Directory UMM :Data Elmu:jurnal:O:Organic Geochemistry:Vol31.Issue12.Dec2000:
Organic Geochemistry 31 (2000) 1533±1543
www.elsevier.nl/locate/orggeochem
Biomarkers in upper Holocene Eastern North Sea and
Wadden Sea sediments
Bart E. van Dongen a,*, W. Irene C. Rijpstra a, Catharina J. M. Philippart b,
Jan W. de Leeuw a, Jaap S. Sinninghe Damste a
a
Department of Marine Biogeochemistry and Toxicology, Netherlands Institute for Sea Research (NIOZ), PO Box 59,
1790 AB Den Burg, Texel, The Netherlands
b
Department of Marine Ecology, Netherlands Institute for Sea Research (NIOZ), PO Box 59, 1790 AB Den Burg,
Texel, The Netherlands
Abstract
Total extracts of sediment cores from ®ve dierent stations in the North Sea and Wadden Sea were analysed for their
biomarker composition. Only sediments of the Skagerrak contained signi®cant amounts of marine biomarkers (mainly
alkenones), other sites contained predominantly terrestrial biomarkers. Bioturbation in the Skagerrak is, however, far
too high to determine sea surface temperature (SST) changes within short time intervals. These results indicate that
biomarkers contained in these sediments are not useful to reconstruct climate ¯uctuations during the upper Holocene.
High amounts of a-, b- and o-hydroxy fatty acids as well as small amounts of a, b-dihydroxy fatty acids were released
from the insoluble organic material of the sediments from the Wadden Sea station, indicating a signi®cant input of the
eelgrass Zostera marina. This was con®rmed by microscopic observations. This is the ®rst time the a,b-dihydroxy fatty
acids have been found in a sediment core and they have proven to be potential biomarkers for these seagrass species.
# 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Biomarkers; Uk37' ; SST; Zostera marina; Zostera noltii; Upper Holocene; North Sea; Sediments
1. Introduction
The North Sea is an epicontinental sea located on the
northwest European passive continental margin and has
been the focus of numerous studies. Besides biological
and physical research a lot of eort has been put into
the reconstruction of the origin of the sedimentary
matter. The Norwegian Sea, the Atlantic Ocean and the
Baltic Sea are the main sources of suspended sediments
supplied to the North Sea. Furthermore, atmospheric
input, primary production, rivers and coastal erosion
add to the suspended and bed load of the North Sea
(McManus and Prandle, 1997). Riverine input is extremely limited (Eisma et al., 1982). Of the organic carbon
that is presently deposited in the North Sea about 20%
* Corresponding author.Tel.: +31-222-319567; fax: +31222-31967.
E-mail address: dongen@nioz.nl (B.E. van Dongen).
is estimated to be of terrigenous origin, the remainder is
supplied by primary production (Beckmann and Liebezeit, 1988) and a small amount by import from the
Norwegian Sea (de Haas and van Weering, 1997).
Biological data series collected from the North Sea
and Wadden Sea have yielded a wealth of information
on the decadal variability within these marine environments during the last few decades (e.g. Kickel et al.,
1989; Beukema, 1990; Witbaard and Klein, 1994).
However, this timespan is too short to determine longterm variations of the North Sea and Wadden Sea ecosystems. Therefore, it is necessary to derive long-term
information from other sources. One of these sources
could be the sedimentary archives. Until now no
attempt has been made to investigate the biomarker
composition of sediment cores from the North Sea area.
Changes in ecosystems may be recorded in the sedimentary biomarker record. For example, the sedimentary unsaturated long-chain ketones, biosynthesized by
the coccolithophorid (Prymnesiophyta) Emiliania hux-
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00125-X
1534
B.E. van Dongen et al. / Organic Geochemistry 31 (2000) 1533±1543
leyi (Volkman et al., 1980) and Geophryrocapsa oceanica
(Volkman et al., 1995) enable the determination of
0
theUk37 (e.g. Brassell et al., 1986; Prahl and Wakeham,
1987; MuÈller et al., 1998) This ratio of C37 alkenones
with two and three double bounds can be used to
reconstruct the sea surface temperatures SSTs. Biomarker distributions may also re¯ect the biologic input
into sediments, giving insight to the composition of the
ecosystem(s) in the recent past. Thus, the biomarker
information contained in sequences of undisturbed
sediments may be used to explore the possible existence
of long-term and short-term variations in the North Sea
ecosystem over time spans of hundreds or thousands of
years. In this paper we report the biomarker composition in sediment cores from ®ve dierent stations in the
North Sea and Wadden Sea and discuss their potential
to reconstruct climate ¯uctuations eecting the North
Sea during the upper Holocene. In addition, we assess
the contribution of seagrass to the biomarker ®ngerprints of Wadden Sea sediments.
2. Study area
The depth of the North Sea gradually decreases from
approximately 200 m in the northern part to less than 30
m in the south. To the Northeast, in the Skagerrak
region, the sea ¯oor steeply slopes down to more than
700 m. In the nutrient-rich waters of the shallow
Southern Bight and along the eastern boundary most of
the primary production occurs. Counterclockwise currents transport ®ne-grained suspended material from the
Southern Bight along the eastern boundaries of the
North Sea towards the Skagerrak (Eisma and Kalf,
1987; Fig. 1). The deep Skagerrak is assumed to be the
ultimate sink for organic matter (van Weering et al.,
1993). A core was taken at a station (3) where the sedimentation rate is extremely high according to de Haas
and van Weering (1997). Other sediment cores were
taken at stations along the eastern boundaries of the
North Sea; the Norwegian channel, Skagerrak, German
Bight and the Wadden Sea. The sediment core from the
Wadden Sea was taken in the `Vlieter'. The `Vlieter' is
an abandoned channel in the Dutch Wadden Sea, which
used to be the main drainage channel connecting the
former `Zuiderzee' and the western Wadden Sea, before
closure of the dike called ``Afsluitdijk'' in 1932.
1997 using the RV `Navicula' (see Fig. 1 and Table 1 for
the exact locations of the sample sites). All the cores
show somewhat lamination and were stored at 4 C. At
least three subsamples were taken from every sediment
core from dierent depths and, if possible dierent layers, and analysed for their biomarker composition.
Additional samples were taken from the core from station 3 from the Skagerrak, to increase the time resolution.
3.2. Extraction and fractionation
The extraction, separation and identi®cation procedures followed are based on those described by Goossens et al. (1989a,b) and are schematically depicted in
Fig. 2. Subsamples were taken from the cores at several
depths (see Tables 1 and 3) and freeze-dried. The freezedried samples were Soxhlet extracted with dichloromethane/methanol (DCM/MeOH, 7.5:1, v/v) for 32 h.
In order to remove elemental sulfur, activated (2N HCl)
copper curls were added. The fresh seagrass Zostera
noltii was dried and ultrasonically extracted with DCM.
