Biomarker stratigraphic records over the last 150 kyears o
the NW African coast at 25
N
Marie-Alexandrine Sicre
a,*, Yann Ternois
b, Martine Paterne
a, Anne Boireau
c,
Luc Beaufort
b, Philippe Martinez
d, Philippe Bertrand
daLaboratoire 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
bCEREGE, CNRS SDU UMR 6635, Europole de l'Arbois, B.P. 80, 13545 Aix-en Provence Cedex 4, France cLaboratoire de Physique et Chimie Marines, Universite Pierre et Marie Curie, CNRS SDU ESA7077, 4 place Jussieu,
75252 Paris Cedex 05, France
dDeÂpartement 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 (25N, 16W, 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 4.5C 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 coccolitho-phorid 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 tem-peratures (SSTs). Production and sedimentation of organic carbon (OC) in this region is in¯uenced by the
supply of terrigenous material and biogenic sedimenta-tion stimulated by the Northeast Trade Winds (NETW). However, the intensity of the upwelling cells is not uni-form 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 (21N), and the South Atlantic Central Waters (SACW), South of Cape Blanc (Tomczak, 1977). MuÈller et al. (1983) have shown that around 25N, OC production and sedimentation have varied considerably over the late Quaternary. Paleopro-ductivity 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
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* Corresponding author: Tel.: +33-1-69-82-43-34; fax: +33-1-69-82-35-68.
allows a more detailed examination of the source mate-rial deposited to the sediment. In this work we report our results on several classes of biomarkers analyzed in the SU94-20bK core (25N, 16W, 1445 m water depth) collected during the Sedorqua cruise in February 1994, on the R/VLe 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 distribu-tion of upwelling cells, today.
Three classes of biomarkers were selected for this study to determine the temporal variations of terrige-nous 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
most widely used class of biomarkers for paleoclimatic and paleoceanographic studies, were measured to esti-mate paleo-SSTs. The C37alkenone unsaturation index UK0
37has 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, 1984; McCarey et al., 1990; Conte et al., 1992), theUK0
37 index, on which paleo-temperature calculations are based does not seem to be aected by food web pro-cesses (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 recon-struction 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; Vil-lanueva 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 Emilia-nia huxleyi as well as the closely relatedGephyrocapsa
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 con-ditions. The abundance of long-chain saturated n -alkanes,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; Villa-nueva 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 over-lying water's planktonic production (Volkman, 1986, ten Haven et al., 1989; Farrimond et al., 1990). Chan-ges 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).
2. Methods
2.1. Oxygen isotope stratigraphy
Oxygen isotope analyses were performed along the SU94-20bK core on planktonic foraminifera Globiger-ina bulloõÈdes(>250mm) on a Finnigan 251 mass spec-trometer calibrated in PDB via NBS19. External precision was 0.07%d18O (1 sigma value). Thed18O 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 ÿ18C. 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 follow-ing the procedure described by Conde (1989). Fractions were stored in 4 ml vial at ÿ18C 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 300C at 10C minÿ1 (60 min). UK0
37 ratios were calculated from chromatographic peak areas. Analytical precision obtained after triplicate injections was calculated to be 0.01 unit ratio. The fractions containingn-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 m0.32 mm i.d., 0.25 mm
®lm thickness, J&W Scienti®c) with a temperature pro-gram from 60 to 300C at a heating rate of 7C 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/
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 140C, 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,
Gephyrocapsasmaller than 3 mm (mainly G. ericsonii), and Gephyrocapsa larger than 3 mm (mainly G. muel-lerae andG. oceanica) were counted. In order to esti-mate the absolute abundance of coccoliths, we used a dierent preparation technique. About 20 mg of dried sediment was weighted, diluted in deionized water buf-fered 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 0.45mm. The membrane, once dried, was moun-ted 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 cocco-liths 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 carbon (%OC) andUK0
37values of the surface sediments and core-tops. Data from other sites occupied during previous cruises (R/VMeteor and Ocean Drilling Pro-gram) are also listed. SSTs were derived from theUK0
37 index and the calibration established by Prahl et al. (1988) (UK0
37=0.034T+ 0.039). These temperature esti-mates 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 calculated from theUK0
19.4C, at
27N, and 25C at9N. South of 15N,
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 recorded by theUK0
37values: between 20 and 25N, SSTs vary in a tight range (20±21C). 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 non-upwelling regime, today. The OC content at the top of the SU94-20bK core was 0.68%.
