Continental Shelf Research 28 (2008) 582–592
A comparison of sedimentary aliphatic hydrocarbon distribution
between East China Sea and southern Okinawa Trough
Woei-Lih Jeng
a,, Chih-An Huh
baInstitute of Oceanography, National Taiwan University, Taipei, Taiwan, ROC bInstitute of Earth Sciences, Academia Sinica, Taipei, Taiwan, ROC
Received 5 July 2007; received in revised form 15 October 2007; accepted 19 November 2007 Available online 4 December 2007
Abstract
In this study, 34 surface sediments were collected from the East China Sea (ECS) shelf, and their aliphatic hydrocarbon concentrations measured. The hydrocarbon distribution of the ECS was compared with that of the southern Okinawa Trough (SOT) to explore for possible relations between the two coastal marine areas. Results from principal component analysis performed on hydrocarbon data of ECS and SOT sediments indicated that the first two components explained 74% of the total variance and that the score plot of the two components showed three groups. The first group defined by the 12 SOT sediments contained three sediments on the ECS inner shelf. Eight sediments except one on the ECS inner shelf formed group II, and the remaining 21 sediments located away from the inner shelf (mainly on the mid-to-outer shelf) formed group III. Even though three inner shelf sediments cluster with group I, the separation between groups I and II suggests that the hydrocarbon relation between the inner shelf and the SOT is very weak. Groups II and III do not cluster together; their separation is supported by previous studies showing that Changjiang River sediment tends to remain in the coastal waters and is transported to the south by the coastal currents along the Chinese coast. Also, group I does not cluster with group III, signifying a great difference between them; this can be attributed to little material exchange between the ECS and the SOT because of the Kuroshio Current. In addition, the SOT and ECS inner shelf had carbon preference index values averaging 2.9270.79 (n ¼ 12) and 4.5670.93 (n ¼ 8), terrestrial/aquatic ratios averaging 9.6474.33 (n ¼ 12) and 50.4758.0 (n ¼ 8), and (nC27+nC29+nC31)/TOC ratios averaging 17317482 107g(g C)1 (n ¼ 12) and 30167868 107g(g C)1 (n ¼ 8), respectively; strongly indicating considerable differences between the two areas. Based on the present results, hydrocarbon export from the ECS inner shelf to the SOT appears to be unlikely. r2007 Elsevier Ltd. All rights reserved.
Keywords: Sediment; Hydrocarbon; East China Sea; Southern Okinawa Trough
1. Introduction
The main circulation patterns of the Changjiang Estuary and adjacent East China Sea (ECS) can be characterized by the northward flow of warm (13 1C) and saline (34 psu) waters of the Taiwan Warm Current (TWC) and a southward flow of the colder (5 1C) and less saline (30 psu) waters of the Yellow Sea Coastal Current (YSCC) (Fig. 1). During high river runoff, one part of the Changjiang plume with the freshest water extends to the south along the coast, and the other part with low salinity
extends offshore on average toward the northeast. How-ever, during low river runoff, the surface plume spreads toward the south only, possibly because of the influence of the TWC and YSCC convergence (Beardsley et al., 1985). The ECS has a very broad continental shelf. The Changjiang River annually discharges 5 108tons of sediment directly into the ECS (Milliman and Meade, 1983). This sediment is confined to the coastal zones of the ECS and ultimately transported south and southwestward by the Changjiang Coastal Water (Milliman et al., 1985); offshore transport has been prevented by tidal currents and by the northward movement of the Taiwan Warm Water (Milliman et al., 1989). DeMaster et al. (1985) have reported that the accumulation rates of ECS shelf sediments are relatively high in muddy areas and low in
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sandy areas. The sedimentation rates on the ECS shelf range from 2 to 0.02 cm yr1, and generally decrease southward along the inner shelf and eastward offshore (Huh and Su, 1999). On the other hand, the southern Okinawa Trough (SOT) has high sedimentation rates located approximately in the lower trough, deeper than 1000 m (range from 0.25 to 0.52 cm yr1estimated from the excess 210Pb profiles of cores), while low sedimentation rates are generally in the upper trough, shallower than 1000 m (Chung and Chang, 1995). Based on results from a site (24148.240N, 122130.000E) of ODP Leg 195 in the SOT,
the sedimentation rate has always been high, reaching 325 cm kyr1(0.325 cm y1) since the late Holocene (ODP, 2001). Therefore, the SOT is apparently an area of focused sedimentation along the path of the Kuroshio Current (KC). Furthermore, the lower slope (41000 m) sediments consist almost entirely of silty mud (Chen et al., 1995); for instance, mud accounts for over 97% (Jeng and Chen, 1995), while the upper slope (o1000 m) sediments are composed of sand with little mud (Chen et al., 1995).
