SAHARAN WIND REGIMES TRACED BY THE Sr–Nd ISOTOPIC COMPOSITION OF SUBTROPICAL ATLANTIC SEDIMENTS: LAST GLACIAL MAXIMUM vs TODAY

PII: S0277—3791(97)00048-6

Quaternary Science Reviews, Vol. 17, pp. 395—409, 1998 ( 1998 Elsevier Science Ltd. Printed in Great Britain. All rights reserved. 0277—3791/98, $19.00

SAHARAN WIND REGIMES TRACED BY THE Sr–Nd ISOTOPIC COMPOSITION OF SUBTROPICAL ATLANTIC SEDIMENTS: LAST GLACIAL MAXIMUM vs TODAY F.E. GROUSSET*, M. PARRA*, A. BORY-‡, P. MARTINEZ*, P. BERTRAND*, G. SHIMMIELD-° and R.M. ELLAM± * De´ partement Ge´ ologie et Oce´ anographie, UMR CNRS 5805, Universite´ Bordeaux I, Avenue des Faculte´ s, 33405 Talence Cedex, France (e-mail: [email protected].) - Marine Geosciences Unit, Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, U.K. ± Isotope Geosciences Unit, Scottish Universities Research and Reactor Centre, East Kilbride G75 0QF, U.K. Abstract—New Nd—Sr isotopic data on the (30 lm lithic particles of surface and Last Glacial Maximum sediments recovered along the African margin between the Equator and the Gibraltar Strait are presented in combination with grain-size measurements. This (30 lm size fraction allows us to eliminate any hemipelagic contribution that could occur in the coarser fractions. In the eolian fraction, both Sr and Nd isotopic tracers reveal the same major northwestern origin (Mauritania, Mali, southern Algeria and Morocco). The Archaean formations of the western Saharan shield could be the source of the very unradiogenic ratios observed here. The more southern regions (Senegal, Guinea) act only as secondary sources. A similar pattern is observed for the LGM. Lithic particles are mostly transported by both Trade and Saharan Air Layer (SAL) winds, along an approximate NE—SW axis; this main feature matches the ‘southern plume’, characterizing the dust transport observed during winter. No significant latitudinal shift of the belt winds is observed between the LGM and today. At the LGM, however, dust fluxes were 2—4 times higher than today, leading to a more ‘Archaean-type’ imprint in the deposits. We do not observe any clear relationship between the latitudinal variability of the upwelling systems identified in this region at the LGM and the location of the major wind systems. Both enhanced aridity on the continent and increased wind speed probably occurred together over western tropical Africa during the Last Glacial period. ( 1998 Elsevier Science Ltd. All rights reserved

INTRODUCTION

derived from the more southerly Saharan—Sahelian region (Rognon et al., 1996), indicating transport by E—W winds. Depending on their altitude, these E—W winds are called: continental trade winds, Saharan Air Layer (SAL) or African Easterly Jets. According to d’Almeida (1986), they mostly originated from source regions located roughly at the same latitude in the Sahelian belt (Mali, Niger, southern Algeria and even Chad). These two major wind systems are still active to-day over this region (Schu¨tz, 1980; Coude´-Gaussen et al., 1987), their relative intensity being seasonally modulated. They are at the origin of two major seasonal dust plumes over the ocean: one centered around 20°N during the summer, the other shifting southward around 10°N during the winter (Kalu, 1979; Moulin et al., 1997). The seasonal latitudinal shifts of about 10° of the ITCZ (Inter Tropical Convergence Zone) from a summer northern location to a winter southern location would control the location of these plumes. In oceanic surface sediments, the distribution of lacustrine diatoms would reflect the transport of aerosols by these two major wind regimes (Pokras and Mix, 1985). According to Kolla et al. (1979), who studied the quartz distribution in sediment cores, the main dust

It is well known that every year, millions of tons of Saharan dust are transported over, and deposited in, the tropical Atlantic (Buat-Me´nard and Chesselet, 1979; Kolla et al., 1979; Sarnthein et al., 1981; Ganor and Mamane, 1982; Duce et al., 1991), with the finest particles even reaching the Caribbean (Prospero, 1981). The study of dust accumulation in marine sediments leads to a better understanding of palaeo-atmospheric circulation. Previous studies have demonstrated that Saharan dust is transported by two major wind systems. Thus, the crust-derived fraction of the Holocene loess that accumulated over the Canary Islands was derived from Morocco (Grousset et al., 1992) and transported from the NE by the ‘marine trade winds’. By contrast, the crust-derived fraction of the Holocene loess accumulated over the Cape Verde Islands was

‡ Present address: Centre des Faibles Radioactivite´s, Laboratoire Mixte CNRS-CEA, Avenue de la Terrasse, 91198 Gif-sur-Yvette, France. ° Present address: Dunstaffnage Marine Laboratory, P.O. Box 3., Oban, Argyll PA34 3AD, U.K.

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transport axis was centered around about 8°N at the LGM and has shifted northward since that period, as far north as about 18°N in the late Holocene. Sarnthein et al. (1981) studied the distribution of the terrigenous silt fraction (%'6 lm of the carbonate-and-opal-free) in both surface and LGM levels. This fraction should reflect the input of African aerosols transported westward, mostly by the SAL winds, across the Atlantic ocean. However, these authors do not observe any clear latitudinal shift of the patterns between these two periods, but do infer an increase, by a factor of 2, in the silt particle flux at LGM, with the latitudinal location remaining roughly the same. Thus, the ITCZ may not have shifted substantially between Glacial and Interglacial periods, and the observed difference would be the result of variable prevailing winds, NE trades carrying white quartz, chlorite and illite from the northern region and E—W SAL winds transporting iron-oxidecoated quartz and kaolinite from the more southern Sahelian zone (Sarnthein and Koopmann, 1979). On the other hand, Ruddiman (1997) recently suggested that, as changes in monsoonally driven source-area aridity had not left a strong dust-flux imprint in tropical Atlantic sediments in the late Pleistocene, sourcearea aridity could not be the main factor controlling dust fluxes. In this scenario, enhanced glacial influxes of dust would be more likely to the due to changes in transport by winter winds, rather than to glacial hyperaridity. Furthermore, Rea (1994) suggested that aridity was the dominating factor that controls the increase in the dust flux emitted by the desert areas at the LGM. Clearly, the origin of LGM lithic particles, their glacial atmospheric context and their marine sedimentary regimes are highly controversial issues. In this context, we studied the (30 lm lithic particles (the carbonate-free fraction) of surface sediments (boxcores) and the LGM levels (piston-cores) recovered along the African margin between the Equator and the Gibraltar Strait. This size fraction ((30 lm) allows us to eliminate any hemipelagic contribution that could occur in the coarser fractions. The purpose of studying the Nd—Sr isotopic composition and the grain-size distribution of these lithic particles was to fingerprint their origins and identify their distribution patterns for both periods (LGM and today). These two approaches might help to constrain better the atmospheric transport patterns and strength, to identify possible latitudinal shifts that could characterize the contrast between glacial and interglacial regimes and thus to assess better the hypothesized temporal latitudinal shifts of the Mauritanian upwelling system.

