Distribution of terrigenous lipids in marine sediments off
northeastern Taiwan
Woei-Lih Jeng
a,*, SaulwoodLin
a, Shuh-Ji Kao
ba
Institute of Oceanography, National Taiwan University, Taipei, Taiwan, ROC
b
Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan, ROC Received12 December 2002
Abstract
Surface sediments on the continental margin off northeastern Taiwan have been analyzed for terrigenous lipids including n-alkanes, n-fatty alcohols, andsterols. Marine input to the sediments is particularly low basedon the average n-C17/n-C29alkane and n-C16/n-C28fatty alcohol ratios, 0.1570.13 and0.1370.06, respectively; this may be due to the fact that marine lipids are more prone to degradation than terrestrial ones. The study area has the highest plant wax n-alkane contribution (average carbon preference index 3.971.2) among the coastal marine areas surrounding Taiwan; lateral particle transport from the southern East China Sea shelf andriver runoff from the east Taiwan coast are considered to be the major contributors. The distributions of plant wax n-alkane and n-alkanol concentrations normalizedto total organic carbon (TOC) in the study area generally show maximum values on the upper slope of the southernmost Okinawa Trough, but not for phytosterols. Linear regression of TOC versus plant wax n-alkane concentrations show a weak relationship (r ¼ 0:64; p ¼ 0:001), andan even weaker relationship (r ¼ 0:42; p ¼ 0:05) between TOC andplant wax n-fatty alcohol concentrations is found. This could be attributed to several factors: (1) a complex input (not a point source) of terrigenous organic matter to the study area, (2) TOC also including marine organic matter, (3) temporal variations in river flow due to flooding, and (4) different rates of degradation for TOC and individual biomarkers. However, in spite of those factors, TOC and phytosterol concentrations are positively linearly correlated(r ¼ 0:85; po0:001), implying that the dilution of phytosterols in terrigenous organic carbon with marine organic carbon with or without the phytosterols follows a nearly constant ratio, which is remarkable. In addition, the predominant source of diploptene in the sediments does not appear to be of higher plant origin.
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1. Introduction
Hydrocarbon distribution as indicators of source matter identification has been an important anduseful tool in organic geochemistry studies. In general, biomarkers for higher plants can be found
in most continental shelf sediments. For instance, n-alkanes in marine sediments that show an odd carbon number predominance in the n-C25—n-C35
region indicate a contribution from land sources (Gearing et al., 1976;Farrington andTripp, 1977;
Keizer et al., 1978;Jeng, 1984). Higher plant waxes tendto contain mainly longer-chain (>C22)
saturatedalcohols (Eglinton andHamilton, 1963, 1967; Kolattukudy, 1970; Tulloch, 1976).
*Corresponding author. Fax: 886-2-23626092. E-mail address:wljeng@oc.ntu.edu.tw (W.-L. Jeng).
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Therefore, longer-chain alcohols also have been usedcommonly as higher-plant markers in sedi-ments (Cranwell, 1984; Volkman et al., 1987). However, in areas of higher productivity, terrige-nous biomarkers are less abundant than marine biomarkers. As an example, Smith et al. (1983)
reportedthat a marine sediment taken from the Peru continental shelf showeda larger contribu-tion from marine sources (probably mainly from phytoplankton andbacteria) andlittle terrigenous influence. Campesterol, stigmasterol and b-sitos-terol are the three common phytosb-sitos-terols generally foundin epicuticular waxes of vascular plants (Scheuer, 1973; Weete, 1976; Wannigama et al., 1981), andare normally consideredto be higher-plant input to sediments (Gagosian et al., 1983;
Shaw andJohns, 1985). However, C29sterols also
couldoriginate from marine organisms such as certain microalgae andperhaps cyanobacteria (Volkman, 1986).
Huang andMeinschein (1976)have shown that the % composition of cholesterol increases as the % composition of b-sitosterol decreases in the sample sequence of river inlet sediments—bay sediments—Gulf sediments—bay and Gulf plank-ton. Calculations basedon the sterol ratios gave the percent of the sterols in the Gulf sediments derived from terrigenous sources.Lee et al. (1979)
observeda decrease in sterol contents seawardin the sediments collected (farther east of the SEEP sampling locations) in the Atlantic Ocean, and concluded that part of the sterols in the slope sediments originate from terrigenous debris.
For the transport of organic matter from the landto the sea, organic geochemical analyses of benthic sediments from a Northern Queensland (Australia) coastal transect indicated that most terrestrial organic material is confinedto near-shore sediments (less than 10 km offnear-shore) (Currie andJohns, 1989). For the transport on the continental margin, Venkatesan et al. (1987)
examinedthe lipidfraction in six sediment cores collectedfrom the Atlantic shelf, slope andthe rise areas, andfoundthat the concentrations of organic compoundclasses decreasedfrom the shelf through the slope to the rise, andestimated that about 50% of the shelf organic matter is exportedto the slope. Similar results were also
obtainedfor sediments from the continental shelf south of New England(Venkatesan et al., 1988). In general, the terrestrial component of the organic matter in unconsolidated sediments de-creases progressively offshore, but this is not so on the Scotian margin (Canada) where organic matter with the greatest marine input is foundin inshore basins (Pocklington et al., 1991).
Lin et al. (1992)investigatedthe organic carbon content in the continental margin sediments off northeastern Taiwan andfoundthat lowest organic carbon concentrations were foundin the shelf sediments while high concentrations were observedin the upper continental slope sediments.
