Directory UMM :Data Elmu:jurnal:O:Organic Geochemistry:Vol31.Issue6.2000:
Organic Geochemistry 31 (2000) 577±588
www.elsevier.nl/locate/orggeochem
Biomarker stratigraphic records over the last 150 kyears o
the NW African coast at 25N
Marie-Alexandrine Sicre a,*, Yann Ternois b, Martine Paterne a, Anne Boireau c,
Luc Beaufort b, Philippe Martinez d, Philippe Bertrand d
a
Laboratoire des Sciences du Climat et de l'Environnement, CNRS SDU UMR 1572, Domaine du CNRS, Avenue de la Terrasse,
91198 Gif-sur-Yvette Cedex, France
b
CEREGE, CNRS SDU UMR 6635, Europole de l'Arbois, B.P. 80, 13545 Aix-en Provence Cedex 4, France
c
Laboratoire de Physique et Chimie Marines, Universite Pierre et Marie Curie, CNRS SDU ESA7077, 4 place Jussieu,
75252 Paris Cedex 05, France
d Âpartement
De
de GeÂologie et OceÂanographie, Universite de Bordeaux I, CNRS SDU UMR 5805, Avenue des faculteÂs,
33405 Talence Cedex, France
Received 27 July 1999; accepted 2 February 2000
(returned to author for revision 15 October 1999)
Abstract
Terrigenous and marine biomarkers were investigated in a core o Northwest Africa in the Northeast Atlantic
(25 N, 16 W, 1445 m depth) to assess changes in the sedimentation pattern of organic carbon (OC) over the last 150
kyears. Alkenone derived temperatures recorded a warming of 4.5 C during the last deglaciation. n-Alkanol Mass
Accumulation Rates (MAR) ¯uctuated in parallel with Northeast Trade Winds (NETW) intensity. OC and sterol
MAR both increased during glacial times indicating enhanced fertility of the ocean in response to intensi®ed NETW.
Alkenone/OC ratios were higher by a factor of two over stages 4±6 than stages 1±3 thus re¯ecting distinct coccolithophorid inputs. This transition coincides with a major change of alkenone producers inferred from coccolith counts.
# 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Biomarkers; Paleoceanography; Alkenones; Alkanols; Sterols; Africa; Upwelling; North Atlantic
1. Introduction
The NW African coast is a region of intense primary
production, today. Wind-driven coastal upwellings
occur under the in¯uence of longshore northeasterlies,
blowing towards the equator, bringing to the surface
cold and nutrient-rich waters. The cold waters of the
Canary Current, ¯owing southwards are another
hydrological feature that in¯uences the sea surface temperatures (SSTs). Production and sedimentation of
organic carbon (OC) in this region is in¯uenced by the
* Corresponding author: Tel.: +33-1-69-82-43-34; fax:
+33-1-69-82-35-68.
E-mail address: marie-alexandrine.sicre@lsce.cnrs-gif.fr (M.A. Sicre).
supply of terrigenous material and biogenic sedimentation stimulated by the Northeast Trade Winds (NETW).
However, the intensity of the upwelling cells is not uniform along the coast. It is stronger o Cape Blanc
leading to high productivity and sedimentation rates.
Upwelled waters are the North Atlantic Central Waters
(NACW), North of Cape Blanc (21 N), and the South
Atlantic Central Waters (SACW), South of Cape Blanc
(Tomczak, 1977). MuÈller et al. (1983) have shown that
around 25 N, OC production and sedimentation have
varied considerably over the late Quaternary. Paleoproductivity estimates derived from OC measurements have
shown that during glacial time productivity was higher
as the result of intensi®ed upwelling in response to
stronger atmospheric circulation (MuÈller et al., 1983).
Although organic matter can be traced in general terms
by measuring OC, the distribution of speci®c biomarkers
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00021-8
578
M.-A. Sicre et al. / Organic Geochemistry 31 (2000) 577±588
allows a more detailed examination of the source material deposited to the sediment. In this work we report
our results on several classes of biomarkers analyzed in
the SU94-20bK core (25 N, 16 W, 1445 m water depth)
collected during the Sedorqua cruise in February 1994,
on the R/V Le Suroit (Fig. 1). This coring site is located
on the continental slope o Cape Bojador which makes
it an ideal site to study the impact of changing NETW
regime on the nature and pattern of OC production and
sedimentation. Several surface sediments were also
taken along the coast to determine the spatial distribution of upwelling cells, today.
Three classes of biomarkers were selected for this
study to determine the temporal variations of terrigenous and marine source inputs of OC and apply this
information to describe past climatic changes over the
last climatic cycle. Alkenones, which are undoubtly the
Fig. 1. Map of the NW African continental margin showing
the locations of the surface sediments and SU94-20bK core
collected during the Sedorqua cruise. Also shown are the sites
of ODP 658A, 659 and 660A and Meteor cores M12392-1 and
M16415-2. The percentage of OC and SST values are indicated
for each site when available.
most widely used class of biomarkers for paleoclimatic
and paleoceanographic studies, were measured to estimate paleo-SSTs. The C37 alkenone unsaturation index
0
UK
37 has previously been shown to be linearly dependent
to growth temperature (Prahl and Wakeham, 1987;
Prahl et al., 1988; Conte and Eglinton., 1993; Sikes and
Volkman, 1993; Sikes et al., 1997; Ternois et al., 1997).
Core-top statistical evaluations con®rmed the existence
of this relationship on a global scale (Sikes et al., 1991;
Rosell-Mele et al., 1995; Sonzogni et al., 1997; MuÈller et
al., 1998). Although several studies demonstrated that
alkenones are not immune to degradation (Marlowe,
0
1984; McCarey et al., 1990; Conte et al., 1992), the UK
37
index, on which paleo-temperature calculations are
based does not seem to be aected by food web processes (Volkman et al., 1980) or early diagenesis (Prahl
et al., 1989; Madureira et al., 1995; Sicre et al., 1999).
Therefore, it has been successfully used for SSTs reconstruction in the past (Brassell et al., 1986; Jasper and
Gagosian, 1989; Prahl et al., 1989; McCarey et al.,
1990; ten Haven and Kroon, 1991; Eglinton et al., 1992;
Lyle et al., 1992; Kennedy and Brassell, 1992a; Kennedy
and Brassell, 1992b; Rostek et al., 1993; Wakeham,
1993; Zhao et al., 1993; Ohkouchi et al., 1994; Sikes and
Keigwin, 1994; Prahl et al., 1995; Schneider et al., 1995;
Zhao et al., 1995; Bard et al., 1997; Madureira et al.,
1997; Rostek et al., 1997; Villanueva et al., 1998a; Villanueva et al., 1998b; Ternois et al., 2000). More
recently, alkenones have been used to track down inputs
from some Haptophyte algae in long-term sedimentary
records (Rostek et al., 1997; Villanueva et al., 1998a;
Villanueva et al., 1998b). These compounds are mostly
biosynthesized by the marine coccolithophorid Emiliania huxleyi as well as the closely related Gephyrocapsa
species (Volkman et al. 1980; Marlowe et al., 1990;
Volkman et al., 1995). Alkenones were also used here to
assess inputs from coccolithophorids.
Few studies have applied biomarkers other than
alkenones as tracers for determining paleoclimate conditions. The abundance of long-chain saturated nalkanes, n-alkanols and fatty acids have been shown to
track terrigenous inputs to marine sediments (Poynter et
al., 1989; Prahl et al., 1989; Madureira et al., 1997;
Ohkouchi et al., 1997; Villanueva et al., 1998a; Villanueva et al., 1998b). We selected high molecular weight
(HWM) n-alkanols (C20-C32) for their abundance and
speci®ty to trace detrital material while 4-desmethyl
sterols (C27-C29) were quanti®ed to evaluate the overlying water's planktonic production (Volkman, 1986,
ten Haven et al., 1989; Farrimond et al., 1990). Changes of SSTs and Mass Accumulation Rates (MAR) of
OC, n-alkanols, alkenones and sterols are discussed in
relation to climatic changes over the last 150 kyears.
Basic data (OC, CaCO3 and opal) including the
chronology have already been published in Martinez et
al. (1996).
M.-A. Sicre et al. / Organic Geochemistry 31 (2000) 577±588
2. Methods
2.1. Oxygen isotope stratigraphy
Oxygen isotope analyses were performed along the
SU94-20bK core on planktonic foraminifera Globigerina bulloõÈdes (>250 mm) on a Finnigan 251 mass spectrometer calibrated in PDB via NBS19. External
precision was 0.07% d18O (1 sigma value). The d18O
chronology determination is described in Martinez et al.