The extracts obtained were concentrated using rotary
evaporation and dried over magnesium sulfate. An aliquot (1±5 mg) of the total extract was derivatized with
diazomethane (CH2N2) in diethylether to convert fatty
acids into their corresponding methyl esters. Subsequently, very polar compounds were removed by column chromatography over silica gel with ethyl acetate as
eluent. The eluate was dried under a stream of nitrogen.
3. Material and methods
3.1. Core material
Sediment cores were taken during the `Dynamo'
cruise (28.05.96±5.06.96) using the RV `Pelagia'. Additional sediment cores were taken in the `Vlieter' in June
Fig. 1. Sampling sites in the North Sea.
1535
B.E. van Dongen et al. / Organic Geochemistry 31 (2000) 1533±1543
Table 1
Location, water depth, type of core, estimated average sedimentation rate and core length analysed of the sediment cores from the
North Sea and Wadden Sea
Area
Station
Position
Water depth (m)
Core typea
Estimated average
sedimentation rate
(mm yearÿ1)
Core length
analysed (cm)
Skagerrak
3
5
7-1c
58 46.290 N±10 08.580 E
57 36.590 N±07 34.790 E
53 01.570 N±05 04.190 E
235
316
6.5
p
p
b
8±10b
1±2b
9±11c
200
100
30
14
17
60 40.720 N±04 35.150 E
54 06.350 N±08 06.080 E
358
22
p
p
2-4b
±
100
100
The ``Vlieter''
(Wadden Sea)
Norwegian Channel
German Bight
a
b
c
p, piston core; b, box core.
de Haas and van Weering (1997).
Unpublished results.
Fig. 2. Extraction and isolation scheme.
This fraction was dissolved in pyridine and bis(trimethylsilyl)tri¯uroacetamide (BSTFA) was added.
This mixture was heated (60 C; 20 min) to convert
alcohols into their corresponding trimethylsilyl ethers.
The compounds present were analysed by means of gas
chromatography (GC) and gas chromatography-mass±
spectrometry (GC±MS).
Dried residues after extraction and aliquots of the
underivatized extracts were saponi®ed by re¯uxing with
1N KOH/MeOH (1 h). After cooling the reaction mixtures were neutralised with 2N HCl/MeOH (1:1; v/v) to
pH 3.5, centrifuged and transferred to a separation
funnel. The residues were washed subsequently with
MeOH/H2O, MeOH and DCM. Each time the supernatants were transferred to the separation funnel. Water
was added and the DCM-layers were separated. The
MeOH/H2O layers were washed with DCM (2). The
combined DCM layers were concentrated using rotary
evaporation, dried over magnesium sulfate and derivatized as described above.
The dried residues after saponi®cation were also
treated with 4N HCl (6 h; 100 C). After cooling the
reaction mixture was neutralised with 16N KOH to pH
8.5, freeze-dried and saponi®ed as above. The extracts
were washed, separated and derivatized as described
above.
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B.E. van Dongen et al. / Organic Geochemistry 31 (2000) 1533±1543
Table 2
Relative abundancea of biomarker classes found in the total
extracts (fraction E1) of samples from the dierent sampling
stations
Station
Compound classes
Alcohols
n-Alkanes
Alkenones
Fatty acids
C22
Hopanoic acids
C3217a,21b (H)
C3217b,21b (H)
Hopanols
C3217b,21b (H)
o-Hydroxy fatty acids
Methyl ketones
Mid-chain alcohols
Sterols
5a-cholestan-3b-ol
desmethyldinosterol
dinosterol
5b-sitostanol
b-sitosterol
Wax esters
a
3
5
7-1c
+
+
++
++ ++
+
+
ÿ
ÿ
14
17
++
+
ÿ
++
+
ÿ
+/ÿ +
+
+
+/ÿ +/ÿ +/ÿ
+
+
+/ÿ
ÿ
+
ÿ
ÿ
+
+
ÿ
ÿ
ÿ
ÿ
+
ÿ
ÿ
+/ÿ
+
ÿ
ÿ
ÿ
+
+
ÿ
+
+
ÿ
ÿ
ÿ
+
ÿ
+
+
ÿ
ÿ
+
+
+
ÿ
ÿ
ÿ
+
+
+
ÿ
+
+
ÿ
+
+
+
+
+/ÿ
+
+
+
ÿ
+
+
+
+
+
ÿ
++ Abundant, + present, +/ÿtrace and ÿ absent.
3.4. Gas chromatography±mass spectrometry (GC±MS)
GC±MS was performed using a Hewlett-Packard
5890 gas chromatograph interfaced to a VG Autospec
Ultima mass spectrometer operated at 70 eV with a
mass range of m/z 50±800 and a cycle time of 1.7 s
(resolution 1000). The gas chromatograph was equipped
with a fused silica capillary column (25 m 0.32 mm)
coated with CP Sil-5 (®lm thickness=0.12 mm) The
carrier gas was helium. The samples were on column
injected at 70 C and subsequently the oven was programmed to 130 C at 20 C/min and then at 4 C/min to
310 C at which it was held for 10 min. Compounds were
identi®ed by comparison of mass spectra and retention
times with those reported in literature.
3.5. Microscopy
The microscopic determination of the seagrass parts
was performed on a Zeiss Axiophot microscope with 40
magni®cation.
Table 3
0
Core depth and Uk37 values of samples from sediment station 3
Core depth (cm)
Uk37' a
25±27
27±28
28±29
29±30
30±31
31±32
50±52
75±77
110±111
189±190
0.58
0.59
0.57
0.58
0.58
0.59
0.58
0.59
0.58
0.59
a
by the HP 6890 were detected by a ¯ame ionisation
detector (FID). Compounds separated by the HP 5890
were detected simultaneously by FID and a sulfurselective ¯ame photometric detector (FPD), using a
stream splitter at the end of the column (split ratio FID:
FPD=1:1). The samples were dissolved in ethyl acetate
and injected at 70 C. Subsequently, the oven was programmed to 130 C at 20 C/min and then at 4 C/min to
310 C at which it was held for 15 min.
0
Uk37 =[C37:2]/[C37:2+C37:3].
4. Results
4.1. General
Cores from the North Sea and Wadden Sea were
analysed for their biomarker composition. Because the
core of the Wadden Sea revealed a signi®cant input of
seagrass the leaf fragments found in the core were analysed microscopically. In order to compare the two species of seagrass present in the Wadden Sea and to verify
which one is present in the core, fresh Z. noltii was also
analysed chemically. The biomarker composition of
Zostera marina was already known (de Leeuw et al.,
1995).