Chen et al. (1992) studying the composition and texture of surface sediments off northeastern Taiwan concluded that in the Okinawa Trough the sediments beneath the path of the main Kuroshio flow are composedof non-biogenic, fined-grain mud.
The circulation patterns of the seas off north-eastern Taiwan have been given elsewhere (Hsu et al., 1998). Briefly, the Kuroshio Current flows northwardalong the east coast of Taiwan. Its subsurface water shoals up off the northeast coast andturns northeastwardalong the East China Sea continental slope. The inputs of terrigenous organic matter to this area are rather complex— from the northwest andsouth together with the runoff of the Lan-yang River. In order to examine the influence of high river flow andthe Kuroshio Current on the lipidandorganic carbon distribu-tions in this area, n-alkanes, n-fatty alcohols and sterols in the sediments were analyzed to determine terrestrial lipiddistribution andcontribution to this area andits relation to organic carbon, which can provide important information for under-standing carbon budget.
2. Materials and methods
Samples were collectedon the continental margin off northeastern Taiwan using a box corer on cruises ORI-417 (April 24–May 1, 1995) and ORI-456 (July 6–12, 1996). A total of 23 box core sediments were collected, and sample locations are given in Table 1. Surface sediments (top 3–5 cm
from subcores) were individually analyzed. Sedi-ment samples were freeze-dried. After adding internal standards (n-C24D50and1-heptadecanol),
sediment samples were extracted with a mixture of benzene andmethanol (1:1, v/v) for 24 h in a Soxhlet apparatus. The lipidextract was then saponifiedby reflux for 3 h with 0.5 N KOH solution in methanol. The non-saponifiable lipids were isolatedby hexane extraction four times and concentratedusing N2 gas. Aliphatic
hydrocar-bons andthe fatty alcohols andsterols containing fraction were isolatedfrom the neutral lipids by silica gel (deactivated with 5% H2O) column
chromatography using hexane anda mixture of dichloromethane/methanol (4/1, v/v), respectively. The lipids between the two fractions were eluted with hexane/dichloromethane (2/3, v/v) and dis-carded. The isolated alcohols and sterols were taken to dryness, redissolved in benzene, and derivatized with N,O-bis(trimethylsilyl)acetamide. For gas chromatography (GC) analysis, an HP
5890 gas chromatograph equippedwith a split/ splitless injector anda flame ionization detector (FID) was used. An SGE (Australia) OCI-5 cool on-column injector also was fittedin the gas chromatograph for quantitation. Separation of aliphatic hydrocarbons was achieved on an SPB-1 capillary column (30 m 0.25 mm i.d.), and that of alkanols/sterols (as TMS ethers) was accomplished by another SPB-1 column (25 m 0.25 mm i.d.). Oven-temperature programming was 45–90C at
15C/min and90–270C at 3C/min for analyzing
aliphatic hydrocarbons, and 45–90C at 15C/min
and90–250C at 3C/min, 30 min at 250C for
analyzing alkanols/sterols. Identification was made with co-injection of authentic standards andgas chromatography–mass spectrometry (GC–MS). The GC–MS analyses were performed with an HP 6890 GC (HP-1MS crosslinkedmethyl siloxane column, 30 m 0.25 mm i.d.) interfaced directly to an HP 5973 quadruple mass selective detector (electron impact, electron energy 70 eV, scannedfrom 50 to 550 D). Basedon replicate analyses by GC–FID, the analytical precision of lipids was calculated to be 2–8%.
Organic carbon was analyzedusing infrared determination of CO2evolvedfrom high
tempera-ture (1400C) combustion with a LECO SC-444
carbon/sulfur analyzer. Approximately 0.25 g of dry sediment was pre-acidified withB2 ml of 6 N HCl to remove carbonate carbon. After drying on a hot-plate for 8 h at B60C, the sample was combustedin a LECO carbon analyzer to measure its organic carbon content. The LECO carbon standard (502-062) was used for calibration. The average analytical precision was 0.88% (1 s.d.).
3. Results and discussion
3.1. Terrestrial lipid contribution and distribution Gas chromatograms of aliphatic hydrocarbons in the study area show a minor unresolved complex mixture (UCM) (Fig. 1), suggesting some input from petroleum or biodegradation sources (Brassell andEglinton, 1980) or from eroded hydrocarbons. In addition, it is noted that the lower molecular weight region (oC23) has a CPI
Table 1
Sampling locations off northeastern Taiwan
Station Latitude (N) Longitude (E) Water depth (m) ORI-417 5 2450.180 12249.080 1522 8 2500.970 12230.840 1438 9 2459.440 12240.240 1465 10 2500.170 12250.160 1518 16 2511.350 12230.880 894 20 2501.590 12218.030 1096 21 2445.320 12222.570 1115 27 2546.130 12307.080 118 34 2439.910 12230.080 462 36 2430.570 12220.70 542 37 2430.180 12212.220 404 42 2531.060 12303.770 805 43 2518.59 12300.510 1360 44 2515.120 12252.100 1465 45 2506.150 12248.250 1526 46 2450.090 12300.120 1665 ORI-456 27 2533.880 12236.720 887 33 2513.940 12234.960 911 34 2522.280 12225.150 451 35 2531.860 12214.980 117 37 2511.930 12207.990 156 39 2449.930 12208.040 612 K 2530.970 12250.980 1211
of 0.99, possibly indicating a contribution from fossil fuels. As shown in Fig. 2, mass chromato-grams of characteristic ions m=e 85, 191, and217 illustrate the distribution of alkanes, triterpanes andsteranes, respectively. For pentacyclic triter-panes (m=e 191), besides diploptene, two com-pounds cannot be unequivocally identified from mass spectra that are similar to those of lupane andoleanane. Sterane (m=e 217) characteristic ions are of very low intensity, andno steranes were found. In addition, generally there were no low-molecular-weight (p3 aromatic rings) polycyclic aromatic hydrocarbons (PAHs) along with their alkylatedhomologues foundin the sediments, nor were high-molecular-weight (X4 aromatic rings) parental (non-alkylated) PAH compounds found. This result indicates probably lack of fossil fuels and their combustion products in the sediments. However, there was one predominant PAH— perylene found(Fig. 3; Table 2), which is con-sidered to be derived from diagenesis (Venkatesan, 1988). For instance, a down-core increase of perylene concentrations (191, 311, 388, 514, 629, 542 ng/g, each section being 4 cm) was observedin a box core taken from station 417-20, reflecting transformation from unknown natural precursors
(Louda and Baker, 1984). From the above results, it can be concluded that the study area is considered to have minimal contamination by hydrocarbons related to petroleum.
