Organic geochemistry of recent marine sediments
from the Nansha Sea, China
Yi Duan*
State Key Laboratory of Gas Geochemistry, Lanzhou Institute of Geology, Chinese Academy of Sciences, Lanzhou, Gansu Province 730000, People's Republic of China
Received 29 May 1998; accepted 21 October 1999 (Returned to author for revision 18 December 1998)
Abstract
Recent marine sediments from two cores, collected on the continental slope of the Nansha Sea, China, have been analyzed for organic matter content,n-alkane and isoprenoid hydrocarbons, fatty acids,n-alkanols and sterols. The organic carbon contents of the two sediment cores average 0.7 and 0.53%, respectively, and are higher than those of other sedimentary environments in this region. The distributions of various lipid compounds indicate that most of the sedimentary organic matter in the two cores is derived from marine plankton and bacteria, with land-derived organic matter present in relatively small amounts. The decrease in relative abundances of shorter chain lipids (n-alkanes,n -fatty acids andn-alkanols) with depth is evident, as is the biochemical conversion of stenols to stanols in some samples. The low pristane/phytane ratios may result primarily from microbial activity, and re¯ect the occurrence of signi®cant diagenetic alteration of organic matter in the two cores.#2000 Elsevier Science Ltd. All rights reserved.
Keywords:Nansha Sea; Organic carbon content; Lipids; Sedimentary environments; Diagenesis
1. Introduction
Studies of the organic geochemistry of recent marine sediments are valuable because they provide informa-tion about paleoenvironments and paleoceanography (Keswani et al., 1984; Venkatesan and Kaplan, 1987). A basic understanding of various lipid sources, their lipid diagenetic evolution, and the accumulation and pre-servation of organic matter by such studies, is helpful in interpreting sedimentary records. To date, such studies have been concerned with sediments of temperate or polar regions (Degens and Mopper, 1976; Venkatesan, 1988). The Nansha Sea of China is located in a low-latitude tropical zone. No previous data have been reported on the organic geochemical contents from sediments of this region, although some studies have been conducted on the organic geochemistry of sinking
particulate material (Duan et al., 1997a; 1998). This paper reports the detailed organic geochemical contents of recent marine sediments from the Nansha Sea. It also discusses the relationship between the organic carbon content and sedimentary environment, and the sources and diagenetic changes of sedimentary organic material in two sediment cores.
2. Materials and methods
2.1. Sampling site
The Nansha Sea is located to the south of the South China Sea. The region has a monsoon-type tropical marine climate, and an annual average ambient tem-perature of 29.2C. The water mass in this area consists mainly of modi®ed water from the West Paci®c Ocean, with salinity averaging slightly more than 33%. Marine organisms are very abundant in this area, and diatoms
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are the major phytoplankton. The area includes a vari-ety of representative topographies, such as the con-tinental shelf, coral island, island shelf, lagoon, continental slope and deep-sea basin features.
2.2. Sampling
Sediment cores of station 103(11 120N, 110 240E) and station 102 (12020N, 111000E) were recovered in the northwestern Nansha Sea in 1990 by the Multi-disciplinary Oceanographic Expedition Team of Acade-mia Sinica (Fig. 1), using a hydraulic piston corer with minimum disturbance. The sediment cores were divided into many sections for various analyses, and all samples used here were kept frozen in the laboratory until analysis. Station 103 was located in the mid continental slope at 1584 m water depth. The sediments consisted of silty clay, with an average carbonate content of 25.4%. Average sedimentation rate for this area is estimated at 2.4 cm/1000 year (Fen and Luo, 1993). Station 102 is located in the lower slope at 2884 m water depth. The sediments were also composed of silty clay, with an average carbonate content of <20% and a sedimentation rate of 1.4±3.1 cm/1000 years (Fen and Luo, 1993).
2.3. Analytical procedures
Sediment samples were dried at room temperature and ground to 80 mesh. The total organic carbon con-tent of each sample was measured with a LECO CS-344 elemental analyzer, after HCl dissolution of carbonates. The concentrations of organic carbon were calculated on a sample dry-weight basis.
Lipid contents were extracted using a Soxhlet appa-ratus with a dichloromethane: methanol (2:1) solvent mixture for 72 h, then saponi®ed overnight with 6% KOH-methanol at room temperature. The correspond-ing neutral and fatty acid fractions were successively recovered with dichloromethane, the latter after acid-i®cation with concentrated HCl to pH 1. The neutral fraction was fractionated by column chromatography
on alumina over silica gel. A variety of lipid fractions, including alkanes, aromatic hydrocarbons, alkanols and sterols, were obtained. Fatty acid compounds were esteri®ed with 10% BF3-methanol, and alkanol and sterol compounds were converted to their silyl ethers by treatment with BSTFA prior to instrumental analysis (Barbe et al., 1989).