Transport processes of particulate matter in the marginal seas were first investigated in the middle Atlantic Bight during the SEEP (Shelf Edge Exchange Processes I and II) program (Walsh et al., 1988;Biscaye et al., 1988) and later in several other seas in the world. For the ECS and the SOT, it is the KEEP (Kuroshio Edge Exchange Processes) program (Wong et al., 2000; Liu et al., 2003). Using the
results of field observations and diagnostic numerical experiments, Yanagi et al. (1996) have concluded that suspended matter is transported from the shelf edge to the inner shelf in summer and from the inner shelf to the shelf edge in autumn (which is maximum) and winter due to the vertical circulation mainly induced by the monsoon wind. Studying time-series sediment trap samples collected at SST-1 (29121.60N, 128113.50E, 1100 m water depth),
Katayama and Watanabe (2003) have concluded that lateral transport through the bottom layer is important in the transport of terrigenous particles from the ECS shelf to the mid-OT and that Changjiang influence is relatively high during winter. As far as organic carbon is concerned, analyzing 11 sediment cores from the mid-ECS,Oguri et al. (2003) have reported that organic carbon accumulation rates range from 0.42 to 3.1 mg C cm2yr1and are higher in the inner shelf edge and the slope areas. Based on the balance of organic carbon budget in the shelf sediments,
Kao et al. (2003) consider that a major fraction of adsorbed carbon from primary production must be exported out of the ECS shelf. They further propose that one possible mechanism is the cross-shelf export of particulate organic matter (POM) and that the most likely site for deposition of exported POM from the ECS is the SOT. Liu et al. (2000) propose a conceptual model illustrating the possible route of transporting sediment. From isotopic evidence, Kao et al. (2003)conclude that a major fraction of the sedimentary organic matter in the SOT may originate from the inner shelf of the ECS.
In the present study, 34 surface sediments from the ECS were analyzed for their aliphatic hydrocarbons. A compar-ison of hydrocarbon distribution in sediments between ECS and SOT (data from Jeng and Huh, 2006) was made in order to understand the significance of cross-shelf export of particulate organic carbon to the SOT. To find the possible relation of hydrocarbon distribution between the two areas, principal component analysis (PCA) was employed since it was a data reduction technique used to simplify data sets while retaining the underlying patterns within the data. This result of comparing hydrocarbon similarity between ECS and SOT may shed some light on the transport of sedimentary lipids between them, and might provide some information about the question ‘‘Does the ECS export organic carbon to the SOT?’’
2. Experimental
In the present study, 34 surface sediments were collected from the ECS shelf on board R/V Ocean Researcher I using a box corer (Fig. 1). The top 3–4 cm of sediment was stored in stainless steel boxes (pre-cleaned with solvent) and kept frozen (20 1C) until analyzed. Prefixes A, B, C, D, E, F and G of sample codes denote cruise numbers 551 (May 29–June 4, 1999), 542 (March 8–11, 1999), 525 (August 17–24, 1998), 493 (July 8–13, 1997), 460 (August 20–26, 1996), 456 (July 6–12, 1996) and 417 (April 24–May 1, 1995), respectively.
Fig. 1. Sample sites on the East China Sea shelf and in the Okinawa Trough with water circulation patterns. Three major currents are the Yellow Sea Coastal Current (YSCC), the Taiwan Warm Current (TWC) and the Kuroshio Current (KC). Sample codes for the Okinawa Trough (OTn) are top row (from left): stations 18 and 5; second row: stations 1, 3, 4 and 22; third row: stations 12, 23, 19 and 9; bottom row: stations 20 and 8.
In the lab, frozen sediments were freeze-dried. Following the addition of an internal standard (n-C24D50), the dried
sediment was extracted with a mixture of dichloromethane and methanol (1:1; v/v) for 24 h in a Soxhlet apparatus. The lipid extract was then saponified by reflux for 3 h with 0.5 N KOH solution in methanol. The non-saponifiable lipids were isolated by hexane extraction four times and concentrated using N2 gas. The aliphatic hydrocarbon
fraction was isolated from the neutral lipids by silica gel (deactivated with 5% H2O) column chromatography using
hexanes.
For gas chromatography (GC) analysis, an HP 5890A gas chromatograph equipped with a split/splitless injector and a flame ionization detector (FID) was used. Separation of aliphatic hydrocarbons was achieved by an SPB-1 capillary column (30 m 0.25 mm i.d. 0.25 mm). Oven temperature programming was 45–90 1C at 15 1C min1 and 90–270 1C at 3 1C min1 for analyzing aliphatic hydrocarbons. Identification was made with co-injection of authentic standards and gas chromatography–mass spectrometry (GC–MS). The GC–MS analyses were performed with an HP 6890 GC (HP-1MS crosslinked methyl siloxane column, 30 m 0.25 mm i.d. 0.25 mm) interfaced directly to an HP 5973 quadrupole mass selective detector (electron ionization, electron energy 70 eV, scanned from 50 to 550 Da).
An SGE (Australia) OCI-5 cool on-column injector was also fitted in the gas chromatograph for obtaining the best quantification. GC peak areas of all hydrocarbons and the internal standard were obtained using an electronic integrator (Chromatopac C-R6A, Shimadzu, Japan). Each hydrocarbon concentration was determined using the internal standard. Based on eight replicate analyses, the analytical precision (expressed as the percent coefficient of variation) of hydrocarbon abundances was calculated to be 2–8%.