The sediments have predominantly n-C25, n-C27,
n-C29, n-C31 and n-C33 paraffins, typical of
terrestrial plant waxes (Eglinton andHamilton, 1967), and a unimodal distribution having a maximum at C31(the abundance order n-C25
on-C27on-C29on-C31). In comparison, the n-alkane
profiles of the Changjiang estuary sediments exhibit a bimodal distribution with the first maximum at n-C31 andthe secondmaximum at
n-C17or other lower molecular weight (on-C23)
n-alkanes (Bigot et al., 1990). The common feature of these two areas is the n-C31 maximum. The
major n-alkanes foundin marine phytoplankton (Blumer et al., 1971), benthic algae (Youngblood et al., 1971), andpelagic Sargassum (Burns and Teal, 1973) are n-C15and n-C17, which are present
in only small amounts in the sedimentary n-alkanes of this area. The ratio of n-C17/n-C29 was
employedby Venkatesan et al. (1987) as a parameter to indicate marine vs. terrigenous input of n-alkanes; this ratio in the 23 samples examined has a wide range of 0.00–0.31 with a mean of
Fig. 1. Gas chromatogram of aliphatic hydrocarbons from station 456-37. Numbers above the peaks refer to carbon number of n-alkanes; Pr=pristane; Ph=phytane; Sq=squalene; Di=diploptene; UCM=unresolved complex mixture.
Fig. 2. Specific ion plots for m=e 85 (alkanes), 191 (triterpanes), and217 (steranes) from the GC–MS analysis of the aliphatic fraction from station 456-37.
0.1570.13 (Table 2). Two surficial sediments with a low level of contamination from the Changjiang estuary andadjacent East China Sea have n-C17
/n-C29 ratios of 0.20 and0.60 (calculatedfrom
histograms given by Bigot et al., 1990). The ratio in the suspended particles from surface waters of the Changjiang has an average of 0.3370.18 (n=13, calculatedfrom n-alkane concentrations given by Sicre et al., 1993). Surface sediments (0–10 cm) on the Amazon shelf have a mean n-C17/
n-C29 ratio of 0.38 (six values being 0.23, 0.33,
0.37, 0.39, 0.41 and0.57 calculatedfrom histo-grams given by Elias andCardoso, 1996). Two sediment core sections (20–40 and 40–60 cm) from the Bering Sea have the n-C17/n-C29 ratio of 0.26
and0.59 (calculatedfrom the peak heights of GC traces given byKennicutt et al., 1991). One surface sediment from the Bellingshausen Sea, Antarctica has a n-C17/n-C29 ratio of 0.80 (calculatedfrom
histogram given by Cripps, 1995). The ratio for sediments from southeast Florida is 6.68 (reef track), 10.26 (Hawk Channel), 2.54 (nearshore Atlantic), 0.46 (bay), and4.15 (ABC channel) (calculatedfrom data given by Snedaker et al., 1995). The top 1 cm sediment from the middle Black Sea has a n-C17/n-C29 ratio of 0.09
(calculatedfrom data given by Wakeham, 1996).
For our samples, the ratio is apparently at the low end; this can probably be attributed to (1) the predominance of the terrestrial n-alkane contribu-tion to the coastal marine sediments, and (2) preferential degradation of marine-derived n-alkanes relative to terrigenous n-n-alkanes (Prahl et al., 1980;Meyers et al., 1984).
To compare with other marine sediments surrounding Taiwan, another parameter is used. Landplant n-alkane contribution to marine sediments is often expressed as carbon preference index, (CPI). Compared with the CPIs of northern andsouthern Taiwan Strait surface sediments and eastern Taiwan shelf (ca. 1–20 km in width) sediments, 2.0, 1.7 and 2.3, respectively (Jeng, 1978, 1979, 1984), the average CPI value of the study area is relatively high, 3.8871.16 (Table 2) indicating a strong terrestrial n-alkane influence. A similar index for higher plant wax contribution to marine sediments is odd–even predominance (OEP) proposedby Scalan andSmith (1970)as a means of reducing the mathematical limitations of the CPI. The Mackenzie River is regarded as providing a homogeneous, well-defined terrige-nous input of n-alkanes to the Mackenzie shelf (Yunker et al., 1991). Five OEP C29 ratios for
Mackenzie shelf (Beaufort Sea) sediments are 2.20,
2.18, 2.11, 3.14 and4.28 (Yunker et al., 1993), with a mean of 2.78. The average of our OEP C29ratios
is 3.7771.09 (Table 2). The difference could be due to stronger terrigenous n-alkane influence in the study area or difference in n-alkane max-imum—C31 for the study area and C29 for the
Mackenzie shelf.