4.2. North Sea stations
3.3. Gas chromatography
GC was performed using a Hewlett Packard 6890 or a
Hewlett Packard 5890 instrument, both equipped with
on-column injectors. A fused silica capillary column (25
m 0.32 mm) coated with CP-Sil-5 (®lm thickness 0.12
mm) using helium as carrier gas. Compounds separated
Analyses of the total extracts (fraction E1; Fig. 2) of
samples from sediment cores of the North Sea stations
revealed mainly biomarkers of terrestrial origin (Fig. 3
and Table 2). Especially C22±C30 alcohols, C24±C32 fatty
acids (both with an even-over-odd carbon number predominance) and C25±C31 n-alkanes (odd-over-even car-
B.E. van Dongen et al. / Organic Geochemistry 31 (2000) 1533±1543
1537
Fig. 3. Gas chromatograms of the total extracts (fraction E1; Fig. 2) from (a) Skagerrak (station 3), (b) Skagerrak (station 5), (c) Norwegian Channel (station 14) and (d) German
Bight (station 17). Numbers indicate carbon chain length and IS= internal standard.
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B.E. van Dongen et al. / Organic Geochemistry 31 (2000) 1533±1543
bon number predominance) were found. Also present,
but in a signi®cant lower amount, was a complex mixture of sterols (predominantly: b-sitosterol, 5b-sitostanol, dinosterol, desmethyldinosterol and 5a-cholestan3b-ol) and hopanols [mainly 17b,21b(H)-bishomohopan-32-ol].
Analyses of samples from the sediment core from
station 3 of the Skagerrak revealed also high amounts of
di- and tri-unsaturated C37±C39 long-chain alkenones.
The analyses of dierent sections of the sediment core 3
0
showed no signi®cant change in Uk37 as shown in Table
3.
Fig. 4. Gas chromatograms of (A) the total extract, fraction E1, (B) the residue hydrolysed under base conditions, fraction E3, and
(C) the residue hydrolysed under acid conditions, fraction E4, of sediments from the Wadden Sea station. Numbers indicate carbon
chain length. i, iso branched, ai, anteiso branched and n, normal b-hydroxy fatty acid.
B.E. van Dongen et al. / Organic Geochemistry 31 (2000) 1533±1543
4.3. Wadden Sea station
Like all sediments investigated from the North Sea
stations, the total extract (fraction E1) of samples from
sediments core of the Wadden Sea station revealed
predominantly terrestrial biomarkers (Fig. 4a). In contrast, signi®cant amounts of o-hydroxy fatty acids
were also found (Table 2). High amounts of C16±C28
o-hydroxy fatty acids were released from the residues
by base hydrolysis (fraction E3; Fig. 4b). High
amounts of C22-C27 a- and C12-C20 b-hydroxy fatty
acids as well as trace amounts of C22±C24 a,b-dihydroxy fatty acids were released from the residues by
acid hydrolysis (fraction E4; Fig. 4c). Since the mass
spectra of the b-hydroxy fatty acids do not contain suf®cient information to distinguish iso (i)-, anteiso (ai)branched or normal (n)-hydroxy fatty acids, these
assignments are based upon relative retention times in
analogy to the identi®cation of hydroxy fatty acids by
Boon et al. (1977).
Further investigation of the sediments from the
Wadden Sea station revealed a signi®cant amount of 2methylmercaptobenzthiazole (MTBT) and N-methylmercaptobenzthiazole (NMTBT) as shown in Fig. 5.
1539
are released after acid treatment (fraction E4; Fig. 7).
The b-hydroxy fatty acids are absent or virtually absent
in all fractions of the living seagrass. The o-hydroxy
fatty acids are only encountered in fraction E2. Only
traces of C22±C24 a,b-dihydroxy fatty acids were found
in fraction E4.
If the quantities of fatty acids and hydroxy fatty acids
in fresh Z. noltii are compared with those in fresh Z.
marina, analysed by de Leeuw et al. (1995), only fraction
E4 reveals any signi®cant dierences (Fig. 7). The Z.
marina E4 fraction contains a signi®cant amount of
4.4. Seagrass
Microscopic observations of leaf fragments found in
the sediments of the Wadden Sea station revealed a signi®cant amount of the eelgrass Z. marina (Fig. 6), which
chemical composition is reported by de Leeuw et al.
(1995). The chemical analysis of the other potential
species, Z. noltii, revealed a similar fatty acid distribution patterns in all fractions. Besides the C16 saturated
fatty acid a signi®cant contribution comes from the
unsaturated C18 fatty acid. The a-hydroxy fatty acids
Fig. 5. Partial gas chromatogram from total extract E1 of
sediment from the Wadden Sea station with (a) a ¯ame ionisation detector and (b) a sulfur-selective ¯ame photometric
detector. *2-methylmercaptobenzthiazole (MTBT); **Nmethylmercaptobenzthiazole (NMTBT).
Fig. 6. Photomicrographs of leaves from (a) Z. marina found
in sediment core of the Wadden Sea station (b) fresh Z. marina
and (c) fresh Z. noltii.
1540
B.E. van Dongen et al. / Organic Geochemistry 31 (2000) 1533±1543
Fig. 7. Overview of the relative quantities of fatty acids and hydroxy fatty acids in fresh Zostera noltii and sediment of the Wadden
Sea station compared with the quantities found for fresh and decomposing Z. marina (de Leeuw et al., 1995).
b-hydroxy fatty acids and a,b-dihydroxy fatty acids but
only traces were found in Z. noltii. Furthermore, the
contribution of unsaturated fatty acids, especially the
unsaturated C18 fatty acid, is relatively higher in the Z.
noltii than in the Z. marina.
5. Discussion
5.1. All stations
In total extracts (fraction E1) of all the sediments
terrestrial biomarkers dominate. This is most likely due
to the oxic conditions experienced by the organic material during frequently occurring transport through
resuspension and sedimentation in the North Sea. This
process likely leads to an enrichment of the more
refractory terrestrial biomarkers relative to the more
labile marine biomarkers (Haddad et al., 1992; Harvey,
1994; Santos et al., 1994; Canuel and Martens, 1996;
Colombo et al., 1997; Budge and Parrish, 1998). Our
samples contain signi®cant though lower amounts of
marine sterols (i.e. dinosterol and desmethyldinosterol)
and hopanols. This indicates a contribution of dino¯agellate algae (Shimizu et al., 1976; Boon et al., 1979;
Withers, 1983) and bacteria (Rohmer et al, 1992; Yunker et al., 1995), respectively. These biomarkers may
provide some palaeo-environmental information. However, the bioturbation, the long residence time in the
basin, the transport through the North Sea, sedimentation and erosion must have caused considerable disturbance of the record. This and the relative small
amounts of the biomarkers found, makes them less useful for palaeo-environmental reconstruction.
5.2. Skagerrak (station 3)
Only the sediments from the Skagerrak (station 3)
revealed a signi®cant amount of unsaturated C37±C39
long-chain alkenones, probably derived from blooms of
Emiliania huxleyi in the area (Buitenhuis et al., 1996).