All sediment samples were determined for their total organic carbon by the dichromate–acid oxidation method (Gaudette et al., 1974) modified by addition of Ag2SO4to
H2SO4 at the rate of 15 g L1. Titration was carried out
with an automatic titrator (Metrohm 702 SM Titrino, Switzerland). The relative standard deviation of TOC determination was generally o1%.
PCA was performed on GC data of aliphatic hydro-carbons. Each compound was used as a variable. Prior to PCA, all hydrocarbon data were standardized by subtract-ing each data from the mean and dividsubtract-ing by the standard deviation. PCA was performed using xlSTAT version 5.1 (Addinsoft, France).
3. Results and discussion
From the GC chromatograms of aliphatic hydrocar-bons, the sediments showed a general picture of similar distribution patterns with n-alkanes predominant along with squalene, diploptene, UCM (unresolved complex mixtures), etc. (Fig. 2). In general, higher-molecular-weight
n-alkanes of terrigenous sources—C25, C27, C29 and C31
(Rieley et al., 1991) dominated over lower-molecular-weight ones of marine sources such as phytoplankton— C15, C17, C19and C21 (Winters et al., 1969;Blumer et al.,
1971). This result can be attributed to n-alkanes derived from marine sources being degraded faster than those from terrigenous ones (Meyers et al., 1984; Gagosian and Peltzer, 1986). Also, land plants have proportionally larger amounts of hydrocarbons than do marine algae. Squalene exhibited a wide concentration range as compared to n-alkanes (Table 1) probably because it is a lipid constituent of most marine organisms and particularly of phytoplank-ton (Bieger et al., 1997;Salas et al., 2006). Also, it possesses relatively high stability like n-C18 and pristane (Jeng and
Huh, 2004). Diploptene (hop-22(29)-ene) is derived from terrestrial higher plants and is also formed by bacteria (Rohmer et al., 1984). If higher plants are the sole source of diploptene, a strong correlation between diploptene and terrestrial higher plant n-alkanes has been demonstrated (Prahl et al., 1992). A positive correlation between diploptene and higher plant n-alkanes was found for 34 ECS sediments (PC25–33¼ 152+13.2 Di, r ¼ 0.96),
strongly indicating a higher plant source to the sediments. UCM, consisting of cyclic and branched alkanes, is known to resist microbial degradation more effectively than n-alkanes and thus has a greater tendency to remain in the environment after n-alkanes have degraded (Gough and Rowland, 1990;Bouloubassi and Saliot, 1993). It has a linkage to degraded or weathered petroleum residues (Venkatesan et al., 1980; Readman et al., 1987). Most marine sediments analyzed generally contained a medium UCM (Fig. 2;Table 1). This does not necessarily reflect the presence of degraded petroleum in the sediments since in some cases concentrations of UCMo10 mg g1 are com-mon in coastal marine environments far from petrogenic hydrocarbon sources (Matsumoto, 1983; Tolosa et al., 1996). In addition, the distribution of the lower molecular weight n-alkanes (oC23) looked like a petrogenic origin
(carbon preference index (CPI))close to 1) which could probably be attributed to recycled organic matter since the mid-to-outer shelf was composed mainly of relict sediments (Niino and Emery, 1961). Further, only five stations gave Pr/Ph ratios close to 1 (A14, 0.82; B7, 0.81; D16, 0.86; D27, 1.21; E49, 1.05; calculated fromTable 1). Combining the UCM and Pr/Ph ratio results, it can be concluded that the petrogenic hydrocarbon contamination on the ECS shelf is minimal.
The sediment textures varied greatly as reflected by the TOC content of sediments ranging from 0.043 to 0.85 g C(100 g)1 (Table 2). To compensate for the grain size difference, all hydrocarbon data were normalized to TOC before performing multivariate analysis.
Prior to PCA, cluster analysis was performed on all variables (e.g. individual aliphatic hydrocarbons) of ECS and SOT sediments (Tables 1 and 3). The resulting dendrogram (not shown) illustrated that UCM, diploptene, and squalene merged with the clusters in the final stages
Fig. 2. Gas chromatograms of the aliphatic hydrocarbon fraction from station A40 (upper) and OT8 (lower). Numbers denote carbon numbers of n-alkanes. I.S., internal standard; Pr, pristane; Ph, phytane; Sq, squalene; Di, diploptene; and UCM, unresolved complex mixtures.