SAMPLES AND METHODS Samples Sample locations are reported in Fig. 1 and Table 1. Aerosol samples: except for two samples collected over the Canary Islands (dA18) and Niger (dNya), all

the aerosol samples were collected aboard research vessels, during oceanographic cruises conducted over the last two decades, off NW Africa between the Canary Islands and the Equator. Loess, fine powdered dust (fesh—fesh) and silt samples were collected in the northwestern regions of the African continent from Morocco to as far south as Guinea, mostly in dune fields (ergs) and Sahelian powdered dust accumulations. Although these deposits are drifted by winds over long distances, we assume that they are mainly derived from their underlying outcrops and thus they provide an average composition of the regional geological units. We also documented the areas that are considered as the main sources of the present-day aerosols (d’Almeida, 1986; Chiapello, 1996; Chiapello et al., 1997). Marine sediment samples: we sampled the first centimeter of both the surface sediments (using multi- and box-cores) and the LGM levels (using piston- and karsten-corers) in cores recovered along the African margin, between the Gibraltar Strait and the Equator. Ages were derived from the organic carbon records measured on all cores and calibrated against the d18O records obtained on planktic foraminifera from three of these cores (Bertrand et al., 1996; Martinez et al., 1996). In the four BOFS cores, the LGM levels were derived from the d18O records obtained on planktic foraminifera (Matthewson et al., 1995). Due to bioturbation processes, surface sediments generally represent a mixture of particles deposited over the last few centuries and frequently the last two millennia. Thus, their isotopic composition will most likely reflect an integration of the historical period, rather than the presentday situation. For both the surface and the LGM levels, we sampled hemipelagic deposits after checking optically that there was no visible evidence of turbidities (hiatus, vertical grain-size gradient, laminae) or of bioturbation (worm burrows). Nonetheless, we cannot rule out the possibility of a hardly visible diffuse bioturbation, mostly due to benthic foraminifera movements (Bornmalm et al., 1997). Methods Grain-size distributions were measured on the lithic fraction, after leaching the carbonate fraction, using a Malvern 2600 E laser sizer, based on the near-forward scattering of a laser beam by particles suspended in water (McCave and Syvitski, 1991). Sufficient sample was weighed out to yield about 100 mg of alumino-silicate material after dissolution of the calcium carbonates and crushed in a grinder. Carbonate was dissolved using mechanical agitation of a pH sodium acetate solution buffered at a pH of 4.5. This carbonate free fraction was then sieved at 30 lm in order to analyze the (30 lm fraction. The carbonate-free material may include some opaline silica, but since the concentrations of Sr and Nd in marine biogenic opal are low (+5 ppm and +3 ppm, respectively, unpublished data), we do not consider this to be

Aerosols A17 SL A9Nya A18 A4n°4 n°10 n°17 n°27 n°59

Possible source Ouj Elra Foum Smara Zouerat Atar Erg Elm Tich Kif Nou Tim MEK-21 MEK-58 ATK-35 Tas Kay Tamba Est Foun Ro Sen Riv Til Lab Ivo

Sample

Canaries Islands off St. Louis (Senegal) Cape Verde Basin Niamey (Niger) Sierra Leone Basin Cape Verde Basin Corsica Corsica Corsica Corsica Corsica

areas Oujda El Rachidia Foum el Hassan Essmarra Zouerat Atar Erg sud Atar El Mroiti Tichit Kiffa Nouakchott Timimoun Sebkra Mekkerane Sebkra Mekkerane Atakor Tassendjanet Kayes Tambacounda Est Dakar Foundiougne continental shelf Senegal river Tillabe´ri Labbe

Name

Latitude +28°N +17°N 11°—14°N 14°N 8°—13°N 0°—3°N 42°N 42°N 42°N 42°N 42°N

Morocco Morocco Morocco Morocco Mauritania Mauritania Mauritania Mauritania Mauritania Mauritania Mauritania Algeria Algeria Algeria Algeria Algeria Mali Senegal Senegal Senegal Senegal Senegal Niger Guinea Ivory Coast

Location

Longitude +04°W +18°30W +26°W +02°30E +13°30W +29°30W +09°E +09°E +09°E +09°E +09°E 33.9 13.9 35.6 32.0 37.9 38.6 23.2 25.9 33.4 31.3 26.3

0.511949 0.511890 0.512062 0.512123 0.511897 0.511980 0.512059 0.511959 0.511913 0.511890 0.512025

(10) (14) (17) (11) (12) (40)

0.512015 (08) 0.511247 0.511855 (11) 0.511894 (08) 0.511807 (13) 0.511901 (10) 0.511894 (11) 0.511838 (31) 0.511999 (12) 0.512013 (08) 0.511493

39.7 30.9 31.4 3.2 28.4 41.9 19.2 29.0 22.0

0.512031 0.511938 0.511761 0.511798 0.511725 0.511717 0.511943 0.511784 0.511856 0.511923 0.511760

(12) (21) (14) (16) (12) (12) (07) (12) (08) (07) (09)

143Nd/144Nd ($2sig]10~6)

29.9 32.7 38.7 40.2 38.0 3.5 3.2 35.5 31.4 31.5 55.9

Nd (ppm)

!13.4 !14.6 !11.2 !10.0 !14.4 !12.8 !11.3 !13.2 !14.1 !14.6 !12.0

!12.1 !27.1 !15.2 !14.5 !16.2 !14.3 !14.5 !13.1 !12.4 !12.1 !23.2

!11.8 !13.6 !17.1 !16.3 !17.8 !17.9 !13.5 !16.6 !15.2 !13.9 !17.1

E(Nd)o

TABLE 1. Sample location, Sr and Nd concentrations and isotopic ratios

95.7 154.3 140.9 90.5 147.3 127.7 128.6 92.5 128.1 116.0 110.2

66.9 74.0 10.6 127.5 97.5 47.7 87.1 37.5

111.6 87.2 90.3 104.6 91.6 23.1 78.1 94.1 78.2 77.8 126.5 21.1 164.7 168.6

Sr (ppm)

0.725225 0.713499 0.721871 0.714660 0.721116 0.720972 0.72071 0.72451 0.71925 0.72426 0.72227

0.713 0.736840 0.718296 0.726326 0.714707 0.715296 0.728581 0.723832 0.723603 0.7146

0.716593 0.726932 0.728197 0.734041 0.735679 0.727284 0.737645 0.737535 0.731493 0.728389 0.720021 0.726253 0.720515 0.724395

(16) (30) (09) (09) (14) (46)

(10) (14) (11) (18) (20) (42) (28) (43)

(22) (08) (09) (18) (37) (19) (17) (16) (11) (16) (10) (13) (16) (35)

87Sr/86Sr ($2sig]10~6)

1 1 1 1 1 1 3 3 3 3 3

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 1 1 1 1 1 1 1 1 2

Source*

F.E. Grousset et al.: Saharan wind regimes

397

cores) cores) cores) cores)

