A geochemical investigation of carboxylic acids released via
sequential treatments of two sur®cial sediments from the
Changjiang delta and East China Sea
Yahya Zegouagh
a, Sylvie Derenne
a, Claude Largeau
a,*, Alain Saliot
b aLaboratoire de Chimie Bioorganique et Organique Physique, UMR CNRS 7573, E.N.S.C.P. 11, rue P. et M. Curie, 75231Paris cedex 05, France
bLaboratoire de Physique et Chimie Marines, ESA CNRS 7077, Universite P. et M. Curie, 75252 Paris cedex 05, France
Abstract
Two surface sediments from the delta of the Changjiang River (the fourth largest river in the world in terms of water discharge and the seventh in terms of sediment discharge) and an adjacent area of the East China Sea were subjected to sequential treatments in order to obtain complete, quantitative and qualitative, information on carboxylic acid moi-eties. Five successive treatments allowed for the release of all acid moieties occurring in each sample: (i) the ``unbound'' acids isolated from extracts, (ii) the OHÿ-labile acids released by classical saponi®cation, (iii) the OHÿ(PTC)-labile acids
obtained via an additional saponi®cation with a phase transfer catalyst, (iv) the H+-labile acids released by a
sub-sequent acid hydrolysis and (v) the tightly bound acids only released by thermolysis. These ®ve fractions correspond to distinct pools whose occurrence re¯ects dierences in the mode of linkage of the acid moieties and/or in the protection provided by the structures to which they are linked. Such analysis of acid moieties provided information on (i) the relative contributions from microalgae, bacteria and higher plants in the ``delta'' and ``seaward'' stations, (ii) the degree of early diagenetic alteration for the acids derived from these dierent sources and (iii) the types of microalgal and bacterial species implicated. These features were also compared with the results of the only previous study where the sequential treatments were applied, on sur®cial sediments from a sharply dierent environment (the Lena River delta, Arctic).#2000 Elsevier Science Ltd. All rights reserved.
Keywords:Changjiang delta; East China Sea; Surface sediments; Acid moieties; Sequential acid release; Early diagenetic alteration
1. Introduction
Fatty acids in sur®cial sediments have been exten-sively studied for information on the sources and the early diagenetic degradation of organic matter (e.g. Meyers and Ishiwatari, 1993; Schubert and Stein, 1997). A number of these studies have examined river estuaries and deltaic systems, due to the high primary productiv-ity and complex mixing processes typical of such areas, and their potential role as a sink for atmospheric CO2.
Fatty acids can be released from sedimentary organic matter by dierent treatments, depending on their mode of occurrence and/or the nature of the structures to which they are linked. Various acid fractions can thus be obtained from a given sample, including the ``unbound'' acids (corresponding to moieties occurring in soluble lipidic components) and the ``bound'' acids (corresponding to moieties linked to insoluble macro-molecular structures). Moreover, the latter can be sepa-rated into several pools: the OHÿ-labile acids, released
by the cleavage of ester-linked moieties via a classical saponi®cation or saponi®cation with a phase transfer catalyst (PTC) (AmbleÁs et al., 1987, 1993) the H+-labile
acids, freed by the cleavage of amide- or ether-linked moieties via acid hydrolysis (Goossens et al., 1986, 1989)
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* Corresponding author. Tel.: 1-4325-7975; fax: +33-1-4325-7975.
and the ``tightly bound'' acids which can only be released through thermal stress (Kawamura and Ishiwatari, 1984; Largeau et al., 1986; Kawamura et al., 1986). The very high resistance to chemical and diagenetic degradation of these ``tightly bound'' moieties is believed to be due to the ecient steric protection of potentially labile functions, such as esters, provided by the macro-molecular structure of sedimentary organic matter (Largeau et al., 1986; Kadouri et al., 1988; Derenne et al.,1989).
A complete examination of these dierent acid frac-tions in sedimentary organic matter, using the above successive treatments, should provide valuable informa-tion on the abundance, sources and diagenetic alterainforma-tion of acid moieties. Nevertheless, most previous studies of carboxylic acids in recent sediments, have focused only on the ``unbound'' acids and the OHÿ-labile acids
released via a classical saponi®cation. To our knowl-edge, just one complete study has been reported, describing three sur®cial arctic sediments from the Lena River delta and adjacent area of the Laptev Sea (Zegouagh et al.,1996). The total amount of carboxylic acids recovered from these samples after sequential treatments was 3- to 4-fold higher than the acids released by the two traditional treatments.
The purpose of the present work was to carry out a detailed study of acid moieties in recent sediments from a markedly dierent environment and to draw compar-isons with the above observations on arctic samples. To this end, two sur®cial sediments from the Changjiang delta and an adjacent zone of the East China Sea were submitted to a detailed analysis of their dierent fatty acid fractions. The Changjiang (Yangtze) River origi-nates from the Qinghai-Xizang Plateau in Tibet. Like the Huanghe, it drains into sedimentary basins of the Chinese Platform and the broad shelf of the East China Sea. The drainage basin of the Changjiang is 1.8106
km2. It is the fourth largest river in the world in terms of
water discharge (ca. 930±980 km3 yearÿ1) and the
seventh in terms of sediment discharge (ca. 480106tons
yearÿ1, including 4.5±6.0
106tons yearÿ1of particulate
organic carbon and 1.8106 tons yearÿ1 of dissolved
organic carbon) (Milliman and Qingming, 1985; Cauwet and Mackenzie, 1993; Meybeck and Ragu, 1995). Sedi-ments in the two areas where the samples were collected correspond to silty clays originating from the rapid accumulation of material discharged by the Changjiang (Mckee et al., 1983).
