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The distribution and composition of hydrocarbons in sediments of the South Mid-Atlantic Ridge
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Xin HUANG1, *, Shuai CHEN2, 3, Xiaoyuan WANG2, 3, Shuwen ZHANG1, Fajin CHEN1, Xiaoqiang PU1
Acta Oceanologica Sinica | 2018, 37(1) : 89 - 96
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Acta Oceanologica Sinica | 2018, 37(1): 89-96
• Marine Geology •
The distribution and composition of hydrocarbons in sediments of the South Mid-Atlantic Ridge
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Xin HUANG1, *, Shuai CHEN2, 3, Xiaoyuan WANG2, 3, Shuwen ZHANG1, Fajin CHEN1, Xiaoqiang PU1
Affiliations
  • 1 Guangdong Province Key Laboratory of Coastal Ocean Variation and Disaster Prediction, Guangdong Ocean University, Zhanjiang 524088, China
  • 2 Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China
  • 3 Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266071, China
Published: 2018-01-25 doi: 10.1007/s13131-018-1160-1
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Sediment samples obtained from the South Mid-Atlantic Ridge are analyzed by a gas chromatography-mass spectrometer (GC-MS) for the abundances and distributions of hydrocarbons. The hydrocarbons in the samples exhibit a bimodal distribution of n-alkanes and are rich in 3-methylalkanes, 8-methylalkanes and 2, 4, (n-1)-trimethylalkanes, which may be the result of metabolic activity of benthic microorganism. Terpanes, hopanes and steranes are all enriched in the samples, which also support the microbial origin of hydrocarbons in samples. Bitumen and hydrocarbons in the samples show a trend that the contents are the highest in the Samples 22V-TVG10 and 26V-TVG05 collected near hydrothermal areas, and the lowest in samples 22IV-TVG01, 22V-TVG11, and 22V-TVG14 collected far from the hydrothermal areas, which suggest the possible influence on the samples by hydrothermal activity.

hydrocarbons  /  South Mid-Atlantic Ridge  /  sediment  /  hydrothermal activity
Xin HUANG, Shuai CHEN, Xiaoyuan WANG, Shuwen ZHANG, Fajin CHEN, Xiaoqiang PU. The distribution and composition of hydrocarbons in sediments of the South Mid-Atlantic Ridge[J]. Acta Oceanologica Sinica, 2018 , 37 (1) : 89 -96 . DOI: 10.1007/s13131-018-1160-1
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1 Introduction
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The study of hydrocarbons in the geologic environment has been paid wide attention by many researchers (Simoneit et al., 1979, 1990, 2004; Chernova et al., 2001; Lein et al., 2003; Venkatesan et al., 2003; Petrova et al., 2010; Peng et al., 2011; Morgunova et al., 2012). The discovery of hydrothermal activity provided a novel impetus to the further study of hydrocarbons in the geologic environment (Zhang et al., 2001; Lein et al., 2003; Simoneit et al., 2004; Peng et al., 2011). In recent years, many researchers tried to judge the origin of hydrocarbons and assess the influence on hydrothermal products by hydrothermal activity. For example, Lein et al. (2003) and Simoneit et al. (2004) analyzed the distribution and composition of hydrocarbons and other organic matter in hydrothermal sulfide and sediment in Rainbow hydrothermal fields, and suggested the microorganisms related to hydrothermal activity might be the origin of organic matter in hydrothermal sulfide and sediment; Morgunova et al. (2012) discovered that the contents of hydrocarbons in hydrothermal sediments was much higher than those in pelagic sediments around Ashadze hydrothermal field, and considered the thermal degradation of macromolecule organic matters in hydrothermal system might be the reason of the enrichment of hydrocarbons in hydrothermal sediments.