Glacial sediments (LGM) Sedorqua K17d‡ K20b‡ Sedorqua 28 ° BOFS K15‡ Sedorqua 32 ° BOFS K11‡ Sedorqua BOFS 29 ° K02‡ Sedorqua 31° BOFS VM22-196 L-DEO (Vema VM27-175 L-DEO (Vema VM22-189 L-DEO (Vema VM30-41K L-DEO (Vema

26°53N 25°02N 24°34N 23°44N 22°31N 21°29N 20°34N 19°29N 19°00N 13°50N 08°48N 04°55N 00°13N

34°20N 32°24N 26°59N 26°53N 25°02N 24°34N 23°44N 22°31N 21°20N 20°34N 20°00N 19°29N 19°00N 13°50N 08°48N 04°55N 00°13N

Latitude

14°41W 16°39W 22°49W 17°16W 22°00W 17°57W 21°07W 17°17W 20°10W 18°57W 22°12W 21°07W 23°03W

07°01W 10°25W 14°13W 14°41W 16°39W 22°49W 17°16W 22°00W 18°15W 21°07W 30°00W 17°17W 20°10W 18°57W 22°12W 21°07W 23°03W

Longitude

2597 1445 4855 1000 4560 1200 4005 1407 3300 3728 4548 2525 3874

730 3165 1245 2597 1445 4855 1000 4560 2002 4005 5000 1407 3300 3728 4548 2525 3874

Depth (m)

24.8 28.5 36.9 27.0 30.7 28.2 32.1 26.3 27.8 39.2 35.1 25.4

0.511853 0.512001 0.511634 0.511964 0.511720 0.511933 0.511723 0.511957 0.511697 0.511819 0.511928 0.512053

0.511911 0.511947 0.511899 0.511940 0.511876 0.511892 0.512024 0.511901 0.511940 0.511832 0.512733 0.511684 0.512058

32.2 30.7 27.4 29.4 24.5 27.1 32.1 21.9 27.4 32.4 34.6 40.0 26.6

(28) (08) (09) (08) (11) (11) (11) (07) (09) (08) (09) (09)

(10) (07) (09) (10) (10) (06) (15) (58) (10) (19) (11) (12) (14)

0.512037 (08) 0.511969 (10)

143Nd/144Nd ($2sig]10~6)

35.3 33.0

Nd (ppm)

* 1"This work; 2"Alle`gre and Ben Othman (1982); 3"Colin (1993); 4"Bory (1997); 5"Grousset and Biscaye (1989). In a previous paper we had already analyzed the bulk fraction of these samples (Grousset et al., 1988). ‡ Cores from DGO (Sedorqua and Dialante cruises). ° R/V Charles Darwin cruise 53 BOFS Leg C (U.K.).

cores) cores) cores) cores)

Cruise

Surface sediments Sedorqua K23‡ K25‡ Sedorqua Sedorqua K22‡ K17d‡ Sedorqua Sedorqua K20b‡ BOFS 28 ° K15‡ Sedorqua 32 ° BOFS Sedorqua K09‡ 29 ° BOFS CV2‡ Dialante K02‡ Sedorqua 31° BOFS VM22-196 L-DEO (Vema VM27-175 L-DEO (Vema VM22-189 L-DEO (Vema VM30-41K L-DEO (Vema

Sample

TABLE 1. (continued)

!14.1 !12.4 !19.5 !13.1 !17.9 !13.7 !17.8 !13.3 !18.3 !15.9 !13.8 !11.4

!15.3 !13.4 !14.4 !13.6 !14.8 !14.6 !12.0 !14.3 !13.6 !15.7 #1.9 !18.6 !11.3

!11.7 !13.0

E(Nd)o

205.3 122.9 126.7 187.3 104.0 199.6 168.5 113.1 158.7 79.1 145.1 112.1 126.2

102.2 112.5 142.2 165.3 200.0 131.0 150.6 142.4 137.7 133.1 121.0 125.0 138.6 138.7 476.8 124.5 111.7

Sr (ppm)

0.716317 0.719334 0.720436 0.719040 0.720932 0.716994 0.717435 0.723080 0.717439 0.719912 0.718480 0.718046 0.717470

0.719567 0.724544 0.719791 0.718151 0.720657 0.720137 0.720385 0.719635 0.720191 0.720367 0.720419 0.721866 0.719497 0.718021 0.704635 0.719184 0.719789 (25) (28) (17) (50) (33) (39) (34) (44) (38) (13) (73) (11) (07)

(19) (19) (10) (09) (11) (19) (12) (66) (08) (19) (27) (27) (53) (18) (16) (13) (11)

87Sr/86Sr ($2sig]10~6)

1 1 4 1 4 1 4 1 4 1 1 1 1

4 4 1 1 1 4 1 4 1 4 5 1 4 1 1 1 1

Source*

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399

F.E. Grousset et al.: Saharan wind regimes

FIG. 1. Sample location. Symbols: Possible source areas (solid circles), marine cores (squares), aerosols (crossed open squares). Along with our sediment cores (solid squares), are some cores from Dasch (1969) and Grousset et al. (1988) (open squares).

for individual analyses on each mass spectrometer. The measured 87Sr/86Sr and 143Nd/144Nd ratios were corrected for mass fractionation by normalizing to 86Sr/ 88Sr"0.1194 and 146Nd/144Nd"0.7219, respectively. Blanks were measured (Sr(10~9 g and Nd( 10~10 g) and are considered to be negligible in all cases. For convenience, 143Nd/144Nd ratios are normalized and reported as follows: e (o)"((143Nd/144Nd/ N$ 0.512636)!1)]104, in which 0.512636 is the Chondritic Uniform Reservoir value (Jacobsen and Wasserburg, 1980). The spike contribution to the isotope ratios for samples was corrected using an iterative calculation. Strontium standard NIST 987 was measured 6 times at Toulouse with an average 87Sr/86Sr" 0.710205 ($0.00002), and 14 times at SURRC with an average 87Sr/86Sr"0.710214 ($0.00002) vs the certified value of 0.710245. Neodymium standard LaJolla was analyzed at Toulouse 5 times with an average 143Nd/144Nd"0.511846 ($0.000015) vs the certified value of 0.511865. This standard was analyzed at SURRC 20 times during instrument installation, when it gave 0.511851 ($0.000012). Neodymium standard Johnson-Matthey was analyzed at SURRC 25 times with an average 143Nd/144Nd"0.511504 ($0.00002) vs the long-term mean value of 0.511503 ($0.000016). Differences between the analyzed and certified values are within the range of the uncertainty of the sample measurements. Possible Effect of Grain-size on Isotopic Compositions

a serious problem. We will refer to this carbonate-free fraction as the lithic fraction. This fraction may sometimes also contain a minor authigenic component, e.g. Fe—Mn—oxides, that has a negligible effect on the isotopic composition of the lithic fraction (Grousset and Biscaye, 1989). &20 mg aliquots of carbonate-free samples (and &150 mg for the BOFS cores) were dissolved with a mixture of teflon-distilled [HF# HClO #HNO ]. Sr and Nd were chemically sepa4 3 rated through ionic chromatographic columns, in a ‘class-1000’ clean laboratory. We followed chemical and mass spectrometer techniques previously described (Grousset et al., 1988; Grousset and Biscaye, 1989; Fitton et al., 1997). TIMS analysis of the samples was done on two different mass spectrometers: analysis of Sr and Nd in the BOFS cores (Table 1) was done on a VG Sector 54-30 multi-detector mass spectrometer at Scottish Universities Research and Reactor Centre (SURRC) [U.K.]; analysis of Sr and Nd on the other samples was done at the Universite´ de Toulouse [France] on a Finnigan MAT 261 multi-detector mass spectrometer. At Toulouse, Sr was mounted as nitrate on a W filament and Nd as nitrate on a Re-filament; at SURRC, Sr was mounted as nitrate onto Ta filament and Nd as nitrate onto Ta—Re—Ta triple filaments. All the measurements, on both mass spectrometers, were made using multiple Faraday cups. We have intercalibrated the results of both mass spectrometers with standards and the results are within the range of errors