2. Experimental
2.1. Sampling
Sur®cial samples (ca. 1 cm) were collected in January 1986, during the DH1 cruise of RV Xiang Yang Hong
09, using a box corer. Sediments were immediately stored in aluminium boxes, frozen and kept at ÿ20C
until freeze-drying.
2.2. Isolation of the dierent acid fractions
The ``unbound'' acids, the OHÿ-labile acids, the
OHÿ(PTC)-labile acids, the H+-labile acids and the
``tightly bound'' acids were isolated, from ca. 150 g of ®nely powdered, freeze-dried samples, using the same treatments as previously described for the arctic sedi-ments (Zegouagh et al., 1996). In brief, (i) the ``unbound'' acids were obtained from sediment extracts (CH2Cl2/MeOH, 3/1 v/v, re¯ux for 3 h with stirring)
after saponi®cation of the extracts under nitrogen (KOH 6% in MeOH/H2O, 9/1 v/v, re¯ux for 6 h,
stir-ring at room temperature overnight) and successive extractions with diethyl ether under basic and acidic conditions; (ii) the OHÿ-labile acids were released by a
classical saponi®cation of the extracted, lipid-free, sedi-ments with 6% KOH; (iii) the OHÿ(PTC)-labile acids
were freed from the insoluble residues, obtained after the above treatment, via saponi®cation (re¯ux under nitrogen for 2 days, 8.5% solution of KOH in H2O)
with a phase transfer catalyst Aliquat 336 (trica-prylmethylammonium chloride, Aldrich); (iv) the H+
-labile acids were released by 1.5 N HCl (re¯ux under nitrogen for 6 h) from the residues obtained after the second saponi®cation step and (v) the insoluble residues from the latter treatment were ®nally subjected to ``o-line'' pyrolysis at 400C for 1 h, under a Helium ¯ow, to
obtain the ``tightly bound'' acids.
2.3. Acid analysis
The ®ve acid fractions thus released from each sedi-ment sample were transformed into methyl esters by re¯uxing for 20 min, under nitrogen, in 2.5 ml of MeOH containing a few drops of acetyl chloride. Gas chroma-tography (GC) analyses of methyl esters were carried out using a 25 m, 0.32 mm i.d. CPSil 5 CB capillary column programmed from 150 to 280C at 4C minÿ1.
The total amount of acids in each fraction and the amounts of individual compounds were determined by using the methyl ester of the C22:1(o9) acid as an
inter-nal standard. Acids were identi®ed by combined Gas chromatography/Mass spectrometry (GC/MS) analyses of methyl esters, using a HP 5890 gas chromatograph (same column and conditions as above) coupled with a HP 5989 mass spectrometer (electronic impact at 70 eV, mass rangem/z40-600, cycle time 0.7 s). Dimethyl dis-ul®de (DMDS) adducts of unsaturated fatty acid methyl esters were prepared as described by Scribe et al. (1988) by treatment (heating at 50C for 2 days in a 10 ml closed
3. Results and discussion
Fig. 1 shows the location of the two sampling sites. Station 18 (salinity=25.2; water depth ca. 10 m) is located o the Changjiang River mouth in the direction of the South Branch, the main branch of the delta (ca. 55% of the total water ¯ow) (Beardsley et al. 1985). Station 9 (salinity=33.2; water depth ca. 40 m) is far-ther oshore in the East China Sea and not in the main direction of the river discharge; it is close to areas with especially high primary productivity as shown by chlor-ophyll-a maxima (Denant et al., 1990). Total organic carbon (TOC) content, as % of sediment dry weight, decreases from station 18 (``delta'': 0.65%) to station 9 (``seaward'': 0.39%). Such contents are comparable with previous measurements in this region (Kennicutt et al., 1987; Bigot et al., 1990). Previous studies on surface sediments from other estuarine or deltaic systems usually showed TOC values around 0.1 to 1% in tem-perate, subtropical and equatorial climates (Van Vleet et al., 1984; Barouxis et al., 1988; Bigot et al., 1989; Lipiatou and Saliot, 1991; Bouloubassi and Saliot, 1993), although much higher values (up to 7.8%) were noted in the Ebro delta area (Grimalt and AlbaigeÂs, 1990). Relatively high values were noted in arctic envir-onments, including the Mackenzie River delta and Mackenzie shelf (Yunker et al., 1993) and the Lena River delta and Laptev Sea (Zegouagh et al.,1996): up to 1.6 and ca. 3%, respectively.
