n-Alkane and pollen reconstruction of terrestrial climate and vegetation for N.W. Africa over the last 160 kyr

Organic Geochemistry 34 (2003) 131–143 www.elsevier.com/locate/orggeochem

n-Alkane and pollen reconstruction of terrestrial climate and vegetation for N.W. Africa over the last 160 kyr Meixun Zhaoa,*, Lydie Dupontb, Geoffrey Eglintonc,d, Mark Teecee a

Department of Earth Sciences, Dartmouth College, Hanover, NH 03755-3571, USA b FB Geowissenschaften, Univ. Bremen, Postfach 330440, 28334 Bremen, Germany c Biogeochemistry Centre, Department of Earth Sciences, University of Bristol, Bristol, BS8 1RJ, UK d Hanse-Wissenschaftskolleg, Delmenhorst, 27753, Germany e Department of Chemistry, SUNY- College of Environmental Science and Forestry, Syracuse, NY 13210, USA Received 13 November 2000; accepted 26 July 2002 (returned to author for revision 2 April 2001)

Abstract Proxy environmental records have been obtained for the last 160 kyr from the well-studied ODP Site 658, ca. 200 km West of Cap Blanc, N.W. Africa (20
50 N, 18
350 W; 2263 m water depth). This collective assessment of several terrigenous proxies (lithogenic fraction, n-alkane content and d13C values, and pollen counts) provides a better understanding of the climate and vegetation history of the N.W. African hinterland. The results indicate that, for this site beneath the main dust pathway of the Saharan Air Layer (SAL), the dust flux increased during dry glacial periods but the amounts of pollen and n-alkanes delivered were lower, in accord with the decreased density of vegetation cover of their source areas. The % ‘C4 pollen’ was also very low, but the heavy d13C values of the n-alkanes accorded with a mainly C4 plant origin. Evidently, the load of SAL-pollen was much reduced, but not the SAL wind strength. # 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction Many climate records have shown that the climate and the vegetation cover of tropical and subtropical Africa (Fig. 1A) have experienced both long-term and short-term changes (Gasse, 2000; deMenocal et al., 2000; Pokras and Mix, 1985). Although most of these records have been derived using biological and other materials preserved in lake sediments, the lake records are not continuous beyond the last glacial/interglacial period, due to intermittent sedimentation (Gasse, 2000; Hooghiemstra, 1988). On the other hand, marine sediments contain wind-transported terrigenous materials

* Corresponding author. Tel.: +1-603-646-2150; fax: +1603-646-3922. E-mail address: [email protected] (M. Zhao).

that provide us with more complete terrestrial climate records. The aridity history of the NW African hinterland has been gained from the record of the freshwater diatoms of the genus Melosira, and often through terrigenous sediment components preserved in marine sediments off NW Africa (Pokras and Mix, 1985). These records generally indicate that the glacial period was much drier, but became much wetter around 15 ka with the onset of the African Humid Period. As far as the winds are concerned, early studies by Sarnthein et al. (1981) indicated that the direction of the SAL remained constant throughout the last 18 kyr, though the speed of this wind system was believed to be lower at the last glacial maximum (LGM). In contrast, most studies have inferred that NETW speed was higher at 18 ka, although this inference was not supported by the results of higher SST and lower productivity during the LGM period (23–18 ka)

0146-6380/03/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0146-6380(02)00142-0

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Fig. 1. Maps of Phytogeography and Wind systems, and % C4 plant contribution by pollen counts and by C29 n-alkane d13C (%) in surface sediments off N.W. Africa. (A). A Map of phytogeographical regions (White, 1983) and wind systems. From North to South: Med, Mediterranean vegetation zone; MST, Mediterranean-Saharan transitional steppes; Sahara, absolute desert, desert and semidesert; Sahel, semi-desert grassland to Acacia wooded grassland; Savanna, dry savannas and woodland; SDF, semi-deciduous forest; RF, tropical rain forest. The major wind systems are: the North East Trades (NETW), low altitude winds stronger in winter; the Saharan Air Layer (SAL), medium altitude, mid-tropospheric winds, strongest in summer; January Trades, low altitude winds carrying dust in winter from Northern Nigeria and Lake Chad areas, when the Intertropical Convergence Zone (ITCZ) is in the southern most position. The shaded areas over the North East Atlantic ocean are those for which high occurrences of atmospheric haze are recorded; northern zone during summer and southern zone during winter. Bathymetry: 200 and 2000 m, shelf edge and break, respectively. The plus symbol denotes the location of ODP Site 658 and snowflake symbols denote the locations of the other cores mentioned in the text. (B) A map of % C4 plant contribution by pollen counts in surface sediments off N.W. Africa. The % C4 pollen estimates are given as Isopol contours. The % C4 is calculated using the main C4 components (Poaceae+0.5 * Cheno-Am)/total pollen. On the continent, the C4 biomass, mostly made up by Cheno-Am and grasses, is approximately indicated by the intensity of shading. (C) A map of % C4 plant contribution by C29 n-alkane d13C in surface sediments off Northwest Africa. % C4 distribution of n-alkanes is calculated from d13C values of C29 n-alkane using a two-component mixing equation with end member values of 34 and 19%, respectively. Sampling sites are indicated by+symbol. On the continent, the Circles indicate major source areas for dust deflation, such as Holocene lake deposits (N.B. not shown but to east of map is another major source area around Lake Chad, centred on ca. l4
N, 11
E). Arrows mark major river inputs of particulate terrigenous organic carbon (104 tonnes y1) (reproduced with permission from Huang et al., 2000).