Several factors can account for this interesting result of high landplant n-alkane inputs to the study area. The most important one is that shorter-chain alkanes, such as n-C17, are more
prone to degradation than longer-chain n-alkanes.
Second, a part of the study area is a region with comparatively high sedimentation rates around Taiwan (Hung andChung, 1994; Chung and Chang, 1995); this tends to create a reducing environment, especially on the upper slope (e.g., mottledandfilamentedauthigenic pyrite has been foundbyChen et al., 1995), which is favorable for preserving organic matter. Third, suspended par-ticulates in runoff of other rivers on the eastern coast carriedby the Kuroshio Current presumably tendto be depositedin the southern part of the study area. Table 2 Hydrocarbon data Sample TOC (102g/g) n-C17 (ng/g) n-C29 (ng/g) n-C17/n-C29Sn-C2533 (ng/g) (Sn-C2533/ TOC) 107 CPIa& OEPb Diploptene (ng/g) Perylene (ng/g) ORI-417 5(0–5 cm) 1.01 95 467 0.20 1820 1800 2.61/2.62 1140 n.d.c 8(0–3 cm) 1.05 108 509 0.21 1970 1880 2.54/2.40 2020 n.d. 9(0–3 cm) 1.09 113 439 0.26 1760 1610 2.49/2.27 518 151 10(0–3 cm) 1.00 10 439 0.02 1690 1690 2.60/2.50 530 n.d. 16(0–5 cm) 0.82 n.d. 394 — 1640 2000 3.47/3.35 169 157 20(0–4 cm) 0.63 32 691 0.05 2450 3880 6.06/5.27 42 191 21(0–4 cm) 0.77 222 418 0.53 1630 2110 2.81/2.76 433 118 27 0.16 1 57 0.02 208 1300 5.49/5.55 10 31 34(0–4 cm) 0.69 32 731 0.04 2620 3800 5.24/5.21 29 740 36(0–4 cm) 0.42 51 502 0.01 1870 4450 5.76/5.69 37 124 37(0–4 cm) 0.29 17 91 0.19 358 1230 3.17/3.46 9 n.d. 42(0–4 cm) 0.41 51 154 0.33 581 1420 3.82/4.24 32 24 43(0–4 cm) 0.73 8 288 0.03 1090 1500 3.69/3.50 182 86 44(0–4 cm) 0.84 84 378 0.24 1460 1940 3.34/3.27 303 102 45 0.87 58 380 0.15 1470 1690 3.23/3.16 529 155 46(0–4 cm) 0.79 59 335 0.18 1300 1650 3.83/3.86 533 204 ORI–456 27 0.74 27 784 0.03 2150 1910 5.17/4.69 92 n.d. 33(0–4 cm) 0.38 n.d. 177 — 552 1450 4.17/3.90 70 n.d. 34(0–4 cm) 0.74 n.d. 827 — 3000 4050 5.91/5.54 74 n.d. 35 0.14 3 43 0.07 135 964 4.13/4.24 15 n.d. 37 0.36 11 341 0.03 998 2770 3.19/3.03 37 n.d. 39(0–4 cm) 0.71 59 477 0.12 1140 1610 3.19/3.14 176 n.d. K(0–4 cm) 0.60 62 252 0.25 763 1270 3.22/2.95 205 n.d. Average 0.15 3.88/3.77 s.d.(1s) 0.13 1.16/1.09 a
Carbon preference index ¼1 2 nC25þ nC27þ nC29þ nC31þ nC33 nC24þ nC26þ nC28þ nC30þ nC32 þnC25þ nC27þ nC29þ nC31þ nC33 nC26þ nC28þ nC30þ nC32þ nC34 : b
odd even predominance n-C29ratio=(n-C27+6n-C29+n-C31)/(4n-C28+4n-C30). c
Prior to discussing the distribution of lipid data, it is advantageous to know the nearby river flow andpredominant ocean currents. The inputs of terrigenous organic matter to the study area are rather complex—from the northwest andsouth together with the runoff of the Lan-yang River (Fig. 4). A small alongshore flow just off north Taiwan may carry materials from the southern East China Sea andTaiwan Strait (west of Taiwan) to this area (arrow 1). Contribution of materials from the Lan-yang River runoff is apparent (arrow 2). Another source is other river runoffs from eastern Taiwan carriedby the Kuroshio Current to the study area (arrow 3). Their respective contributions of terrigenous lipids
to this area will be comparedin the following discussion.
In coastal regions receiving predominantly detrital input of organic material essentially from a point source, plant wax n-alkane concentrations normalizedto TOC show a general decrease with distance offshore, e.g., Washington State, USA (Prahl andCarpenter, 1984). The trendcan be attributedto dilution of the biomarkers in terrestrial organic carbon, with an increasing proportion of marine organic carbon containing none of the terrestrial n-alkanes. The spatial distribution of land plant derived n-alkanes (sum of n-C25, n-C27, n-C29, n-C31and n-C33) normalized
to TOC (to eliminate the grain size effect) is given
Fig. 4. Map showing river inputs andocean currents in the study area. Arrow 1: A small alongshore flow just off north Taiwan carrying materials from the southern East China Sea andTaiwan Strait (west of Taiwan) to the study area. Arrow 2: Direct inputs of terrigenous materials from the Lan-yang River. Arrow 3: Other river runoffs from eastern Taiwan carriedby the Kuroshio Current to the study area.
in Fig. 5. The major distribution features are that (1) the westernmost four stations in the study area have low values, (2) the four stations with highest values are on the upper slope (shown in solid squares) near Taiwan, and(3) the stations on the lower slope show low values. The distribution suggests a major terrestrial contribution of n-alkanes to the area from Taiwan. This is supported by the fact that the immediate shelf north of the study area has no net deposition. For instance, we have collectedsix other surface sediments (located from 120100E to 123E andfrom 25400N to
25500N) from the immediate shelf area and have
foundthat the samples contain shell debris and have low concentrations of mud(0.03–0.77%).