The slow-spreading Mid-Atlantic Ridge, which accounts for about 40% of the total length of global mid-ocean ridges, stretches from 87°N (≈330 km from the North Pole) to 54°S. The Mid-Atlantic Ridge is divided into the North Mid-Atlantic Ridge and the South Mid-Atlantic Ridge by the Romanche Trench near the equator. The South Mid-Atlantic Ridge turns towards the Atlantic -Indian Ridge near 54°S, crosses the Crozet Plateau, and continues eastwards to the Southwest Indian Ridge.
In recent years, scientists have discovered several hydrothermal areas in the South Mid-Atlantic Ridge. In 2009, two new hydrothermal areas between the 13°–14°S segments of the South Mid-Atlantic Ridge were found during the DY115-21 cruise, and hydrothermal sulfide chimney samples were obtained (Tao et al., 2011). In 2010–2012, the DY115-22 and DY115-26 cruises continued to investigate the South Mid-Atlantic Ridge, and a variety of hydrothermal sulfides, sediment, and rocks were collected. During these cruises, the sampling locations (except 22V-TVG14) are centered along the South Mid-Atlantic Ridge between 12° and 15°S (Fig. 1), where the spreading rate is about 3.4 cm/a (DeMets et al., 1994).
In this study, we measured the abundance and distribution of hydrocarbons of ten sediment samples collected along the South Mid-Atlantic Ridge, and compared the hydrocarbons in sediments collected at different distances from the hydrothermal field. Our goal is to identify the main source of hydrocarbons in the samples, and assess the influence on hydrocarbons by hydrothermal activities in the sediments.
2 Sampling and analyses
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In 2010–2012, samples were collected using a TV-grab during the DY115-22 and DY115-26 cruises aboard the R/V Dayang Yihao conducted by the China Ocean Mineral Resources Research and Development Association along the South Mid-Atlantic Ridge. The sites, water depths and descriptions of samples are summarized in Table 1 and Fig. 1.
Calcite was the dominating mineral component in all samples (Fig. 2), and a small amount of kaolinite, which might be terrgenous input (Simoneit, 1977), was detected in 22II-TVG04 and 22V-TVG10.
After collection, the samples were placed in bags and stored at –20°C until analysis. Sediment (200 g) from each sample was placed into dry acid-cleaned glass beakers, and dried at 40°C for 48 h. The dried sediment was powdered in an agate mortar to 100 meshes and dried for at 40°C for a further 24 h.
The extraction and analysis of hydrocarbons were performed at the Lanzhou Center for Oil and Gas Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences. The extraction process was as follows: bitumen was extracted using a Soxhlet extractor with chloroform for 72 h. n-hexane was used to remove asphaltene and solubilize organic matter. Soluble organic matter was separated by a column chromatography (silica-gel 60; inner diameter 5 mm; length 35 mm), and then analyzed by a gas chromatography-mass spectrometry (GC-MS).
A 6890N gas chromatography analyzer (HP 6890 GC plus trace-MS selective detector) with a 30-m DB-5MS fused silica capillary column (inner diameter 0.2 mm; film thickness 0.2 μm) was used. The carrier gas was helium. The GC temperature program used was as follows: injection at 80°C, 2 min isothermal; from 80 to 290°C at 4°C/ min; 20 min isothermal. The MS (5973N) was operated in EI model at 70 eV.
3 Results
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3.1 Bitumen
The concentration of bitumen in the samples ranged from 20.23×10–6 to 87.74×10–6 (Table 2), and the concentration was the highest in Sample 22V-TVG10, followed by Sample 26V-TVG05. The concentrations of bitumen in Samples 22V-TVG10 and 26V-TVG05 were corresponded to the values in carbonate sediments (59.88×10–6) of Logatchev hydrothermal field on the North Mid-Atlantic Ridge (Peng et al., 2011).