3.2. SU94-20bK core
3.2.1. d18O and UK0
37derived SST records Fig. 2a provides completed18O andUK0
37records of the glacial/interglacial stages over the last 150 kyears, extending from stage 6 to the Holocene.UK0
37 ratio and
d18O pro®les show similar long term changes over the last climatic cycle (r2=0.67;n=83). SSTs vary from a low value of 16.7C at the last glacial to highest of 23.3C at the Eemien (Fig. 2b). They increase from a mean value of 17.4C for the last glacial to a mean
value of21.8C for the Holocene. SSTs at the Eemien
are warmer than mean Holocene SSTs by 1.5C. The amplitude of the last glacial/interglacial warming at our coring site is about 4.5C. 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 chan-ges. In the southern BOFS 31K marine core (19N,
20W, 3300 m water depth), Chapman et al. (1996) reported UK0
37 based SSTs of 20.5C for the Holocene and of 17C 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 21.5C in the Holocene dropping to 20.5C towards the top of the core. The temperature change associated with the last deglaciation is 3±4C (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 SU94-20bK 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 (25N 16W; 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 phyto-plankton is labile and degrades rapidly in the water column and in the ®rst centimeters of the surface sedi-ment. 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
Table 1
Coordinates, water depth (in meters), percentage of organic carbon (% OC), andUK0
37values measured in surface sediments of the NW
African margina
Sample Program Site location Depth (in meter) OC (in %) UK0
37 SSTs (UK
0
37) SSTs (Levitus)
SU94-24S Sedorqua 2658N 1401W 765 0.80 0.70 19.4 20.2
SU9417dS Sedorqua 2653N 1441W 2597 0.55 0.73 20.3 20.2
M12392-1 Meteor 2516N 1605W 2575 0.35 n.db ± 20.4
SU94-20bk Sedorqua 2501N 1639W 1445 0.68 0.77 21.5 20.4
SU9421S Sedorqua 2453N 1631W 750 0.74 0.75 20.9 20.4
SU94-15S Sedorqua 2344N 1716W 1000 1.13 0.74 20.6 20.5
SU94-11S Sedorqua 2129N 1757W 1200 2.92 0.73 20.3 20.1
SU94-7S Sedorqua 2111N 1852W 3010 0.90 0.75 20.9 20.6
658A ODP 2075N 1858W 2263 2.67 0.72 20.0 20.7
SU94-2S Sedorqua 1929N 1717W 1407 1.98 0.75 20.9 20.7
659 ODP 1804N 2101W 3070 0.1 n.d. ± 22.4
660A ODP 1000N 1914W 4332 n.d. 0.86 24.1 26.0
M16415-2 Meteor 934N 1964W 3851 n.d. 0.89 25.0 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.