Further, the Keelung shelf offshore northern Taiwan exhibits a rocky bottom with sparsely scattered sandy sediments. These facts suggest that the adjacent shelf has little sediment for export to the study area. However, three sediment-trap moorings deployed in the canyon located between the two highest values of plant wax n-alkanes normalizedto TOC stations (Fig. 5) in the north have been shown to have high apparent mass fluxes, 5–72 gm2d1 (Hung andChung, 1998). The particle flux couldresult from a combined flow of an anticyclonic eddy (whose center located at 122350E, 25200N, details given byTang et al.,
1999) anda southeastwardfilament (a small alongshore flow) occasionally appearing
where between the eddy and northern Taiwan (Chern et al., 1990). This suggests that the major mode of sediment transport is suspended particle flow from the southern East China Sea continental shelf. Another possible contribution to this area (particularly the two highest stations in the south) couldbe the deposition of particles carriedby the northboundKuroshio Current from the south.
For better illustration of terrigenous n-alkane distribution, CPI is used because it is a non-mass dependent and dimensionless quantity. It is used to indicate the degree of diagenesis of straight-chain geolipids, and is a numerical representation of how much of the original biological chain-length specificity is preservedin geological lipids
(Meyers andIshiwatari, 1995). Since all samples examined are surface sediment, the difference in CPIs shouldreflect more different input sources than diagenetic change. As shown in Fig. 6, it is notedthat three northernmost stations exhibit high CPIs, which are not seen in Fig. 5. Sample 456-37 (at the northwest corner of the study area), locatedon the Keelung shelf, has been investigated for grain size effect on CPI. The unfractionated sample has a CPI value of 3.19, andtheo30 mesh fraction has a CPI value of 5.30. This is not in accordwith the general observation that CPIs decrease as particle-size decreases (Thompson and Eglinton, 1978;Meyers andIshiwatari, 1995). It is thought that the larger particle size (>30 mesh)
fraction is made up of eroded older (relict) sediment containing degraded lipids with low CPIs andthat the smaller particle size (o30 mesh) fraction contains recent sediment with high CPIs. One important feature is that the three stations nearest to Taiwan exhibit low CPIs (3.19, 3.19 and 3.17) andthat the highest CPIs are farther from shore. This does not seem to be reasonable if the sediment source is Taiwan.
Geographically, the Lan-yang River, which discharges its sediment (ca. 8.0 Mt/yr, Water Resources Bureau, 1998) directly into the study area, plays a major role in the distribution of terrigenous lipids in the study area, and the CPI signature of its suspended sediments is expected to reflect that of marine sediments. Three samples of total suspended matter from the river have CPIs of 1.38 (Fig. 7), 1.43 and1.46, which are lower than those of the marine sediments. The result is consistent with a recent report that modern carbon in the POC discharged from the main channel of the river waso30% (Kao andLiu, 1996). In other words, the particulates in the river water are mainly oldmaterial, which tends to have lower CPIs. This leads us to consider that, on the two-endmember assumption, the marine sediments examinedare a mixture of two sources—high CPI sediment from either northwest or south of the study area and low CPI sediment from the nearby Lan-yang River. Those samples closest to Taiwan wouldhave much higher CPIs if there were no input from the Lan-yang River. However, com-paratively lower CPIs for the lower slope sedi-ments can be attributedto contribution of old organic carbon from the Lan-yang River.
Terrestrial hydrocarbons (for example, the plant wax alkanes andretene) in Washington shelf sediments have been found to be highly concen-trated in the sand-sized, low-density fraction (particleso1.9 g/ml) of bulk sediments (Prahl and Carpenter, 1983). However, the sediments off northeastern Taiwan have somewhat high concen-trations of plant wax alkanes, but have compara-tively low sandcontents except some stations locatedin the north. For instance, two samples taken from this area (lower slope) have sand contents of 2.2% and2.5% (Jeng andChen, 1995). Deploying ten moorings for sediment traps in the
northern half of our study area,Chung andHung (2000) foundthat the major components of the trap samples in the canyon were silt andsand, but those on the slope were mainly silt. This difference can be attributed to different depositional envir-onments. The low sandpercentage couldbe due to the effect of the Kuroshio Current which turns northeastward in the study area. Coarse suspended particles from the East China Sea shelf are blocked by the Kuroshio andnot depositedon the immediate shelf off north Taiwan, and fine particles cross the shelf andsettle on the slope of the southern Okinawa Trough (Chen et al., 1992). The distribution of n-fatty alcohols shows a general distribution pattern maximizing at C22(for
the samples on the lower slope in particular,
Fig. 8) or >C22. Cranwell (1981) suggests that
decomposer organisms may be the source of the n-C22 alcohol present in the sediments; however,
Jeng andChen (1995), studying the grain size effect, show that bacteria do not seem to be the source of the C22 alcohol. Recent study indicates
that the C22 alcohol can be contributedfrom
freshwater andmarine sources (Volkman et al., 1998; andreferences therein). Lan-yang River suspended matter has an n-alkanol distribution with a maximum at C22(Fig. 7), which may play a
role in the marine sediments. The low-molecular-weight alkanols (n-C14–n-C22), which may be
formed by hydrolysis of esterified alcohols derived from a wide variety of organisms, such as zooplankton (Boon andde Leeuw, 1979), are considered to be from marine sources. The major alcohols of zooplankton are C16:0, C20:1, andC22:1
(Kattner andKrause, 1989). Higher plants gen-erally have their n-alkanol distribution >C22
(Kolattukudy, 1970; Wannigama et al., 1981). Linear regression between n-C22OH andother
biomarker n-alkanols (Table 4) shows that the relationship is better correlatedfor C20, C24, C26,
andC28than for C14, C16, andC18n-alkanols. This
result appears to indicate that the sediments have receivedmore n-C22OH from terrigenous sources
than from marine sources. In addition, Lan-yang River suspended matter also contains a high concentration of phytol (Fig. 7), which may contribute to the marine sediments. Linear regres-sion of phytol against other biomarker n-alkanols
Fig. 7. (upper) Gas chromatogram of the aliphatic fraction from the Lan-yang River suspended matter. Numbers above peaks refer to carbon number of n-alkanes; Sq=squalene; I.S.=internal standard (n-C24D50). CPI2533=1.38. (lower) Gas chromatogram of the
alkanol/sterol fraction from the Lan-yang River suspended matter. Numbers above peaks refer to carbon number of n-alkanols; I.S.=internal standard (n-C17OH).