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from ODP site 658C (Zhao et al., 1995, 2000). Also, results from a nearby site, SU94–11K, have revealed a very similar SST profile over the last 70 kyr (Ternois et al., 2000; Sicre et al., 2001). This unexpected rise in SST over the LGM was accompanied at both sites by lower total organic carbon and alkenone fluxes (Jordan et al., 1996; Martinez et al., 1999; Zhao et al., 2000; Sicre et al., 2001). In addition, terrigenous lipid biomarker inputs of C25–C33 n-alkanes into ODP site 658C (Zhao et al., 2000) and of C20–C32 n-alkanols into SU94-11K (Ternois et al., 2000; Sicre et al., 2001) were also lower during the LGM. At both sites, these minima were followed by a rapid rise to maxima at ca. 13 ka and subsequent fall after 10 ka to intermediate levels. The lower terrigenous flux at the LGM has been ascribed to migration of the north African sub-Saharan vegetation belt during the maximum of the African Aridity (Hooghiemstra, 1989; Ternois et al., 2000; Zhao et al., 2000) Pollen studies using marine sediments (Hooghiemstra, 1988, 1989; Hooghiemstra et al., 1992) also indicate that the Saharan desert expanded in both north and south directions during the LGM. The NETW intensified, especially between 36 and 24
N, and took western Mediterranean trade wind indicator pollen as far south as 10
N, while the SAL had a stationary position over site 658 between 17 and 21
N. Between 22 and 16
N the marine pollen record registered changes in both the vegetation of the western Mediterranean area and of the graminae vegetation of the savanna region in the south. In contrast, in the middle Holocene, following the African Humid Period and prior to about 4 ka BP, geomorphic and biostratigraphic evidence has revealed that NW Africa was extensively vegetated. Lakes were present as far north as 27
N and grasslands to 23
N, while no desert plants reached further south than 20
N (Joussaume et al., 1999). In summary, there is reasonable agreement on the interpretation of the chronostratigraphic records for the various proxies in terms of the succession and timing of the main wet (W) and dry (D) periods on the N.W. African continent over the last 160 kyr. They are given at the top of Fig. 2. Recently, Huang et al. (2000) have shown that the d13C values of leaf-wax n-alkanes and the % C4 pollen in surface sediments recovered from a wide area of the Northeastern Atlantic seafloor off northwestern Africa reveal clear patterns (Fig. 1B and C) that reflect the consistent and systematic differences in the proportions of terrestrial C3 and C4 plant input. This mapping procedure, which utilizes both pollen distributions and biomarker d13C data, promises to be a robust tool for assessing late Quaternary N.W. African phytogeographic changes. The specific goal of the present paper is to use these dual analytical approaches to attempt to reconstruct elements of paleoclimate and paleo-phytogeography of a continental landmass by chronostratigraphic

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analysis of a deep sea sediment core at a site that has been receiving persistent offshore aeolian dust transport. The specific site chosen is ODP Site 658, for which several papers have been published using different terrestrial and marine proxies. ODP Site 658 (Fig. 1A) was cored below the main upwelling cell on the continental slope west of Cap Blanc (20
450 N, 18
350 W; 2263 m water depth). High biological productivity coupled with relatively high terrigenous input has produced high sedimentation rate records containing important paleoproxy components (Zhao et al., 2000, and references therein). In this paper we report data for several terrigenous proxies (lithogenic, pollen and n-alkane contents; d13C of n-C29 and C31 alkanes) over the last 160 kyr, complementing and extending the results reported previously for the marine and terrigenous proxies at Site 658 over the last 35 kyr (Zhao et al., 2000). The principal new information comes from the d13C record for C29 and C31 n-alkanes for the last 160 kyr, and the direct comparison of the biomarker proxy and the pollen proxy from the same site. This reveals that there are major changes in the balance of C3 versus C4 vegetation on the N.W. African continent during this time, reflecting significant changes of climate. This approach has potential applications globally. Thus, very large amounts of dust are transported into the deep sea from several major continental areas in addition to NW Africa. For example, west and northwestward into the S. Atlantic from S. Africa, southeastward into the Arabian Sea from Sudan and Saudi Arabia, eastward from the Gobi desert in China into the East China Sea and the NW Pacific Ocean (Prospero et al., 2002). Furthermore, it has been demonstrated that dust transport was enhanced during the glacial periods, due to stronger winds and increased continental aridity (Kohfeld and Harrison, 2001). Thus, terrestrial signals preserved in marine sediments reflected phytogeographic changes over large areas on the continents which were dust source regions. The deep-sea dust records can therefore give broad assessments of the changing climate and vegetation conditions on continents (e.g., C3 vs C4 vegetation) in the past (Poynter et al., 1989b; Bird et al., 1995).