Table 1
Hydrocarbon concentrations (ng g1) for East China Sea (ECS) sediments
Sample Aliphatic hydrocarbonsa
C15 C16 C17 Pr C18 Ph C19 C20 C21 A10 – 1.66 4.27 5.95 3.99 1.42 2.96 2.37 7.78 A14 – – 6.80 3.13 12.2 3.83 13.5 12.6 20.3 A24 – 5.19 32.1 41.8 15.1 5.81 16.6 7.91 11.8 A28 0.372 0.521 0.397 – 2.97 0.818 3.15 3.07 3.72 A31 2.17 7.09 15.7 13.1 13.1 4.99 12.4 10.3 16.0 A35 – – 3.67 2.60 8.64 – 10.1 10.5 16.4 A37 4.11 10.3 24.4 18.4 19.0 6.36 17.0 12.7 21.0 A40 0.789 4.90 12.7 10.3 11.7 2.33 9.34 8.97 10.1 A42 3.36 13.3 30.4 22.2 26.9 9.89 21.1 17.0 22.6 B7 – – 2.10 1.68 5.39 2.07 5.26 5.60 7.42 B8 – 12.5 38.7 45.6 33.9 13.4 32.8 28.1 35.1 C6 – – – – 11.2 – 25.8 24.8 41.7 C7 2.88 14.7 42.9 50.0 27.8 9.85 39.7 23.1 37.8 C8 – 4.34 17.2 11.8 17.5 6.36 16.8 13.7 19.7 C10 – – 3.66 1.30 9.34 – 13.2 13.6 21.4 C12 – – – – – – 10.3 18.4 34.4 C14 – – – – – – 11.5 16.3 29.4 CA – – 2.29 – 8.74 3.01 13.2 13.9 19.8 CB – – 1.93 1.13 4.82 1.76 5.24 5.06 7.35 CC – – – – 1.72 – 2.37 2.91 3.21 D7 25.4 32.1 70.8 62.6 42.5 16.7 50.6 32.7 57.3 D12 – 3.11 12.1 8.05 13.4 5.26 10.2 9.46 11.1
Table 1 (continued )
Sample Aliphatic hydrocarbonsa
C15 C16 C17 Pr C18 Ph C19 C20 C21 D16 0 0.918 5.33 2.40 6.84 2.78 4.86 4.50 4.77 D17 2.56 8.09 18.7 14.1 15.5 3.01 11.6 9.79 12.9 D20 – 3.48 11.4 8.08 11.8 4.60 8.14 7.21 8.33 D21 – – – – 4.30 0.513 5.92 7.07 9.00 D22 – 4.64 10.8 6.88 10.3 3.93 7.18 6.39 7.66 D27 – 1.35 5.02 4.13 6.50 3.40 5.26 4.67 5.67 E45 – – – – – – 0.958 1.83 2.95 E49 – 1.20 3.35 1.60 3.67 1.52 2.35 2.16 2.30 F1 44.6 48.6 94.8 120 66.6 27.8 62.2 60.8 71.4 F3 46.4 48.4 93.2 141 65.3 26.0 60.2 69.1 66.6 F6 40.3 41.7 116 91.0 59.3 23.1 60.7 44.1 65.0 G6 – – 20.6 21.3 26.1 9.56 32.7 28.7 42.1 C22 C23 C24 C25 C26 C27 C28 Sq C29 A10 2.67 4.68 3.10 4.62 3.00 6.82 6.97 76.0 14.6 A14 17.8 31.3 21.0 39.6 28.6 65.0 32.9 134 136 A24 7.91 13.5 9.42 15.0 10.2 22.6 12.4 161 48.4 A28 2.95 4.91 3.17 4.21 4.21 7.86 6.27 41.6 12.7 A31 13.1 21.2 13.1 24.7 15.4 37.2 24.1 93.4 80.4 A35 14.6 26.0 16.6 37.8 19.8 63.3 27.5 125 144 A37 16.2 28.6 18.8 36.8 22.6 56.1 33.7 197 117 A40 9.38 14.3 9.67 17.4 13.8 30.7 22.2 83.2 60.2 A42 21.2 32.8 22.5 35.1 23.1 53.3 28.6 138 106 B7 7.54 10.5 7.42 9.01 6.39 10.2 7.95 92.5 17.1 B8 36.7 44.9 40.8 51.3 34.6 56.9 32.5 194 98.0 C6 47.4 89.6 58.3 144 66.4 237 104 100 565 C7 40.3 80.0 47.3 125 56.4 218 89.1 133 523 C8 21.4 35.9 27.0 42.0 22.9 55.6 30.7 51.8 127 C10 22.8 36.6 31.8 50.9 37.6 73.3 48.3 160 140 C12 45.3 89.0 69.1 156 90.2 262 135 141 587 C14 36.6 70.0 39.9 111 51.9 196 80.4 108 485 CA 22.2 37.4 32.7 52.2 40.9 71.1 45.8 65.1 118 CB 6.15 9.95 6.33 10.8 6.19 14.0 7.03 62.1 27.0 CC 3.79 5.79 3.88 5.49 3.00 6.35 3.53 24.7 11.3 D7 54.0 90.7 58.2 123 63.8 213 105 119 512 D12 12.0 19.1 13.9 30.0 20.7 55.5 45.0 57.2 109 D16 4.80 8.32 5.12 8.00 3.94 10.8 5.54 50.3 22.2 D17 12.0 18.5 13.1 23.7 14.8 34.7 22.3 87.1 76.0 D20 7.66 12.4 9.93 16.4 8.57 21.0 12.5 70.1 43.1 D21 8.88 14.6 10.1 21.4 10.5 25.9 11.2 145 53.5 D22 6.69 10.8 7.10 12.3 6.39 14.0 12.8 78.4 25.8 D27 5.36 9.08 7.37 10.7 9.75 20.0 24.7 64.0 38.6 E45 3.42 5.86 3.93 6.36 4.00 9.80 4.61 12.5 17.9 E49 2.28 4.09 2.43 3.64 2.05 5.11 2.14 6.89 8.85 F1 70.5 85.6 57.9 83.9 58.9 116 73.4 111 201 F3 69.3 83.2 56.2 84.5 62.3 118 75.0 116 205 F6 65.4 95.8 67.0 131 74.9 205 98.3 275 457 G6 41.3 70.5 46.7 90.7 54.8 147 97.9 161 312 C30 C31 C32 Di C33 C34 C35 UCM CPI A10 12.9 14.2 2.45 13.1 6.14 0.672 6.26 388 1.71 A14 60.1 167 21.4 57.7 79.6 9.46 47.8 2330 3.09 A24 42.9 51.0 8.64 28.8 19.9 2.28 24.5 1100 1.97 A28 12.6 11.1 3.84 6.92 4.98 1.12 6.25 – 1.41 A31 52.3 93.7 15.3 49.0 44.3 6.91 45.0 3410 2.40 A35 80.8 186 17.3 71.3 80.1 6.92 40.2 2180 3.26 A37 71.0 137 20.3 60.2 62.0 6.95 61.3 2980 2.56 A40 43.0 63.5 13.0 23.1 33.8 6.31 23.8 852 2.06 A42 41.6 126 19.0 42.3 60.3 9.26 44.2 3990 2.98 B7 11.2 17.4 4.68 10.7 9.59 1.78 10.9 658 1.83 B8 43.8 123 22.4 54.7 71.4 9.07 34.4 – 2.56