(Table 5) shows that phytol is better correlated with C14, C16, C18, andC20than with C24, C26, and
C28 n-alkanols, indicating that the sedimentary
phytol receives more contribution from marine sources.
The ratio of n-C16OH to n-C28OH was usedby
Venkatesan et al. (1987) as an index to show marine vs. terrigenous input of n-alkanols to the sediments. The ratios of the sediments range from 0.03 to 0.24 (with a single outlier of 3.70) with a mean of 0.1370.06 (Table 3), indicating the predominance of higher plant n-alkanols in the area, which is in agreement with the n-alkane distribution. To show the spatial distribution of terrestrial alkanols in the area, the sum of n-C24OH+n-C26OH+n-C28OH normalizedto TOC
was calculated(Table 3) andshown inFig. 9. The spatial distribution pattern of n-fatty alcohols
matches well with that of n-alkanes with respect to the occurrence of high values (shown in solid squares).
Three phytosterols—campesterol, stigmasterol, and b-sitosterol—are generally foundin epicuti-cular waxes of vasepicuti-cular plants (Scheuer, 1973;
Weete, 1976). They are a major part of the sterol fraction in the sediments analyzed (Fig. 8) and account for, on average, 24.3% of the total sterols (Table 6). The spatial distribution of the phytos-terol concentrations normalizedto TOC (Fig. 10) is totally different from those of alkanes and n-alkanols. The four stations with the highest values for n-alkanes and n-alkanols do not exhibit the highest values for phytosterols. Instead, the high-est value spot is locatedjust off northeastern Taiwan. It is seen from Fig. 10 that phytosterols are randomly distributed. The cause of this
Fig. 8. Gas chromatogram of the alkanol/sterol fraction for sample 417-45. Numbers above peaks refer to carbon number of n-alkanols; I.S.=internal standard (n-C17OH). Letters above peaks refer to sterols: (A) 24-nor-5a-cholest-22E-en-3b-ol, (B)
27-nor-24-methylcholesta-5,22E-dien-3b-ol, (C) 27-nor-24-methyl-5a-cholest-22E-dien-3b-ol, (D) cholesta-5,22E-dien-3b-ol (22-dehydrocholes-terol), (E) 5a-cholest-22E-en-3b-ol, (F) cholest-5-en-3b-ol (choles(22-dehydrocholes-terol), (G) 5a-cholestan-3b-ol (cholestanol), (H) 24-methylcholesta-5,22E-dien-3b-ol (diatomsterol), (I) 24-methyl-5a-cholest-22E-en-3b-ol, (J) 24-methylcholesta-5,24(28)-dien-3b-ol (24-methylenecho-lesterol), (K) 24-methylcholest-5-en-3b-ol (campesterol)+24-methyl-5a-cholest-24(28)-en-3b-ol(?), (L) 24-methyl-5a-cholestan-3b-ol, (M) 23,24-dimethylcholesta-5,22E-dien-3b-ol, (N) 24-ethylcholesta-5,22E-dien-3b-ol (stigmasterol), (O) 24-ethyl-5a-cholest-22E-en-3b-ol, (P) 24-ethylcholest-5-en-3b-ol (sitosterol), (Q) 24-ethyl-5a-cholestan-3b-ol+unknown, and(R) 4a,23,24-trimethyl-5a-cholest-22E-en-3b-ol (dinosterol). Dipl.=diplopterol.