3.2 n-alkanes
The concentrations of n-alkanes were between 2.22×10-3 and 1.88×10–6 (Table 3). They were the highest in Samples 22V-TVG10 and 26V-TVG05, and the lowest in Sample 22V-TVG13. n-alkane lengths ranged from carbon number 15 to 33 (n-pentadecane was not detected in Sample 22V-TVG14). The n-alkanes in the samples exhibited a bimodal pattern (Fig. 3), and the CPI (carbon preference index) in the samples ranged from 1.04 to 1.52. The ratios between short-chain (≤ C21) and long-chain (>C21) n-alkanes ranged from 0.26 to 1.42. The short-chain n-alkanes possessed obvious advantage of even carbon number with C16 or C18 as the maximum (C17 in Sample 22V-TVG13), and the values of OEP17 (odd-even-predominance at C17, Table 3) were between 0.66 and 1.05. However, the long-chain n-alkanes possessed the obvious advantage of an odd carbon number with C29 or C31 as the maximum, and the values of OEP29 (odd-even-predominance at C29, Table 3) were between 1.80 and 3.56.
3.3 Branched alkanes
The concentrations of three kinds of branched alkanes (3-methylalkane, 8-methylalkanes and 2, 4, (n-1)-trimethylalkanes) in all samples were high, and the concentration of the latter was the highest in all samples except 22V-TVG11.
The 3-methylalkanes ranged from C16–C24, and exhibited an unimodal pattern with maximum carbon number at 19 (Table 3). The structures of mass spectra of 3-methylalkanes were similar with the same carbon number n-alkanes. The mass spectra exhibited a base peak at m/z=57 (mass-to-charge ratio), and intense key ion at m/z =211, 225, 239, 267, 281, 295, 309 and so on, with typical ethyl [(M+(Mass)=29)] cleavage (Fig. 4a).
The 8-methylalkanes ranged from C17 to C31, with maximum carbon number at 19 and only odd carbon numbered homologs (Table 3). The mass spectra of 8-methylalkanes exhibited a base peak at m/z=127, and intense key ion at m/z=239, 267, 295, 323, 351, 379, 407 and so on (Fig. 4b).
The 2, 4, (n-1)-trimethylalkanes ranged from C14 to C30, with maximum carbon number at 18 and only even carbon numbered homologs (Table 3). The mass spectra exhibited a base peak at m/z=85, and intense key ion at m/z=197, 225, 253, 281, 309, 337 and so on, with typical isopropyl (M+=29) cleavage (Fig. 4c).
The concentrations of phytane and pristane were between 0.037×10–6 and 0.74×10–6, and their ratios ranged from 0.69 to 1.01. Squalene was detected in high concentration in all samples. The concentrations in samples 22V-TVG10 and 26V-TVG05 were 0.43×10–6 and 0.79×10–6 respectively, which demonstrates that the samples were affected by microorganism. Nor-pristane was present in some samples, but the content was lower than those of phytane and pristane.
3.4 Cyclic biomarkers
The cyclic biomarkers present in all samples were tricyclic terpanes, hopanes and steranes, and most samples had similar triterpane compositions. The peaks in the chromatogramat m/z 191 (Fig. 5) indicate the presence of tricyclic, tetracyclic and pentacyclic triterpanes. The dominant terpanes were hopanes (17α(H), 21β(H)-hopane series) and moretanes (17β(H), 21α(H)-hopane series). The 17α(H), 21β(H)-hopane series ranges from C27 to C35 (C28 absent) with a maxima of C29 and C30.
C27, C28 and C29 steranes were also detected, and the content followed the same distribution law in all samples: C29>C27>C28 (Fig. 6). The ratios between C27 and C29 steranes were 0.50–0.87. C30 4-methylsteranes detected in the samples probably derive from methanotrophs (Suzuki et al., 1987); Peng et al. (2011) reported that C30 4-methylsteranes lived around hydrothermal vents, and might be an indicator of hydrothermal activity. Diasteranes (C27 to C29) and pregnane were present in low amounts; they usually are present in highly mature condensates (Suzuki et al., 1987).