(1996).
2.2. Biomarker analyses
The SU94-20bK sediment core was subsampled on
board and frozen at ÿ18 C. All the surface sediments
were taken with a box-corer and frozen on board
shortly after recovery. About 5 g aliquots of frozen
sediment were freeze-dried for biomarker analyses.
Extractable lipids were isolated by solvent extraction in
an ultrasonic bath for 10 min. The two ®rst extractions
were performed with methylene chloride/methanol (2:1,
v/v) and the third one with methanol. The extracts were
combined, concentrated by rotary evaporation and
transferred into 4 ml vials. Lipids were fractionated into
compound classes by silica gel chromatography following the procedure described by Conde (1989). Fractions
were stored in 4 ml vial at ÿ18 C until used for gas
chromatographic analyses.
Alkenones were analyzed on a Delsi DI 200 gas
chromatograph equipped with a fused silica CP-Sil-5
capillary column (50 m 0.32 mm i.d., 0.25 mm ®lm
thickness, Chrompack) and a ¯ame ionization detector.
Each alkenone fraction was injected three times. Helium
was used as the carrier gas (25 ml minÿ1). The oven
temperature was programmed from 100 to 300 C at
0
10 C minÿ1 (60 min). UK
37 ratios were calculated from
chromatographic peak areas. Analytical precision
obtained after triplicate injections was calculated to be
0.01 unit ratio. The fractions containing n-alkanols and
sterols were treated with bis(trimethylsilyl)-tri¯uoroacetamide (BSTFA) in 1% trimethylchlorosilane
(TMCS) to form the trimethylsilyl ether derivatives
(TMS). TMS derivatives were analyzed on a fused silica
DB5 capillary column (30 m 0.32 mm i.d., 0.25 mm
®lm thickness, J&W Scienti®c) with a temperature program from 60 to 300 C at a heating rate of 7 C minÿ1
(45 min). Individual n-alkanols, alkenones and sterols
were quanti®ed by comparison of chromatographic
peak areas with that of 5a-cholestane. This standard was
added to each fraction prior to gas chromatographic
injection. Biomarkers were indenti®ed on selected samples
by gas chromatography ± mass spectrometry (GC/MS).
GC/MS analyses were performed on a Varian 3400 gas
chromatograph coupled to a Varian Saturn Ion Trap
mass spectrometer. Operating GC conditions for GC/
579
MS analyses were the same as described above for each
class of biomarkers. Operating conditions of the mass
spectrometer were as follows: ion source temperature at
140 C, electron energy at 70eV, and scanning from 40 to
600 a.m.u. at 0.6 scan sÿ1.
2.3. Counting of coccoliths
Two types of count were made on the samples from
the SU94-20bK core. One was performed in order to
estimate the relative abundance of the diverse coccolith
taxa important to the production of alkenones (mainly
E. huxleyi and Gephyrocapsa sp.) and other coccolith
species. This count was made on smear slides based on
a total count of at least 300 coccoliths. E. huxleyi,
Gephyrocapsa smaller than 3 mm (mainly G. ericsonii),
and Gephyrocapsa larger than 3 mm (mainly G. muellerae and G. oceanica) were counted. In order to estimate the absolute abundance of coccoliths, we used a
dierent preparation technique. About 20 mg of dried
sediment was weighted, diluted in deionized water buffered to pH 8 in a 500 ml ¯ask and homogenized. A 20
ml aliquot of this suspension was ®ltered on a 47 mm
cellulose membrane (MicronSep) having a nominal pore
size of 0.45 mm. The membrane, once dried, was mounted with Canadian Balsam on a microscope slide. The
number of coccoliths per ®eld view of was counted
microscopically at 1250 magni®cation. At least 400
coccoliths were counted on an appropriate number of
®elds view of 0.023 mm2. The concentration of coccoliths per g of dried sediment was computed from this
count.
3. Results and discussion
3.1. Surface sediments
Table 1 gives the coordinates, percentage of organic
0
carbon (%OC) and UK
37 values of the surface sediments
and core-tops. Data from other sites occupied during
previous cruises (R/V Meteor and Ocean Drilling Pro0
gram) are also listed. SSTs were derived from the UK
37
index and the calibration established by Prahl et al.
0
(1988) (UK
37 =0.034T+ 0.039). These temperature estimates are reported in Table 1 together with mean
annual temperatures obtained from the Levitus atlas
(Levitus, 1994). %OC and SST values at each location
are also shown in Fig. 1. These two parameters are used
to determine the spatial distribution of the upwelling
cells along the coast. As can be seen from Table 1, SSTs
0
calculated from the UK
37 index are close to modern mean
annual SSTs obtained from the Levitus atlas (Levitus,
1994), except at lower latitude sites (ODP 660A and
M16415-2) where they are colder by 1±2 C than the
atlas values. Alkenone-derived SSTs were between
580
M.-A. Sicre et al. / Organic Geochemistry 31 (2000) 577±588
Table 1
0
Coordinates, water depth (in meters), percentage of organic carbon (% OC), and UK
37 values measured in surface sediments of the NW
a
African margin
0
0
Sample
Program
Site location
Depth (in meter)
OC (in %)
UK
37
SSTs (UK
37 )
SSTs (Levitus)
SU94-24S
SU9417dS
M12392-1
SU94-20bk
SU9421S
SU94-15S
SU94-11S
SU94-7S
658A
SU94-2S
659
660A
M16415-2
Sedorqua
Sedorqua
Meteor
Sedorqua
Sedorqua
Sedorqua
Sedorqua
Sedorqua
ODP
Sedorqua
ODP
ODP
Meteor
26 58N 14 01W
26 53N 14 41W
25 16N 16 05W
25 01N 16 39W
24 53N 16 31W
23 44N 17 16W
21 29N 17 57W
21 11N 18 52W
20 75N 18 58W
19 29N 17 17W
18 04N 21 01W
10 00N 19 14W
9 34N 19 64W
765
2597
2575
1445
750
1000
1200
3010
2263
1407
3070
4332
3851
0.80
0.55
0.35
0.68
0.74
1.13
2.92
0.90
2.67
1.98
0.1
n.d.
n.d.
0.70
0.73
n.db
0.77
0.75
0.74
0.73
0.75
0.72
0.75
n.d.
0.86
0.89
19.4
20.3
±
21.5
20.9
20.6
20.3
20.9
20.0
20.9
±
24.1
25.0
20.2
20.2
20.4
20.4
20.4
20.5
20.1
20.6
20.7
20.7
22.4
26.0
26.0
a
SSTs from the Levitus atlas (Levitus, 1994) and those derived from alkenones using the Prahl et al. (1988) calibration are also
reported.
b
n.d.: No data.
19.4 C, at 27 N, and 25 C at 9 N. South of 15 N,
SSTs re¯ect the warm waters of the Equatorial Atlantic.
The surface cooling, which characterizes the upwelling
areas by the rise of cold intermediate waters, is not well
0
recorded by the UK
37 values: between 20 and 25 N, SSTs
vary in a tight range (20±21 C). However, the OC data
allow one to conclude that three sites only, located o
Cape Blanc, are indicative of higher production (658A,
SU94-2S and SU94-11S) with OC >2%. At all other
sites modern OC values (< 1%) indicate low planktonic
production. As shown in Fig. 1, the SU94-20bK core is
located in the area of low %OC thus re¯ecting a nonupwelling regime, today. The OC content at the top of
the SU94-20bK core was 0.68%.
3.2. SU94-20bK core
0
3.2.1. d18O and U K
37 derived SST records 0
Fig. 2a provides complete d18O and UK
37 records of the
glacial/interglacial stages over the last 150 kyears,
0
extending from stage 6 to the Holocene. UK
37 ratio and
18
d O pro®les show similar long term changes over the
last climatic cycle (r2=0.67; n=83). SSTs vary from a
low value of 16.7 C at the last glacial to highest of
23.3 C at the Eemien (Fig. 2b). They increase from a
mean value of 17.4 C for the last glacial to a mean
value of 21.8 C for the Holocene. SSTs at the Eemien
are warmer than mean Holocene SSTs by 1.5 C. The
amplitude of the last glacial/interglacial warming at our
coring site is about 4.5 C. This dierence is compatible
with the d18O change during this transition (2.08%)
assuming that the d18O dierence between the LGM
and the Holocene is only due to ice volume (1.2%:
Labeyrie et al., 1987; Shackelton, 1987) and SST changes. In the southern BOFS 31K marine core (19 N,
20 W, 3300 m water depth), Chapman et al. (1996)
0
reported UK
37 based SSTs of 20.5 C for the Holocene
and of 17 C for the LGM, using the same calibration.