3.1. Abundance of acid fractions
The abundances of the ®ve fractions isolated via the successive treatments, from each surface sediment, are reported in Table 1. To summarize, we observe that: all treatments, except the extraction of ``unbound'' fatty acids from station 18 (``delta''), released substantial amounts of fatty acids. Notably, the classical isolation method, only aording the ``unbound'' and OHÿ-labile
acids, would have liberated only ca. 50% of the total fatty acids occurring in these sediments. Both the OHÿ(PTC)-labile acids and the OHÿ-labile acids are
abundant in both cases. In contrast, the ``tightly bound'' acids, released via thermal stress, occur in relatively low amounts. Similar features were previously noted for the Lena River delta and Laptev Sea sediments when sub-mitted to the same successive treatments (Zegouagh et al.,1996). The total amount of fatty acids released by the dierent treatments is close for both samples (around 30 mg/g of dry sediment, Table 1). This total value is substantially lower when compared to those (ca. 100±300mg/g) obtained, via sequential treatments, from the Lena River delta and Laptev Sea samples (Zegouagh et al., 1996). Quantitative information on carboxylic acids in the other previous studies was only concerned with the ``unbound'' acids. Similarly low values (ca. 5
mg/g) to that obtained for station 9 (``seaward'') were observed for most open estuarine environments (e.g. Corbet et al., 1978; Bigot et al., 1989), including the
Changjiang delta and the East China Sea (Lajat et al., 1990a). In contrast, much higher contents of ``unbound'' acids, in the 50±300mg/g range, were noted in more restricted marine environments (e.g. Farrington and Quinn,1973).
3.2. Composition of acid fractions
The various fatty acid fractions were examined by GC and GC/MS to determine the nature and abundance of the dierent series (Table 2), their distribution (Table 3) and the abundance of individual compounds (Appendix).
In addition to information on source organisms, these analyses allowed a comparison of the acid fractions released by the sequential treatments for a given sedi-ment, as well as the results corresponding to the ``delta'' and ``seaward'' samples for a given fraction.
3.2.1. Normal, saturated fatty acids
These comprise, by far, the most abundant series in all the fractions (Table 2). Their distribution (Table 3, Fig. 2) exhibits (i) a maximum corresponding to palmi-tic acid (C16:0), except for the H+-labile fraction from
station 9 (``seaward'') where maxima at both C16 and
C18are observed, and (ii) a predominance of the
even-carbon-numbered compounds in the C12±C20range, with
Carbon Preference Indexes (CPI) ranging from ca. 4 to 14. Long chain, C20+ fatty acids are either absent or only
occur in minor amounts (at most ca. 4% of the totaln -saturated acids) in a few fractions (Fig. 2). No C24+ acids
were released through any of the successive treatments.
3.2.2. Normal, unsaturated fatty acids
These occur in each fraction but their relative abun-dance shows sharp variations: it is especially low in the OHÿ(PTC)-labile fractions but rather large in the OHÿ
-labile and ``unbound'' fractions (Table 2). High percen-tages of unsaturated fatty acids were previously noted in the ``unbound'' fraction from surface sediments in sandy intertidal areas (Volkman et al.,1980), the coastal zone of the Mahakam prodelta (Barouxis et al.,1988), the Laptev Sea (Fahl and Stein, 1997) and rapidly accumulating Table 1
Amounts (mg/g of dry sediment) and relative abundances (%, bracketed values) of the acid fractions obtained via sequential treatments of the two sediments
Station
Acid fractions 18 ``delta'' 9 ``seaward''
``Unbound'' ea 5.2 (19)
OHÿ-labile 15.5 (49) 9.15 (33)
OHÿ(PTC)-labile 9.2 (29) 8.5 (31)
H+-labile 4.7 (15) 3.1 (11)
``Tightly bound'' 2.25 (7) 1. 65 (6)
b 31.65 27.6
a Very low amount (<1mg/g of dry sediment). b
, total amount of carboxylic acids (mg/g of dry sediment) released by each sample.
Table 2
Nature and abundance (mg/g of dry sediment) of the dierent series of acids occurring in the various fractions obtained via sequential treatments of the two sedimentsa
Station
Acid fractions Series 18 ``delta'' 9 ``seaward''
n-Saturated eb(ca.80) 3.8 (73)
``Unbound'' n-Unsaturated eb(ca.20) 1.1 (21)
Branched c 0.3 (6)
n-Saturated 10.3 (66) 7.1 (78)
OHÿ-labile n-Unsaturated 3.7 (24) 1.15 (13)
Branched 1.6 (10) 0.9 (9)
n-Saturated 8.7 (95) 7.2 (85)
OHÿ(PTC)-labile n-Unsaturated 0.1 (1) 0.2 (2)
Branched 0.4 (4) 1.1 (13)
n-Saturated 4.1 (87) 2.8 (91)
H+-labile n-Unsaturated 0.3 (6.5) 0.2 (6)
Branched 0.3 (6.5) 0.1 (3)
n-Saturated 1.95 (86.5) 1.5 (91)
``Tightly bound'' n-Unsaturated 0.2 (9) 0.1 (6)
Branched 0.1 (4.5) 0.05 (3)
a The bracketed values correspond to the relative contribution (%) of each series to the considered fraction. b See note (a) Table 1; approximate quanti®cation could be obtained only for the main series.
coastal sediments (Haddad et al., 1992). The unsatu-rated acids released by the Changjiang delta and East China Sea sediments mostly correspond to C16and C18
compounds (Table 3). Unsaturated fatty acids are encountered in bacteria, microalgae and also zoo-plankton. Nevertheless, double bond location can allow a dierentiation between such sources. The DMDS adducts of the dierent acid fractions were therefore prepared and analysed by GC/MS to determine speci®c isomers. Several isomers were thus identi®ed, as illu-strated in Table 4 for the OHÿ-labile fractions.