Table 3
Fatty alcohol data
Sample TOC (102g/g) Phytol (ng/g) n-C16OH (ng/g) n-C22OH (ng/g) n-C28OH (ng/g) n-C16OH/ n-C28OH Sn-C24,26,28 (ng/g) (Sn-C24,26,28/ TOC) 107 ORI-417 5(0–5 cm) 1.01 981 96 695 682 0.14 1770 1750 8(0–3 cm) 1.05 1740 215 880 881 0.24 2220 2120 9(0–3 cm) 1.09 1100 139 489 632 0.22 1650 1520 10(0–3 cm) 1.00 1110 148 863 701 0.21 1810 1810 16(0–5 cm) 0.82 370 88 437 529 0.17 1420 1740 20(0–4 cm) 0.63 350 72 903 1560 0.05 3940 6250 21(0–4 cm) 0.77 1370 94 842 595 0.16 1590 2060 27 0.16 66 15 194 154 0.10 350 2190 34(0–4 cm) 0.69 204 59 857 1800 0.03 4380 6340 36(0–4 cm) 0.42 65 46 758 1040 0.04 2740 6510 37(0–4 cm) 0.29 34 30 374 162 0.19 378 1300 42(0–4 cm) 0.41 138 34 480 259 0.13 652 1590 43(0–4 cm) 0.73 431 53 694 418 0.13 1120 1530 44(0–4 cm) 0.84 941 81 894 618 0.13 1630 1940 45 0.87 863 72 750 553 0.13 1460 1680 46(0–4 cm) 0.79 450 76 750 562 0.14 1460 1850 ORI–456 27 0.74 861 62 739 1120 0.06 2930 3960 33(0–4 cm) 0.38 250 38 461 279 0.14 771 2030 34(0–4 cm) 0.74 291 51 740 1540 0.03 4140 5590 35 0.14 47 17 180 91 0.19 235 1680 37 0.36 36 185 89 50 3.70 148 411 39(0–4 cm) 0.71 888 77 433 673 0.11 1510 2130 K(0–4 cm) 0.60 451 40 599 367 0.11 966 1610 Average 0.13 s.d.(1s) 0.06a aExclusive of 456–37. Table 4
Linear regressions between n-C22OH andother biomarker
n-alkanols for the 23 marine sediments
Regression r p n-C22OH=4.77 n-C14OH+517 0.24 0.26 n-C22OH=0.890 n-C16OH+544 0.18 0.40 n-C22OH=4.65 n-C18OH+263 0.58 0.004 n-C22OH=4.45 n-C20OH+131 0.84 o 0.001 n-C22OH=0.561 n-C24OH+344 0.74 o 0.001 n-C22OH=0.411 n-C26OH+382 0.69 o 0.001 n-C22OH=0.360 n-C28OH+374 0.70 o 0.001
r=correlation coefficient, p=significance value.
Table 5
Linear regressions of phytol against other biomarker n-alkanols for the 23 marine sediments
Regression r P Phytol=24.6 n-C14OH+74.1 0.63 0.001 Phytol=6.05 n-C16OH+97.2 0.64 0.001 Phytol=11.6 n-C18OH311 0.73 o 0.001 Phytol=7.60 n-C20OH256 0.73 o 0.001 Phytol=1.07 n-C22OH90.2 0.54 0.007 Phytol=0.266 n-C24OH+440 0.18 0.42 Phytol=0.180 n-C26OH+466 0.15 0.48 Phytol=0.162 n-C28OH+460 0.16 0.47
difference is unclear. Part of the reason might be that the three phytosterols originally thought to be terrigenous markers also couldbe producedby marine organisms (Volkman, 1986).
3.2. Correlation between TOC and terrigenous lipid biomarkers
Linear regression of TOC versus concentrations of landplant wax n-alkanes (sum of n-C25, n-C27,
n-C29, n-C31and n-C33) in sediments is expected to
give a strong positive linear correlation if a point source exists in a region of interest, such as that for the Columbia River drainage basin (Hedges and Prahl, 1993). The relationship for the sediments in
the study area is positive but weak (r ¼ 0:64; p ¼ 0:001). Linear regression of TOC against concentrations of landplant wax n-alkanols (sum of n-C24OH, n-C26OH and n-C28OH) also shows
positive linear correlation (r ¼ 0:42; p ¼ 0:048), which is weaker. This result couldbe attributedto several factors: (1) a complex input (not a point source) of terrigenous organic matter to the study area, (2) sediment receiving a blend of eroded and recent lipids (discussed earlier), (3) TOC also including marine organic matter, (4) temporal variations in river flow due to flooding, and (5) different rates of degradation for TOC and individual biomarkers. However, there is a strong positive linear relationship between TOC andthe
concentration of phytosterols (sum of campesterol, stigmasterol andsitosterol), r ¼ 0:85 (po0:001). The strong positive correlation couldmean that the mixing or dilution of phytosterols in terrestrial organic carbon with marine organic carbon with or without the phytosterols follows a nearly constant ratio, which is especially remarkable for the study area with a complex input.