2. Methods 2.1. Age model The 658A/B age model was established by correlating a high resolution d18O stratigraphy of Cibicidoides wuellerstorfi with those of several radiocarbon dated cores from the N. Atlantic (Winn et al., 1991; Tiedemann, 1991). This correlation revealed a large stratigraphic gap spanning from ca. 50 to 74.5 ka. The Hole 658C age model was initially established by cross-correlation of

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Fig. 2. Proxy records for the last 160 kyr for Site ODP 658. Marine isotope stage boundaries are labeled on the top. ‘‘W’’ is for wet period and ‘‘D’’ is for dry period. LGM is the last glacial maximum and PGM is the penultimate glacial maximum. From bottom to top: (A) d18O (%) stratigraphy of Cibicidoides wuellerstorfi for Hole 658A/B (Winn et al., 1991; Tiedemann, 1991). (B) Content of the lithogenic fraction for Hole 658C calculated by subtracting carbonate from the total (dashed line, % dry weight), and record of fresh water diatom Melosira from core V-30–49 (solid line, Melosira/g 105; Pokras and Mix, 1985). (C) The content of total odd numbered n-alkanes (C2733) for Hole 658C (dashed line, mg g1 dry weight); and the content of pollen for Holes 658A/B (solid line, # of grains cm3). (D) n-Alkane flux for Hole 658C (dashed line, mg cm2 kyr1), and pollen flux for Holes 658A/B (solid line, # of grains cm2 yr1). (E) The content of NETW carried pollen (dashed line, # of grains cm3) and SAL carried pollen for Site 658A/B (solid line, # of grains cm3). (F) % C4 plant pollen for Holes 658A/B calculated using % C4=(0.5*Chen-Am+Poaceae)/total. (G) d13C of the average values of C29 and C31 n-alkane (%) for Site 658C, measured by gas chromatography-isotope ratio mass spectrometry. The average values of these two n-alkanes have been assigned as 34% for pure C3 and 19% for pure C4 plant materials.

M. Zhao et al. / Organic Geochemistry 34 (2003) 131–143

the Hole 658C magnetic susceptibility record with those of Hole 658A/B (Zhao et al., 1993), and by correlating 0 the high resolution UK 37 SST record of Hole 658C with other N. Atlantic high resolution climate records (Zhao ’ et al., 1995). However, the Hole 658C UK 37 record did not reveal the large gap that was in Hole 658A/B. For direct comparison of the Site 658 records from the three holes, we have decided to modify the previous age scale to establish a new ‘‘common’’ age scale for Site 658. For the first 4.1 m (composite depth) of the core (0–23 ka calendar age scale), we have used the age model of deMenocal et al. (2000), which is derived from accelerator mass spectrometry (AMS) radiocarbon dating of Globigerinoides bulloides over 18 levels. It gives an average sedimentation rate of approximately 18 cm/kyr. The effect of the deMenocal et al. (2000) age model has been to compress part of the sediment record, previously 18– 7 ka, into 15–7 ka, and to introduce a non-depositional or erosional hiatus from 17.20 to 14.80 cal. ka B.P., which is believed to span the first phase of the deglaciation. The 4-cm interval of sediment comprising the hiatus was sampled for some of the proxies but these points have not been included in the plots, since they cannot be assigned ages pending better AMS constraint of the hiatus. The last glacial maximum (LGM) and the Heinrich 2 event remain essentially unchanged in their position on the age plot (Zhao et al., 2000). The Hole 658AB age model was modified using the Hole 658C age model so that data sets from all three cores can be compared directly. For Hole 658A/B, from the depth of 4.1 (23 ka) to 8.8 mbsf (50 ka), sample age was calculated by linear interpolation. The gap was kept from 8.8 to 9.1 mbsf (74.5 ka). For Hole 658C, sample age was linearly calculated from 4.1 to 9.1 mbsf without a gap. Below the depth of 9.1 mbsf, the Hole 658A/B age-scale was used. Hole 658A/B d18O record (Tiedemann, 1991) is shown in Fig. 2A to help identify the marine isotope boundaries. 2.2. Analytical methods 2.2.1. Lithogenic component proxy The lithogenic fraction was estimated by subtraction of carbonate from the dry sediment weight. This was done because the lithogenic components in Hole 658C were not measured directly. However, the calcium carbonate content was measured at 10 cm resolution by coulometry. Since the opal content was measured only at certain intervals, and averages only about 5% (Tiedemann, 1991; deMenocal et al., 2000), we decided to use the residual sediment percentage obtained after subtraction of carbonate as the lithogenic fraction. Thus, this proxy is used with caution since it is only a rough approximation. 2.2.2. Pollen analysis This was carried out at ca. 4 kyr resolution on samples of about 15 cm3 from Hole 658A/B. The volume