3.2.3. Branched acids
These were identi®ed in all the fractions isolated from the two sediments, although only in moderate relative amounts (ca. 3 to 13% of total), (Table 2). Previous studies showed the presence of branched acids in the ``unbound'' and OHÿ-labile fractions from various
sur-®cial marine sediments (Saliot et al.,1980; Gillan and Sandstrom, 1985; Barouxis et al., 1988; Grimalt and AlbaigeÂs, 1990; Lajat et al., 1990a; Canuel and Martens, 1993; Johns et al., 1994). Branched acid abundance ranged in these studies from ca. 2 to 30% of total acids with the highest values noted for samples collected in summer from rapidly accumulating coastal sediments.
3.3. Comparison of acid fractions
Important variations in the relative abundance and/or distribution of acids are apparent in the various fractions released through sequential treatments of each sample
(Tables 2 and 3, Figs. 2 and 3). Such quantitative and qualitative variations are anticipated when comparing the OHÿ- and the H+-labile acids (ester-linked moieties
cleaved by basic treatments versus amide- or ether-linked moieties cleaved by acid treatments) and also when comparing the acids released by the above hydro-lyses and the ``tightly bound'' counterparts (poorly pro-tected moieties versus non-hydrolysable moieties protected within a macromolecular network). Large dierences are also observed, however, between the OHÿ- and the OHÿ(PTC)-labile fractions. These
dier-ences indicate that two distinct pools of ester-linked acids occur for each sediment and the second one is only released in the presence of a phase transfer catalyst. Accordingly, as previously observed in the case of the Laptev Sea and Lena River delta samples (Zegouagh et al.,1996), it appears that the ®ve acid fractions released from the Changjiang delta and East China Sea sedi-ments all correspond to distinct acid pools. In addition, common features are observed for these two sediments: (i) a sharp decrease in the relative abundance of the unsaturated acids from the OHÿ- to the OHÿ(PTC)-labile
pool, (ii) the occurrence of linoleic acid in the OHÿ-labile
fraction whereas it is no longer observed in the sub-sequent fractions and (iii) a simpler distribution of the branched acids in the ``tightly bound'' fraction when compared to the other pools.
In contrast, large dierences can be noted by com-parison with the acids from the Laptev Sea and Lena River delta sediments (Zegouagh et al., 1996): (i) the latter samples exhibit the highest relative abundances Table 3
Distribution of the dierent series of acids occuring in the various fractions abtained via sequential treatments of the two sedimentsa
Station
Acid fraction Series 18 ``delta'' 9 ``seaward''
n-Saturated C14±C18(C16) C12±C18(C16)
``Unbound'' n-Unsaturated C18:1 C16:1, C18:1*,C18:2b
Branched t iC15, aiC15*,iC17, iC17:1
n-Saturated C12±C21(C16) C12±C18(C16)
OHÿ-labile n-Unsaturated C
14:1, C16:1±C18:1*, C18:2b C14:1, C16:1*±C18:1, C18:2b
Branched iC15*,aiC15,iC17,iC17:1 iC15*,aiC15,iC17, iC17:1
n-Saturated C12±C24(C16) C12±C18(C16)
OHÿ(PTC)-labile n-Unsaturated C
16:1, C18:1* C16:1, C18:1*
Branched iC15,aiC15*,iC17,iC17:1 iC15,aiC15*,iC17,iC17:1
n-Saturated C12±C18(C16) C12±C18(C16,C18)
H+-labile n-Unsaturated C
16:1, C18:1* C16:1, C18:1*
Branched iC15,aiC15*,iC17,iC17:1 iC15,aiC15*,iC17,iC17:1
n-Saturated C12±C20(C16) C12±C23(C16)
``Tightly bound'' n-Unsaturated C16:1, C18:1* C16:1, C18:1*
Branched aiC15*, iC17 aiC15*,iC17
a The maximum of the series is indicated in parentheses or by an asterisk; oleic acid (C
18:1o9) is always the main C18
mono-unsaturated acid; i, iso acids; ai, anteiso acids; t, trace amounts.
Fig. 2. Distribution (%) of then-saturated fatty acids in the acid fractions obtained via sequential treatments of the two surface sediments: A, ``unbound'' acids; B, OHÿ-labile acids; C, OHÿ(PTC)-labile acids; D, H+-labile acids and E, ``tightly bound'' acids. The
for unsaturated fatty acids in the ``tightly bound'' fractions, (ii) linoleic acid occurs not only in the OHÿ
-labile fractions but also in the ``tightly bound'' frac-tions and (iii) no unsaturated acids are detected in the H+-labile fractions and no branched acids in the
OHÿ(PTC)- and H+-labile fractions. These contrasting
composition patterns presumably re¯ect dierences in the organic matter discharged by the Changjiang and the Lena, and/or in the phytoplanktonic and bacterial populations living in these two environments, and/or dierent diagenetic conditions prevalent within the sediments.
3.4. Sources of the acid moieties and comparison of the two stations
Qualitative and quantitative analyses showed similar features, for a given pool, when comparing the ``delta'' and ``seaward'' stations. In fact, no large dierences can be observed between these two sediments. Their acid moieties originate from the same type of sources and the relative contributions of these dierent sources to acids, discussed below, are similar in both cases.
The bulk of the available information on fatty acids in living organisms is limited to the ``unbound'' and
Fig. 3. Comparison of the OHÿ-labile (A) and OHÿ(PTC)-labile (B) acid fractions released from the surface sample of station 9: ~saturated acids;*unsaturated acids;&branched acids;^internal standard.