The long-chain alkenones are only found in this area
probably due to the extremely high sedimentation rates
in combination with the depth. Analyses of dierent
sections in the sediment core showed an average SST of
11±12 C without any change in temperature variation,
indicating no apparent short-and long-term variation in
SST. Dauwe et al. (1998) recently reported the presence
of fauna up to 20 cm depth in the sediment causing a lot
of disturbance and mixing in the Skagerrak. This will
0
make it impossible to determine Uk37 and SST changes
within a short period of time (yearly to decennial). The
absence of long-term variations (decennial to centennial) in SST can be caused by the fact that there are
no SST changes over a longer period of time or they are
too small to be noticed.
5.3. Wadden Sea station
The high amounts of o-hydroxy fatty acids, in fractions E1 and E3, and a- and b-hydroxy fatty acids, in
fraction E4, indicates a high input of seagrass in the
sediment of the `Vlieter'. The composition of the
hydroxy fatty acids is similar to that reported by de
Leeuw et al. (1995) in decomposing eelgrass Z. marina.
The relative small amounts of a,b-dihydroxy fatty acids,
found in fraction E4, is worthy of note since a,b-dihydroxy fatty acids have only been encountered inZ. marina and Z. muelleri. Hence they may be highly speci®c
B.E. van Dongen et al. / Organic Geochemistry 31 (2000) 1533±1543
biomarkers for these seagrasses as proposed by de
Leeuw et al. (1995). Fresh Z. noltii, the other seagrass
present in the Wadden Sea (Phillippart, 1994), also
contains traces of a,b-dihydroxy fatty acids in fraction
E4. This makes it chemically impossible to distinguish Z.
marina from Z. noltii. The leaf fragments found in the
sediment core of the Wadden Sea station were microscopically analysed (Fig. 6) and when compared with
leaves from fresh Z. marina and Z. noltii, the presence of
Z. marina was con®rmed. This makes the presence of
remnants of Z. noltii in the sediment core less likely.
De Leeuw et al. (1995) also analysed decomposing Z.
marina, about 12.5 years old. These fractions were
compared with the same fractions from the sediment
core of the Wadden Sea station. Especially fraction E4
gave a completely dierent picture. In this fraction of
the sediment from the Wadden Sea station the ahydroxy fatty acids are relatively low and the a,b-dihydroxy fatty acids are only present in trace amounts. The
presense of a signi®cant input of b-hydroxy fatty acids
indicates the presence of bacterial biomass, because isoand anteiso-b-hydroxy fatty acids occur as amide-bound
moieties of lipopolysaccharides in gram-negative bacteria (Goossens et al., 1986). This could explain that not
only the Z. marina contributed to this fraction but there
was also a signi®cant contribution of fatty acids and bhydroxy fatty acids coming from bacteria. It must also be
considered that the sediments of the Wadden Sea station
analysed covered the last 25 years but theZ. marina present is probably much older material (as will be explained
below) and therefore the a,b-dihydroxy fatty acids and
the a-fatty acids can be more biodegraded.
Until the early 1930s, the Z. marina was present in
large quantities in the Wadden Sea (Oudemans et al.,
1870; van Goor, 1919; van Eerde, 1942; de Jonge and
Ruiter, 1996). After the epidemic that struck the North
Atlantic population of seagrass in the early thirties,
known as the `wasting disease' (Reigersman et al., 1939;
den Hartog, 1987) and the closure of the `Zuiderzee' in
1932 the Z. marina was almost wiped out and failed to
return in the Wadden Sea (Michaelis et al., 1971; Short
et al., 1988). Nowadays there are only a few small beds
left near the island `Terschelling' (Dijkema et al., 1989;
Philippart, 1994). This means that the Z. marina present
in the `Vlieter' can originate from these beds transported
by the ¯ow from the Vlie basin towards the Marsdiep
basin and passing the `Vlieter' (Ridderinkhof et al.,
1990), if it is fresh, new, seagrass. However, the amounts
of eelgrass found in the sediments of the Wadden Sea
station are relatively high and it is unlikely that it originates from the few small beds near `Terschelling'. An
alternative explanation is that it is old material, which
was deposited somewhere else, washed away and redeposited in the `Vlieter' much later. After the closure of
the `Zuiderzee', increased the ¯ow in some channels in
the Wadden Sea resulted in a lot of erosion. Glim et al.
1541
(1987) for instance reported an increased ¯ow in the
`Doove Balg', a channel near the `Vlieter' causing a lot of
erosion near the `Meerwaard', a large old Z. marina bed.
The high amounts of terrestrial biomarkers and the
high input of eelgrass in the sediments suggest that the
`Vlieter' is one of the major depositional sites of the
Wadden Sea ecosystem. This is further supported by the
input of 2-methylmercaptobenzthiazole and N-methylmercaptobenzthiazole (Fig. 5). Benzothiazoles have
gained widespread application in industrial processes.
They are well-known vulcanisation accelerators (Fiehn
et al., 1994) in the rubber industry, manufacture of
rubber tyres, and can be found in street runo (Spies et
al., 1987). MBTB is an impurity that appears to be
environmentally stable. NMBTB is not a well-known
impurity but may be formed by a methyl shift of MBTB
during the vulcanisation process. Where these compounds originate from is not clear. It is possible they
come from the ``Afsluitdijk'' on which a highway is
present. Another possibility is that they are originating
from somewhere else and through transport (rainwater,
rivers, etc.) end up in the `Vlieter'. The appearance of
these compounds in the sediment in combination with
the high sedimentation rates makes it more likely that
the `Vlieter' is a major depositional site of the Wadden
Sea ecosystem.
6. Conclusions
1. Except for the Skagerrak, where alkenones are
present, the North Sea sediments contain predominantly terrestrial biomarkers.
2. Bioturbation in the Skagerrak is far too high to
determine SST within a short period of time
(yearly to decennial) and the changes in averaged
SST (over approximately the last 200 years) seem
to be too small to be noticed.
3. The sediments from these stations are thus not
useful to reconstruct climate ¯uctuations during
the upper Holocene.
4. High amounts of a-, b- and o-hydroxy fatty acids
and traces of a,b-dihydroxy fatty acids indicate a
signi®cant input of the eelgrass Z. marina in the
Wadden Sea site. This is the ®rst time that these
a,b-dihydroxy fatty acids are found in a sediment
core and that they have proven to be potential
biomarkers for these classes of eelgrass.
5. The `Vlieter' is a major depositional site of the
Wadden Sea system.
Acknowledgements
C.J.N. Winter is gratefully acknowledged for his
assistance with collecting of the core samples and G.
1542
B.E. van Dongen et al. / Organic Geochemistry 31 (2000) 1533±1543
Nieuwland for his help with taking of the microscopic
photos. We wish to thank Dr. H.J. Lindeboom, Dr. S.
Schouten and Dr. G.J.M. Versteegh for helpful discussions. The research was ®nancially supported by the
Netherland-Bremen Oceanography Program (NEBROC). Sampling of the sediment cores was performed
within the Frame-work of an EU-project (DYNAMO;
Fair-ct95-0710). This is NIOZ contribution 3414.