3.3. Source of diploptene
Both diploptene (hop-22(29)-ene, Fig. 1) and diplopterol (hopan-22-ol,Fig. 8) were foundin the sediments; their EI mass spectra are given in
Fig. 11. Diploptene is derived from terrestrial higher plants andis also formedby bacteria. If it is derived from higher plants, a strong correlation
between diploptene and terrestrial higher plant n-alkanes has been demonstrated (Prahl et al., 1992). A plot of diploptene versus SC2533n-alkanes (odd
carbon only) for our samples shows scatter (Fig. 12), indicating no correlation. The diploptene in the study area is not predominantly derived from terrestrial higher plants, but from bacteria. It is also known that diplopterol is present in many bacteria andthat concentrations up to 1600 mg/g dry wt have been found in some methylotrophs, although small amounts of diplopterol also have been foundin some cyanobacteria, mosses and ferns (Rohmer et al., 1984). Moreover, diplopterol plottedagainst diploptene exhibits strong positive linear correlation (r ¼ 0:93; po0:001; Fig. 12). This result suggests that the diploptene in the study area may be mainly derived from diplopterol
Table 6
Sterol and diplopterol data
Sample TOC (g/100 g) Campesterol (ng/g) Stigmasterol (ng/g) Sitosterol (ng/g) Totala Phytosterols (ng/g) (Total phytosterols/ TOC) 107 Total sterols (ng/g) Diplopterol (ng/g) ORI-417 5(0–5 cm) 1.01 257 432 1000 1690 1670 6270 3050 8(0–3 cm) 1.05 416 630 1260 2360 2200 9340 2960 9(0–3 cm) 1.09 398 587 1080 2060 1890 7870 401 10(0–3 cm) 1.00 287 479 1010 1780 1780 6640 892 16(0–5 cm) 0.82 313 455 772 1540 1880 6190 79 20(0–4 cm) 0.63 116 79 282 477 757 2200 55 21(0–4 cm) 0.77 214 377 727 1320 1710 5540 492 27 0.16 25 13 32 70 438 449 26 34(0–4 cm) 0.69 112 101 362 575 833 2450 85 36(0–4 cm) 0.42 84 117 332 533 1270 2130 51 37(0–4 cm) 0.29 36 33 92 161 555 869 13 42(0–4 cm) 0.41 62 118 246 426 1040 1520 38 43(0–4 cm) 0.73 94 190 390 674 923 2780 130 44(0–4 cm) 0.84 191 346 669 1210 1440 4960 232 45 0.87 213 344 685 1240 1430 4860 376 46(0–4 cm) 0.79 117 197 502 816 1030 3320 786 ORI–456 27 0.74 220 191 544 955 1290 4550 90 33(0–4 cm) 0.38 86 111 403 501 1320 1810 71 34(0–4 cm) 0.74 178 178 434 790 1070 3220 108 35 0.14 15 22 44 81 579 371 12 37 0.36 107 162 392 661 1840 2610 38 39(0–4 cm) 0.71 495 502 1100 2090 2950 8180 187 K(0–4 cm) 0.60 113 174 399 686 1140 2660 128 a Phytosterols=campesterol+stigmasterol+sitosterol.
through dehydration in early diagenesis. In addi-tion, many active submarine hydrothermal activ-ities in the study area have been observed in the southern Okinawa Trough (Lee et al., 1998). Furthermore, an extraordinarily high concentra-tion (23 730 nl/l) of dissolved methane in the bottom water (depth 1440 m) was found at 2504.180N, 12235.200E (KEEP-MASS, 1992)
although other high levels of dissolved methane in the bottom waters were also foundin the study area (Chen, 1994). Subsequent collection of bottom water (within 2 m of sea bottom) and sediment simultaneously using a specially-designed sampler for methane study was made closest to this site. Abnormally high to high dissolved
methane concentrations in bottom water were observedat stations P (2503.120N, 12234.230E; water depth 1283 m), Q (2504.370N, 12235.320E; water depth 1395 m) and R (2504.880N, 12240.070E; water depth 1380 m)—11 100, 1500 and320 nl/l, respectively (Chen, 1994). As shown in Fig. 13, relatively high diploptene and diplo-pterol concentrations in sediments also were obtainedat stations Q (5440 and7550 ng/g, respectively) andR (1380 and842 ng/g, respec-tively). If stations Q andR are includedin
Fig. 12, the regression equation is diplopterol =38.7+1.40 diploptene (r ¼ 0:97; po0:001). This leads us to consider some relation between methane, diplopterol and diploptene. As a
consequence, it is speculatedthat diplopterol in the sediments could be synthesized by methylo-trophic bacteria using methane as their carbon source.
4. Conclusions
Comparedwith other coastal areas, the sediments off northeastern Taiwan contain comparatively
Fig. 11. Electron impact mass spectra of diploptene (upper) and diplopterol (lower) from station 417-8. Note that the present silylation procedure using BSA alone did not silylate diplopterol, and the spectrum shown here represents its free form.
high proportions of terrigenous n-alkanes and n-alkanols. The highest concentrations of plant waxes were foundon the upper slope of the southernmost Okinawa Trough near Taiwan. The major terrigenous inputs to this area are particle fluxes from the southern East China Sea shelf andriver runoff from the east Taiwan coast. However, contribution from the Lan-yang River
input is less important. TOC andphytosterol concentrations are positively correlated(r ¼ 0:85; po0:001).
In general, the present results show that the n-alkane and n-alkanol concentrations from mar-ine sources are smaller than those from terrigenous sources. Therefore, biasedconclusions may be obtainedif only n-alkanes and n-alkanols are
considered. In effect, our data do indicate that some marine-derived lipids outweigh terrigenous ones such as phytol andsterols. The observed lower concentrations of marine lipids in the sediments are attributed to the fact that
terrige-nous lipids are often preferentially preserved due to sequestration andthat marine lipids are more prone to degradation in the marine environ-ment especially those compounds derived from plankton.
Fig. 13. Chromatograms containing diploptene (upper) and diplopterol (lower) from station Q with high concentration of dissolved methane in overlying bottom water. Note that the retention time of diploptene differs from that ofFig. 1since the analysis was made earlier using an SE-30 column. Numbers above peaks refer to carbon number of n-alkanes (upper) and n-alkanols (upper). Letters above peaks refer to sterols; their identifications as inFig. 8.
Acknowledgements
We thank the captain, crew andtechnicians of the R/V Ocean Researcher I for help with sediment collection. Our special thanks go to Dr. M. C. Kennicutt, II andDr. J. K. Volkman for constructive comments andsuggestions. This study was financially supported by the National Science Council, Republic of China (grant no. NSC85-2611-M002A-020-K2).
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