The SSTs record along the nearby ODP 658C core is
similar to BOFS 31K although sedimentation rates are
dierent, due to the closer proximity of the former to
the coast. SSTs show a maximum value of 21.5 C in the
Holocene dropping to 20.5 C towards the top of the
core. The temperature change associated with the last
deglaciation is 3±4 C (Zhao et al., 1995). Despite a 5
dierence in latitude, SSTs in these two cores are rather
similar to SU94-20bK core.
3.2.2. Organic carbon and biomarker records
MAR of OC and inorganic components of the SU9420bK core have been discussed elsewhere by Martinez et
al. (1996). The highest OC MAR are found during stage
2, and to a lesser extent during stages 4 and 6, while
during warm stages they are often less than 50 mg mÿ2
kyearsÿ1 (Fig. 3a). They generally correlate well with
biogenic carbonate and opal (Martinez et al., 1996)
suggesting that changes in the sedimentary OC are likely
to re¯ect mostly primary production, as reported also
by MuÈller et al. (1983) in the nearby M12392-1 core
(25 N 16 W; 2575 m depth) (Fig. 1). Even though a
major fraction of the OC sequestered in the sediments is
ultimately derived from marine organisms inhabiting
the surface waters, most of the OC produced by phytoplankton is labile and degrades rapidly in the water
column and in the ®rst centimeters of the surface sediment. Lipid biomarkers are more refractory compared
to other components such as sugars or proteins and can
thus be used to trace marine and continental inputs.
Madureira et al. (1997) recently showed that even in
sediment with low OC (< 1%) biomarkers were present
M.-A. Sicre et al. / Organic Geochemistry 31 (2000) 577±588
581
0
18
Fig. 2. (a) Downhole pro®les of UK
37 index and d O values (in % PDB) along the SU94-20bK core, over the last 150 kyears. (b) SST
0
0
index
and
the
calibration established by Prahl et al. (1988) (UK
estimates obtained from the UK
37 =0.034T+ 0.039). Shaded areas
37
indicate cold isotope stages.
at measurable concentrations. In an earlier study, Prahl
et al. (1989) also successfully measured fatty acid, sterols
and alkenone concentrations in sediments with OC
values between 0.2 and 0.4%. Terrigenous biomarkers
are thought to be better preserved than marine biomarkers during their transit through the water column
and burial in sediments. They have long been thought to
be more refractory as they become incorporated in soil
matrices and humic substances, thus they reach the sea
¯oor without undergoing as much alteration as marinederived compounds. Also, among marine biomarkers,
preferential degradation in the water column and surface sediment may also lead to varying degrees of sediment preservation. These factors constitute a major
limitation for quantitative interpretation of biomarkers
contained in sediments a fact that must be kept in
mind.
3.2.3. Compounds of terrigenous origin
The homologous series of n-alkanols were analyzed
to provide a description on the temporal changes of
terrestrial material. The n-alkanols generally range from
n-C17 to n-C34 and display a bimodal distribution (from
n-C17 to n-C20 and from n-C21 to n-C36), with a strong
even carbon number predominance in the HMW range,
typical of terrestrial plant wax signals (Simoneit, 1977).
HMW n-alkanols have thus been used to diagnose
source inputs in aeolien dust on a regional scale (Gagosian and Peltzer, 1987). In this study we calculated the
MAR of the sum of individual n-alkanols from C20 to
C32, with the exception of C31, co-eluting with a hopanol. The n-alkanol and OC MAR show a strong similarity (Figs. 3a and 3b). They are high in sediments
deposited during the last glacial period reaching
40 mg.mÿ2 kyearsÿ1. During glacial stages 4 and 6,
582
M.-A. Sicre et al. / Organic Geochemistry 31 (2000) 577±588
Fig. 3. OC and biomarker MAR (Mass Accumulation Rate) pro®les against 14C age along the SU94-20bK core, over the last 150
P
P
kyears. (a) Organic carbon MAR (OC). (b)
C20 ÿC32 n-alkanol MAR. (c)
C27 ÿC29 4-desmethyl sterol MAR. (d) C37 alkenone
MAR. Shaded areas indicate cold isotope stages. List of the sterols quanti®ed: 24-nor-24-methyl-cholesta-5,22-dienol, cholesta-5,22dienol, 5a-cholest-22-enol, cholest-5-enol, 5a-cholestanol, 24-methyl-cholesta-5,22-dienol, 24-methyl-5a-cholesta-22-enol, 24-ethylcholesta-5,22-dienol, 24-ethyl-cholest-5-enol, 24-ethyl-5a-cholestanol, 24-ethyl-cholesta-5,24(28)-dienol.
the n-alkanol MAR are not as high, yet are signi®cantly
stronger than during stage 5 when MAR are the lowest
(5 mgmÿ2 kyearsÿ1). Earlier work suggested that river
discharge did not contribute to the terrigenous supply
in this part of the NW African margin (Sarnthein et al.,
1981). Therefore, we can reasonably assume that nalkanol inputs are primarily a function of aeolian
transport. The sediments of the continental margin of
NW Africa are in an area of major dust outbreaks
from the NETW and the Harmattan. During glacial
time, atmospheric circulation intensi®ed over the Eurafrican margin and the Saharan desert was drier than
today (Sarnthein, 1978; Koopmann, 1979). Sediment
deposits similar to modern NETW type deposits dominated o the coast, between 20 N and 28 N (Sarnthein
et al., 1981). Marine-continental correlations of pollen
records provide further evidence for the stronger intensity of the NETW during glacial periods (Hooghiemstra, 1989). Higher glacial n-alkanol in¯uxes are
coherent with more ecient aeolien transport resulting
from the strengthening of the NETW. The general
consistency between higher plant n-alkanol MAR
and pollen records further supports an aeolien source of
n-alkanols.
3.2.4. Compounds of marine origin
The coastal upwelling and Canary Current are both
driven by NETW. Changes in their intensity have a
direct eect on primary production. The close similarity
between the OC and n-alkanol MAR pro®les most likely
lie with the fact that both records have varied mainly in
response to NETW intensity. The OC pro®le in the
nearby but deeper core M12392-1 published by MuÈller
et al. (1983) is similar to the OC pro®le along the SU9420bK core (Fig. 4a in Martinez et al., 1996). The glacial
OC content in both cores reaches a value of 3% suggesting that primary production was no more intense
farther seawards than over the shelf (M12392-1 water
depth: 2575 m; SU94-20bK water depth: 1445 m). The
marine production pattern and its temporal evolution
over the last climatic cycle was investigated in more
detail through the downcore pro®les of alkenone and
sterol MAR.
The coccolithophorid production as revealed by C37
alkenone MAR exhibits distinct features as compared
to OC (Figs. 3a and d). High alkenone MAR occur
during stage 2 and decrease in Holocene sediments, but
in contrast to OC, they remain high during stages 4
and 6. The most striking dierence between these two
M.-A. Sicre et al. / Organic Geochemistry 31 (2000) 577±588
stratigraphic records occurs during stage 5: while OC
MAR decline to their lowest values, the alkenone MAR
display high amplitude ¯ux events. The correlation
between OC and C37 alkenones is weak along the entire
core (r2=0.18; n=91), but improves signi®cantly when
distinguishing stages 1±3 (r2= 0.84) and stages 4±6 (r2=
0.63) (Fig. 4). The alkenone/OC ratio value obtained
over stages 1±3 is about 2 times lower than that found
from stages 4±6 indicating that the coccolithophorid
production contributed less to the OC production than
in sediments older than 60±70 kyears. Thus, the pattern
of coccolithophorid production would have been speci®cally dierent from that of other primary producers
beyong 60±70 kyears. This was investigated further by
looking at the sterol pro®le.
As opposed to alkenones, which are restricted to certain species of Prymnesiophyte algae, sterols originate
from a variety of planktonic species (Volkman, 1986).
Therefore, the downcore record of total sterol MAR
was used here as a rough indicator of the total primary
production and compared to the alkenone one. The
bulk of the sterols identi®ed along the core were the
C27±C29 4-desmethyl-sterols listed in ®gure caption 3.