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was determined by water displacement. Wet samples were split over a 250-mm sieve. The smaller fraction was decalcified with HCl and treated with KOH. Lycopodium marker tablets containing a known number of spores were added together with the HCl. Silt and clay were removed with HF, the sample was acetolysed (heated to 90
C for 5 min in a mixture of 9 parts acetic anhydride and 1 part concentrated H2SO4) and finally particles smaller than 10 mm were removed by ultrasonic sieving. Pollen grains were mounted in glycerine jelly. The slides were analysed by L. Dupont and H. Stalling (Dupont, 1989). Pollen identification was regularly carried out using a magnification of 400–1000 times. In this paper, pollen are divided in NETW- and SAL-pollen based on the following assumptions: 1) Pollen are blown onto the ODP Site 658 by either the NETW or the SAL; 2) We have a reasonable estimate of the pollen brought by the NETW, in the form of the relative abundances of the European and Mediterreanean elements (Pinus, Asteraceae, Artemisia and Ephedra); 3) The remaining part of the pollen has to be carried by the SAL. The % C4 is calculated using the main C4 components (Poaceae+0.5 * Cheno-Am)/total pollen. 2.2.3. n-Akane analyses For n-alkane analyses of Hole 658C, a complete suite of U-channel minicores was sampled at every 2–4 cm. About 0.5 g of freeze-dried sediment was extracted with solvents and analyzed by Gas Chromatography (GC) (Zhao et al., 1995). For the determination of d13C of C29 and C31 n-alkanes, about 5 g of dry sediment was extracted in a sonication bath with 6 ml of DCM/ MeOH (3:1 by volume) for 5 times. The supernatants were combined and dried under a stream of nitrogen gas. The residue was fractionated using silica gel column chromatography and the hydrocarbon fraction was measured by GC and GC–MS (Gas Chromatography– Mass Spectrometry) for quantification of concentration and by GC–IRMS (Gas Chromatography–Isotope Ratio-Mass Spectrometry) for the estimation of d13C values of the C29 and C31 n-alkanes (Collister et al., 1994). Calibration was made with a CO2 standard introduced at the beginning and end of each GC–IRMS run. Errors for d13C were estimated at  0.5%.

3. Results and discussion 3.1. Climate and vegetation zones of N.W. Africa The atmospheric circulation patterns over the coast off NW Africa are dominated by the easterly, middle-altitude Saharan Air Layer (SAL) and the northeasterly, lowaltitude northeast trade winds (NETW), which can reach to ca. 23
N in the summer and to ca. 5
N in January, depending on the main position of the the Intertropical

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Convergence Zone (ITCZ) (Tetzlaff and Wolter, 1980; and Fig. 1A). This circulation pattern also means that precipitation is highly seasonal over northwest Africa. The summer monsoon brings rain to the southern Sahara and Sahel during the boreal summer, while the NETW brings rain to the northern Sahara during the winter (Thompson, 1965). Both the low-level entraining airflows (NETW) and the upper tropospheric transporting flows (SAL) contribute terrigenous material to the marine environment that can be used to study the continental climate and vegetation (Tetzlaff and Wolter, 1980; Sarnthein et al., 1981, 1982; Ruddiman et al., 1989). Phytogeographical zones in northwestern Africa range from the Mediterranean forest (Med in Fig. 1A) in the north to the tropical rain forest (RF) in the south (White, 1983). North of the Saharan desert, the vegetation consists predominantly of C3 plants, with the exception of some halophytic species of the Chenopodiaceae. Grasses of the Mediterranean-Saharan transitional steppes (MST), just south of the Atlas Mountains, are mostly C3 plants. In the sparse vegetation of the Sahara, halophytes and other herbs count among the C4 plants (Dowton, 1975; Winter et al., 1976; Raghavendra and Das, 1978; Ehleringer et al., 1977). The grasses of the Sahel, savanna including woodland, and semi-deciduous forest (SDF) are C4 plants, while the woody species are C3. In the tropical rain forest (RF), woody C3 species vastly dominate over C4 grasses (White, 1983). Thus, C4 plants are mainly found in the Sahel and the savanna, but also in the Sahara, a distribution that has been attributed to high temperatures during the growth season in conjunction with moisture stress (Vogel et al., 1978; Teeri and Stowe, 1976). CAM (crassulacean acid metabolism)-plants do not form a significant constituent of the vegetation of northwestern Africa (Winter and Smith, 1996). 3.2. Dust, aridity and the lithogenic record Specific sources of the wind-blown dust reaching the N.E. Atlantic coast have been inferred from various proxies including lithology, the major- and trace-element contents of the silt and clay components, fresh-water diatom distributions and pollen counts. The origin of the dust load has been commonly assigned to three main deflation regions (Pye, 1987; see Fig. 1). 1). The Atlas Mountains and coastal plain, from which dust is carried southwestwards almost parallel to the coast by the low level North East Trade winds. 2). The southern Sahara and Sahel, from which dust is raised in the northern hemisphere summer by easterly winds into the higherlevel flow of the SAL and is carried westwards into the Atlantic beyond the continental margin between 10 and 25
N. 3). The southeastern edge of the Sahara and the Sahel (alluvial plains of Niger, Faya Largeau and Chad;