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 bio-markers 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 marine-derived compounds. Also, among marine biomarkers, preferential degradation in the water column and sur-face sediment may also lead to varying degrees of sedi-ment 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. Then-alkanols generally range from
n-C17ton-C34and display a bimodal distribution (from
n-C17ton-C20and fromn-C21ton-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 (Gago-sian and Peltzer, 1987). In this study we calculated the MAR of the sum of individualn-alkanols from C20 to C32, with the exception of C31, co-eluting with a hopa-nol. Then-alkanol and OC MAR show a strong simi-larity (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, Fig. 2. (a) Downhole pro®les ofUK0
37index andd18O values (in%PDB) along the SU94-20bK core, over the last 150 kyears. (b) SST
estimates obtained from theUK0
37 index and the calibration established by Prahl et al. (1988) (UK
0
37=0.034T+ 0.039). Shaded areas
then-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 n -alkanol 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 Eur-african margin and the Saharan desert was drier than today (Sarnthein, 1978; Koopmann, 1979). Sediment deposits similar to modern NETW type deposits domi-nated o the coast, between 20N and 28N (Sarnthein et al., 1981). Marine-continental correlations of pollen records provide further evidence for the stronger inten-sity of the NETW during glacial periods (Hooghiem-stra, 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 andn-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 SU94-20bK core (Fig. 4a in Martinez et al., 1996). The glacial OC content in both cores reaches a value of 3% sug-gesting 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
Fig. 3. OC and biomarker MAR (Mass Accumulation Rate) pro®les against14C age along the SU94-20bK core, over the last 150
kyears. (a) Organic carbon MAR (OC). (b) PC20ÿC32n-alkanol MAR. (c) PC27ÿC294-desmethyl sterol MAR. (d) C37alkenone
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 C37alkenones 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 cer-tain 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 zoo-plankton 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
included in the total sterol concentration. Bacteria do not produce 4-desmethyl-sterols (Ourisson et al., 1979), but they can promote the hydrogenation of5-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 -methyl-sterols 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 distribu-tion 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 dur-ing glacial stage 2 and continuously decreased until the disappearance of diatoms from 7 to 5 kyears. Accumu-lation 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 C37alkenone concentrations (in ng/g) vs percentage of organic carbon (%OC). Solid diamonds refer
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 abun-dant nutrient conditions of glacial times, diatoms and/ or other primary producers would outcompete cocco-lithophorids; a second period during stage 5 which indicates optimium growth conditions for coccolitho-phorid 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/oxida-tion 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 lat-eral 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 selec-tive 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 possi-bility of a major change of alkenone producers to explain the alkenone/OC values prior and after the iso-tope transition 3/4. Coccolith abundances were deter-mined from major coccolithophorid species present in the core, namelyE. huxleyi, G. mullerae, G. oceanicaand
G. ericsonii. The coccolith abundances per taxa are shown in Figs. 5b±d together with total counts (Fig. 5e) and C37±C38alkenone abundances (Fig. 5a). Although there is no report on the occurrence of alkenone inG. 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) C37and C38alkenone concentrations inmg/g. (b) Coccolith absolute abundances ofE. huxleyi.(c) Coccolith absolute abundances
species of the genus Gephyrocapsaincluding G. proto-huxleyiand G.ericsonii are likely to be contributors of alkenones to contemporary sediments. E. huxleyi is believed to have evolved from the genusGephyrocapsa
during the late Pleistocene and to be phylogenetically related to G. ericsonii via G. protohuxleyi (McIntyre, 1970). Total coccolith counts con®rm that coccolitho-phorid 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
Gephyrocapsabecome prevalent species beyond 60 kyears, i.e. at the reversal of the dominance pattern of the coc-colithophorid 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. LargerGephyrocapsasuch asG. muellerae and G. ocea-nicaalso 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 at 140, 110, 85 and 75 kyears. It is noteworthy that thePC37Me/
P
C38(Me+Et)
andPC37Me/PC38(Et)remain constant throughout the
core (Ternois, 1996) thus indicating a stable distribution of alkenones despite changing taxa abundances.
4. Conclusions
UK0
37 andd18O values exhibited similar long term var-iations over the last climatic cycle. SSTs derived from
UK0
37 values using the calibration established by Prahl et al. (1988) ranged from a mean value of17.4C for the
last glacial to a mean value of21.8C for the Holocene thus recording a 4.5C warming during the last degla-ciation, 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 dier-ent. The coccolithophorid production (calcareous pro-duction) appeared to be favored during warm periods as indicated by a high pulse of alkenones during stage 5
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 24-methyl-5a -cholest-22-enol/24-methyl-cholesta-5,22-die-nol 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 con-situtents 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 dom-inates in sediments younger than this time boundary,
Gephyrocapsasp. took over in sediments older than 60 kyears. SmallerGephyrocapsa,mostly represented byG. 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 ofE. huxleyi. Finally these data incline to be cau-tious when using alkenones to estimate primary pro-duction 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.
Associate EditorÐB.R.T. Simoneit
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