Dasch (1969) reported increasing 87Sr/86Sr ratios with decreasing grain-size in carbonate-free sediment samples. A grain-size effect on Sr-isotope composition must therefore be eliminated or at least considered. This possibility has already been discussed for the Saharan-derived aerosols (Grousset et al., 1992; Rognon et al., 1996). We minimized this potential grain-size effect by studying the same grain-size fraction ((30 lm) in all samples. The choice of this size range is justified below (Results section). However, for the BOFS samples (see Table 1), we analyzed the carbonate-free bulk samples: nonetheless we do not suspect any significant bias in these four samples as they were collected far away from the continent ('500 km off the coasts), thus precluding any significant input of coarse ('30 lm) particles, making the sediments themselves effectively (30 lm. By contrast, possible grain-size effects on Nd isotopic compositions are of little concern because the Nd isotope ratios are almost not affected by grain size variations, either in riverine sediments (Goldstein et al., 1984) or in aerosols and loess (Grousset et al., 1992). RESULTS Grain-size Distributions The grain-size distributions of source samples, aerosol and marine sediment samples reveal three

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40% of '63 lm particles at the LGM, and less than 20% today; such a high value at the LGM could be explained by an advected hemipelagic contribution. Indeed, at the LGM the low sea-level stand was located at about !120 m (Fairbridge, 1961) and, thus, dune fields invaded the narrow continental shelf and might have directly fed the slope with coarse particles. This is confirmed by the fact that the size of '80% of the particles from both the Saharan dune fields and the sediments from the Senegal shelf, range from &60 to &100 lm. For this reason, in order to fingerprint only the wind-borne particles directly derived from the atmosphere, isotopic analyses have been conducted only on the (30 lm grain-size fraction. Moreover, most of the dust particles collected over this oceanic region are smaller than &30 lm (Beltagy et al., 1972; Schu¨tz, 1980; Coude´-Gaussen, 1989; Wefer and Fischer, 1993; Ratmeyer et al., 1997), which is in agreement with the fact that in the surface sediments more than 80% of the particles are smaller than 30 lm (mode centered around 20 lm).

Isotopic Results Sr/ 86Sr ratios and eNd (o) distributions

87

FIG. 2. Grain-size distribution of the carbonate-free fraction of some characteristic samples: uni-modal aerosols and sand dune fields (a), present-day (c) and LGM (b) marine sediments; some rare LGM samples display a bi-modal distribution, explained by a mixture of aerosols and hemipelagic deposits, advected from the continental shelf (d).

major features (Fig. 2). A first family (Fig. 2a) displays a uni-modal distribution, with more than 80% of the particles ranging between &70 and &90 lm. This sandy population is very well sorted, is characteristic of the Saharan sand dune fields and of the continental shelf deposits, and is very different from the aerosol distribution in which the mode is centered around &20 lm. A second family also displays a uni-modal distribution, with a mode centered around &20— 25 lm (Fig. 2b—c). In some samples, this mode shifts slightly toward the coarser diameters, up to &30— 40 lm. The fine particles ((10 lm) account for less than 20%. These silty populations characterize most of the marine sediments. A third bi-modal family (Fig. 2d) characterizes a few LGM marine sediments from the continental slope of Mauritania, the two previous modes being superimposed (e.g. LGM levels in cores K11 and K20). In this last group of samples, we suspect that such coarse particles (coarse mode) could not have been introduced by atmospheric transport, but more likely reflect the addition of particles advected by gravitational processes from the upper slope or the external shelf. In core K20 (Table 1), we observe about

In considering the surface sediments, we have augmented the new 87Sr/86Sr ratios measured in this study, with previously published Sr isotope data (Dasch, 1969; Grousset and Biscaye, 1989). In the detrital fraction of the surface sediments, 87Sr/86Sr ratios range from &0.715 to &0.725 (with an average of 0.7194 for 25 values) and are thus rather homogeneous if compared to the values observed in the entire Atlantic Ocean: &0.7033 to &0.7450 (Biscaye et al., submitted). Moreover, there is no obvious geographic 87Sr/86Sr zonation. Over the same region, aerosol Sr isotope ratios define a similar range (&0.7135 to &0.7253). Data from the literature (Biscaye et al., 1974; Grousset and Biscaye, 1989), even though they also fall within this range, are not directly comparable because they were obtained on bulk aerosols and are thus modified by the carbonate fraction. The geographic distribution of e (o) in the detrital N$ fraction of surface sediments is displayed in Fig. 3a. Previous data from the literature obtained on both sediments (Grousset et al., 1988) and aerosols (Goldstein et al., 1984; Grousset et al., 1988) are also included. The general pattern displayed in Fig. 3a corresponds roughly to that obtained previously, albeit at much lower resolution (Grousset et al., 1988). Sixteen e (o) N$ values range from !11.3 to !18.6 (with an average of !13.8). Over the same region, present-day aerosols display a narrower range (!12.8 to !14.6). These values are much less radiogenic than the nearby Mediterranean Sea sediments, !2 to !10 (Frost et al., 1986), that are significantly influenced by the local active volcanism. A similar local volcanic influence is observed in core V27-175 (e (o)"#1.9; 87Sr/86Sr N$

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401

FIG. 3. (a) Present-day distribution of e (o). (b) Distribution of e (o) at the LGM. Symbols: Possible source areas (solid circles), marine cores N$ N$ (data from the literature: open squares; our data: solid squares), aerosols (crossed squares), Senegal river (solid diamond). Isoline !20 is drawn according to data from Chardon (1996) and Potrel et al. (1996).