the Bode´le´ depression), from which dust is uplifted by the NETW in the northern hemisphere winter and carried southwestwards to a wide area of the Atlantic and the Gulf of Guinea between 2 and 15
N. However, efforts to identify dust sources using field and meteorological observations have resulted in much of N. Africa being considered as a source. More specific findings come from satellite data (reviewed by Prospero et al., 2002). In particular, the Total Ozone Mapping Spectrometer (TOMS) sensor on the NIMBUS-7 satellite has been used to map the global distribution of major atmospheric dust sources. Goudie and Middleton (2001) suggested two major source areas for the Saharan dust: the Bode´le´ depression and Western Saharan source, which is a large swathe of country covering portions of Mauritania, Mali and southern Algeria. Prospero et al. (2002) point out ‘‘TOMS shows that there are dominant sources and that these present a remarkably consistent pattern both from the standpoint of the geometry of the individual sources and the seasonal changes in their shape and distribution’’. Many of these intense sources are located north of about 15
N and are associated with regions of extensive alluvial deposits (Prospero et al., 2002). Their study shows that there are many factors which affect the production and transport of dust. They conclude that the flux of dust ‘‘is not so much an indicator of aridity as it is of recent or present transition to aridity’’. Nevertheless, to a first approximation, the supply of terrigenous material as aeolian dusts is highly dependent on the aridity of the source regions and on the winds’ strength. Much of the sedimentary areas of aeolian input into the NE Atlantic off NW Africa has been documented in the DIRTMAP database (Kohfeld and Harrison, 2001). As indicated in Section 1.2, the aridity history of the NW African hinterland has been estimated using the marine record of the freshwater diatoms of the genus Melosira and the flux of terrigenous materials (Pokras and Mix, 1985). The lithogenic record for 658C shown in Fig. 2B represents the residual sediment percentage obtained after subtraction of carbonate. It has been shown that this fraction is largely eolian in origin (deMenocal et al., 2000). Thus, to a first approximation, higher values in the last glacial period (e.g., 20–30 ka, Fig. 2B) reflect higher dust input and a drier N.W. African continent. Lower values during the last deglaciation and the early Holocene indicate decreasing dust input and more humid conditions on the continent. The lithogenic record agrees fairly well with another aridity record derived from counts of the fresh water diatom Melosira in core V30-49 (Figs. 1A, 2B, Pokras and Mix, 1985), at least for the last 100 kyr. Thus, these two records and the pollen (Section 3.2 below) are used to identify the main dry and wet periods for the N.W. African continent, as labeled in Fig. 2.

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3.3. Pollen record The distribution of pollen in seafloor sediments provides independent information about inputs from contemporary and recent vegetation (Hooghiemstra et al., 1986, Hooghiemstra, 1988; Calleja et al., 1993). For example, the C4 plant distribution north of the Sahel is reflected in the percentage distribution of pollen from the Chenopodiaceae and Amaranthaceae (Cheno-Am, C3 and C4) in the marine surface sediments of the eastern Atlantic (Huang et al., 2000). The distribution of the C4 plants south of the Sahara is seen in that of the Poaceae (mainly C4 grasses). Thus, the % C4 pollen has been defined as the summed percentages of Poaceae and half of the Cheno-Am (Fig. 1B). This loading allows approximately for the distribution of C3 species among the Cheno-Am, specifically among the non-halophytic species of these families. Within the Poaceae, the amount of C3 species among the tropical savanna grasses is negligible. The pollen record of ODP Site 658 has been reported previously (Dupont, 1989). Over the last 160 kyr, the pollen content (Fig. 2C) and flux (Fig. 2D) were lower during the dry glacial period and higher during the wet interglacial periods. Thus, the contents of total pollen and the SAL and the NETW pollen (Fig. 2E) all show lower values during marine isotope stage 6 (MIS6) and MIS2. The highest values are in MIS 5e and 3. Pollen delivery to any oceanic site is mainly determined by the vegetation cover of the source region which is in turn proportional to the humidity, and by the dust flux which is related to aridity and wind strength. The lithogenic and freshwater diatom contents (Fig. 2B) show that during the glacial (MIS 2 and 6) the continent was drier and dust flux was higher as a result of continental aridity and stronger winds. Thus, the lower pollen flux was mainly caused by the concomitant sparse vegetation cover on the N.W. African continent. With regard to the C3 vs C4 plant cover, the estimate of % ‘C4-pollen’ (Fig. 2F) was the lowest during the last two peak glacial times (the penultimate glacial maximum, or PGM, ca. 135–145 kyr. B.P. and the LGM, ca. 20 kyr. B.P.). The low ‘C4-pollen’ percentages in the LGM and PGM could result from one or more of the following situations: a) Production of less total pollen in the SAL source region, since the region was drier, as indicated by other proxy data (lithogenic components and freshwater diatoms); b) a slightly wetter NETW source region, which would produce more pollen (mainly C3) for delivery. c) a smaller proportion of C4 plants in the SAL source region, but this is not consistent with other climate proxies (drier, lower pCO2); d) a stronger NETW (carrying mainly C3 pollen), and/or weaker SAL (carrying mainly C4 pollen). A stronger NETW during the LGM has been inferred from studies of pollen and lithogenic minerals in marine