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www.elsevier.nl/locate/orggeochem
Biomarkers in upper Holocene Eastern North Sea and
Wadden Sea sediments
Bart E. van Dongen a,*, W. Irene C. Rijpstra a, Catharina J. M. Philippart b,
Jan W. de Leeuw a, Jaap S. Sinninghe Damste a
a
Department of Marine Biogeochemistry and Toxicology, Netherlands Institute for Sea Research (NIOZ), PO Box 59,
1790 AB Den Burg, Texel, The Netherlands
b
Department of Marine Ecology, Netherlands Institute for Sea Research (NIOZ), PO Box 59, 1790 AB Den Burg,
Texel, The Netherlands
Abstract
Total extracts of sediment cores from ®ve dierent stations in the North Sea and Wadden Sea were analysed for their
biomarker composition. Only sediments of the Skagerrak contained signi®cant amounts of marine biomarkers (mainly
alkenones), other sites contained predominantly terrestrial biomarkers. Bioturbation in the Skagerrak is, however, far
too high to determine sea surface temperature (SST) changes within short time intervals. These results indicate that
biomarkers contained in these sediments are not useful to reconstruct climate ¯uctuations during the upper Holocene.
High amounts of a-, b- and o-hydroxy fatty acids as well as small amounts of a, b-dihydroxy fatty acids were released
from the insoluble organic material of the sediments from the Wadden Sea station, indicating a signi®cant input of the
eelgrass Zostera marina. This was con®rmed by microscopic observations. This is the ®rst time the a,b-dihydroxy fatty
acids have been found in a sediment core and they have proven to be potential biomarkers for these seagrass species.
# 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Biomarkers; Uk37' ; SST; Zostera marina; Zostera noltii; Upper Holocene; North Sea; Sediments
1. Introduction
The North Sea is an epicontinental sea located on the
northwest European passive continental margin and has
been the focus of numerous studies. Besides biological
and physical research a lot of eort has been put into
the reconstruction of the origin of the sedimentary
matter. The Norwegian Sea, the Atlantic Ocean and the
Baltic Sea are the main sources of suspended sediments
supplied to the North Sea. Furthermore, atmospheric
input, primary production, rivers and coastal erosion
add to the suspended and bed load of the North Sea
(McManus and Prandle, 1997). Riverine input is extremely limited (Eisma et al., 1982). Of the organic carbon
that is presently deposited in the North Sea about 20%
* Corresponding author.Tel.: +31-222-319567; fax: +31222-31967.
E-mail address: dongen@nioz.nl (B.E. van Dongen).
is estimated to be of terrigenous origin, the remainder is
supplied by primary production (Beckmann and Liebezeit, 1988) and a small amount by import from the
Norwegian Sea (de Haas and van Weering, 1997).
Biological data series collected from the North Sea
and Wadden Sea have yielded a wealth of information
on the decadal variability within these marine environments during the last few decades (e.g. Kickel et al.,
1989; Beukema, 1990; Witbaard and Klein, 1994).
However, this timespan is too short to determine longterm variations of the North Sea and Wadden Sea ecosystems. Therefore, it is necessary to derive long-term
information from other sources. One of these sources
could be the sedimentary archives. Until now no
attempt has been made to investigate the biomarker
composition of sediment cores from the North Sea area.
Changes in ecosystems may be recorded in the sedimentary biomarker record. For example, the sedimentary unsaturated long-chain ketones, biosynthesized by
the coccolithophorid (Prymnesiophyta) Emiliania hux-
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00125-X
1534
B.E. van Dongen et al. / Organic Geochemistry 31 (2000) 1533±1543
leyi (Volkman et al., 1980) and Geophryrocapsa oceanica
(Volkman et al., 1995) enable the determination of
0
theUk37 (e.g. Brassell et al., 1986; Prahl and Wakeham,
1987; MuÈller et al., 1998) This ratio of C37 alkenones
with two and three double bounds can be used to
reconstruct the sea surface temperatures SSTs. Biomarker distributions may also re¯ect the biologic input
into sediments, giving insight to the composition of the
ecosystem(s) in the recent past. Thus, the biomarker
information contained in sequences of undisturbed
sediments may be used to explore the possible existence
of long-term and short-term variations in the North Sea
ecosystem over time spans of hundreds or thousands of
years. In this paper we report the biomarker composition in sediment cores from ®ve dierent stations in the
North Sea and Wadden Sea and discuss their potential
to reconstruct climate ¯uctuations eecting the North
Sea during the upper Holocene. In addition, we assess
the contribution of seagrass to the biomarker ®ngerprints of Wadden Sea sediments.
2. Study area
The depth of the North Sea gradually decreases from
approximately 200 m in the northern part to less than 30
m in the south. To the Northeast, in the Skagerrak
region, the sea ¯oor steeply slopes down to more than
700 m. In the nutrient-rich waters of the shallow
Southern Bight and along the eastern boundary most of
the primary production occurs. Counterclockwise currents transport ®ne-grained suspended material from the
Southern Bight along the eastern boundaries of the
North Sea towards the Skagerrak (Eisma and Kalf,
1987; Fig. 1). The deep Skagerrak is assumed to be the
ultimate sink for organic matter (van Weering et al.,
1993). A core was taken at a station (3) where the sedimentation rate is extremely high according to de Haas
and van Weering (1997). Other sediment cores were
taken at stations along the eastern boundaries of the
North Sea; the Norwegian channel, Skagerrak, German
Bight and the Wadden Sea. The sediment core from the
Wadden Sea was taken in the `Vlieter'. The `Vlieter' is
an abandoned channel in the Dutch Wadden Sea, which
used to be the main drainage channel connecting the
former `Zuiderzee' and the western Wadden Sea, before
closure of the dike called ``Afsluitdijk'' in 1932.
1997 using the RV `Navicula' (see Fig. 1 and Table 1 for
the exact locations of the sample sites). All the cores
show somewhat lamination and were stored at 4 C. At
least three subsamples were taken from every sediment
core from dierent depths and, if possible dierent layers, and analysed for their biomarker composition.
Additional samples were taken from the core from station 3 from the Skagerrak, to increase the time resolution.
3.2. Extraction and fractionation
The extraction, separation and identi®cation procedures followed are based on those described by Goossens et al. (1989a,b) and are schematically depicted in
Fig. 2. Subsamples were taken from the cores at several
depths (see Tables 1 and 3) and freeze-dried. The freezedried samples were Soxhlet extracted with dichloromethane/methanol (DCM/MeOH, 7.5:1, v/v) for 32 h.
In order to remove elemental sulfur, activated (2N HCl)
copper curls were added. The fresh seagrass Zostera
noltii was dried and ultrasonically extracted with DCM.