&0.7046), in which some volcanogenic particles were observed, probably inherited from the nearby Cape Verde volcanic islands through turbiditic currents. This sample cannot therefore be representative of eolian material. Although their contribution is probably negligible in this region, we have also to consider local rivers as possible sources of lithic particles. Potential riverine sources include the minor local Senegal River, which enters the Atlantic at the latitude of the Cape Verde Islands (e (o)"!13.1), and the remote Niger River, N$ which has e (o) of about !10.5 (Goldstein et al., N$ 1984). Fluvial particles could be advected by oceanic bottom currents flowing westward towards the region studied here (Biscaye, 1965) and thereby influence the sediment samples. Finally, in the detrital fraction of the LGM sediments, 87Sr/86Sr ratios (Table 1) range from &0.717 to &0.723 (with an average of 0.72 for 14 values). The e (o) values of the LGM sediments (Fig. 3b) range N$ from !11.4 down to !19.5 (with an average of !15), thereby revealing a significantly less radiogenic Nd than for the surface sediments. 87

Sr/ 86Sr ratios vs Sr concentrations

In considering the Sr isotope data, it is useful to plot 87Sr/86Sr vs 1000/Sr (Fig. 4). Inverting the concentration is avantageous because mixtures between two end-members are constrained to lie upon straight lines

FIG. 4. 87Sr/86Sr ratios plotted against 1000/Sr of surface and lastglacial maximum (LGM) sediments and both literature values and measured values of (30 lm fractions of aerosol, loess and desertic samples from possible source areas. Symbols: possible source areas (solid circles; and open diamond for sample named Tim in Table 1), marine surface sediments (solid squares), marine LGM sediments (open squares), aerosols (crossed squares), Archaean Rocks (data from Potrel et al., 1996; Chardon, 1996) (solid triangles), Precambrian granitoids (data from Alle`gre and Ben Othman, 1980) (reversed solid triangles), sea-water (black cross), Cape Verde basalts (open circle), Senegal river (solid diamond). For end-member ‘A’: see Results section.

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(instead of hyperbolae), making the identification of binary mixing straight forward. On this plot, the data define two distinct fields (called Field I and Field II in Fig. 4). Both fields have a common end-member (low 87Sr/86Sr ratios and high Sr concentration) labelled ‘A’ in Fig. 4. Field I links end-member ‘A’ to the most radiogenic Sr-rich source samples (Morocco, Mauritania and Mali). Field II links ‘A’ to more southerly source samples (Senegal, Guinea). Oceanic surface sediments, as well as most of the aerosols, fall into Field I. LGM sediments located north of the Cape Verde islands also fall into Field I, but a few LGM sediments located between Cape Verde islands and the Equator, together with one aerosol collected off Saint-Louis (Senegal), tend to shift toward Field II. Sr/ 86Sr ratios plotted against eNd (o)

II, along with dust and loess samples from Senegal and from coastal Mauritania (Nouakchott, named ‘Nou’ on Table 1) and one aerosol collected offshore SaintLouis-Senegal (SL). As most of the LGM sediment compositions match the Senegal-and-coastal-Mauritania (Nouakchott) trend (Field II), we could consider that they derived from this southern area. However, they display more radiogenic 87Sr/86Sr ratios and also unradiogenic e (o) values N$ when compared to these sources.

DISCUSSION Are bottom sediments reliable imprints of the eolian input?

87

In this kind of diagram (Fig. 5), mixtures obtained by mixing particles from two end-members, define hyperbolae, the shape of which are determined by the Sr and Nd concentrations and isotopic compositions of the two end-members (Langmuir et al., 1978; Faure, 1986). The sediment samples plot within (calculated) hyperbolic envelopes encompassing the northwestern source-areas (Field I: Morocco, Mali, Mauritania) and the southern source-areas (Field II: Senegal), respectively. These envelopes are hyperbolic mixing curves calculated by mixing a volcanic end member and a desert-derived dust component. Almost all the surface sediment and aerosol samples plot in Field I, clearly reflecting their ‘northwestern’ origin. On the other hand, most of the LGM sediment samples plot in Field

From the data obtained with sediment traps moored at 20°N and 20°W, Wefer and Fischer (1993) concluded that high eolian inputs coincide with enhanced biological productivity and thus may favour high and rapid particle sedimentation in the boreal summer off Cap Blanc. Thus, the particle transfer rate from the atmosphere down to the surface sediments should be short enough to avoid any strong advection by currents or any significant chemical exchange during their short residence time in the water column. Taking into account both the present-day horizontal speed of the watermasses at about 20°N and the residence time of the particles in the water column, a possible advection of these particles could not exceed 3 degrees in latitude (Jeandel, pers. com.; Siegel et al., 1990). This assumption is confirmed by other studies which demonstrated

FIG. 5. 87Sr/86Sr vs e (o) of surface and LGM sediments and both literature values and measured values of (30 lm fractions of aerosol, loess N$ and desertic samples from possible source areas. Symbols as in Fig. 4.

F.E. Grousset et al.: Saharan wind regimes

that the mineralogical composition of surface sediments reflected the aerosol composition (Blank et al., 1985). Two other studies of the vertical evolution of e (o) in the tropical Atlantic water column, also N$ revealed that the surface sediment composition reflected the aerosol composition (Jeandel et al., 1995; Tachikawa et al., 1997; Bory, 1997), as aerosols were transferred mainly as large particles, which sink rapidly through the water column. Thus, we consider that the isotopic composition of the surface sediments closely reflects the isotopic composition of the present-day aerosols. This assumption is verified by the fact that e (o) values of aerosols correspond with the geoN$ graphic distribution of the surface sediment e (o) N$ values (Fig. 3a). As far as the 87Sr/86Sr ratios are concerned, it has also been demonstrated that the aerosol imprint was retained in the underlying sediments (Biscaye et al., 1974). For the LGM samples, it could be argued that their 87Sr/86Sr ratios may be affected by post-depositional exchange with interstitial water. However, the possibility of Sr mobility in the lithic fraction is challenged by the observations of Perry and Turekian (1974), who showed that Sr isotopes do not begin to move and equilibrate between phases in P—¹ conditions corresponding to less than thousands of metres of burial. Present-day dust: origin, transport trajectories, fluxes The average e (o) value in the surface sediment of N$ the North Atlantic Ocean is about &!11 (Goldstein et al., 1984; Grousset et al., 1988). This is typically the range of values that we observe at the boundaries of the studied area, off the Gibraltar Strait and in the Equator region (Fig. 3a). Between these two regions, characterized by the Atlantic background values, e (o) N$ values become less radiogenic, with the highest values ranging between !14 and !18, between the Cape Verde Islands and the African coasts. Such high values require a relatively unradiogenic source on the continent. Such values are found in the region stretching from Nouakchott to the western tip of Algeria, which corresponds to Archaean outcrops of northeastern Mauritania (the Reguibat Rise (Potrel et al., 1996)) and to surrounding Palaeozoic formations. In this province, Archaean rocks can display e (o) as low as N$ !54, with an average of about !35 (Chardon, 1996; Potrel et al., 1996). In the superimposed dune fields accumulated over these formations (samples Nou, Atar, Zouerat on Fig. 1), e (o) values range from N$ &!17 to &!18, which reflect the unradiogenic composition of their source rocks. This region is clearly the source of unradiogenic Nd which controls the NESW 143Nd/144Nd zonation observed in the oceanic surface sediments (Fig. 3). Moreover, present-day aerosols derived from this region are enriched in illitemicas, a clay-mineral which is abundant in these Archaean rocks (Chamley, 1988). In Fig. 4, we identified a second end-member called ‘A’ (low 87Sr/86Sr ratios and high Sr concentration),