4 Discussion
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4.1 The origin of hydrocarbons
The compositions of hydrocarbons in the samples were similar between each other, suggesting the similar source of hydrocarbons in the samples. The distributions and compositions of the n-alkanes in the samples exhibit an odd to even predominance of high molecular mass compounds and show a bimodal distribution (Fig. 7), which are the characteristic of terrigenous input, and indicate a terrigenous source of the high molecular mass n-alkanes (Elias et al., 1997). Simoneit (1977) confirmed that seafloor sediments were affected by eolian dusts at 35°N and 30°S in the South Atlantic. Therefore, the terrigenous input in this study might be also derived from eolian dusts.
The low molecular mass n-alkanes show an even to odd predominance with maxima at C16 and C18 (Fig. 7); this result suggests that the benthic microorganisms may be the main source of low molecular mass n-alkanes in the samples (Simoneit et al., 2004).
The isoprenoid alkenes in the samples may have originated from cell membranes of bacteria or archaea (Kates, 1997). The steranes in the samples may be the result of submarine macromolecular (e.g., cholesterol) thermal alteration; hopanes likely originate from microbial macromolecules (Simoneit et al., 2004). The presence of these typical biomarkers supports the biological origin of organic matter in the samples.
4.2 The influence of hydrothermal activity
Hydrothermal systems are characterized by sufficiently high nutritious and energetic potentials to form and maintain the occurrence of numerous biological communities constituting the so-called “oasis of life” (Yamanaka and Sakata, 2004; Li et al., 2011; Morgunova et al., 2012). The biomass and organic content in sediment near hydrothermal vents are usually several orders of magnitude higher (Yamanaka and Sakata, 2004; Morgunova et al., 2012). The concentration of bitumen can be used as biomass and productivity indicators (Morris and Culkin, 1976; Simoneit et al., 2004). The contents of bitumen in Samples 22V-TVG10 and 26V-TVG05 (similar to the values (59.88×10–6) in carbonate sediments of Logatchev hydrothermal field), were much higher than those in sample 22V-TVG14 which was collected far from hydrothermal field (Table 2). Huang et al. (2017) analyzed the sediment core samples from the northern Okinawa Trough, and discovered that the contents of bitumen in samples affected by hydrothermal activity were higher than the value in samples without the influence of hydrothermal activity. Therefore, the samples which were collected near hydrothermal field (e.g., Samples 22V-TVG10 and 26V-TVG05) might be under the influence of hydrothermal activity.
Peng et al. (2011) pointed out that n-hexadecane and n-octadecane in sediment around hydrothermal field might be related to sulfur metabolism; the n-alkanes (especially n-hexadecane and n-octadecane) in Samples 22V-TVG10 and 26V-TVG05 were higher than those in other samples, suggesting that part of n-alkanes might be derived from sulfur metabolism microorganisms, and also indicating that the samples, especially the samples near the hydrothermal field, may be affected by hydrothermal activity.
However, several samples (e.g., 22IV-TVG04) near the hydrothermal fields had no the characteristics as Samples 22V-TVG10 and 26V-TVG05, which might be related to a sedimentation rate, a hydrothermal plume altitude and a current direction (Brault et al., 1984).
Therefore, hydrocarbons in the samples may mainly origin from metabolic activity of benthic microorganism; meanwhile, the compositions of hydrocarbons in samples make us difficult to ignore the possible influence by hydrothermal activity.
5 Conclusions
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Comparison of the distribution characteristics of hydrocarbons in sediment near hydrothermal fields and in sediment far from hydrothermal fields reveals the influence of hydrothermal activity on sediment around hydrothermal fields. The distributions and compositions of n-alkanes in the samples indicate that the hydrocarbons in the samples may mainly origin from metabolic activity of benthic microorganism. The contents of bitumen in samples collected near hydrothermal field were much higher than those in sample collected far from the hydrothermal field, which shows that the biomass is higher in sediment near hydrothermal field, and suggests that the samples, especially the samples collected near the hydrothermal field, mat be affected by hydrothermal activity. The distribution and composition of hydrocarbons in the samples may be the result of a combination of submarine microorganisms and submarine macromolecular thermal alteration.