The extracts obtained were concentrated using rotary
evaporation and dried over magnesium sulfate. An aliquot (1±5 mg) of the total extract was derivatized with
diazomethane (CH2N2) in diethylether to convert fatty
acids into their corresponding methyl esters. Subsequently, very polar compounds were removed by column chromatography over silica gel with ethyl acetate as
eluent. The eluate was dried under a stream of nitrogen.
3. Material and methods
3.1. Core material
Sediment cores were taken during the `Dynamo'
cruise (28.05.96±5.06.96) using the RV `Pelagia'. Additional sediment cores were taken in the `Vlieter' in June
Fig. 1. Sampling sites in the North Sea.
1535
B.E. van Dongen et al. / Organic Geochemistry 31 (2000) 1533±1543
Table 1
Location, water depth, type of core, estimated average sedimentation rate and core length analysed of the sediment cores from the
North Sea and Wadden Sea
Area
Station
Position
Water depth (m)
Core typea
Estimated average
sedimentation rate
(mm yearÿ1)
Core length
analysed (cm)
Skagerrak
3
5
7-1c
58 46.290 N±10 08.580 E
57 36.590 N±07 34.790 E
53 01.570 N±05 04.190 E
235
316
6.5
p
p
b
8±10b
1±2b
9±11c
200
100
30
14
17
60 40.720 N±04 35.150 E
54 06.350 N±08 06.080 E
358
22
p
p
2-4b
±
100
100
The ``Vlieter''
(Wadden Sea)
Norwegian Channel
German Bight
a
b
c
p, piston core; b, box core.
de Haas and van Weering (1997).
Unpublished results.
Fig. 2. Extraction and isolation scheme.
This fraction was dissolved in pyridine and bis(trimethylsilyl)tri¯uroacetamide (BSTFA) was added.
This mixture was heated (60 C; 20 min) to convert
alcohols into their corresponding trimethylsilyl ethers.
The compounds present were analysed by means of gas
chromatography (GC) and gas chromatography-mass±
spectrometry (GC±MS).
Dried residues after extraction and aliquots of the
underivatized extracts were saponi®ed by re¯uxing with
1N KOH/MeOH (1 h). After cooling the reaction mixtures were neutralised with 2N HCl/MeOH (1:1; v/v) to
pH 3.5, centrifuged and transferred to a separation
funnel. The residues were washed subsequently with
MeOH/H2O, MeOH and DCM. Each time the supernatants were transferred to the separation funnel. Water
was added and the DCM-layers were separated. The
MeOH/H2O layers were washed with DCM (2). The
combined DCM layers were concentrated using rotary
evaporation, dried over magnesium sulfate and derivatized as described above.
The dried residues after saponi®cation were also
treated with 4N HCl (6 h; 100 C). After cooling the
reaction mixture was neutralised with 16N KOH to pH
8.5, freeze-dried and saponi®ed as above. The extracts
were washed, separated and derivatized as described
above.
1536
B.E. van Dongen et al. / Organic Geochemistry 31 (2000) 1533±1543
Table 2
Relative abundancea of biomarker classes found in the total
extracts (fraction E1) of samples from the dierent sampling
stations
Station
Compound classes
Alcohols
n-Alkanes
Alkenones
Fatty acids
C22
Hopanoic acids
C3217a,21b (H)
C3217b,21b (H)
Hopanols
C3217b,21b (H)
o-Hydroxy fatty acids
Methyl ketones
Mid-chain alcohols
Sterols
5a-cholestan-3b-ol
desmethyldinosterol
dinosterol
5b-sitostanol
b-sitosterol
Wax esters
a
3
5
7-1c
+
+
++
++ ++
+
+
ÿ
ÿ
14
17
++
+
ÿ
++
+
ÿ
+/ÿ +
+
+
+/ÿ +/ÿ +/ÿ
+
+
+/ÿ
ÿ
+
ÿ
ÿ
+
+
ÿ
ÿ
ÿ
ÿ
+
ÿ
ÿ
+/ÿ
+
ÿ
ÿ
ÿ
+
+
ÿ
+
+
ÿ
ÿ
ÿ
+
ÿ
+
+
ÿ
ÿ
+
+
+
ÿ
ÿ
ÿ
+
+
+
ÿ
+
+
ÿ
+
+
+
+
+/ÿ
+
+
+
ÿ
+
+
+
+
+
ÿ
++ Abundant, + present, +/ÿtrace and ÿ absent.
3.4. Gas chromatography±mass spectrometry (GC±MS)
GC±MS was performed using a Hewlett-Packard
5890 gas chromatograph interfaced to a VG Autospec
Ultima mass spectrometer operated at 70 eV with a
mass range of m/z 50±800 and a cycle time of 1.7 s
(resolution 1000). The gas chromatograph was equipped
with a fused silica capillary column (25 m 0.32 mm)
coated with CP Sil-5 (®lm thickness=0.12 mm) The
carrier gas was helium. The samples were on column
injected at 70 C and subsequently the oven was programmed to 130 C at 20 C/min and then at 4 C/min to
310 C at which it was held for 10 min. Compounds were
identi®ed by comparison of mass spectra and retention
times with those reported in literature.
3.5. Microscopy
The microscopic determination of the seagrass parts
was performed on a Zeiss Axiophot microscope with 40
magni®cation.
Table 3
0
Core depth and Uk37 values of samples from sediment station 3
Core depth (cm)
Uk37' a
25±27
27±28
28±29
29±30
30±31
31±32
50±52
75±77
110±111
189±190
0.58
0.59
0.57
0.58
0.58
0.59
0.58
0.59
0.58
0.59
a
by the HP 6890 were detected by a ¯ame ionisation
detector (FID). Compounds separated by the HP 5890
were detected simultaneously by FID and a sulfurselective ¯ame photometric detector (FPD), using a
stream splitter at the end of the column (split ratio FID:
FPD=1:1). The samples were dissolved in ethyl acetate
and injected at 70 C. Subsequently, the oven was programmed to 130 C at 20 C/min and then at 4 C/min to
310 C at which it was held for 15 min.
0
Uk37 =[C37:2]/[C37:2+C37:3].
4. Results
4.1. General
Cores from the North Sea and Wadden Sea were
analysed for their biomarker composition. Because the
core of the Wadden Sea revealed a signi®cant input of
seagrass the leaf fragments found in the core were analysed microscopically. In order to compare the two species of seagrass present in the Wadden Sea and to verify
which one is present in the core, fresh Z. noltii was also
analysed chemically. The biomarker composition of
Zostera marina was already known (de Leeuw et al.,
1995).