Among them, cholesterol is a major sterol in zooplankton grazers. However, since cholesterol accounts
for only 15% of the total sterols and its MAR exhibits a
similar pro®le to other sterols along the core, it was
583
included in the total sterol concentration. Bacteria do
not produce 4-desmethyl-sterols (Ourisson et al., 1979),
but they can promote the hydrogenation of 5-stenols
to 5a(H)- and 5b(H)- stanols (Gaskell and Eglinton,
1975). Precise assignment of individual sterol to one
particular species is not univocal (Volkman, 1986), thus
detailed interpretation of species distributions from
sterols is not possible. Only the presence of 4a-methylsterols can be con®dently assigned to dino¯agellate
production, but they were not found at quanti®able
levels in the core. The downcore pro®le of sterol MAR
is similar to that of OC (r2=0.66) and dierent from the
alkenone pro®le. Sterol MAR maximize during the last
glacial period (Fig. 3c) and decline drastically beyond 30
kyears. As opposed to alkenones, the sterol MAR
remain at low levels throughout stage 5. This distribution shows common trends to the diatom derived opal
MAR pro®le published by Martinez et al. (1996). In the
nearby core M12392-1, Abrantes et al. (1994) reported
stage 5 as the period of lower diatom production.
Highest diatom accumulation rates were observed during glacial stage 2 and continuously decreased until the
disappearance of diatoms from 7 to 5 kyears. Accumulation rates of diatoms were also high above the 6/5
isotope stage boundary and sharply decrease at about
120 kyears. The sterol MAR pro®le of SU94-20bK
seems to mainly depict the production of diatoms. The
Fig. 4. Cross-correlation plots of C37 alkenone concentrations (in ng/g) vs percentage of organic carbon (%OC). Solid diamonds refer
to the data from the top of the core to stage 3. Open squares are the data from stages 4±6. Also given are the equations of the linear ®t
and R2 values calculated over each of these two time intervals.
584
M.-A. Sicre et al. / Organic Geochemistry 31 (2000) 577±588
alkenone pro®le is thus clearly dierent from the sterol
one and indicates that coccolithophorid production may
have had two distinct regimes: one period of lower
alkenone/OC ratios coinciding with enhanced primary
production, which leads us to think that under abundant nutrient conditions of glacial times, diatoms and/
or other primary producers would outcompete coccolithophorids; a second period during stage 5 which
indicates optimium growth conditions for coccolithophorid when waters are warmer.
Alternatively, higher alkenone/OC ratios over stages
4±6 may result in part from the better preservation of
alkenones than sterols and OC. The redox conditions
are a major factor determining the preservation of
organic compounds. The in¯uence of reduction/oxidation chemistry can be addressed by examining the sterol
composition. The conversion of stenol into stanol is a
major diagenetic pathways which involves the reduction
of double bonds either in the ring system or on the lateral chain of the sterols in the water column and after
deposition (Nishimura and Koyama, 1977; Nishimura
1978; Gagosian et al., 1980). The 24-methyl-5a-cholest-
22-enol/24-methyl-cholesta-5, 22-dienol ratio is used
here to refer to the saturation of the ring system. Its
temporal evolution indicates that more alteration occurs
on sterols at warmer SSTs, mainly during the warmest
episode of the last interglacial stage 5 (Figs. 3e and f).
Assuming that other marine constituents may also have
undergone stronger alteration, it is possible that selective preservation may partly be responsible for the
observed higher alkenone contribution to OC from
stages 4±6.
Alkenone concentrations were then compared to the
calcareous nanofossil occurrence to examine the possibility of a major change of alkenone producers to
explain the alkenone/OC values prior and after the isotope transition 3/4. Coccolith abundances were determined from major coccolithophorid species present in
the core, namely E. huxleyi, G. mullerae, G. oceanica and
G. ericsonii. The coccolith abundances per taxa are
shown in Figs. 5b±d together with total counts (Fig. 5e)
and C37±C38 alkenone abundances (Fig. 5a). Although
there is no report on the occurrence of alkenone in G.
ericsonii, Marlowe et al. (1990) suggested that living
Fig. 5. Abundances of coccoliths of the three major coccolithophorid identi®ed along the SU94-20bK core over the last 150 kyears.
(a) C37 and C38 alkenone concentrations in mg/g. (b) Coccolith absolute abundances of E. huxleyi. (c) Coccolith absolute abundances
of G. mullerae and G. oceanica. (d) Coccolith absolute abundances of smaller Gephyrocapsa mainly G. ericsonii. (e) Total abundances
of coccoliths expressed in coccoliths/g. Shaded areas indicate cold isotope stages.
M.-A. Sicre et al. / Organic Geochemistry 31 (2000) 577±588
species of the genus Gephyrocapsa including G. protohuxleyi and G. ericsonii are likely to be contributors of
alkenones to contemporary sediments. E. huxleyi is
believed to have evolved from the genus Gephyrocapsa
during the late Pleistocene and to be phylogenetically
related to G. ericsonii via G. protohuxleyi (McIntyre,
1970). Total coccolith counts con®rm that coccolithophorid production is higher over stages 4±6 than from
stages 1±3. Fig. 5b also shows that E. huxleyi is the
dominant species in sediments younger than 60 kyears,
when alkenone/OC values are rather low, and that it
becomes a minor coccolithophorid species in sediments
where the alkenone/OC ratios are high. The smaller
Gephyrocapsa become prevalent species beyond 60 kyears,
i.e. at the reversal of the dominance pattern of the coccolithophorid population, an event that has a global
extension (Thierstein et al., 1977), when alkenones/OC
values are higher. Our results are consistent with those
reported by MuÈller et al. (1997) in the South Atlantic.
Larger Gephyrocapsa such as G. muellerae and G. oceanica also increase during interglacial stage 5 when SSTs
were warmer but coccolith counts of these species do no
match alkenone concentration ¯uctuations. If no species
monitored here can clearly account for the alkenone
pro®le, some sort of similarity is found in the absolute
abundances of the small Gephyrocapsa and alkenone
pro®les: maxima of small Gephyrocapsa around 140,
110, 80 and 75 kyears are in relative synchronism with
the alkenone concentration peaks P
at 140, 110,
P 85 and 75
kyears.
It
is
noteworthy
that
the
C
/
C38(Me+Et)
37Me
P
P
and C37Me/ C38(Et) remain constant throughout the
core (Ternois, 1996) thus indicating a stable distribution
of alkenones despite changing taxa abundances.
4. Conclusions
585
whereas siliceous production may have been more
important during glacial times. This may possibly
explain higher alkenone/OC ratio values from stages 4±
6 than from the present day to stage 3. However, the 24methyl-5a-cholest-22-enol/24-methyl-cholesta-5,22-dienol ratio record indicates stronger alteration of sterols,
mainly during stage 5. Based on the correlation between
OC and sterols, we can assume that other marine consitutents may have undergone similar alteration and
thus hypothesize that selective preservation may also
contribute to higher alkenone/OC ratios from stages 4±
6. A change of alkenone producers was also envisaged
as an alternative explanation for a higher contribution
of alkenones to sedimentary OC. The comparison
between alkenone abundances and coccolith counts
indicates a major temporal change of the coccolith taxa
assemblages around 60 kyears. While E. huxleyi dominates in sediments younger than this time boundary,
Gephyrocapsa sp. took over in sediments older than 60
kyears. Smaller Gephyrocapsa, mostly represented by G.
ericsonii, better matched the alkenone pro®le than larger
ones. Overall our results suggest that besides a change
from siliceous to calcareous production, the observed
shift in the alkenone/OC ratios could be indicative of a
change in the species producing alkenones, if the
amount of these compounds per cell was dierent from
that of E. huxleyi. Finally these data incline to be cautious when using alkenones to estimate primary production in the past.
Acknowledgements
Samples were provided by the Sedorqua program
which was supported ®nancially by French research
institutions (CNRS INSU, and MESR). This is LSCE
contribution number 0341.