Table 4
Double bond location in then-monounsaturated fatty acids of the OHÿ-labile fraction obtained from the two sediments and relative
abundances (%) of the dierent isomersa
Station Carbon number
C14 C16 C17 C18
C14:1o8 (2.2) C16:1o5 (2.5) C17:1o6 (0.7) C18:1o7 (8)
18 ``delta'' C16:1o7 (10.6) C17:1o8 (0.6) C18:1o9 (55.6)
C16:1o10 (2.2) C17:1o9 (0.6)
C17:1o11 (3.3)
C14:1o8 (0.9) C16:1o5 (3.4) C17:1o11 (1.7) C18:1o7 (8.5)
9 ``seaward'' C14:1o9 (1.3) C16:1o7 (34.3) C18:1o9 (24.3)
C16:1o9 (11.5)
C16:1o10 (1.7)
a Linoleic acid (C18:2o6,o9) accounts for ca. 14 and 12% of the total unsturated acids of this fraction in the case of the ``delta''
OHÿ-labile pools. Indeed, only a few studies have been
reported on the acid moieties released from various types of biological samples through sequential treat-ments similar to those we applied to surface seditreat-ments in the present work. However, previous studies revealed that (i) the H+-labile acids of 13 species of bacteria,
selected to represent various groups, also exhibit the same main features as observed for the ``unbound'' and OHÿ-labile acids of these bacteria (namely the abundant
presence of branched iso and anteiso acids, especially in the gram-positive bacteria, and of a- and b -hydroxy-acids in the gram-negative species) (Goossens et al., 1986), (ii) the ``tightly bound'' acids of the green microalga Botryococcus brauniiare not fundamentally dierent of the acids in the other pools and correspond to functions eciently protected by steric hindrance in high mole-cular weight (probably highly cross-linked) macro-molecular structures (Largeau et al., 1986) and (iii) several elongation pathways probably occur, for a given species, in higher plants (Kolattukudy et al., 1976) so that the long chain fatty acids typical of such organisms are probably found in dierent pools. Therefore, it seems that the major features of the acids in a type of biological samples (bacteria, microalgae and higher plants) are likely found both in the classically considered pools and in the additional pools released through the sequential treatments. Accordingly, the lack of extensive studies on the composition of all the dierent acid pools in living organisms do not preclude from discussing our observa-tions in terms of sources. Moreover, the main interest of a detailed study of all the pools in sediment samples is to ensure that the most protected acids are actually ana-lysed so that the observed features well re¯ect the nature of the source(s) and are not highly biased by degradation.
3.4.1. Microalgae
In microalgae, normal saturated fatty acids dominate all the fractions and palmitic acid is the main constituent in most cases. However, the abundant presence of this acid does not provide, alone, information on organic matter sources. Indeed, palmitic acid is abundant and ubiquitous in higher plant waxes, microalgae and bacteria (e.g. Volkman et al.,1988; Scribe et al.,1991; Meyers and Eadie, 1993) and it is often an abundant component of marine sediments (Barouxis et al.,1988; Bigot et al., 1989; Sun and Wakeham, 1994). Then-saturated acids released via sequential treat-ments from the ``delta'' and ``seaward'' samples exhibit a marked predominance of medium chain length, even-car-bon-numbered C12±C20fatty acids. Such a feature, in recent
sediments, is considered as re¯ecting a large autochthonous input of microalgae-derived organic matter (Taylor et al., 1984; Venkatesan, 1988; Grimalt and AlbaigeÂs, 1990; Johns et al.,1994; Sun and Wakeham, 1994).
The distribution of the unsaturated acids also sup-ports an abundant contribution of microalgal material and provides further information on potential sources.
Fatty acids in a number of microalgal groups, including diatoms, are dominated by oleic acid (C18:1 o9). This
isomer exhibits a high relative abundance, when com-pared to the other unsaturated acids in both stations (Tables 3 and 4). A large microalgal contribution to these sediments is also re¯ected by the presence of the C16:1o7 isomer, well known as a typical acid of diatoms
(Venkatesan, 1988; Volkman et al., 1988; Visot and Marty, 1993; Dunstan et al.,1994). These features are consistent with previous studies which showed a major contribution of diatoms to total phytoplanktonic biomass for the Changjiang delta and adjacent areas of the East China Sea (Ning et al.,1988; Denant et al., 1990).
The above observations, added to the information on the extent of bacterial and higher plant inputs discussed in next sections, indicate a major autochthonous (dia-tom) contribution of microalgae-derived components to acid moieties in the Changjiang delta and East China Sea sur®cial samples. This algal material appears only slightly altered, at least in the ``marine'' station. It is well documented that the resistance of fatty acids to diage-netic degradation is strongly in¯uenced by their degree of unsaturation. Polyunsaturated fatty acids are thus espe-cially sensitive (Goossens et al.,1986; Farrington et al., 1988; Wakeham and Ertel, 1988; Wakeham and Canuel, 1990) and thus their presence in surface sediments is commonly considered as re¯ecting a contribution of liv-ing algal cells and/or of freshly deposited, moderately altered, algal material (Canuel and Martens, 1993). The mode of occurrence of acid moieties is also known to be an important factor and the ``unbound'' pool often shows a high degree of alteration (e.g. Taylor et al., 1984; WuÈnsche et al.,1988; Lajat et al.,1990b; Meyers and Ishiwatari, 1993). Accordingly, the presence of substantial amounts of linoleic acid in the ``unbound'' pool from station 9 (ca. 10% of total unsaturated acids) points to input of some relatively fresh algal material. In contrast, only monounsaturated acids were detected in the ``unbound'' fractions from the Laptev Sea and Lena River delta samples (Zegouagh et al.,1996), indicating a higher level of bacterial reworking in the water column and the upper layer of the sediment for the latter environment.