Acknowledgements
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The authors thank the crews of the COMRA cruise (DY115-22 and DY115-26) for their help with sampling operations, as well as Meng Qianxiang for his help with sampling analysis.
Funding
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  • The National Basic Research Program (973 Program) of China under contract No. 2013CB429700; the National Natural Science Foundation of China under contract Nos 41476044, 41325021, 41676008 and 41476010; the Special Fund for the Taishan Scholar Program of Shandong Province under contract No. ts201511061; the AoShan Talents Program supported by Qingdao National Laboratory for Marine Science and Technology under contract No. 2015ASTP-0S17; the Program for Key Basic Research of the Ministry of Science and Technology under contract No. 2016YFC1401403; the Foundation for Distinguished Young Teacher in Higher Education of Guangdong under contract No. Yq2014004; the Guangdong Natural Science Foundation of China under contract No. 2016A030312004; the International Science and technology cooperation project of China under contract No. GASI-IPOVAI-04; the Open Fund of the Key Laboratory of Marine Geology and Environment, Chinese Academy of Sciences under contract No. MGE2015KG04; the Open Fund of the Key Laboratory of Submarine Geosciences, State Oceanic Administration of China under contract No. KSLG1503; the Program for Scientific Research Start-up Funds of Guangdong Ocean University under contract No. E15169.
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Year 2018 volume 37 Issue 1
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Article Info
doi: 10.1007/s13131-018-1160-1
  • Receive Date：2017-05-11
  • Online Date：2026-04-13
  • Published：2018-01-25
Article Data
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History
  • Received：2017-05-11
  • Accepted：2017-05-31
Funding
The National Basic Research Program (973 Program) of China under contract No. 2013CB429700; the National Natural Science Foundation of China under contract Nos 41476044, 41325021, 41676008 and 41476010; the Special Fund for the Taishan Scholar Program of Shandong Province under contract No. ts201511061; the AoShan Talents Program supported by Qingdao National Laboratory for Marine Science and Technology under contract No. 2015ASTP-0S17; the Program for Key Basic Research of the Ministry of Science and Technology under contract No. 2016YFC1401403; the Foundation for Distinguished Young Teacher in Higher Education of Guangdong under contract No. Yq2014004; the Guangdong Natural Science Foundation of China under contract No. 2016A030312004; the International Science and technology cooperation project of China under contract No. GASI-IPOVAI-04; the Open Fund of the Key Laboratory of Marine Geology and Environment, Chinese Academy of Sciences under contract No. MGE2015KG04; the Open Fund of the Key Laboratory of Submarine Geosciences, State Oceanic Administration of China under contract No. KSLG1503; the Program for Scientific Research Start-up Funds of Guangdong Ocean University under contract No. E15169.
Affiliations
    1 Guangdong Province Key Laboratory of Coastal Ocean Variation and Disaster Prediction, Guangdong Ocean University, Zhanjiang 524088, China
    2 Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China
    3 Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266071, China

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表12种不同金属材料的力学参数
科
Family		 属数
Number of
genus		 种数
Number of
species		 占总种数比例
Percentage of
total species (%)		 属
Genus		 种数
Number of
species		 占总种数比例
Percentage of total
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鹅膏菌科Amanitaceae 2 11 5.26 鹅膏菌属 Amanita 10 4.78
小菇科 Mycenaceae 2 12 5.74 丝盖伞属 Inocybe 5 2.39
多孔菌科 Polyporaceae 8 14 6.70 蜡蘑属 Laccaria 5 2.39
红菇科 Russulaceae 3 23 11.00 小皮伞属 Marasmius 6 2.87
小菇属 Mycena 11 5.26
光柄菇属 Pluteus 5 2.39
红菇属 Russula 17 8.13
栓菌属 Trametes 5 2.39
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