0
18
UK
37 and d O values exhibited similar long term variations over the last climatic cycle. SSTs derived from
0
UK
37 values using the calibration established by Prahl et
al. (1988) ranged from a mean value of 17.4 C for the
last glacial to a mean value of 21.8 C for the Holocene
thus recording a 4.5 C warming during the last deglaciation, consistent with previous studies. OC MAR
indicated a signi®cant rise during the last glacial in
response to a strengthening of the NETW as revealed by
land-derived n-alkanol MAR. This increase resulted
from a higher supply of eolian dust from the continent
and primary production of the surface ocean. However,
while sterol MAR followed similar trends as OC MAR,
recording highest accumulation during glacial stages
(mostly stages 2 and 3) and a sharp decrease during the
last interglacial, the alkenones pro®le was quite dierent. The coccolithophorid production (calcareous production) appeared to be favored during warm periods as
indicated by a high pulse of alkenones during stage 5
Associate EditorÐB.R.T. Simoneit
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www.elsevier.nl/locate/orggeochem
Biomarker stratigraphic records over the last 150 kyears o
the NW African coast at 25N
Marie-Alexandrine Sicre a,*, Yann Ternois b, Martine Paterne a, Anne Boireau c,
Luc Beaufort b, Philippe Martinez d, Philippe Bertrand d
a
Laboratoire des Sciences du Climat et de l'Environnement, CNRS SDU UMR 1572, Domaine du CNRS, Avenue de la Terrasse,
91198 Gif-sur-Yvette Cedex, France
b
CEREGE, CNRS SDU UMR 6635, Europole de l'Arbois, B.P. 80, 13545 Aix-en Provence Cedex 4, France
c
Laboratoire de Physique et Chimie Marines, Universite Pierre et Marie Curie, CNRS SDU ESA7077, 4 place Jussieu,
75252 Paris Cedex 05, France
d Âpartement
De
de GeÂologie et OceÂanographie, Universite de Bordeaux I, CNRS SDU UMR 5805, Avenue des faculteÂs,
33405 Talence Cedex, France
Received 27 July 1999; accepted 2 February 2000
(returned to author for revision 15 October 1999)
Abstract
Terrigenous and marine biomarkers were investigated in a core o Northwest Africa in the Northeast Atlantic
(25 N, 16 W, 1445 m depth) to assess changes in the sedimentation pattern of organic carbon (OC) over the last 150
kyears. Alkenone derived temperatures recorded a warming of 4.5 C during the last deglaciation. n-Alkanol Mass
Accumulation Rates (MAR) ¯uctuated in parallel with Northeast Trade Winds (NETW) intensity. OC and sterol
MAR both increased during glacial times indicating enhanced fertility of the ocean in response to intensi®ed NETW.
Alkenone/OC ratios were higher by a factor of two over stages 4±6 than stages 1±3 thus re¯ecting distinct coccolithophorid inputs. This transition coincides with a major change of alkenone producers inferred from coccolith counts.
# 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Biomarkers; Paleoceanography; Alkenones; Alkanols; Sterols; Africa; Upwelling; North Atlantic
1. Introduction
The NW African coast is a region of intense primary
production, today. Wind-driven coastal upwellings
occur under the in¯uence of longshore northeasterlies,
blowing towards the equator, bringing to the surface
cold and nutrient-rich waters. The cold waters of the
Canary Current, ¯owing southwards are another
hydrological feature that in¯uences the sea surface temperatures (SSTs). Production and sedimentation of
organic carbon (OC) in this region is in¯uenced by the
* Corresponding author: Tel.: +33-1-69-82-43-34; fax:
+33-1-69-82-35-68.
E-mail address: marie-alexandrine.sicre@lsce.cnrs-gif.fr (M.A. Sicre).
supply of terrigenous material and biogenic sedimentation stimulated by the Northeast Trade Winds (NETW).
However, the intensity of the upwelling cells is not uniform along the coast. It is stronger o Cape Blanc
leading to high productivity and sedimentation rates.
Upwelled waters are the North Atlantic Central Waters
(NACW), North of Cape Blanc (21 N), and the South
Atlantic Central Waters (SACW), South of Cape Blanc
(Tomczak, 1977). MuÈller et al. (1983) have shown that
around 25 N, OC production and sedimentation have
varied considerably over the late Quaternary. Paleoproductivity estimates derived from OC measurements have
shown that during glacial time productivity was higher
as the result of intensi®ed upwelling in response to
stronger atmospheric circulation (MuÈller et al., 1983).
Although organic matter can be traced in general terms
by measuring OC, the distribution of speci®c biomarkers
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00021-8
578
M.-A. Sicre et al. / Organic Geochemistry 31 (2000) 577±588
allows a more detailed examination of the source material deposited to the sediment. In this work we report
our results on several classes of biomarkers analyzed in
the SU94-20bK core (25 N, 16 W, 1445 m water depth)
collected during the Sedorqua cruise in February 1994,
on the R/V Le Suroit (Fig. 1). This coring site is located
on the continental slope o Cape Bojador which makes
it an ideal site to study the impact of changing NETW
regime on the nature and pattern of OC production and
sedimentation. Several surface sediments were also
taken along the coast to determine the spatial distribution of upwelling cells, today.
Three classes of biomarkers were selected for this
study to determine the temporal variations of terrigenous and marine source inputs of OC and apply this
information to describe past climatic changes over the
last climatic cycle. Alkenones, which are undoubtly the
Fig. 1. Map of the NW African continental margin showing
the locations of the surface sediments and SU94-20bK core
collected during the Sedorqua cruise. Also shown are the sites
of ODP 658A, 659 and 660A and Meteor cores M12392-1 and
M16415-2. The percentage of OC and SST values are indicated
for each site when available.
most widely used class of biomarkers for paleoclimatic
and paleoceanographic studies, were measured to estimate paleo-SSTs. The C37 alkenone unsaturation index
0
UK
37 has previously been shown to be linearly dependent
to growth temperature (Prahl and Wakeham, 1987;
Prahl et al., 1988; Conte and Eglinton., 1993; Sikes and
Volkman, 1993; Sikes et al., 1997; Ternois et al., 1997).
Core-top statistical evaluations con®rmed the existence
of this relationship on a global scale (Sikes et al., 1991;
Rosell-Mele et al., 1995; Sonzogni et al., 1997; MuÈller et
al., 1998). Although several studies demonstrated that
alkenones are not immune to degradation (Marlowe,
0
1984; McCarey et al., 1990; Conte et al., 1992), the UK
37
index, on which paleo-temperature calculations are
based does not seem to be aected by food web processes (Volkman et al., 1980) or early diagenesis (Prahl
et al., 1989; Madureira et al., 1995; Sicre et al., 1999).
Therefore, it has been successfully used for SSTs reconstruction in the past (Brassell et al., 1986; Jasper and
Gagosian, 1989; Prahl et al., 1989; McCarey et al.,
1990; ten Haven and Kroon, 1991; Eglinton et al., 1992;
Lyle et al., 1992; Kennedy and Brassell, 1992a; Kennedy
and Brassell, 1992b; Rostek et al., 1993; Wakeham,
1993; Zhao et al., 1993; Ohkouchi et al., 1994; Sikes and
Keigwin, 1994; Prahl et al., 1995; Schneider et al., 1995;
Zhao et al., 1995; Bard et al., 1997; Madureira et al.,
1997; Rostek et al., 1997; Villanueva et al., 1998a; Villanueva et al., 1998b; Ternois et al., 2000). More
recently, alkenones have been used to track down inputs
from some Haptophyte algae in long-term sedimentary
records (Rostek et al., 1997; Villanueva et al., 1998a;
Villanueva et al., 1998b). These compounds are mostly
biosynthesized by the marine coccolithophorid Emiliania huxleyi as well as the closely related Gephyrocapsa
species (Volkman et al. 1980; Marlowe et al., 1990;
Volkman et al., 1995). Alkenones were also used here to
assess inputs from coccolithophorids.
Few studies have applied biomarkers other than
alkenones as tracers for determining paleoclimate conditions. The abundance of long-chain saturated nalkanes, n-alkanols and fatty acids have been shown to
track terrigenous inputs to marine sediments (Poynter et
al., 1989; Prahl et al., 1989; Madureira et al., 1997;
Ohkouchi et al., 1997; Villanueva et al., 1998a; Villanueva et al., 1998b). We selected high molecular weight
(HWM) n-alkanols (C20-C32) for their abundance and
speci®ty to trace detrital material while 4-desmethyl
sterols (C27-C29) were quanti®ed to evaluate the overlying water's planktonic production (Volkman, 1986,
ten Haven et al., 1989; Farrimond et al., 1990). Changes of SSTs and Mass Accumulation Rates (MAR) of
OC, n-alkanols, alkenones and sterols are discussed in
relation to climatic changes over the last 150 kyears.
Basic data (OC, CaCO3 and opal) including the
chronology have already been published in Martinez et
al. (1996).
M.-A. Sicre et al. / Organic Geochemistry 31 (2000) 577±588
2. Methods
2.1. Oxygen isotope stratigraphy
Oxygen isotope analyses were performed along the
SU94-20bK core on planktonic foraminifera Globigerina bulloõÈdes (>250 mm) on a Finnigan 251 mass spectrometer calibrated in PDB via NBS19. External
precision was 0.07% d18O (1 sigma value). The d18O
chronology determination is described in Martinez et al.
(1996).