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sediments (Sarnthein and Koopmann, 1980; Sarnthein et al., 1981; Hooghiemstra, 1988, 1989). However, the estimate for Site 658 NETW pollen input (Fig. 2E) is itself low. On the other hand, it has been suggested that the NETW direction also changed during the glacial time, swinging away from the continent (Ternois et al., 2000). In summary, although our data are of low resolution with no very clear glacial/interglacial trends, they would accord with a combination of situations a and b. 3.4. The biomarker compounds The contents of long-chain n-alkanes (C27, C29, C31 and C33) also represent the terrigenous input into marine sediments. These biomarkers are specific to higher land plant leaf waxes (Eglinton and Hamilton, 1967) and are in eolian dust (e.g. Simoneit et al., 1977; Gagosian et al., 1981; Huang et al., 1993, 2000) and fluvial particulates (Bird et al., 1995; Pelejero et al., 1999). The content of the n-C29 homologue, nonacosane, has been used in several marine stratigraphic studies as a proxy for leaf wax input to infer the variations in terms of changing terrigenous input related to wind strength (Poynter et al., 1989a,b; Ishiwatari et al., 1994; Madureira et al., 1997). In the cores studied, the content of n-C29 was high in glacial times, in parallel with higher percentages of terrigenous sediment and consistent with a higher input of dust. Taking into consideration the wind systems and climate of the N.W. African continent, the amount of n-alkanes delivered to a site off N.W. Africa is determined by: 1). The total amount of dust input, which is generally controlled by wind strength and the size and long-term persistence of the areas serving as dust sources, which are in turn dependent on both the aridity of these regions and the availability of suitable sediments for deflation. During dry periods dust input is generally higher. 2). The organic contents of the dusts, which are determined by the density and type of contemporary vegetation and organic matter (OM) in soils and in dried up lake beds. Since vegetation is generally controlled by precipitation and to a lesser degree by temperature, then contemporary vegetation cover and soil OM content will be high during warm and humid periods, but lower during dry and cold periods. In addition, the carbon isotope values of the biomarkers can provide further information regarding continental climate and vegetation. Plants use two main carbon fixation pathways during photosynthesis: Calvin-Benson (C3) and Hatch-Slack (C4) cycles (e.g., O’Leary, 1981). C3 plants (trees, shrubs and cool-climate grasses) generally have lighter d13C values (22 to 33%) for bulk organic matter than those (9 to 16%) of C4 plants (many tropical grasses and sedges). Thus, the d13C values of TOC have been used to reconstruct the changes in C3 and C4 plant abundance from

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paleosols (Krishnamurthy and DeNiro, 1982; Guillet et al., 1988), and from marine sediments (France-Lanord and Derry, 1994). However, organic matter in lake and marine sediments is a mixture of both terrestrial and aquatic sources which often have different isotopic values (Westerhausen et al., 1992; Meyers and Ishiwatari, 1993; Huang et al., 1999b). Compound specific isotopic analyses of the land plant n-alkanes preserved in marine and lake sediments have shown promise for identifying their origins and for the study of changes in terrestrial C3 and C4 plant cover in the late Quaternary (Huang et al., 1993, 1999a,b; Collister et al., 1994; Bird et al., 1995; Kuypers et al., 1999; Yamada and Ishiwatari, 1999; Brincat et al., 2000; Pearson and Eglinton, 2000; Zhao et al., 2000). However, future, more extensive use of d13C values will undoubtedly benefit from improved understanding of the climatic and environmental processes that govern the production, isotope values and release of n-alkanes in the source regions, and their delivery processes to, and their preservation in, the sediments. 3.4.1. The n-alkane content record The record of n-alkane content of 658C for the last 35 kyr has been previously reported (Zhao et al., 2000). The extended, high-resolution record over the last 160 kyr now given in Fig. 2C shows some large and abrupt changes, e.g. at 9, 14, 40, 60–70 and 130–134 ka. These must have been caused mainly by sudden changes in the vegetation cover of the source regions and/or the wind strength. In view of the relatively low resolution of the pollen and n-alkane d13C records, only the general patterns of the n-alkane data are discussed here. For the n-alkane content of 658C (Fig. 2C), there is an overall negative correlation with the lithogenic content (Fig. 2B), except during the late Holocene (5–0 ka) and part of MIS 5 (130–122 ka). Thus, the alkane contents are higher during the warm and wetter interglacials and lower during the cold and dry glacials. The n-alkane content for 658C and the total pollen content for 658A/ B show similar trends (Fig. 2C), which suggests that both the n-alkane and pollen fluxes to Site 658 are mainly controlled by vegetation cover on the continent. Thus, during the glacial periods, climatic aridity results in sparse vegetation cover and consequently lower plant biomass and a reduced amount of contemporary, freshly-biosynthesized leaf wax available for wind transportation. Pokras and Mix (1985) interpreted the effect of the African Arid Period as bringing about the southward movement of the northern extent of the tall grassland vegetation belt of the savanna. Ternois et al. (2000) reached similar conclusions from their study of two cores off N.W. Africa (SU94-11K and SU94-20bK). The leaf wax load carried by the wind systems out to the ocean will, of course, also contain older, sedimentary leaf waxes which have been lifted off in the dust from