with Euclidean distances of 6.246, 7.034 and 8.717, respectively, indicating that the three components had very weak relation with other hydrocarbons. As a consequence, these three components were excluded for performing PCA. The score plot of the first two components explaining 74% of the total variance is shown inFig. 3. To facilitate discussion, peripheral samples of all
OT samples were connected by line segments, forming a polygon; those in positive PC1 and negative PC2 as well as those in negative PC1 were, respectively, connected in the same way. Therefore, three groups are visible in the score plot, those virtually with a positive score on PC1 and with low positive and low negative scores on PC2 (group I), those with a positive score on PC1 and a high negative score on PC2 (group II), and those with negative scores on PC1 (group III). Of the three groups, group I contained 12 OT samples and three inner shelf samples (C8, D7 and F6). With the exception of sample D27, group II comprised seven samples on the inner shelf. The majority of samples (21) on the ECS shelf located away from the inner shelf constituted group III. In addition, samples F1 and F3, situated in high positive scores on PC1 and high positive scores on PC2, were fairly peculiar and might be considered outliers.
It is of importance to note that group I defined by 12 OT samples includes three samples on the inner shelf; this is especially unreasonable because group I clusters with distant samples instead of nearby samples. This requires more discussion. The Lanyang River (nearest river to the SOT, not shown) has been reported to be not a major hydrocarbon source for OT sediments, but to a less extent it does influence two stations OT 1 and OT 12 closest to the river mouth based on the individual TAR (terrigenous/ aquatic ratio) and CPI values (Jeng and Huh, 2006). This also reflects in OT1 in Fig. 3 that the sample is situated relatively away from the cluster of other OT samples. That is, sample OT1 has a low similarity to the other OT
Table 1 (continued ) C30 C31 C32 Di C33 C34 C35 UCM CPI C6 102 778 68.0 156 357 24.4 108 9250 5.46 C7 129 747 77.1 141 330 20.8 101 6400 5.05 C8 23.0 174 23.4 42.3 75.9 9.16 32.0 6560 4.05 C10 43.5 168 29.1 48.2 83.0 14.2 53.1 5490 2.85 C12 120 796 82.3 142 340 31.6 128 7920 4.49 C14 77.0 648 57.4 98.2 280 18.8 85.2 5120 5.82 CA 43.1 138 28.3 39.9 69.2 13.8 49.5 2960 2.48 CB 8.93 31.3 5.38 24.5 14.0 1.69 14.2 1300 3.10 CC 4.05 13.9 2.05 6.19 6.33 1.19 5.98 – 2.89 D7 108 687 82.2 167 298 21.3 97.6 9420 4.61 D12 46.8 131 36.7 35.0 66.4 21.2 32.0 1300 2.35 D16 5.33 25.7 3.73 14.6 11.4 1.54 6.46 – 3.60 D17 27.0 90.9 18.9 42.6 44.9 8.30 29.3 1940 2.89 D20 20.7 49.7 9.11 33.1 22.4 4.36 13.7 1210 2.64 D21 20.4 62.0 12.6 41.1 29.6 7.94 18.3 1290 3.02 D22 6.88 29.9 4.56 26.0 12.0 0.972 11.0 911 2.73 D27 31.8 56.6 36.7 20.2 42.2 14.3 17.4 1250 1.48 E45 4.42 22.3 3.05 2.64 9.43 1.30 3.62 – 3.54 E49 2.12 11.0 1.18 2.30 4.60 0.455 2.55 – 3.77 F1 81.3 239 41.5 75.6 129 20.7 46.4 4580 2.63 F3 64.4 247 48.8 70.4 134 19.0 96.8 6660 2.75 F6 104 607 90.1 166 308 28.8 112 5820 4.12 G6 76.8 431 51.2 110 219 18.1 79.7 3530 3.84 a
Pr, pristane; Ph, phytane; Sq, squalene; Di, diploptene; UCM, unresolved complex mixtures; CPI, carbon preference index ¼ (1/2)[(nC25+nC27+
nC29+nC31+nC33)/(nC24+nC26+nC28+nC30+nC32)+(nC25+nC27+nC29+nC31+nC33)/(nC26+nC28+nC30+nC32+nC34)].