3.4.2. Bacteria
Branched acids are abundant in a number of bac-teria, comprising up to 80% of total acids (Volkman et al., 1980; Parkes and Taylor, 1983; Shaw and Johns, 1986). These bacterial acids chie¯y are made up of medium chain length, iso or anteiso saturated com-pounds, generally characterized by a maximum at C15
or C17and by the exclusive occurrence (or very strong
predominance) of odd-carbon-numbered compounds. Similar distributions were observed for the branched acids identi®ed in the present study (Table 3). Howver, the total amounts of branched acids for both samples (ca. 2 mg/g of dry sediment) and their abundance with respect to total acids (ca. 7%) point to relatively low bacterial contributions. Nevertheless, the presence of this type of acids in the dierent fractions indicates that bacteria-derived moieties contributed to all of the acid pools.
Most bacteria living in sediments do not produce substantial amounts of oleic acid whereas the C18:1o7
isomer, vaccenic acid, predominates in a number of species (Cranwell, 1978; Perry et al., 1979; Gillan and Sandstrom, 1985). This acid also occurs in other marine organisms but is far less abundant than in bacteria (Barouxis et al., 1988). Accordingly, like the branched acids, vaccenic acid is considered as a characteristic bacterial marker and it was detected in the OHÿ-labile
fractions (Table 4). Bacterial contributions are also re¯ected by the presence of low amounts of the C16:1
o10 isomer. The occurrence of the n-C17:1 acids in the
OHÿ-labile fraction may re¯ect the contribution of
anaerobic fermentating bacteria (Wilkinson, 1988). Normal, odd-carbon-numbered, medium chain length (maxima at C15 and C17), saturated fatty acids
are also considered as bacterial markers ( Mendoza et al., 1987a; Goossens et al.,1989; Haddad et al.,1992; Meyers and Ishiwatari, 1993). Accordingly, the strong dominance of the even-carbon-numbered homologs, re¯ected by CPI values, also supports a relatively weak bacterial contribution for acid moieties in the two sediments. Moreover, the complete lack of hydro-xyacids in the dierent fractions obtained from the Changjiang and East China Sea samples points to a relatively low diversity in the bacterial populations liv-ing in the water column and in the surface layer of these sediments, in particular a low contribution of gram negative species (a- andb-hydroxyacids are typical acids of bacteria (Cardoso and Eglinton, 1983; Shaw and Johns, 1985; Mendoza et al.,1987b; Fukushima et al., 1992a) but they are not ubiquitous (WuÈnsche et al., 1988; Goossens et al., 1989; Ogura et al., 1990) and occur in some species or groups of species, especially gram negative bacteria).
Finally, the presence of a monounsaturated C17
branched (iso) acid suggests that sulphate-reducing bacteria contributed to acid moieties in both samples. It
can be speculated that such a feature, added to the relatively low level of bacterial reworking mentioned above, would re¯ect a high primary productivity induced by the large nutrient supply from the Chang-jiang in these two areas. It is well established that a large phytoplanktonic production is associated with fast sinking rates of microalgal debris and therefore with reduced mineralization in the water column. Under such conditions, large amounts of metabolizable algal organic matter reaches the sea ¯oor. A shoaling of the anoxic zone is thus promoted and allows for the growth of sul-phate-reducers in the upper layer of the sediment.
3.4.3. Higher plants
Epicuticular waxes of higher plants are characterized by long chain saturated fatty acids, in the C22±C34
range, with an even predominance (Hitchcock and Nichols, 1971; Kolattukudy et al.,1976; Tulloch, 1976; Rieley et al.,1991; Ficken et al.,1998). The presence of saturated C20+ fatty acids with an even predominance,
in marine sediments, is therefore generally considered as an indicator of an allochthonous terrigenous contribu-tion (e.g. Saliot et al., 1980; Shaw and Johns, 1986; Venkatesan et al., 1987; Grimalt and AlbaigeÂs, 1990; Saliot et al., 1991). The C20ÿ/C20+ or the C16:0/C26:0
ratios have been used to assess autochthonous/alloch-thonous inputs in recent sediments (e.g. Venkatesan et al., 1987; Venkatesan, 1988; Ogura et al., 1990). Sub-stantial amounts of terrigenous, saturated, long chainn -fatty acids were previously observed in the ``unbound'' fraction isolated from some surface sediments in delta areas (Barouxis et al.,1988; Bigot et al.,1989; Lajat et al., 1990a). In the Ebro delta more than 50% of the total ``unbound'' fatty acids correspond to C20+
nor-mal, saturated compounds in open sea zones in front of the river mouth (Grimalt and AlbaigeÂs, 1990). In con-trast, only low amounts of this type of acid were noted in rapidly accumulating coastal sediments (Haddad et al., 1992; Canuel and Martens, 1993). In the present study, only minor amounts of C20+ fatty acids were
detected in a few fractions and no C24+ homologs were
observed for stations 9 and 18 in any acid fraction. These distributions indicate a negligible contribution from higher plants to acid moieties in the two sediments.
