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Organic Geochemistry 31 (2000) 375±388
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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
a

Laboratoire de Chimie Bioorganique et Organique Physique, UMR CNRS 7573, E.N.S.C.P. 11, rue P. et M. Curie, 75231
Paris cedex 05, France
b
Laboratoire 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 moieties. 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 subsequent acid hydrolysis and (v) the tightly bound acids only released by thermolysis. These ®ve fractions correspond to
distinct pools whose occurrence re¯ects di€erences 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 di€erent 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 di€erent 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 extensively 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 productivity and complex mixing processes typical of such areas,
and their potential role as a sink for atmospheric CO2.

* Corresponding author. Tel.: +33-1-4325-7975; fax: +331-4325-7975.
E-mail address: clargeau@ext.jussieu.fr (C. Largeau).

Fatty acids can be released from sedimentary organic

matter by di€erent 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 macromolecular structures). Moreover, the latter can be separated 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)

0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00007-3

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Y. Zegouagh et al. / Organic Geochemistry 31 (2000) 375±388

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 ecient steric protection of potentially labile
functions, such as esters, provided by the macromolecular structure of sedimentary organic matter
(Largeau et al., 1986; Kadouri et al., 1988; Derenne et
al., 1989).
A complete examination of these di€erent acid fractions in sedimentary organic matter, using the above
successive treatments, should provide valuable information on the abundance, sources and diagenetic alteration
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 knowledge, 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 di€erent environment and to draw comparisons 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 di€erent fatty
acid fractions. The Changjiang (Yangtze) River originates 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. 480106 tons
yearÿ1, including 4.5±6.0106 tons yearÿ1 of particulate
organic carbon and 1.8106 tons yearÿ1 of dissolved
organic carbon) (Milliman and Qingming, 1985; Cauwet
and Mackenzie, 1993; Meybeck and Ragu, 1995). Sediments 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 ÿ20 C
until freeze-drying.
2.2. Isolation of the di€erent 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 sediments (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, stirring 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, sediments 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 (tricaprylmethylammonium 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 400 C 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 sediment 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 chromatography (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 280 C at 4 C 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 internal 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 range m/z 40-600, cycle time 0.7 s). Dimethyl disul®de (DMDS) adducts of unsaturated fatty acid methyl
esters were prepared as described by Scribe et al. (1988)
by treatment (heating at 50 C for 2 days in a 10 ml closed
tube) with DMDS and iodine and immediately analyzed
by GC/MS under the same conditions as above.

Y. Zegouagh et al. / Organic Geochemistry 31 (2000) 375±388

377

3. Results and discussion

3.1. Abundance of acid fractions

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 farther o€shore 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 chlorophyll-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 temperate, 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 environments, 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.

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 a€ording 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 submitted to the same successive treatments (Zegouagh et
al., 1996). The total amount of fatty acids released by
the di€erent 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±300 mg/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

Fig. 1. East China Sea and sampling locations.

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Y. Zegouagh et al. / Organic Geochemistry 31 (2000) 375±388

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''

``Unbound''
OHÿ-labile
OHÿ(PTC)-labile
H+-labile
``Tightly bound''

ea
15.5 (49)
9.2 (29)
4.7 (15)
2.25 (7)

b

31.65

9 ``seaward''
5.2 (19)
9.15 (33)
8.5 (31)
3.1 (11)
1. 65 (6)
27.6

a

Very low amount (