Table 2
Total organic carbon (TOC in g C (100 g)1) in East China Sea and
southern Okinawa Trough sediments
East China Sea Southern Okinawa Trough Sample TOC Sample TOC Sample TOC
A10 0.059 CA 0.20 OT1 0.65 A14 0.34 CB 0.10 OT3 0.56 A24 0.18 CC 0.057 OT4 0.79 A28 0.046 D7 0.49 OT5 0.75 A31 0.45 D12 0.23 OT8 0.71 A35 0.21 D16 0.082 OT9 0.73 A37 0.31 D17 0.17 OT12 0.81 A40 0.15 D20 0.098 OT18 0.82 A42 0.26 D21 0.13 OT19 0.59 B7 0.063 D22 0.11 OT20 0.60 B8 0.31 D27 0.064 OT22 0.85 C6 0.42 E45 0.047 OT23 0.76 C7 0.42 E49 0.043 C8 0.16 F1 0.46 C10 0.24 F3 0.43 C12 0.45 F6 0.52 C14 0.33 G6 0.46
samples. If sample OT1 is excluded fromFig. 3, the area of group I is greatly reduced, and samples C8, D7 and F6 are not included in group I. In other words, the relation between these three samples and group I depends entirely on sample OT1, and it can be concluded that this relation is very weak, or even does not exist at all.
It is of great interest to examine the hydrocarbon distribution in the southwestern part of the ECS since it is an important area proposed for transporting POM from the ECS to the SOT byLiu et al. (2000). In their conceptual model, the possible route of transporting sediment is from the Changjiang River mouth, southward along the
south-eastern coast of China, eastward across northern Taiwan Strait, along offshore northern Taiwan to the SOT. Considering the samples collected from the Changjiang River mouth to the SOT (22 samples on the shelf west of longitude 1231 and 12 in the SOT), it was found that only three samples (C8, D7 and F6) clustered with group I (OT samples) and the other 19 samples do not. This suggests that the hydrocarbon relation between the inner shelf and the SOT is actually weak, implying that the possibility of transporting hydrocarbons along the route of the con-ceptual model to SOT is very low. This result can be quite reliable because n-alkanes are relatively unreactive and
Table 3
Hydrocarbon concentrations (ng g1) for Okinawa Trough (OT) sediments (fromJeng and Huh, 2006)
Sample Aliphatic hydrocarbonsa
C15 C16 C17 Pr C18 Ph C19 C20 C21 OT1 1.94 29.3 97.6 170 110 45.9 116 85.1 87.5 OT3 – – 23.8 28.1 61.4 23.2 55.7 50.3 49.2 OT4 – – 42.7 49.4 81.1 26.2 96.1 75.4 82.4 OT5 – – 20.1 17.9 56.7 17.9 66.6 70.7 82.1 OT8 – 32.7 90.4 118 88.7 28.7 95.1 71.5 77.8 OT9 – – 18.3 17.1 55.7 15.8 89.5 83.1 95.9 OT12 80.3 63.8 103 147 89.1 32.0 102 65.5 64.7 OT18 10.7 42.9 103 101 103 36.3 79.5 73.8 73.1 OT19 11.2 32.2 67.2 89.8 65.5 20.5 52.2 47.7 56.5 OT20 10.8 28.2 52.7 34.1 52.0 15.7 40.2 28.6 36.0 OT22 – 14.5 80.6 113 101 37.9 104 78.8 84.5 OT23 – – 14.9 15.6 49.4 17.4 54.6 56.5 60.7 C22 C23 C24 C25 C26 C27 C28 Sq C29 OT1 96.4 137 114 171 111 241 165 217 418 OT3 53.2 70.6 65.0 98.6 54.8 136 87.6 178 253 OT4 88.7 125 98.6 156 96.7 243 168 199 473 OT5 86.7 122 97.3 149 95.4 220 157 271 382 OT8 82.3 119 96.8 150 96.3 221 145 161 421 OT9 108 135 116 156 97.6 214 139 317 392 OT12 65.1 103 97.6 160 102 240 176 184 457 OT18 72.3 107 77.7 148 79.7 214 149 230 451 OT19 69.8 94.7 73.5 110 73.0 168 88.3 100 319 OT20 41.2 99.0 77.3 178 94.8 346 121 99.0 690 OT22 86.6 125 99.6 145 101 236 141 28.2 433 OT23 64.4 94.7 78.6 133 76.4 195 127 171 359 C30 C31 C32 Di C33 C34 C35 UCM CPI OT1 195 564 150 450 273 65.9 167 8930 2.35 OT3 102 378 77.9 874 167 25.9 121 3410 2.82 OT4 202 634 178 1130 295 76.3 186 8080 2.46 OT5 148 561 162 363 280 72.0 179 4450 2.46 OT8 168 573 153 917 263 69.8 182 8760 2.53 OT9 161 574 131 1310 260 55.8 165 8790 2.61 OT12 172 616 121 586 278 49.1 203 7420 2.72 OT18 106 606 127 626 265 44.4 206 – 3.23 OT19 97.9 456 80.0 915 206 26.4 94.9 – 3.25 OT20 114 885 91.1 79.9 371 26.4 101 – 5.24 OT22 195 618 167 716 316 76.9 206 10700 2.53 OT23 147 549 115 1210 230 34.8 177 3540 2.81
aPr, pristine; Ph, phytane; Sq, squalene; Di, diploptene; UCM, unresolved complex mixtures; CPI, carbon preference index ¼ (1/2)[(nC
25+nC27+
unlikely to be affected by any mechanism that would be selective for alternating homologs in the sequence (Hedges and Prahl, 1993).