Lipid fractions were analyzed by gas chromato-graphy±mass spectrometry (GC±MS), using an HP5890A GC interfaced to a HP5988A MS. The GC column used was fused silica, 50 m long, 0.32 mm internal diameter, wall coated with SE-54. The tem-perature was programmed from 80 to 200C at 6C minÿ1, then ramped to 300C at 3C minÿ1. Helium was used as the carrier gas. MS conditions included EI ioni-zation at 70 eV with an ion source temperature at 200C. Identi®cations of individual lipid compounds were based on retention times of authentic standards and by comparison of their mass spectra with published MS data (e.g. Matsuda and Koyama, 1977; Volkman et al., 1990).
3. Results and discussion
3.1. Organic carbon
The mean organic carbon contents in core 103 and 102 samples are 0.7 and 0.53% (Table 1), respectively. These values are higher than the mean 0.2% identi®ed for modern deep ocean sediments (Degens and Mopper, 1976) and 0.26±0.32% for DSDP samples (Keswani et al., 1984). Organic carbon contents in modern marine sediments are controlled by many factors (e.g. biological productivity, sediment nature, sedimentation rate, sedi-mentary environment, diagenesis, etc.). In the Nansha Sea, the sedimentary environments of the atoll lagoon and coral island all have low organic carbon contents with mean values of 0.18 and 0.42%, respectively (Song and Li, 1995). The sedimentary organic carbon content of the continental shelf is also low, with a mean of 0.25% (unpublished data). These results indicate that the sedimentary environment of the continental slope at the sites of cores 103 and 102, particularly the middle slope, favors the accumulation and preservation of organic matter compared with other sedimentary envir-onments of the study area. This recognition is consistent with the observation of the distribution of particulate organic matter and diatom fossils in the Nansha Sea. Previous data have indicated that the continental slope contains higher particulate organic matter in sea water than does the continental shelf (Huang et al., 1991) and more abundant diatom fossils in sediments than con-tinental shelf and coral island (Yu, 1991).
(Table 1). This trend has also been described previously for other marine sediment cores, and it has been pro-posed as an eect of organic matter biodegradation (Waples and Sloan, 1980; Keswani et al., 1984; Venka-tesan, 1988).
3.2. n-Alkanes and isoprenoid hydrocarbons
The compositions ofn-alkanes in the samples from cores 103 and 102 exhibit bimodal distributions with C18 and C27, C29, or C31as maxima (Fig. 2). The short chain
n-alkanes (C15±C20) are most probably derived from algae and bacteria (Han and Calvin, 1969). The long chain n-alkanes (C20±C35) in sample 01 from core 103 and sample 05, 07 and 08 from core 102 show little odd-carbon predominance (Table 2 and Fig. 2). Such dis-tributions of higher n-alkanes are similar to those reported in Peru coastal marine sediments by Volkman et al. (1983) and are considered to originate from microbes (Albro and Dittmer, 1967; Davis, 1968; John-son and Calder, 1973; Volkman et al., 1983; Venkatesan and Kaplan, 1987). However, the long chain n-alkanes in other samples of cores 103 and 102 have a distinct odd-even predominance, and CPI20±34 values fall in a narrow range of 1.81±2.09, which is below the CPI20±34 range of 4±10 reported for land plants (Clark and Blu-mer, 1967; Caldicott and Eglinton, 1973) and 6.9±8.2 reported for Ellesmere Lake sediments (Rieley et al., 1991). This comparison suggests that thesen-alkanes are only partially derived from higher land plants. Land plantn-alkanes may have been transported to the sites of cores 103 and 102 by advectively ¯owing continental shelf currents and by aeolian transport and deposition (Simoneit, 1977). A rapid decrease of C15±20/C21±35 ratios with increasing depth is observed. This change may be due to the preferential biodegradation of short chainn-alkanes which enhances the relative contribution
of long chain homologues (Johnson and Calder 1973; Brassell et al., 1978, 1983). Furthermore, a dierence in the distributions of long chainn-alkanes between cores 103 and 102 samples is also observed. The former (Core 103), with exception of sample 01, has a major peak at C31; the latter (Core 102) at C29. This re¯ects source dierences, although diagenesis can aect the carbon number maximum in the range of long chainn-alkanes. In this case the diagenetic eects in the two studied regions do not dier signi®cantly (Fen and Luo, 1993). Previous studies show that then-alkane distributions of grasses are dominated byn-C31,whereas those of trees and shrubs are dominated by n-C29 (Simoneit et al., 1984; Heras et al., 1989). This indicates that the middle slope may contain more grass-derived n-alkanes than the lower slope.