dried-up lake beds, areas of desiccated soils, and from coastal sediments exposed at times of low sea level. The large depressions in both the Bode´le´ and Western Saharan source areas contain ancient lake beds that exhibit signs of intense deflation in the Holocene (PetitMaire, 1991). This load will respond rather crucially to the aridity, since deflation of dust is highly dependant on the moisture content of the soil being eroded (Pye, 1987). Hence, the relationship between aridity in the Sahel/Sahara and the aeolian supply of leaf waxes (alkane contents) to marine sediments is likely to depend non-linearly on these two inputs—the supply of ‘‘young’’ alkanes from contemporary vegetation and of ‘‘old’’ alkanes in dust deflated from desiccated alluvial sediments. Both inputs will be somewhat proportional to wind strength. 3.4.2. The n-alkane d13C record The n-alkane d13C record for 658C over the last 35 kyr has been previously reported (Zhao et al., 2000). The extended record for the last 155 kyr reported herein covers two glacial/interglacial cycles. As a first approximation, the average d13C values (Fig. 2G) for glacials are heavier than those of the interglacials, but these heavier values extend into the early interglacials (e.g., early MIS5e and the early Holocene). The heaviest d13C values are recorded during the MIS6 to MIS5e transition (130 ka) and the MIS4 to MIS3 transition (63 ka). Thus, the d13C record shows that a higher proportion of n-alkanes were of C4 plant origin during the glacial periods and the early interglacial periods. For example, the n-alkane d13C values at 130 kyr. BP correspond approximately to 85% C4 contribution. For Site 658, n-alkanes are delivered by both the SAL and the NETW. Hence, the d13C values of the sedimentary n-alkane content are determined by the relative contributions of alkanes from the two wind systems which are, in turn, dependent on the vegetation cover of the continental source regions. Based on the d13C values of n-alkanes extracted from modern sediments in the vicinity of the site, it is estimated that at present the SAL delivers more alkane to Site 658 than the NETW does (Zhao et al., 2000). The downcore variation of the d13C values of n-alkanes should reflect the changing contributions from the SAL and the NETW. Based on previous phytogeographical studies (Dowton, 1975; Winter et al., 1976; Ehleringer et al., 1977; Raghavendra and Das, 1978; White, 1983), the NETW pick-up region is presently mostly covered by C3 plants, while that of the SAL region vegetation is mostly C4 dominated. Over time the proportions may vary, but the C4 dominance of the SAL region has probably persisted through the last few glacial–interglacial cycles (Dupont, 1989). Thus, several explanations can be advanced for the high C4 contributions inferred from the heavy d13C

M. Zhao et al. / Organic Geochemistry 34 (2003) 131–143

values found for the glacial periods and early interglacials: 1). A vegetational shift to include more C4 plants in the NETW dust source region. 2). A relative decrease of the NETW compared to that of the SAL just after the peak glacial. This would accord with the low relative values of pollen from the Mediterranean area and the North Sahara (‘NETW pollen’; Fig. 2E). 3). A shift to even more C4 vegetation in the SAL dust source region. 4). Increased SAL flux during the drier period as indicated by the higher lithogenic input during the peak glacial periods, followed by decreased dust deflation consequent upon a denser vegetation cover during deglaciation into the early interglacial period. 5). Enhanced erosion of paleolake deposits in the SAL deflation areas, providing an increased flux of C4-rich n-alkanes from the circum lacustrine paleo-grassland. 6). Exposure of coastal sediments during glacial low sea level stands. This would result in the weathering, transportation and advection of reworked previously sedimented old alkanes. Our view is that a combination of explanations 4, 5 and 6 could likely account for the C4 increase during these intervals, but the data are not sufficient to allow us to reach firm conclusions. 3.5. Comparison of pollen and n-alkane records at Site 658 As discussed above, the Site 658 stratigraphic records of the content of pollen and n-alkanes provide a reasonably consistent picture of the vegetation cover on the continent. During the humid periods, continental vegetation cover was denser, and the fluxes of both pollen and n-alkanes were higher, even though dust deflation and wind strength may have been lower. In contrast, during the dry periods, vegetation cover was sparse and pollen and alkane fluxes were lower, even though dust deflation and wind strength were higher. Thus, for Site 658, both pollen and alkane contents of the sediments appear to have been largely determined by the density of the vegetation cover of the African continent. However, the n-alkane d13C record (Fig. 2G) and the ‘C4-pollen’% record (Fig. 2F) appear to give conflicting climate information during the peak glacial periods (the PGM, ca. 135–145 ka and the LGM, ca. 18–23 ka) regarding the C3 vs C4 vegetation. Pollen data suggest a low C4 plant contribution but the d13C values indicate a high percentage of C4 derived n-alkanes. This conflict can be resolved if one considers the detailed mechanisms of the preservation of pollen and alkanes in soils and their subsequent delivery as dusts by deflation and eolian transport. Pollen grains are produced and delivered directly rather quickly to environments such as the deep-sea floor. A study of the pollen content of aerosol filters showed that the large majority of the pollen grains are fresh and have not been fossilised (Lavik et al., personal communication). Pollen grains can be