One marked feature is that groups II and III are, respectively, located in positive and negative PC1, signify-ing their difference in hydrocarbon distribution. This is supported by the literature result that Changjiang sedi-ments have been transported to the south via the coastal current along the Chinese coast and that offshore transport has been prevented by tidal currents and by the northward movement of the TWC (Milliman et al., 1989). Another reason may be that the inner shelf consists largely of muddy sediments, which are considered recent deposits, and the mid-to-outer shelf is composed chiefly of sandy sediments (Huh and Su, 1999), which are known as relict sediments (Niino and Emery, 1961). In addition, stations F1 and F3 located near the inner shelf have no relation with group II, which might be attributed to the influence of the TWC (Fig. 1). Furthermore, sample locations G6, F1 and F3 are very close to each other geographically (Fig. 1), but sample G6 separates clearly from samples F1 and F3 in the score plot (Fig. 3), suggesting that the deposition of Changjiang River sediments terminates at station G6.
As expected, groups I and III are separated from each other in Fig. 3, likely attributable to little material exchange between them. The KC flows northward and turns northeastward when approaching the ECS shelf although it intrudes onto the shelf in the winter and spring time (Chern and Wang, 1992;Hsueh et al., 1992).
Note that the SOT contains 12 stations within a relatively small area, but the hydrocarbon distribution of the samples varies widely. For example, the areas covered by groups I and III in Fig. 3 are comparable, but their respective geographical areas in the SOT and on the mid-to-outer shelf differ greatly (Fig. 1). The reason can be explained as follows. The lipid inputs by suspended matter/
sediment transport to the seas off northeastern Taiwan can derive from three sources (Fig. 1). From the northwest, a small alongshore flow just off northern Taiwan (south of station D27) along with the eddy flow (Tang et al., 1999) may carry materials from the southern ECS and Taiwan Strait to the SOT. The Lanyang River (from the west) exerts small influence on the SOT sediments as discussed earlier. Another source from south is other river runoffs from eastern Taiwan carried by the Kuroshio Current to the SOT.
Terrestrial higher plant wax contribution to sediment or soil is generally expressed by the CPI of n-alkanes. Higher CPI25–33values indicate greater contribution from vascular
plants (Rieley et al., 1991); CPI25–33values close to one are
thought to indicate greater input from marine microorgan-isms, recycled organic matter, and/or petroleum (Kennicutt et al., 1987). The present CPI25–33values of the ECS ranged
widely from 1.41 to 5.82 with an average of 3.12 (calculated from Table 1), suggesting non-uniform distribution of terrigenous hydrocarbons. In general, high CPI values were found on the inner shelf, and low values on the mid-to-outer shelf. This suggests that the Changjiang River contributes more to the former than to the latter, consistent with the literature result indicated earlier. On the other hand, the average CPI value (3.12, n ¼ 34) of the ECS was comparable to that (average 2.92, n ¼ 12) of the SOT. These two averages are considered to be relatively high, suggesting that fossil fuel contamination is minimal. However, it should be noted that CPI values would become smaller if there is any addition of ‘old’ hydro-carbons (e.g., CPI of petroleum close to 1;Bray and Evans, 1961) to the sediments from pollution. Some samples were found to contain different proportions of ‘old’ hydro-carbons as evidenced by the presence of 17(a)H,21(b)H-hopanes. This fossil component could be contributed from fossil fuels through contamination (Albaige´s and Albrecht, -8 -6 -4 -2 0 2 4 6 8 -8 -6 -4 -2 0 2 4 6 8 -- axis PC-1 (47 %) --> -- axis PC-2 (27 %) --> III I II
Fig. 3. Score plot of PCA for ECS and SOT sediments. Sample codes for ECS samples have prefixes of A–G, and those for SOT samples have a prefix of OT. Group I contains 12 SOT sediments and three sediments on the inner shelf, group II consists of eight but one sediments on the inner shelf, and group III is composed of 21 sediments mainly on the mid-to-outer shelf.
