)和C19 (c, d)沉积柱中可还原性硫化物(CRS)质量分数及其δ34SCRS值 |
Fig. 4 Age model and depth profile of TS/TOC at C19图4 C19站位年龄和TS/TOC比值剖面图 |
Tab. 2 AMS 14C dating of planktonic foraminifera at C19表2 C19站位浮游有孔虫AMS 14C测试结果 |
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 |
注: 0 cal a BP=1950AD |
Fig. 5 Relationship between TS and TOC for sediments at C14. Cited from Li et al (2016)图5 C14站位中的TS和TOC质量分数散点图 |
The authors have declared that no competing interests exist.
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