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the nature of which has to be discussed. This endmember ‘A’ is very close in isotopic composition to sea-water. Sea-water might be a suitable end-member if we had kept the carbonate fraction, but since we leached our samples specifically to remove the seawater signature in the carbonate fraction, this is not an appealling possibility. Another possibility could be a volcanogenic end-member, but ‘A’ also displays a radiogenic value (&0.709). The only possibility is provided by plotting the field of values obtained for the Archaean rocks (Chardon, 1996; Potrel et al., 1996), which goes exactly across point ‘A’. Now, if we agree with the fact that the main source of dust is located in the northen part of the western Sahara (western Algeria, ex-Spanish Sahara, Southern Morocco), we have to explain why we observe an apparent NE—SW distribution of the dust inherited from these regions. North of &20°N, such a NE—SW oriented gradient could be partly explained by advection in the NE—SW Canaria surface current (Melia, 1984); south of &20°N, it requires, however, a similarly oriented dominant wind regime. Thus, if we look at the present-day airmass trajectories responsible for the transport of present-day dust, we observe that the dominant pathway is a NE—SW direction over both the continent (Chiapello et al., 1997) and the tropical Atlantic (Kalu, 1979; Moulin et al., 1997), matching the feature described above for e (o) (Fig. 3a). Moreover, for many authors, one of the N$ most important sources of dust in northwest Africa is precisely located in the northwestern Archaean province (region ‘dA1’ in d’Almedia, 1986; ‘sector d3’ in Chiapello et al., 1997). Such a northwestern Saharan origin matches precisely the contours of the nothwestern Archaean province described above. According to Chiapello et al. (1997), dust events of northwestern and western Saharan origins (southwestern Algeria, ex-Spanish Sahara, southern Morocco) are the most frequent nowadays. This dominant regime, typically a ‘trade pattern’, would explain why such a NE—SW elongated feature is recorded in the oceanic surface sediments. LGM dust: origin, transport trajectories, fluxes The e (o) of the LGM sediments are also less N$ radiogenic than the so-called ‘Atlantic background (!11)’, ranging from &!13 to &!20. Paradoxically, they display the same geographic pattern as the one observed for the surface sediments, with the same major NE—SW negative gradient, roughly located at the same latitudes. Sarnthein et al. (1981) did not observe any clear northward shift of the patterns between these two periods, but more likely an increase by a factor 2 in the silt particle flux at LGM, the latitudinal location remaining roughly the same. We have already observed a similar increase in core K20 (Table 1; Bertrand et al., 1996) and even in the remote Cape Verde abyssal plain (30°W; depth: 5000 m, CV2 in Fig. 1) (Grousset et al.,

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1989). Kolla et al. (1979) studied the temporal evolution of quartz (%), in a core located &300 km off the Senegal coast (core V22-196, Fig. 1). They observed about 40% of quartz in the two last glacial maxima (stages 2 and 6), compared to 10—20% during the interglacial periods. In terms of the flux of quartz, the LGM fluxes were &3 to &6 times higher than today. In the more southerly cores (as far south as the Equator), mean LGM dust fluxes are &2 to &3 times higher than today (Franiois et al., 1990; Ruddiman, 1997). Such a factor, 2—3, does not characterize solely the LGM, but is characteristic of at least the last 8 climatic cycles in this area (Parkin and Shackleton, 1973). It is interesting to point out that a similar pattern was observed in another core also located in the tropical latitudes, but in the eastern Pacific Ocean (Boyle, 1983). Was it possible that advection processes could have artificially increased apparent fluxes? Although most of the material must have initially been delivered by winds, there was then possibly local-scale redistribution on the sea-floor. Thus, flux increase evaluations should account for sea-floor redistribution (Ruddiman, pers. com.). In terms of Sr isotopic composition, most of the LGM and present-day samples are very similar, both lying within Field I (Fig. 4). Only the three southern samples (south of 10°N) display slightly different values (shifting through Field II): they are thus more influenced by the Senegal—Guinea province. When we compare present-day surface and LGM samples on the e (o) vs 87Sr/86Sr plot (Fig. 5), most of the LGM N$ values tend to shift through Field II, toward the bottom-left part of the plot (Fig. 5), where a possible third end-member might exist. The Senegal—Guinea province thus cannot be the unique source for these deposits. A possible candidate for such an end-member, characterized by both unradiogenic Sr and Nd isopic compositions, could be the old Archaean (Potrel et al., 1996; Chardon, 1996) and Proterozoic (Alle`gre and Ben Othman, 1980; Ben Othman et al., 1984) formations of the Saharan shield. Samples with even lower negative e (o) values are N$ observed along the margin over &200 km from the coasts. For example, along the longitude 18°W, e (o) N$ ranges between !15 and !20 at LGM, and between !14 and !15 at the present day. This less radiogenic LGM pattern could reflect an increased contribution of the Archaean-derived source region at the LGM. Such an increase would require increased weathering/ erosion processes on the continent, but it is difficult to imagine that it would preferentially affect the Archaean outcrops and not the surrounding giant Paleozoic formations. Moreover, it is difficult to admit that it would preferentially affect the most unradiogenic (87Sr/86Sr(0.71) outcrops (end-member ‘A’ suggested on Fig. 4). A more plausible explanation can be proposed. In the LGM sediments, lower (e (o) down to !19.5) N$ could be explained by an increased dust input, which would then prevail upon the Atlantic background

(e (o)&!11), which largely originates from the periN$ Atlantic rivers (Goldstein et al., 1984) and is mostly composed of (10 lm particles (Biscaye, 1965). Off the Mauritanian coasts there are more than 40% of clay minerals in the surface sediments and more than 60% south of 15°N (Johnson, 1979). Indeed, a simple mixing calculation demonstrates that a doubling of the dust flux can induce the observed e (o) shift (from !14 to N$ !18). An argument confirms this explanation: the most negative value (e (o)"!18.6) is observed at N$ about 6°N (core VM22-189; Fig. 1) on the top of a seamount, less affected by possible advections, thus recording a purer eolian signal. It must be pointed out that, when we look at the ratio of the LGM to present interglacial fluxes (Ruddiman, 1997), we observe the same NE—SW elongation axis, roughly linking Mauritania to the northeastern tip of Brazil. Thus, this LGM NE—SW direction is reflected by both qualitative (e (o)) and quantitative (terrigenous fluxes) parameters. N$ Finally, overlapping this NE—SW trade dust signal, a SAL dust signal can be recognized at more than about 250 km offshore, west of +21°W, south of 23°N (Fig. 3b). Elongated along an E—W axis, the e (o) N$ values range from !12 to !15, reflecting a more Sahelian origin (i.e. Field II, in Fig. 5). This SAL signal is deposited precisely where previous studies recognized it in the grain-size (Sarnthein et al., 1981) and clay mineral (Lange, 1982) features. LGM vs today According to Ruddiman (1997), the winter dust plume would have been increased at the LGM compared to nowadays and this change, involving wind changes, could have been induced: 1) by a southward shift; 2) or by an increase in aridity and/or by increasing the deflation efficiency or 3) by increasing their velocity. Are there arguments favoring one of these possible explanations? A southward shift in transport trajectories According to Kolla et al. (1979) who studied the quartz distribution in sediment cores, the main dust transport axis was centered around &8°N at the LGM and shifted northward since that period, as far north as &18°N in the late Holocene. Such a shift, however, is neither reflected by our isotopic data nor by Ruddiman’s accumulation rate maps (1997), nor by the '6 lm coarse particle distribution (Sarnthein et al., 1981). What do we know about wind trajectories? The present-day wind regime has largely been described in previous works (Kalu, 1979; Sarnthein et al., 1981; Chamley, 1988; Coude´-Gaussen, 1989; Rognon et al., 1996). Nowadays, the most intense events come from the more southern Sahelian source and occur only during the coldest period (December and January) (Chiapello et al., 1997; Ratmeyer et al., 1997). They are induced by the so-called continental trade winds and