1979;Shaw et al., 1985;Readman et al., 1996) from boat traffic or a spill. The addition of ‘old’ hydrocarbons will lower some CPI values observed in ECS sediments. The purpose of this study is to compare hydrocarbon distribu-tion between ECS and SOT and to explore if they are related. For comparing the two areas, eight samples (C14, C12, C10, C8, C7, C6, D7 and F6;Fig. 1) on the ECS inner shelf and 12 samples in the SOT were chosen; their average CPIs were, respectively, 4.5670.93 and 2.9270.79, sug-gesting that the hydrocarbon sources to the two areas were quite different. The result is quite dependable because the CPI is a diagenetically insensitive ratio (Hedges and Prahl, 1993), implying that the CPI would not change over time during transport.
The ratio of terrigenous-to-aquatic n-alkanes were estimated as
TAR ¼ ðnC27þnC29þnC31Þ=ðnC15þnC17þnC19Þ.
This ratio is valuable for determining changes in relative contributions of organic matter from land and aquatic flora although it may over-represent the absolute amounts from terrigenous sources (Meyers, 1997). This ratio was adopted in the present study to estimate the relative contribution of terrigenous and marine sources to ECS and SOT sediments. The average TAR for the ECS inner shelf was 50.4758.0 (n ¼ 8), and that for the SOT was 9.6474.33 (n ¼ 12) (calculated from Tables 1 to 3). This suggests that the two areas receive different hydrocarbon inputs. The TAR values are expected to be larger for SOT sediments than for ECS sediments (since the SOT is much deeper than the ECS and since aquatic components degrade faster) if they have the same source. The present result is reversed and suggests the ECS and SOT having different sources. Note that this ratio is one of the factors used for comparing the differences of the multiple study sites. Higher plant n-alkanes were the predominant components in the sediments from the ECS and SOT; to correct for the grain size effect, (nC27+nC29+nC31)/TOC
was used to estimate the input of higher plant n-alkanes to the two areas (Prahl, 1985). The SOT and ECS inner shelf gave (nC27+nC29+nC31)/TOC averages of 17317482
107g(g C)1 (n ¼ 12) and 30167868 107g (g C)1 (n ¼ 8) (calculated from Tables 1 to 3), respectively, indicating different inputs of higher plant n-alkanes to the two areas. In the HMW region, the C27 and C29
n-alkanes are diagnostic of waxes from trees and shrubs, and the n-C31 alkane is diagnostic of grass waxes
(Cranwell, 1973). The nC31/(nC27+nC29+nC31) ratio was
employed to see if any difference existed between the two areas. The average ratios of nC31/(nC27+nC29+nC31) for
the SOT and ECS inner shelf were, respectively, 0.47870.012 (n ¼ 12) and 0.48370.018 (n ¼ 8) (calculated from Tables 1 to 3), showing no difference between the two averages. This can be attributed to n-alkane distribu-tion maximizing at C31, which makes the change in C31
insensitive.
Seven stations (not shown) between Taiwan and 12 SOT sample sites were selected for collecting total suspended matter (2 m below sea surface) on July 23–26, 2000 (cruise #590). The average CPI of the seven samples was 1.0170.12. This low CPI average reflects the contribution from a nearby river (Lanyang River), which is known to contain ‘‘old’’ hydrocarbons in its suspended matter and sediments (Jeng and Kao, 2002; Jeng and Huh, 2006). In addition, one of our cruises (#525 on August 17–24, 1998) to the ECS happened to be the high runoff period of the Changjiang River. The runoff of the river had a very different color than the offshore seawater of the ECS. Two stations—C14 and C15 (31119.960N, 122130.090E, not
shown) representing the runoff of the river were occupied, and one sample of total suspended matter (2 m below sea surface) for each station was collected. CPI values of the two stations were 1.55 and 1.32, which differed greatly from those of off northeast Taiwan. These results indicate that transport of suspended sediment from the Changjiang River mouth to the SOT is quite unlikely.
In conclusion, most sediments on the ECS inner shelf did not group with those in the SOT, indicating dissim-ilarity in hydrocarbon distribution between the two areas. Moreover, striking differences in CPI, TAR and (nC27+nC29+nC31)/TOC values were found between SOT
and ECS inner shelf. In addition, hydrocarbon distribution differences in suspended matter from Changjiang River runoff and off northeastern Taiwan were also observed, suggesting that hydrocarbon sources to the two areas were different. Based on these results, it can be concluded that hydrocarbon contribution from the ECS inner shelf to the SOT appears to be minimal.
Acknowledgments
Our special thanks go to the captain, crew and technicians of the R/V Ocean Researcher I for help with sediment collection. We are grateful to Prof. Philip A Meyers (University of Michigan) and one anonymous reviewer for constructive comments and suggestions. This study was supported by a National Science Council Grant (NSC88-2611-M-002-017-K2) of the Republic of China.
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