4.2. North Sea stations
3.3. Gas chromatography
GC was performed using a Hewlett Packard 6890 or a
Hewlett Packard 5890 instrument, both equipped with
on-column injectors. A fused silica capillary column (25
m 0.32 mm) coated with CP-Sil-5 (®lm thickness 0.12
mm) using helium as carrier gas. Compounds separated
Analyses of the total extracts (fraction E1; Fig. 2) of
samples from sediment cores of the North Sea stations
revealed mainly biomarkers of terrestrial origin (Fig. 3
and Table 2). Especially C22±C30 alcohols, C24±C32 fatty
acids (both with an even-over-odd carbon number predominance) and C25±C31 n-alkanes (odd-over-even car-
B.E. van Dongen et al. / Organic Geochemistry 31 (2000) 1533±1543
1537
Fig. 3. Gas chromatograms of the total extracts (fraction E1; Fig. 2) from (a) Skagerrak (station 3), (b) Skagerrak (station 5), (c) Norwegian Channel (station 14) and (d) German
Bight (station 17). Numbers indicate carbon chain length and IS= internal standard.
1538
B.E. van Dongen et al. / Organic Geochemistry 31 (2000) 1533±1543
bon number predominance) were found. Also present,
but in a signi®cant lower amount, was a complex mixture of sterols (predominantly: b-sitosterol, 5b-sitostanol, dinosterol, desmethyldinosterol and 5a-cholestan3b-ol) and hopanols [mainly 17b,21b(H)-bishomohopan-32-ol].
Analyses of samples from the sediment core from
station 3 of the Skagerrak revealed also high amounts of
di- and tri-unsaturated C37±C39 long-chain alkenones.
The analyses of dierent sections of the sediment core 3
0
showed no signi®cant change in Uk37 as shown in Table
3.
Fig. 4. Gas chromatograms of (A) the total extract, fraction E1, (B) the residue hydrolysed under base conditions, fraction E3, and
(C) the residue hydrolysed under acid conditions, fraction E4, of sediments from the Wadden Sea station. Numbers indicate carbon
chain length. i, iso branched, ai, anteiso branched and n, normal b-hydroxy fatty acid.
B.E. van Dongen et al. / Organic Geochemistry 31 (2000) 1533±1543
4.3. Wadden Sea station
Like all sediments investigated from the North Sea
stations, the total extract (fraction E1) of samples from
sediments core of the Wadden Sea station revealed
predominantly terrestrial biomarkers (Fig. 4a). In contrast, signi®cant amounts of o-hydroxy fatty acids
were also found (Table 2). High amounts of C16±C28
o-hydroxy fatty acids were released from the residues
by base hydrolysis (fraction E3; Fig. 4b). High
amounts of C22-C27 a- and C12-C20 b-hydroxy fatty
acids as well as trace amounts of C22±C24 a,b-dihydroxy fatty acids were released from the residues by
acid hydrolysis (fraction E4; Fig. 4c). Since the mass
spectra of the b-hydroxy fatty acids do not contain suf®cient information to distinguish iso (i)-, anteiso (ai)branched or normal (n)-hydroxy fatty acids, these
assignments are based upon relative retention times in
analogy to the identi®cation of hydroxy fatty acids by
Boon et al. (1977).
Further investigation of the sediments from the
Wadden Sea station revealed a signi®cant amount of 2methylmercaptobenzthiazole (MTBT) and N-methylmercaptobenzthiazole (NMTBT) as shown in Fig. 5.
1539
are released after acid treatment (fraction E4; Fig. 7).
The b-hydroxy fatty acids are absent or virtually absent
in all fractions of the living seagrass. The o-hydroxy
fatty acids are only encountered in fraction E2. Only
traces of C22±C24 a,b-dihydroxy fatty acids were found
in fraction E4.
If the quantities of fatty acids and hydroxy fatty acids
in fresh Z. noltii are compared with those in fresh Z.
marina, analysed by de Leeuw et al. (1995), only fraction
E4 reveals any signi®cant dierences (Fig. 7). The Z.
marina E4 fraction contains a signi®cant amount of
4.4. Seagrass
Microscopic observations of leaf fragments found in
the sediments of the Wadden Sea station revealed a signi®cant amount of the eelgrass Z. marina (Fig. 6), which
chemical composition is reported by de Leeuw et al.
(1995). The chemical analysis of the other potential
species, Z. noltii, revealed a similar fatty acid distribution patterns in all fractions. Besides the C16 saturated
fatty acid a signi®cant contribution comes from the
unsaturated C18 fatty acid. The a-hydroxy fatty acids
Fig. 5. Partial gas chromatogram from total extract E1 of
sediment from the Wadden Sea station with (a) a ¯ame ionisation detector and (b) a sulfur-selective ¯ame photometric
detector. *2-methylmercaptobenzthiazole (MTBT); **Nmethylmercaptobenzthiazole (NMTBT).
Fig. 6. Photomicrographs of leaves from (a) Z. marina found
in sediment core of the Wadden Sea station (b) fresh Z. marina
and (c) fresh Z. noltii.
1540
B.E. van Dongen et al. / Organic Geochemistry 31 (2000) 1533±1543
Fig. 7. Overview of the relative quantities of fatty acids and hydroxy fatty acids in fresh Zostera noltii and sediment of the Wadden
Sea station compared with the quantities found for fresh and decomposing Z. marina (de Leeuw et al., 1995).
b-hydroxy fatty acids and a,b-dihydroxy fatty acids but
only traces were found in Z. noltii. Furthermore, the
contribution of unsaturated fatty acids, especially the
unsaturated C18 fatty acid, is relatively higher in the Z.
noltii than in the Z. marina.
5. Discussion
5.1. All stations
In total extracts (fraction E1) of all the sediments
terrestrial biomarkers dominate. This is most likely due
to the oxic conditions experienced by the organic material during frequently occurring transport through
resuspension and sedimentation in the North Sea. This
process likely leads to an enrichment of the more
refractory terrestrial biomarkers relative to the more
labile marine biomarkers (Haddad et al., 1992; Harvey,
1994; Santos et al., 1994; Canuel and Martens, 1996;
Colombo et al., 1997; Budge and Parrish, 1998). Our
samples contain signi®cant though lower amounts of
marine sterols (i.e. dinosterol and desmethyldinosterol)
and hopanols. This indicates a contribution of dino¯agellate algae (Shimizu et al., 1976; Boon et al., 1979;
Withers, 1983) and bacteria (Rohmer et al, 1992; Yunker et al., 1995), respectively. These biomarkers may
provide some palaeo-environmental information. However, the bioturbation, the long residence time in the
basin, the transport through the North Sea, sedimentation and erosion must have caused considerable disturbance of the record. This and the relative small
amounts of the biomarkers found, makes them less useful for palaeo-environmental reconstruction.
5.2. Skagerrak (station 3)
Only the sediments from the Skagerrak (station 3)
revealed a signi®cant amount of unsaturated C37±C39
long-chain alkenones, probably derived from blooms of
Emiliania huxleyi in the area (Buitenhuis et al., 1996).