2.2. Biomarker analyses
The SU94-20bK sediment core was subsampled on
board and frozen at ÿ18 C. All the surface sediments
were taken with a box-corer and frozen on board
shortly after recovery. About 5 g aliquots of frozen
sediment were freeze-dried for biomarker analyses.
Extractable lipids were isolated by solvent extraction in
an ultrasonic bath for 10 min. The two ®rst extractions
were performed with methylene chloride/methanol (2:1,
v/v) and the third one with methanol. The extracts were
combined, concentrated by rotary evaporation and
transferred into 4 ml vials. Lipids were fractionated into
compound classes by silica gel chromatography following the procedure described by Conde (1989). Fractions
were stored in 4 ml vial at ÿ18 C until used for gas
chromatographic analyses.
Alkenones were analyzed on a Delsi DI 200 gas
chromatograph equipped with a fused silica CP-Sil-5
capillary column (50 m 0.32 mm i.d., 0.25 mm ®lm
thickness, Chrompack) and a ¯ame ionization detector.
Each alkenone fraction was injected three times. Helium
was used as the carrier gas (25 ml minÿ1). The oven
temperature was programmed from 100 to 300 C at
0
10 C minÿ1 (60 min). UK
37 ratios were calculated from
chromatographic peak areas. Analytical precision
obtained after triplicate injections was calculated to be
0.01 unit ratio. The fractions containing n-alkanols and
sterols were treated with bis(trimethylsilyl)-tri¯uoroacetamide (BSTFA) in 1% trimethylchlorosilane
(TMCS) to form the trimethylsilyl ether derivatives
(TMS). TMS derivatives were analyzed on a fused silica
DB5 capillary column (30 m 0.32 mm i.d., 0.25 mm
®lm thickness, J&W Scienti®c) with a temperature program from 60 to 300 C at a heating rate of 7 C minÿ1
(45 min). Individual n-alkanols, alkenones and sterols
were quanti®ed by comparison of chromatographic
peak areas with that of 5a-cholestane. This standard was
added to each fraction prior to gas chromatographic
injection. Biomarkers were indenti®ed on selected samples
by gas chromatography ± mass spectrometry (GC/MS).
GC/MS analyses were performed on a Varian 3400 gas
chromatograph coupled to a Varian Saturn Ion Trap
mass spectrometer. Operating GC conditions for GC/
579
MS analyses were the same as described above for each
class of biomarkers. Operating conditions of the mass
spectrometer were as follows: ion source temperature at
140 C, electron energy at 70eV, and scanning from 40 to
600 a.m.u. at 0.6 scan sÿ1.
2.3. Counting of coccoliths
Two types of count were made on the samples from
the SU94-20bK core. One was performed in order to
estimate the relative abundance of the diverse coccolith
taxa important to the production of alkenones (mainly
E. huxleyi and Gephyrocapsa sp.) and other coccolith
species. This count was made on smear slides based on
a total count of at least 300 coccoliths. E. huxleyi,
Gephyrocapsa smaller than 3 mm (mainly G. ericsonii),
and Gephyrocapsa larger than 3 mm (mainly G. muellerae and G. oceanica) were counted. In order to estimate the absolute abundance of coccoliths, we used a
dierent preparation technique. About 20 mg of dried
sediment was weighted, diluted in deionized water buffered to pH 8 in a 500 ml ¯ask and homogenized. A 20
ml aliquot of this suspension was ®ltered on a 47 mm
cellulose membrane (MicronSep) having a nominal pore
size of 0.45 mm. The membrane, once dried, was mounted with Canadian Balsam on a microscope slide. The
number of coccoliths per ®eld view of was counted
microscopically at 1250 magni®cation. At least 400
coccoliths were counted on an appropriate number of
®elds view of 0.023 mm2. The concentration of coccoliths per g of dried sediment was computed from this
count.
3. Results and discussion
3.1. Surface sediments
Table 1 gives the coordinates, percentage of organic
0
carbon (%OC) and UK
37 values of the surface sediments
and core-tops. Data from other sites occupied during
previous cruises (R/V Meteor and Ocean Drilling Pro0
gram) are also listed. SSTs were derived from the UK
37
index and the calibration established by Prahl et al.
0
(1988) (UK
37 =0.034T+ 0.039). These temperature estimates are reported in Table 1 together with mean
annual temperatures obtained from the Levitus atlas
(Levitus, 1994). %OC and SST values at each location
are also shown in Fig. 1. These two parameters are used
to determine the spatial distribution of the upwelling
cells along the coast. As can be seen from Table 1, SSTs
0
calculated from the UK
37 index are close to modern mean
annual SSTs obtained from the Levitus atlas (Levitus,
1994), except at lower latitude sites (ODP 660A and
M16415-2) where they are colder by 1±2 C than the
atlas values. Alkenone-derived SSTs were between
580
M.-A. Sicre et al. / Organic Geochemistry 31 (2000) 577±588
Table 1
0
Coordinates, water depth (in meters), percentage of organic carbon (% OC), and UK
37 values measured in surface sediments of the NW
a
African margin
0
0
Sample
Program
Site location
Depth (in meter)
OC (in %)
UK
37
SSTs (UK
37 )
SSTs (Levitus)
SU94-24S
SU9417dS
M12392-1
SU94-20bk
SU9421S
SU94-15S
SU94-11S
SU94-7S
658A
SU94-2S
659
660A
M16415-2
Sedorqua
Sedorqua
Meteor
Sedorqua
Sedorqua
Sedorqua
Sedorqua
Sedorqua
ODP
Sedorqua
ODP
ODP
Meteor
26 58N 14 01W
26 53N 14 41W
25 16N 16 05W
25 01N 16 39W
24 53N 16 31W
23 44N 17 16W
21 29N 17 57W
21 11N 18 52W
20 75N 18 58W
19 29N 17 17W
18 04N 21 01W
10 00N 19 14W
9 34N 19 64W
765
2597
2575
1445
750
1000
1200
3010
2263
1407
3070
4332
3851
0.80
0.55
0.35
0.68
0.74
1.13
2.92
0.90
2.67
1.98
0.1
n.d.
n.d.
0.70
0.73
n.db
0.77
0.75
0.74
0.73
0.75
0.72
0.75
n.d.
0.86
0.89
19.4
20.3
±
21.5
20.9
20.6
20.3
20.9
20.0
20.9
±
24.1
25.0
20.2
20.2
20.4
20.4
20.4
20.5
20.1
20.6
20.7
20.7
22.4
26.0
26.0
a
SSTs from the Levitus atlas (Levitus, 1994) and those derived from alkenones using the Prahl et al. (1988) calibration are also
reported.
b
n.d.: No data.
19.4 C, at 27 N, and 25 C at 9 N. South of 15 N,
SSTs re¯ect the warm waters of the Equatorial Atlantic.
The surface cooling, which characterizes the upwelling
areas by the rise of cold intermediate waters, is not well
0
recorded by the UK
37 values: between 20 and 25 N, SSTs
vary in a tight range (20±21 C). However, the OC data
allow one to conclude that three sites only, located o
Cape Blanc, are indicative of higher production (658A,
SU94-2S and SU94-11S) with OC >2%. At all other
sites modern OC values (< 1%) indicate low planktonic
production. As shown in Fig. 1, the SU94-20bK core is
located in the area of low %OC thus re¯ecting a nonupwelling regime, today. The OC content at the top of
the SU94-20bK core was 0.68%.
3.2. SU94-20bK core
0
3.2.1. d18O and U K
37 derived SST records 0
Fig. 2a provides complete d18O and UK
37 records of the
glacial/interglacial stages over the last 150 kyears,
0
extending from stage 6 to the Holocene. UK
37 ratio and
18
d O pro®les show similar long term changes over the
last climatic cycle (r2=0.67; n=83). SSTs vary from a
low value of 16.7 C at the last glacial to highest of
23.3 C at the Eemien (Fig. 2b). They increase from a
mean value of 17.4 C for the last glacial to a mean
value of 21.8 C for the Holocene. SSTs at the Eemien
are warmer than mean Holocene SSTs by 1.5 C. The
amplitude of the last glacial/interglacial warming at our
coring site is about 4.5 C. This dierence is compatible
with the d18O change during this transition (2.08%)
assuming that the d18O dierence between the LGM
and the Holocene is only due to ice volume (1.2%:
Labeyrie et al., 1987; Shackelton, 1987) and SST changes. In the southern BOFS 31K marine core (19 N,
20 W, 3300 m water depth), Chapman et al. (1996)
0
reported UK
37 based SSTs of 20.5 C for the Holocene
and of 17 C for the LGM, using the same calibration.