The above features cannot re¯ect a bias resulting from the presence of long chain acids in a pool that was not analysed, since all acid pools were recovered. They also do not re¯ect preferential degradation of these acids since the resistance of fatty acids to early diage-netic alteration increases with chain length (WuÈnsche et al., 1988; Lajat et al., 1990b; Haddad et al., 1992; Mey-ers and Eadie, 1993; Colombo et al., 1997; Laureillard et al., 1997; Wakeham et al., 1997). The degradation of ``unbound''n-saturated C20+ acids in some recent
sedi-ments is ca. 10 times lower when compared to C20-
the organic matrix is important for acid preservation and ``unbound'' acids in higher plant debris are more resistant than the same compounds in microalgal biomass (e.g. Prahl, 1985; Venkatesan et al., 1987; Volkman et al., 1987; Haddad et al., 1992; Sun and Wakeham, 1994).
The absence of a signi®cant contribution of car-boxylic acids originating from higher plants is also sup-ported by the complete lack of hydroxyacids in all the fractions. o-Hydroxyacids, along with mid-chain di-and trihydroxyacids, are major constituents of cutins, suberins and higher plant epicuticular waxes (Tulloch, 1976). Their abundance was shown to re¯ect the extent of terrestrial contribution to marine and coastal sediments (Fukushima et al.,1992b) and they were detected in var-ious sediments with documented higher plant inputs (Cardoso and Eglinton, 1983; WuÈnsche et al., 1988; Goossens et al.,1989; Fukushima et al.,1992a).
3.4.4. Marine versus terrestrial sources
Previous studies of organic matter in marine surface sediments in deltas and adjacent areas have revealed contrasting pictures of the extent of terrestrial in¯uence. In the case of the Changjiang delta, examination of contributions to whole sedimentary organic matter, as re¯ected by bulk d13C measurements and C/N atomic
ratios, showed terrestrial inputs extending up to 100 km seaward (Kennicutt et al.,1987). In contrast, when fatty acid moieties are considered, terrestrial input from higher plants appears very weak, even for the ``delta'' station (18) located in the main direction of the river discharge, despite the enormity of this river system, and the fact that sequential treatments allowing for a com-plete release of all acid moieties were applied in the present study. Instead, the fatty acid moieties released from the two samples are characterized by a major input of autochthonous material of microalgal (mostly diatom) origin. These somewhat unexpected features likely re¯ect the combination of two main factors: a high primary productivity triggered by a large in¯ux of nutrients from the Changjiang (Aller et al., 1985) and a high level of degradation of the discharged material. Indeed, (i) a large part of the organic matter discharged by rivers corresponds to highly degraded material derived from soils (Hedges et al., 1986; Ittekkot, 1988; Jae et al., 1995; Peulve et al., 1996; Goni et al.,1997) and14C measurements recently showed an apparent age
of ca. 8000 years for the organic matter from the Mississipi River (Goni et al., 1997) and (ii) this organic matter probably suers further degradation following discharge (Hedges et al., 1997). As a result the Changjiang may only provide low amounts of acid moieties of terrestrial origin to sediments in the delta and adjacent areas of the East China Sea, highly diluted by microalgae-derived moieties. The similarities observed between the ``delta'' and ``seaward'' stations are most likely due to this large overprinting.
A similar lack of a signi®cant input of acid moieties from higher plants was previously observed, in arctic environments, for surface sediments from the Lena River delta and adjacent areas of the Laptev Sea (Zegouagh et al.,1996), even though the Lena River is the eighth largest river in the world in terms of water discharge (525 km3 yearÿ1) and its annual supply of
suspended sediments to the Laptev Sea amounts to 17.6106 tons. Taken together, these observations on
two contrasting environments suggest that a negligible contribution of higher plant components to acid moi-eties is not an uncommon feature for the sediments deposited in the deltas of major rivers and adjacent marine areas.
4. Conclusions
Two surface sediments from the delta of the Chang-jiang and an adjacent area of the East China Sea were submitted to sequential treatments to obtain a complete release of their acid moieties. The main conclusions of qualitative and quantitative analyses of these acid fractions are:
i. Five distinct acid pools occur in these sur®cial samples. The traditional isolation method, aording the ``unbound'' and OHÿ-labile acids only, would have
allowed for the identi®cation of only ca. 1/2 of the total acids present in the sediments. The occurrence of these discrete pools re¯ects dierences in the mode of linkage of acids and/or in the degree of protection provided by the structures to which the acids are linked.
ii. Autochthonous organic matter derived from microalgae, especially from diatoms, provided a major contribution to acid moieties in the two samples. This algal biomass apparently underwent moderate bacterial reworking, owing to fast deposition rates triggered by high primary productivity.
iii. Bacterial contributions to acids were low in both samples, although bacteria-derived moieties were observed in all of the pools. Analysis of acids suggests that bacterial populations were characterized by a rela-tively limited diversity and sulphate-reducing bacteria were probably implicated.
Acknowledgements
We thank Drs. G.H. Yu and J-M. Martin for leading the coopereative research between State Oceanic
Administration of China and CNRS (France). Mr. Y. Pouet is acknowledged for assistance in GC/MS experi-ments. Dr. M.B. Yunker and an anonymous referee are acknowledged for reviews and comments.