The long-chain alkenones are only found in this area
probably due to the extremely high sedimentation rates
in combination with the depth. Analyses of dierent
sections in the sediment core showed an average SST of
11±12 C without any change in temperature variation,
indicating no apparent short-and long-term variation in
SST. Dauwe et al. (1998) recently reported the presence
of fauna up to 20 cm depth in the sediment causing a lot
of disturbance and mixing in the Skagerrak. This will
0
make it impossible to determine Uk37 and SST changes
within a short period of time (yearly to decennial). The
absence of long-term variations (decennial to centennial) in SST can be caused by the fact that there are
no SST changes over a longer period of time or they are
too small to be noticed.
5.3. Wadden Sea station
The high amounts of o-hydroxy fatty acids, in fractions E1 and E3, and a- and b-hydroxy fatty acids, in
fraction E4, indicates a high input of seagrass in the
sediment of the `Vlieter'. The composition of the
hydroxy fatty acids is similar to that reported by de
Leeuw et al. (1995) in decomposing eelgrass Z. marina.
The relative small amounts of a,b-dihydroxy fatty acids,
found in fraction E4, is worthy of note since a,b-dihydroxy fatty acids have only been encountered inZ. marina and Z. muelleri. Hence they may be highly speci®c
B.E. van Dongen et al. / Organic Geochemistry 31 (2000) 1533±1543
biomarkers for these seagrasses as proposed by de
Leeuw et al. (1995). Fresh Z. noltii, the other seagrass
present in the Wadden Sea (Phillippart, 1994), also
contains traces of a,b-dihydroxy fatty acids in fraction
E4. This makes it chemically impossible to distinguish Z.
marina from Z. noltii. The leaf fragments found in the
sediment core of the Wadden Sea station were microscopically analysed (Fig. 6) and when compared with
leaves from fresh Z. marina and Z. noltii, the presence of
Z. marina was con®rmed. This makes the presence of
remnants of Z. noltii in the sediment core less likely.
De Leeuw et al. (1995) also analysed decomposing Z.
marina, about 12.5 years old. These fractions were
compared with the same fractions from the sediment
core of the Wadden Sea station. Especially fraction E4
gave a completely dierent picture. In this fraction of
the sediment from the Wadden Sea station the ahydroxy fatty acids are relatively low and the a,b-dihydroxy fatty acids are only present in trace amounts. The
presense of a signi®cant input of b-hydroxy fatty acids
indicates the presence of bacterial biomass, because isoand anteiso-b-hydroxy fatty acids occur as amide-bound
moieties of lipopolysaccharides in gram-negative bacteria (Goossens et al., 1986). This could explain that not
only the Z. marina contributed to this fraction but there
was also a signi®cant contribution of fatty acids and bhydroxy fatty acids coming from bacteria. It must also be
considered that the sediments of the Wadden Sea station
analysed covered the last 25 years but theZ. marina present is probably much older material (as will be explained
below) and therefore the a,b-dihydroxy fatty acids and
the a-fatty acids can be more biodegraded.
Until the early 1930s, the Z. marina was present in
large quantities in the Wadden Sea (Oudemans et al.,
1870; van Goor, 1919; van Eerde, 1942; de Jonge and
Ruiter, 1996). After the epidemic that struck the North
Atlantic population of seagrass in the early thirties,
known as the `wasting disease' (Reigersman et al., 1939;
den Hartog, 1987) and the closure of the `Zuiderzee' in
1932 the Z. marina was almost wiped out and failed to
return in the Wadden Sea (Michaelis et al., 1971; Short
et al., 1988). Nowadays there are only a few small beds
left near the island `Terschelling' (Dijkema et al., 1989;
Philippart, 1994). This means that the Z. marina present
in the `Vlieter' can originate from these beds transported
by the ¯ow from the Vlie basin towards the Marsdiep
basin and passing the `Vlieter' (Ridderinkhof et al.,
1990), if it is fresh, new, seagrass. However, the amounts
of eelgrass found in the sediments of the Wadden Sea
station are relatively high and it is unlikely that it originates from the few small beds near `Terschelling'. An
alternative explanation is that it is old material, which
was deposited somewhere else, washed away and redeposited in the `Vlieter' much later. After the closure of
the `Zuiderzee', increased the ¯ow in some channels in
the Wadden Sea resulted in a lot of erosion. Glim et al.
1541
(1987) for instance reported an increased ¯ow in the
`Doove Balg', a channel near the `Vlieter' causing a lot of
erosion near the `Meerwaard', a large old Z. marina bed.
The high amounts of terrestrial biomarkers and the
high input of eelgrass in the sediments suggest that the
`Vlieter' is one of the major depositional sites of the
Wadden Sea ecosystem. This is further supported by the
input of 2-methylmercaptobenzthiazole and N-methylmercaptobenzthiazole (Fig. 5). Benzothiazoles have
gained widespread application in industrial processes.
They are well-known vulcanisation accelerators (Fiehn
et al., 1994) in the rubber industry, manufacture of
rubber tyres, and can be found in street runo (Spies et
al., 1987). MBTB is an impurity that appears to be
environmentally stable. NMBTB is not a well-known
impurity but may be formed by a methyl shift of MBTB
during the vulcanisation process. Where these compounds originate from is not clear. It is possible they
come from the ``Afsluitdijk'' on which a highway is
present. Another possibility is that they are originating
from somewhere else and through transport (rainwater,
rivers, etc.) end up in the `Vlieter'. The appearance of
these compounds in the sediment in combination with
the high sedimentation rates makes it more likely that
the `Vlieter' is a major depositional site of the Wadden
Sea ecosystem.
6. Conclusions
1. Except for the Skagerrak, where alkenones are
present, the North Sea sediments contain predominantly terrestrial biomarkers.
2. Bioturbation in the Skagerrak is far too high to
determine SST within a short period of time
(yearly to decennial) and the changes in averaged
SST (over approximately the last 200 years) seem
to be too small to be noticed.
3. The sediments from these stations are thus not
useful to reconstruct climate ¯uctuations during
the upper Holocene.
4. High amounts of a-, b- and o-hydroxy fatty acids
and traces of a,b-dihydroxy fatty acids indicate a
signi®cant input of the eelgrass Z. marina in the
Wadden Sea site. This is the ®rst time that these
a,b-dihydroxy fatty acids are found in a sediment
core and that they have proven to be potential
biomarkers for these classes of eelgrass.
5. The `Vlieter' is a major depositional site of the
Wadden Sea system.
Acknowledgements
C.J.N. Winter is gratefully acknowledged for his
assistance with collecting of the core samples and G.
1542
B.E. van Dongen et al. / Organic Geochemistry 31 (2000) 1533±1543
Nieuwland for his help with taking of the microscopic
photos. We wish to thank Dr. H.J. Lindeboom, Dr. S.
Schouten and Dr. G.J.M. Versteegh for helpful discussions. The research was ®nancially supported by the
Netherland-Bremen Oceanography Program (NEBROC). Sampling of the sediment cores was performed
within the Frame-work of an EU-project (DYNAMO;
Fair-ct95-0710). This is NIOZ contribution 3414.
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