The SSTs record along the nearby ODP 658C core is
similar to BOFS 31K although sedimentation rates are
dierent, due to the closer proximity of the former to
the coast. SSTs show a maximum value of 21.5 C in the
Holocene dropping to 20.5 C towards the top of the
core. The temperature change associated with the last
deglaciation is 3±4 C (Zhao et al., 1995). Despite a 5
dierence in latitude, SSTs in these two cores are rather
similar to SU94-20bK core.
3.2.2. Organic carbon and biomarker records
MAR of OC and inorganic components of the SU9420bK core have been discussed elsewhere by Martinez et
al. (1996). The highest OC MAR are found during stage
2, and to a lesser extent during stages 4 and 6, while
during warm stages they are often less than 50 mg mÿ2
kyearsÿ1 (Fig. 3a). They generally correlate well with
biogenic carbonate and opal (Martinez et al., 1996)
suggesting that changes in the sedimentary OC are likely
to re¯ect mostly primary production, as reported also
by MuÈller et al. (1983) in the nearby M12392-1 core
(25 N 16 W; 2575 m depth) (Fig. 1). Even though a
major fraction of the OC sequestered in the sediments is
ultimately derived from marine organisms inhabiting
the surface waters, most of the OC produced by phytoplankton is labile and degrades rapidly in the water
column and in the ®rst centimeters of the surface sediment. Lipid biomarkers are more refractory compared
to other components such as sugars or proteins and can
thus be used to trace marine and continental inputs.
Madureira et al. (1997) recently showed that even in
sediment with low OC (< 1%) biomarkers were present
M.-A. Sicre et al. / Organic Geochemistry 31 (2000) 577±588
581
0
18
Fig. 2. (a) Downhole pro®les of UK
37 index and d O values (in % PDB) along the SU94-20bK core, over the last 150 kyears. (b) SST
0
0
index
and
the
calibration established by Prahl et al. (1988) (UK
estimates obtained from the UK
37 =0.034T+ 0.039). Shaded areas
37
indicate cold isotope stages.
at measurable concentrations. In an earlier study, Prahl
et al. (1989) also successfully measured fatty acid, sterols
and alkenone concentrations in sediments with OC
values between 0.2 and 0.4%. Terrigenous biomarkers
are thought to be better preserved than marine biomarkers during their transit through the water column
and burial in sediments. They have long been thought to
be more refractory as they become incorporated in soil
matrices and humic substances, thus they reach the sea
¯oor without undergoing as much alteration as marinederived compounds. Also, among marine biomarkers,
preferential degradation in the water column and surface sediment may also lead to varying degrees of sediment preservation. These factors constitute a major
limitation for quantitative interpretation of biomarkers
contained in sediments a fact that must be kept in
mind.
3.2.3. Compounds of terrigenous origin
The homologous series of n-alkanols were analyzed
to provide a description on the temporal changes of
terrestrial material. The n-alkanols generally range from
n-C17 to n-C34 and display a bimodal distribution (from
n-C17 to n-C20 and from n-C21 to n-C36), with a strong
even carbon number predominance in the HMW range,
typical of terrestrial plant wax signals (Simoneit, 1977).
HMW n-alkanols have thus been used to diagnose
source inputs in aeolien dust on a regional scale (Gagosian and Peltzer, 1987). In this study we calculated the
MAR of the sum of individual n-alkanols from C20 to
C32, with the exception of C31, co-eluting with a hopanol. The n-alkanol and OC MAR show a strong similarity (Figs. 3a and 3b). They are high in sediments
deposited during the last glacial period reaching
40 mg.mÿ2 kyearsÿ1. During glacial stages 4 and 6,
582
M.-A. Sicre et al. / Organic Geochemistry 31 (2000) 577±588
Fig. 3. OC and biomarker MAR (Mass Accumulation Rate) pro®les against 14C age along the SU94-20bK core, over the last 150
P
P
kyears. (a) Organic carbon MAR (OC). (b)
C20 ÿC32 n-alkanol MAR. (c)
C27 ÿC29 4-desmethyl sterol MAR. (d) C37 alkenone
MAR. Shaded areas indicate cold isotope stages. List of the sterols quanti®ed: 24-nor-24-methyl-cholesta-5,22-dienol, cholesta-5,22dienol, 5a-cholest-22-enol, cholest-5-enol, 5a-cholestanol, 24-methyl-cholesta-5,22-dienol, 24-methyl-5a-cholesta-22-enol, 24-ethylcholesta-5,22-dienol, 24-ethyl-cholest-5-enol, 24-ethyl-5a-cholestanol, 24-ethyl-cholesta-5,24(28)-dienol.
the n-alkanol MAR are not as high, yet are signi®cantly
stronger than during stage 5 when MAR are the lowest
(5 mgmÿ2 kyearsÿ1). Earlier work suggested that river
discharge did not contribute to the terrigenous supply
in this part of the NW African margin (Sarnthein et al.,
1981). Therefore, we can reasonably assume that nalkanol inputs are primarily a function of aeolian
transport. The sediments of the continental margin of
NW Africa are in an area of major dust outbreaks
from the NETW and the Harmattan. During glacial
time, atmospheric circulation intensi®ed over the Eurafrican margin and the Saharan desert was drier than
today (Sarnthein, 1978; Koopmann, 1979). Sediment
deposits similar to modern NETW type deposits dominated o the coast, between 20 N and 28 N (Sarnthein
et al., 1981). Marine-continental correlations of pollen
records provide further evidence for the stronger intensity of the NETW during glacial periods (Hooghiemstra, 1989). Higher glacial n-alkanol in¯uxes are
coherent with more ecient aeolien transport resulting
from the strengthening of the NETW. The general
consistency between higher plant n-alkanol MAR
and pollen records further supports an aeolien source of
n-alkanols.
3.2.4. Compounds of marine origin
The coastal upwelling and Canary Current are both
driven by NETW. Changes in their intensity have a
direct eect on primary production. The close similarity
between the OC and n-alkanol MAR pro®les most likely
lie with the fact that both records have varied mainly in
response to NETW intensity. The OC pro®le in the
nearby but deeper core M12392-1 published by MuÈller
et al. (1983) is similar to the OC pro®le along the SU9420bK core (Fig. 4a in Martinez et al., 1996). The glacial
OC content in both cores reaches a value of 3% suggesting that primary production was no more intense
farther seawards than over the shelf (M12392-1 water
depth: 2575 m; SU94-20bK water depth: 1445 m). The
marine production pattern and its temporal evolution
over the last climatic cycle was investigated in more
detail through the downcore pro®les of alkenone and
sterol MAR.
The coccolithophorid production as revealed by C37
alkenone MAR exhibits distinct features as compared
to OC (Figs. 3a and d). High alkenone MAR occur
during stage 2 and decrease in Holocene sediments, but
in contrast to OC, they remain high during stages 4
and 6. The most striking dierence between these two
M.-A. Sicre et al. / Organic Geochemistry 31 (2000) 577±588
stratigraphic records occurs during stage 5: while OC
MAR decline to their lowest values, the alkenone MAR
display high amplitude ¯ux events. The correlation
between OC and C37 alkenones is weak along the entire
core (r2=0.18; n=91), but improves signi®cantly when
distinguishing stages 1±3 (r2= 0.84) and stages 4±6 (r2=
0.63) (Fig. 4). The alkenone/OC ratio value obtained
over stages 1±3 is about 2 times lower than that found
from stages 4±6 indicating that the coccolithophorid
production contributed less to the OC production than
in sediments older than 60±70 kyears. Thus, the pattern
of coccolithophorid production would have been speci®cally dierent from that of other primary producers
beyong 60±70 kyears. This was investigated further by
looking at the sterol pro®le.
As opposed to alkenones, which are restricted to certain species of Prymnesiophyte algae, sterols originate
from a variety of planktonic species (Volkman, 1986).
Therefore, the downcore record of total sterol MAR
was used here as a rough indicator of the total primary
production and compared to the alkenone one. The
bulk of the sterols identi®ed along the core were the
C27±C29 4-desmethyl-sterols listed in ®gure caption 3.