Pristane and phytane occur in the studied samples. The pristane/phytane ratios vary from 0.44 to 0.87 and average 0.62. The values are far lower than those in Antarctic marine sediments (pristane/phytane ratios53) reported by Venkatesan and Kaplan (1987). The pris-tane/phytane ratio is generally thought to be a guide to depositional conditions, where values below one repre-sent anoxic conditions and values above one re¯ect oxic conditions (Didyk et al., 1978). However, methanogenic microbes can also generate phytane (Brassell et al,1981; Venkatesan and Kaplan, 1987), and these microbes live in sediments that lack oxygen (and sulfate), which are common in most sub-bottom parts of the sea¯oor. Therefore, low pristane/phytane ratios in the studied samples could result mainly from microbial activity.
3.3. Fatty acids
The distributions of individual fatty acids are shown in Fig. 3. Then-fatty acids in the upper samples of the two cores have trimodal distributions with a dominant Table 1
Analytical data ofn-alkanes, isoprenoid hydrocarbons,n-fatty acids and organic carbon content (%); NA=not analyzed
maximum at C16, a secondary maximum at C9 and a third maximum at C22 or C24, whereas other samples contain bimodal distributions with maxima at C16and at C22or C26. The ratios of CPI16±30in the two cores range from 4.77 to 11.77 (Table 2), indicating that then -fatty acids in the samples are dominated by even car-bon-numbered components.
Then-fatty acids of <C12in sediments are not often reported. The existence of abundant C8 to C11 n-fatty acids in the upper sediments of cores 103 and 102 may
indicate that they are derived mainly from bacteria, because surface or shallow sediments contain abundant bacteria (Hayashi and Takii, 1977). The relatively high abundances of short-chainn-fatty acids (C12to C20) in the two cores re¯ect planktonic and bacterial contributions to the sediment (Chuecas and Riley, 1969; Heras et al., 1989), whereas the relatively low abundances of long-chainn-fatty acids (>C20) in the two cores re¯ect small amounts of land-plant inputs to the sediment (Brown et al., 1972; Gaskell et al., 1975a,b; Matsuda and Koyama,
Table 2
Analytical data ofn-alkanols and sterols
Station Sample number
Sub-bottom depth (cm)
n-Alkanols Sterols
nC9ÿ20 nC21ÿ28
C27 stanol
stenol C28 stanol
stenol C29 stanol
stenol C27
% C28
% C29
% C30
%
103 01 19±29 1.41 0.53 0.46 0.33 44.5 28.5 18.8 8.2
02 48±58 0.61 0.21 0.26 0.27 55.2 16.0 18.3 10.6
03 77±87 0.94 0.32 0.25 0.30 54.6 20.0 17.1 8.5
04 164±174 0.61 0.40 0.27 0.21 44.3 28.4 20.4 7.2
102 05 35±65 2.83 0.34 0.19 0.47 39.0 32.5 18.4 10.1
06 100-132 2.27 0.15 0.23 0.33 55.1 20.4 18.6 6.0
07 210±240 2.36 0.35 0.23 0.39 35.6 31.9 28.6 4.0
08 341±351 1.14 0.12 0.20 0.08 34.4 16.8 45.1 3.1
1977). This source recognition of n-fatty acids is sup-ported by carbon isotopic studies (Duan et al., 1997b). The meand13C values for C
16n-fatty acids in sample 02 and 03 are ÿ28.2% and ÿ27.9%, respectively, con-sistent with a source from the lipid fraction (ÿ28%) of marine plankton (Degens, 1969). Individual long-chain
n-fatty acids have light isotopic composition compared with the C16 and C18 n-fatty acids, which range from ÿ31 to ÿ30.7% which indicates that they come from low-latitude tropical land-plants. Mean isotopic values of C14and C15n-fatty acids areÿ31.8%andÿ31.4%, respectively, i.e. lighter than those for othern-fatty acids and may represent the carbon isotopic composition of methanogenic bacteria in the Nansha sea marine sedi-ments. The unsaturated (C16:1,18:1) and iso (C14,15) fatty acids are also present in the samples and are thought to originate from bacteria (Matsuda and Koyama, 1977; Perry et al., 1979; Volkman et al., 1980). However, carbon isotopic compositions of C16and C18unsaturated fatty acids indicate that they may have the same source as corresponding saturated acids (Duan et al., 1997b). Although a mean 2.5%dierence also exists between the C16 and C18unsaturated versus n-saturated acids, this value may be due to isotopic fractionation accompanying desaturation in biosynthetic processes (Fang et al., 1993). As sediment depth increases, decreases are observed in C8±20/C21±30 ratios and unsaturated acid abun-dances in the two cores (Table 2 and Fig. 3). These changes may be due to diagenesis. The early diagenetic changes of fatty acids have been investigated previously (Rhead et al., 1971; Matsuda and Koyama, 1977; Cranwell, 1981) and show that short-chain acids and unsaturated acids are less stable than the long-chain and
n-saturated acids. This datum provides further evidence for these observations.