F.E. Grousset et al.: Saharan wind regimes

are considered to be responsible for most of the dust transport to the ocean (Le´zine et al., 1995). This last hypothesis is in agreement with higher influx of windblown pollens, interpreted as indicating stronger trade winds (Melia, 1984; Hooghiemstra, 1989). On the other hand, during the summer period, the African easterly jets reaching their maximum strength in the summer, would be responsible for the transport of the main pollen release from southern Sahara and Sahel zones (Hooghiemstra, 1989), which is in agreement with the increase in '6 lm coarse particles reported by Sarnthein et al. (1981). At the LGM, due to the extension of the ice-sheets in the Northern Atlantic, the subsequent cooling (Rind, 1987) and the southward shift of the westerlies (Rognon and Coude´-Gaussen, 1996), the wind regimes characterizing northwest Africa could have been intensified significantly (Sarnthein et al., 1981; Ruddiman, 1997). The LGM increase in eolian sedimentation occurred in both winter—spring and summer dust plumes (Balsam et al., 1995); this would explain why we do not identify any clear latitudinal shift. Finally, atmospheric global circulation models (AGCMs) do not simulate any shift between the LGM and today, but more likely a wind speed strengthening (Joussaume, 1993). An increase in source-area aridity? According to Rea (1994), more dust would reflect more aridity, but coarser grains would reflect stronger winds. Thus, at the LGM the dust flux increase could be the sole result of an aridity increase. There are some arguments favoring this hypothesis. At the LGM, dune fields were more extended in latitude over the Saharan—Sahel area (Sarnthein, 1978), which could be the result of increased aridity. The most efficient sources of fine particles are, however, more likely to be located in the semi-desertic regions, surrounding the main deserts (Pye, 1987; Rea, 1994). Over the last 30 years, the increase in dustiness over West Africa was caused by a decrease in rainfall (N’Tchayi et al., 1994). The subsequent decline in vegetation facilitates soil erosion and thus air dustiness. An increase in the deflation efficiency might have contributed too (Marticorena, 1995). This possibility has been suggested by Balsam et al. (1995), who observed a higher iron oxide content in sediments at the LGM, which could reflect an increased deflation of the Sahara and Sahel dust during glacial time. Similarly, it has been proposed that variations in Chinese dust flux to the North Pacific could reflect to some degree variations in the intensity of vertical dust lifting events (Pye and Zhou, 1989). Unfortunately, we do not have relevant data to address this problem seriously. If we follow Rea (1994), more dust would reflect more aridity. Indeed, increased dust fluxes were observed in the LGM marine sediments at different latitudes: a factor 4 for quartz at &25°N (Thiede et al., 1982), a factor 3 for Al at &20°N (Matthewson et al., 1995), thus supporting the aridity enhancement hypothesis.

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On the other hand, a few arguments against aridity have to be considered. If aridity had been the only reason for the dust flux increase, both wind regimes (northeast trades and African easterly winds) would have transported more dust particles and we would observe the imprints of both the northern summer plume and the southern winter plume (as described in Kalu, 1979). Pokras and Mix (1985) observed that LGM sediments from both plumes were highly enriched in the freshwater diatom Melosira, when compared to the lower Holocene (up to a factor 10). Such an enrichment could reflect an enhanced aridity on the continent, as already suggested by Diester-Haas (1976). However, their abundance is very similar nowadays and at the LGM. Finally, if aridity was predominant, time-series might display a precession variability (23 ky) and if northern glaciations were the dominant controlling factor, time-series might be more likely to display an obliquity variability (41 ky). Matthewson et al., (1995) who studied a core from the Cape Verde Terrace (at roughly 20°N—20°W) observed only the precession frequency by carrying out a cross spectral analysis on the Zr/Rb ratio, a grain-size proxy, and concluded that aridity was the governing factor. Thus, considering these drawbacks, we have to envisage other simultaneous forcing parameters. An alternative (but not mutually exclusive) explanation could be an increase in the wind velocity Increased winds should transport coarser particles (Rea, 1994). However, the grain-size distributions of our samples (Fig. 2) do not show any strong grain-size increase at the LGM: most of the present-day and LGM samples display an eolian mode between 10 and 40 lm. This is typically what has been found in a series of sediment traps moored at &500 km off the Mauritanian coasts, in which the present-day lithogenic fraction reaches more than 50% of the total flux and is largely composed of quartz grains of &10—50 lm size (Wefer and Fischer, 1993). At the LGM the coarsest sediments are certainly observed between 10 and 20°N. For example, in core VM22-196 (Fig. 1), the LGM mode ranges from &20 to &60 lm (see Fig. 2b). Although Saharan sands and aerosols from central Sahara would display a uni-modal distribution centered around &40—50 lm (Schu¨tz, 1980), we have observed much coarser grains (around &60—100 lm) in the southern Algeria sand dunes and the Senegal continental shelf (Fig. 2a). Increased winds could have brought these coarse particles to the ocean. We cannot be sure, however, that this coarse mode reflects only an increased wind-speed, as these cores are the closest to the margin and thus could be suspected of contamination by hemipelagic inputs (Fabre, 1995) Increased winds should also transport heavier particles. In cores K02 and CV2 (Fig. 1), we have observed a significant increase in the Ti/Al ratios in the LGM levels (&0.06) compared to the present-day ratios