Among them, cholesterol is a major sterol in zooplankton grazers. However, since cholesterol accounts
for only 15% of the total sterols and its MAR exhibits a
similar pro®le to other sterols along the core, it was
583
included in the total sterol concentration. Bacteria do
not produce 4-desmethyl-sterols (Ourisson et al., 1979),
but they can promote the hydrogenation of 5-stenols
to 5a(H)- and 5b(H)- stanols (Gaskell and Eglinton,
1975). Precise assignment of individual sterol to one
particular species is not univocal (Volkman, 1986), thus
detailed interpretation of species distributions from
sterols is not possible. Only the presence of 4a-methylsterols can be con®dently assigned to dino¯agellate
production, but they were not found at quanti®able
levels in the core. The downcore pro®le of sterol MAR
is similar to that of OC (r2=0.66) and dierent from the
alkenone pro®le. Sterol MAR maximize during the last
glacial period (Fig. 3c) and decline drastically beyond 30
kyears. As opposed to alkenones, the sterol MAR
remain at low levels throughout stage 5. This distribution shows common trends to the diatom derived opal
MAR pro®le published by Martinez et al. (1996). In the
nearby core M12392-1, Abrantes et al. (1994) reported
stage 5 as the period of lower diatom production.
Highest diatom accumulation rates were observed during glacial stage 2 and continuously decreased until the
disappearance of diatoms from 7 to 5 kyears. Accumulation rates of diatoms were also high above the 6/5
isotope stage boundary and sharply decrease at about
120 kyears. The sterol MAR pro®le of SU94-20bK
seems to mainly depict the production of diatoms. The
Fig. 4. Cross-correlation plots of C37 alkenone concentrations (in ng/g) vs percentage of organic carbon (%OC). Solid diamonds refer
to the data from the top of the core to stage 3. Open squares are the data from stages 4±6. Also given are the equations of the linear ®t
and R2 values calculated over each of these two time intervals.
584
M.-A. Sicre et al. / Organic Geochemistry 31 (2000) 577±588
alkenone pro®le is thus clearly dierent from the sterol
one and indicates that coccolithophorid production may
have had two distinct regimes: one period of lower
alkenone/OC ratios coinciding with enhanced primary
production, which leads us to think that under abundant nutrient conditions of glacial times, diatoms and/
or other primary producers would outcompete coccolithophorids; a second period during stage 5 which
indicates optimium growth conditions for coccolithophorid when waters are warmer.
Alternatively, higher alkenone/OC ratios over stages
4±6 may result in part from the better preservation of
alkenones than sterols and OC. The redox conditions
are a major factor determining the preservation of
organic compounds. The in¯uence of reduction/oxidation chemistry can be addressed by examining the sterol
composition. The conversion of stenol into stanol is a
major diagenetic pathways which involves the reduction
of double bonds either in the ring system or on the lateral chain of the sterols in the water column and after
deposition (Nishimura and Koyama, 1977; Nishimura
1978; Gagosian et al., 1980). The 24-methyl-5a-cholest-
22-enol/24-methyl-cholesta-5, 22-dienol ratio is used
here to refer to the saturation of the ring system. Its
temporal evolution indicates that more alteration occurs
on sterols at warmer SSTs, mainly during the warmest
episode of the last interglacial stage 5 (Figs. 3e and f).
Assuming that other marine constituents may also have
undergone stronger alteration, it is possible that selective preservation may partly be responsible for the
observed higher alkenone contribution to OC from
stages 4±6.
Alkenone concentrations were then compared to the
calcareous nanofossil occurrence to examine the possibility of a major change of alkenone producers to
explain the alkenone/OC values prior and after the isotope transition 3/4. Coccolith abundances were determined from major coccolithophorid species present in
the core, namely E. huxleyi, G. mullerae, G. oceanica and
G. ericsonii. The coccolith abundances per taxa are
shown in Figs. 5b±d together with total counts (Fig. 5e)
and C37±C38 alkenone abundances (Fig. 5a). Although
there is no report on the occurrence of alkenone in G.
ericsonii, Marlowe et al. (1990) suggested that living
Fig. 5. Abundances of coccoliths of the three major coccolithophorid identi®ed along the SU94-20bK core over the last 150 kyears.
(a) C37 and C38 alkenone concentrations in mg/g. (b) Coccolith absolute abundances of E. huxleyi. (c) Coccolith absolute abundances
of G. mullerae and G. oceanica. (d) Coccolith absolute abundances of smaller Gephyrocapsa mainly G. ericsonii. (e) Total abundances
of coccoliths expressed in coccoliths/g. Shaded areas indicate cold isotope stages.
M.-A. Sicre et al. / Organic Geochemistry 31 (2000) 577±588
species of the genus Gephyrocapsa including G. protohuxleyi and G. ericsonii are likely to be contributors of
alkenones to contemporary sediments. E. huxleyi is
believed to have evolved from the genus Gephyrocapsa
during the late Pleistocene and to be phylogenetically
related to G. ericsonii via G. protohuxleyi (McIntyre,
1970). Total coccolith counts con®rm that coccolithophorid production is higher over stages 4±6 than from
stages 1±3. Fig. 5b also shows that E. huxleyi is the
dominant species in sediments younger than 60 kyears,
when alkenone/OC values are rather low, and that it
becomes a minor coccolithophorid species in sediments
where the alkenone/OC ratios are high. The smaller
Gephyrocapsa become prevalent species beyond 60 kyears,
i.e. at the reversal of the dominance pattern of the coccolithophorid population, an event that has a global
extension (Thierstein et al., 1977), when alkenones/OC
values are higher. Our results are consistent with those
reported by MuÈller et al. (1997) in the South Atlantic.
Larger Gephyrocapsa such as G. muellerae and G. oceanica also increase during interglacial stage 5 when SSTs
were warmer but coccolith counts of these species do no
match alkenone concentration ¯uctuations. If no species
monitored here can clearly account for the alkenone
pro®le, some sort of similarity is found in the absolute
abundances of the small Gephyrocapsa and alkenone
pro®les: maxima of small Gephyrocapsa around 140,
110, 80 and 75 kyears are in relative synchronism with
the alkenone concentration peaks P
at 140, 110,
P 85 and 75
kyears.
It
is
noteworthy
that
the
C
/
C38(Me+Et)
37Me
P
P
and C37Me/ C38(Et) remain constant throughout the
core (Ternois, 1996) thus indicating a stable distribution
of alkenones despite changing taxa abundances.
4. Conclusions
585
whereas siliceous production may have been more
important during glacial times. This may possibly
explain higher alkenone/OC ratio values from stages 4±
6 than from the present day to stage 3. However, the 24methyl-5a-cholest-22-enol/24-methyl-cholesta-5,22-dienol ratio record indicates stronger alteration of sterols,
mainly during stage 5. Based on the correlation between
OC and sterols, we can assume that other marine consitutents may have undergone similar alteration and
thus hypothesize that selective preservation may also
contribute to higher alkenone/OC ratios from stages 4±
6. A change of alkenone producers was also envisaged
as an alternative explanation for a higher contribution
of alkenones to sedimentary OC. The comparison
between alkenone abundances and coccolith counts
indicates a major temporal change of the coccolith taxa
assemblages around 60 kyears. While E. huxleyi dominates in sediments younger than this time boundary,
Gephyrocapsa sp. took over in sediments older than 60
kyears. Smaller Gephyrocapsa, mostly represented by G.
ericsonii, better matched the alkenone pro®le than larger
ones. Overall our results suggest that besides a change
from siliceous to calcareous production, the observed
shift in the alkenone/OC ratios could be indicative of a
change in the species producing alkenones, if the
amount of these compounds per cell was dierent from
that of E. huxleyi. Finally these data incline to be cautious when using alkenones to estimate primary production in the past.
Acknowledgements
Samples were provided by the Sedorqua program
which was supported ®nancially by French research
institutions (CNRS INSU, and MESR). This is LSCE
contribution number 0341.
0
18
UK
37 and d O values exhibited similar long term variations over the last climatic cycle. SSTs derived from
0
UK
37 values using the calibration established by Prahl et
al. (1988) ranged from a mean value of 17.4 C for the
last glacial to a mean value of 21.8 C for the Holocene
thus recording a 4.5 C warming during the last deglaciation, consistent with previous studies. OC MAR
indicated a signi®cant rise during the last glacial in
response to a strengthening of the NETW as revealed by
land-derived n-alkanol MAR. This increase resulted
from a higher supply of eolian dust from the continent
and primary production of the surface ocean. However,
while sterol MAR followed similar trends as OC MAR,
recording highest accumulation during glacial stages
(mostly stages 2 and 3) and a sharp decrease during the
last interglacial, the alkenones pro®le was quite dierent. The coccolithophorid production (calcareous production) appeared to be favored during warm periods as
indicated by a high pulse of alkenones during stage 5
Associate EditorÐB.R.T. Simoneit
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