Appendix
Table 5
Concentration of individual acids (mg/g of dry sediment) in the various fractions obtained via sequential treatments of the two sediments
Acid fraction Series C12 C13 C14 C15 C16 C17 C18 C19 C20 C21 C22 C23 C24 P
Station 18 ``delta''a
n-saturated 0.50 ± 1.56 0.52 6.03 0.15 1.18 0.09 0.15 0.12 ± ± ± 10.3 OHÿ-labile n-monounsaturatedb ± ± ± ± 0.47- ± 2.63 ± ± ± ± ± ± 3.7
n-Diunsaturated ± ± ± ± ± ± 0.6 ± ± ± ± ± ± 1.6 Branched (i)c ± ± ± 0.57 ± 0.21 ± ± ± ± ± ± ± ±
Branched (ai) ± ± ± 0.68 ± 0.14 ± ± ± ± ± ± ± ± n-Saturated 0.04 ± 0.70 0.40 5.69 0.15 1.20 0.13 0.08 ± 0.21 ± 0.10 8.7 OHÿ(PTC)-labile n-Monounsaturatedb ± ± ± ± 0.07 ± 0.03 ± ± ± ± ± ± 0.1
Branched (i) ± ± ± 0.12 ± 0.05 ± ± ± ± ± ± ± 0.4 Branched (ai) ± ± ± 0.23 ± ± ± ± ± ± ± ± ± ± n-Saturated 0.46 ± 1.06 0.23 1.76 0.16 0.43 ± ± ± ± ± ± 4.1 H+-labile n-Monounsaturatedb ± ± ± ± 0.25 ± 0.05 ± ± ± ± ± ± 0.3
Branched (i) ± ± ± 0.07 ± 0.08 ± ± ± ± ± ± ± 0.3 Branched (ai) ± ± ± 0.10 ± 0.05 ± ± ± ± ± ± ± ± n-Saturated 0.36 0.14 0.41 0.17 0.69 0.05 0.11 0.01 0.01 ± ± ± ± 1.95 ``Tightly bound'' n-Monounsaturatedb ± ± ± ± 0.15 ± 0.05 ± ± ± ± ± ± 0.2
Branched (i) ± ± ± 0.10 ± ± ± ± ± ± ± ± ± 0.1
Branched (ai) ± ± ± ± ± ± ± ± ± ± ± ± ± ±
Station 9 ``seaward''
n-Saturated 0.05 ± 0.22 0.11 3.01 0.20 0.21 ± ± ± ± ± ± 3.8 ``Unbound'' n-Monounsaturatedb ± ± ± ± 0.25 ± 0.56 ± ± ± ± ± ± 1.1
n-Diunsaturated ± ± ± ± ± ± 0.18 ± ± ± ± ± ± 0.3 Branched (i) ± ± ± 0.09 ± 0.10 ± ± ± ± ± ± ± ± Branched (ai) ± ± ± 0.11 ± ± ± ± ± ± ± ± ± ± n-Saturated 1.28 ± 2.10 0.38 2.85 0.11 0.38 ± ± ± ± ± ± 7.1 OHÿ-labile n-Monounsaturatedb ± ± ± ± 0.41 ± 0.66 ± ± ± ± ± ± 1.15
n-Diunsaturated ± ± ± ± ± ± 0.08 ± ± ± ± ± ± 0.9 Branched (i) ± ± ± 0.40 ± 0.13 ± ± ± ± ± ± ± ± Branched (ai) ± ± ± 0.37 ± ± ± ± ± ± ± ± ± ± n-Saturated 0.84 ± 1.02 0.40 3.02 0.12 1.80 ± ± ± ± ± ± 7.2 OHÿ(PTC)-labile n-Monounsaturatedb ± ± ± ± 0.12 ± 0.08 ± ± ± ± ± ± 0.2
Branched (i) ± ± ± 0.57 ± 0.34 ± ± ± ± ± ± ± 1.1 Branched (ai) ± ± ± 0.19 ± ± ± ± ± ± ± ± ± ± n-Saturated 0.05 ± 0.30 0.12 1.14 0.07 1.12 ± ± ± ± ± ± 2.8 H+-labile n-Monoununsaturatedb ± ± ± ± 0.11 ± 0.09 ± ± ± ± ± ± 0.2
Branched (i) ± ± ± 0.04 ± 0.03 ± ± ± ± ± ± ± 0.1 Branched (ai) ± ± ± 0.03 ± ± ± ± ± ± ± ± ± ± n-Saturated 0.19 0.07 0.29 0.12 0.45 0.04 0.26 0.01 0.02 0.01 0.03 0.01 ± 1.5 ``Tightly bound'' n-Monounsaturatedb ± ± ± ± 0.05 ± 0.05 ± ± ± ± ± ± 0.1
Branched (i) ± ± ± 0.05 ± ± ± ± ± ± ± ± ± 0.05
Branched (ai) ± ± ± ± ± ± ± ± ± ± ± ± ± ±
a Due to the very low total amount of carboxylic acids (<1mg/g of dry sediment) in the ``unbound'' fraction of this section, the
abundance of individual compounds could not be precisely determined.
b The dierent isomers, occurring for most monounsaturated acids, often coelute in direct GC analyses and they can only be separated
and identi®ed via GC/MS examination of their DMDS adducts. However, reliable quantitive information can be hardly derived from the analysis of these adducts. Therefore, when several isomers were found for a given carbon number only their total amount, obtained via analysis before DMDS addition, is reported.
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