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(&0.04), this increase being correlated with an increase in the coarse fraction (Bertrand et al., 1996). Volcanic rocks from western Sahara and the lateritic formation of the Sahelian zone are particularly enriched in ilmenite (Dumon, 1977). Titanium is known to be preferentially concentrated in the coarse fraction, as heavy minerals (ilmenite density &4.72). Thus, at the LGM, these coarse particles were much heavier than today. Therefore, a high Ti/Al ratio could reflect intensified wind transport, as previously suggested for dust fluxes (Boyle, 1983). Good correlations between Ti/Al and the aluminosilicate fluxes, and even with the quartz content (Kolla et al., 1979) have already been attributed to both more arid source regions and increased intensity (Boyle, 1983). It is interesting to point out that Boyle (1983) studied a core located in the same latitudinal band, but off the Peru coast: this coherence could therefore reflect synchronous climate variations in different oceans. Another wind-speed proxy can be considered: the Zr/Rb ratio indicates an increase in grain-size (Matthewson et al., 1995) and grain density at the LGM, and thus could be considered as a wind velocity proxy. Thus, the wind speed had to be significantly intensified at LGM. Finally, the southwesterly imprint of the dust fallout in both glacial and interglacial periods identified in this study could, however, be presumably caused by stronger (Ruddiman, 1997) or longer (Boyle, 1983) winter transport. Indeed, geographical patterns displayed on Fig. 1 roughly fit that previously observed for aerosols collected exclusively in March and April (Grousset et al., 1988), and with the winter and spring patterns simulations produced in model (Moulin et al., 1997). Due to the fact that one-centimetre thick sediment core samples integrate a few centuries, it may not be possible, however, to identify any short term variation, either the seasonal waning and waxing of the ITCZ or any decadal trends of dust exportation such as the one driven nowadays by the North Atlantic oscillation (Moulin et al., 1997). This last explanation, strong winter winds, is the hypothesis which best explains most of the data. However, as airborne particle fluxes were also much higher than today, an increase in the aridification of the source area cannot be ruled out and we must conclude that both increases in source-area aridity and in wind velocity occurred together during the LGM. On a global scale, this conclusion is consistent with the fact that LGM dust fluxes are enriched by factors 10-to-20 in Greenland and Antarctic ice-cores (Thompson and Mosley-Thompson, 1981; Petit et al., 1981; Biscaye et al., 1997), which implies not only a general aridification of the desert areas, but also a more intense atmospheric circulation (Joussaume, 1993). Our data allow us to propose that both processes were enhanced simultaneously at the LGM, which is an adequate way of reconciling the increased aridity advocates (Kolla et al., 1979; Rea, 1994; Matthewson et al., 1996) with the increased wind strength advocates (Sarnthein et al., 1981; Boyle, 1983; Ruddiman, 1997).

Finally, arguments advanced for increased glacial aridity are not very strong, but the lack of much change in grain-sizes is an argument against increased wind strength. Thus, a last possible explanation could be that the season of generally strong winds lengthened, carrying more dust of about the same size. This would mean that there was an increase, not only in wind strength, but also in glacial winter regime duration (Ruddiman, pers. com.). Implications for upwellings It has been suggested that in the southern Ocean, aerosols could have a fertilizing influence on the surface waters (Martin, 1990) and could have enhanced the biological productivity by a factor +5 at the LGM in the Southern Atlantic (Kumar et al., 1995). The possible influence of the dust input has not been investigated so far in this area. From the data they obtained with sediment traps moored at 20°N and 20°W, Wefer and Fischer (1993) concluded that in the boreal summer, high eolian inputs coincided with enhanced biological productivity. The idea of increased fertilization by windblown nutrients has also been envisaged by Ruddiman (1997) in the tropical eastern Atlantic. On the other hand, the high productivity characterizing this region can be also due to the strong upwelling activity. In the hypothesis of Le´zine et al. (1995), the NE—SW marine trade winds would predominantly generate coastal upwellings. At the LGM, African aerosols transported westward (mostly by the SAL winds) across the Atlantic ocean, could have strongly influenced the intensity of biological production in the upwelling zone (Sarnthein et al., 1981), triggering a strong enhancement of the carbon fluxes (Muller and Suess, 1979). However, such a systematic enhancement of carbon fluxes at the LGM has to be reconsidered, as in some cores from the Mauritanian margin (e.g. K11), we observed a decrease in carbon fluxes at the LGM (Bertrand et al., 1996; Martinez et al., 1996). Thus, we cannot easily link wind and upwelling patterns. The dust distribution that we observed (Fig. 3) could, however, be explained by both processes. Further work needs to be done if we are to address fully the short scale variability of this upwelling system.

CONCLUSIONS Along the NW African margin, the eolian input of the oceanic sediments is mostly concentrated in the 10—40 lm grain-size range. In the eolian fraction of the sediments deposited between the Canary Islands and the equator, both Sr and Nd isotopic tracers reveal the same major northwestern origin (Mauritania, Mali, southern Algeria and Morocco). The Archaean formations of the western Saharan shield could supply the very unradiogenic Nd ratios characterizing this fraction off Africa. The more southern regions (Senegal,

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Guinea) act only as secondary sources. A similar pattern is observed for the Last Glacial Maximum. Nowadays, the isotopic composition of oceanic sediments reveals that lithic particles are transported by both Trade and SAL winds, along an average NE—SW axis, linking roughly Dakar to the northeastern tip of Brazil. The same main transport direction has also been identified by looking at airmass trajectories (Chiapello, 1996). This main feature matches the southern plume, characterizing the dust transport observed during winter. No significant latitudinal shift of the wind belts is observed between the LGM and today; the sole major difference between these two periods is that, at the LGM, the dust fluxes were 2—4 times higher than today, leading to a more Archaean-type imprint in the deposits. Thus, if the winds are the major upwelling driving force, the latitudinal variability of the upwelling systems identified in this region at the LGM (Bertrand et al., 1996), cannot be explained by simple latitudinal shifts of the major wind systems. Many arguments (isotopic tracers, geographic patterns, dust fluxes, grain-size distribution, particle density proxies) tend to demonstrate that, at the LGM, both enhanced aridity on the continent and increased wind speed occurred together over the western tropical Africa and confirm that transports were enhanced mostly during winter, as recently proposed by Ruddiman (1997).

ACKNOWLEDGEMENTS We thank the officers and crew of the the R/V Le Suroit who helped us to recover sediment cores during the Sedorqua cruise (March 1994). We thank all the colleagues who provided us with aerosols (R. Chester, J. Prospero), loess (G. Coude´-Gaussen, P. Rognon), powdered soils (G. Bergametti, G. Berliet, T. Biscaye, G. Coude´-Gaussen, P. Rognon) or marine sediment samples (G. Shimmield, I.N. McCave) from various potential source areas of northwestern Africa and the L-DEO Deep-Sea Sample Repository, and its curator, Rusty Lotti, who kindly provided us with sediment samples from the 0°-to-15°N latitudinal band (support for the curating facilities of the L-DEO Deep-Sea Sample Repository is provided by the NSF through grant OCE94-02150 and the Office of Naval Research through grant N00014-96-I-0186). Special thanks to G. Berliet, who collected soil samples at each stage bivouac, during the 1996 Paris-Dakar car rally! Thanks are due to A. Matthewson (Univ. of Edinburgh, U.K.) who provided the d18O stratigraphy on BOFS cores. We are grateful to P. Pe´demay and to A. Kelly and V. Gallagher, for their help in the Sr-Nd chemistry carried out at University Bordeaux I (France) and at the SURRC, respectively. We are also grateful to M. Roy-Barman for access to the mass spectrometer at the University Paul Sabatier (Toulouse, France). We thank P. Buat-Me´nard, W.

Ruddiman and M. Sarnthein for their very helpful and pertinent reviews, C. Jeandel for fruitful discussions and A. Gonialves who polished up the English language. Aloys Bory’s research was funded by the European Commission as part of a Community Training Project. This work was mostly funded by the French Centre National de la Recherche Scientifique.

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