3.4. n-Alkanols and sterols
Then-alkanols show an even/odd predominance with major peaks at C14, C16, C18,or C22(Fig. 4). TheC9±20/
C21±28range from 1.14 to 2.83 for core 102 (Table 2) indicates that short-chain n-alkanols are predominant. In contrast, core 103 (ratio from 0.61 to 1.14) contains more abundant longer chainn-alkanols. Longer chainn -alkanols are usually thought to originate from terrige-nous plants (Eglinton and Hamilton, 1967; Cranwell and Volkman, 1981), although carbon isotopic data indicate that they can also derive from marine organ-isms and bacteria (Duan et al., 1997c). Shorter chainn -alkanols are considered to be a source indicator for aquatic organisms, such as plankton or submerged macrophytes (Ogura et al., 1989). Therefore, the n -alkanol distribution patterns in the samples show a mixed input of both marine and terrigenous organic matter. Variations in thenC9±20/nC21±28ratios with the depth (Table 2) occur as part of an overall general decrease, which may result from diagenetic processes.
zooplankton lipids. C28sterols include 24-methylcholest-5,22-dien-3b-ol, 24-methyl-5a-cholest-22-en-3b-ol, 24-methylcholest-5-en-3b-ol, and 24-methyl-5a -cholestan-3b-ol, which comprise 16±32.5% of the total sterols. 24-Methylcholest-5, 22-dien-3b-ol is a major component of C28 sterols in the samples, and has previously been identi®ed as major constituent of diatoms (Volkman, 1986). Diatom fossils are abundant in the two core samples (Tu et al., 1993), so diatoms are a likely source of this sterol in the regions studied. C29sterols are also abundant in samples 01±07 and predominate in sample
08 (Table 2). In general, C29sterols are thought to ori-ginate from higher plants, but they may also derive from diverse algal species (Volkman, 1986). Most C29sterols in the two sediment sections studied are likely derived from marine algae, because long-chain fatty acids in these samples, which represent contributions from higher plants, are low in abundance.
Stanols occur in relatively low abundance. Thestanol/
stenol ratios of C27to C29in cores 103 and 102 range from 0.12 to 0.50 (Table 2). Most of these values are higher than those reported from Antarctic marine sediments
Fig. 4. Distribution patterns ofn-alkanols from core 102 and core 103 samples normalized to the major components.
(Venkatesan, 1988). Stanols can originate either from direct biogenic inputs (Nishimura and Koyama, 1977) or from hydrogenation of sterols by microorganisms (Gaskell and Eglinton, 1975a,b). A C27stanol/stenol ratio of 0.1±0.2 represents the constitution of stenols and stanols in plankton (Wakeham and Canuel, 1990). Higherstanol/stenol ratios in some samples indicate that diagenetic hydrogenation of stenols could have occurred in the two core sections.
Dinosterol is identi®ed by its characteristic mass spectrum, which is consistent with that reported by Volkman et al. (1990), and is also present in all samples, accounting for 3.1±10.6% of the total sterols. This sterol is speci®cally synthesized by dino¯agellates (Boon et al., 1979), and dino¯agellate fossils have been identi®ed in these samples (Tu et al., 1993). Therefore, its occurrence indicates that dino¯agellates also contribute to the deposited organic matter in the region.
4. Conclusion
The data show that sediments of cores 103 and 102 from the continental slope of the Nansha Sea contain a higher concentration of organic carbon compared with core samples from other sedimentary environments in this region, which re¯ects better accumulation and preservation of organic matter on the continental slope. An apparent decrease in the percentage of organic carbon with burial depth is observed and is likely due to diagenetic processes.
The distributions of various lipid compounds supported by stable carbon isotopic data show that the organic matter in both cores is primarily derived from marine sources. The input of organic matter from terrigenous sources into the two locales appears to be relatively small. Terrigenous material in the continental slope zone may have been transported by advectively ¯owing continental shelf currents and aeolian deposition of airborne particles.
The relative abundances of the shorter to longer chain lipids clearly decrease with burial depth. This change could be due to the greater refractive stability of the longer chain lipids and may also re¯ect the existence of signi®cant diagenetic alteration of organic matter in the two cores. The low pristane/phytane ratios may result mainly from microbial activity.
Associate EditorÐB. Simoneit
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