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resuspended from soils into moving air masses (Calleja et al., 1993), but they are not well preserved in dry, aerated dust or soil, as under oxic conditions pollen is readily attacked by bacteria and certain yeasts (Havinga, 1967). On the other hand, organic compounds such as the n-alkanes can persist in soils for thousands of years (Huang et al., 1996). Thus, even during the dry period when organic production is low, sediments being deflated would still contain significant amounts of n-alkanes retained from the plant debris previously incorporated from the abundant vegetation of earlier times. Hence, during the peak glacial periods when the African continent was very dry, the SAL dust would have contained much less pollen, but could still have contained significant quantities of n-alkanes. The much increased SAL dust flux would ensure that SAL n-alkanes dominated the record. For example, the glacial d13C values of ca. 24% indicate. ca. 70% C4 origin (Fig. 2G). Thus, at least for Site 658, pollen delivery reflects mainly the contemporary vegetation cover of the source region, which is primarily controlled by local precipitation. Pollen flux can be used to infer continental humidity. During periods with extremely sparse vegetation cover, wind strength plays only a secondary role in delivering the pollen to the sea floor. Carbon isotope values of sedimentary n-alkanes, on the other hand, are controlled by the vegetation and by the organic content of soils of the source area from which the dust flux is derived. Thus, the n-alkanes in the dusts can be derived from both contemporary vegetation and the debris of past vegetation in deflated soils. Hence, the isotope values can only provide information regarding the relative contribution from SAL (mainly C4) and NETW (mainly C3), if one assumes that the vegetation types of the two source regions remained relatively constant over time. 3.6. Comparison of terrigenous proxy records at Site 658 and other sites Published core records of the changing pollen fluxes between the glacial and interglacial periods can be broadly divided into two groups. The first group are located within the main dust plume, such as ODP Site 658 and M16017 (21
14.70 N, 17
48.20 W; Hooghiemstra; Fig. 1A). In these cores, pollen fluxes are relatively high (up to 1 million pollen grains per m2 per year), and pollen counts are higher during the warm stages but lower during the cold stages. As discussed above for ODP Site 658, the conclusion is that for sites situated beneath the dust plume the density of land vegetation cover in the source area mainly controls the pollen flux. The second group of records is for cores located outside the main dust plume (see Figs. 1 and 3 and references cited therein). In these cores, the pollen fluxes are

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Fig. 3. Flux of total pollen and spores (# of grains m2 a1) over the last 160 kyrs for ODP Site 658, located just under the center of the summer dust plume, compared with those for four other sites around West Africa outside this plume (for locations see Fig. 1A). Highest fluxes are found for ODP Site 658 off Cape Blanc, and are more than one order of magnitude higher than the fluxes off Guinea (GIK16415, 9
340 N 19
060 W; Dupont and Agwu 1992), Liberia (GIK16776 3
440 N 11
240 W; Jahns et al. 1998), Gabon (GIK16867 2
120 S 5
060 E; Dupont et al. 1998) and in the equatorial Atlantic (GIK16772 1
210 S 11
580 W; Marret, 1994). The solid vertical lines mark the marine isotope stage boundaries, and the MIS stages are labeled on the top.

relatively low (less than 30,000 pollen grains per m2 per year, that is less than 3 pollen grains per cm2 per year), but show large changes on glacial/interglacial time scales, with much higher fluxes during the glacial periods and much lower fluxes during the interglacial periods. Thus, although the pollen content of dust was high during the warm periods, few pollen grains could reach these sites as the wind was not strong enough. However, even though the pollen content was low during the cold and dry glacial periods, the much enhanced dust load carried on the strong wind systems would still carry more pollen for long distances. Thus, for these sites, the strength and direction of the wind system are the main factors determining pollen flux. Similar considerations should apply more generally to the delivery of terrigenous lipid biomarkers to sites in the N.E. Atlantic Ocean. Thus, for cores located beneath the main SAL dust plume such as ODP Site 658 and SU94–11K, the n-alkane (Zhao et al., 2000) and n-alkanol (Ternois et al., 2000) fluxes were respectively lower during the dry glacial periods and higher during the more humid interglacial periods. However, for cores located outside of the main dust plumes such as T88-9P near 48
N and 25
W (Madureira et al., 1997), the

n-alkane flux was higher during the dry glacial periods and lower during the more humid interglacial periods. For core SU94-20bK, which is beneath the main NETW flow but largely outside the SAL plume, the terrigenous n-alkanol flux was also higher during the glacial time, in agreement with previous lithological analyses of marine dust deposits (Sarnthein et al., 1981) and palaeoisopollen maps (Hooghiemstra, 1988, 1989) which indicate intensified glacial NETW (Sicre et al., 2000; Ternois et al., 2000).

4. Concluding remarks 1. Comparison of the stratigraphic records of pollen and n-alkanes from ODP Hole 658 over the last 160 kyr. shows that both pollen and n-alkane inputs were low during the extremely dry portions of the glacial periods. We ascribe these findings to the lower pollen and n-alkane content in the dusts, even though dust deflation was higher. On the other hand, both pollen and n-alkane inputs increased during the wet interglacial periods, reflecting higher pollen and n-alkane contents even though dust deflation was lower. Thus, we infer

M. Zhao et al. / Organic Geochemistry 34 (2003) 131–143

that, for sites beneath the main dust pathway of the SAL, the amounts of pollen and n-alkanes delivered are determined mainly by the density of vegetation cover of the source areas. 2. During the peak dry glacial times, the % ‘C4 pollen’ in Site 658 was very low, though their d13C values indicated that the n-alkanes were mostly from C4 plants. Taken together, these two records appear to indicate that the amounts of pollen, but not alkanes, from the SAL deflation regions were much reduced. Presumably, the content of n-alkanes of paleo-vegetational origin preserved in soils and dried up lake beds in the Saharan source areas was mainly of C4 type. The weathering and erosion of old n-alkanes from the coastal sediments loaded with past SAL dust and then exposed during lower sea level stands could also have played a role. 3. In contrast to site 658, data from locations not beneath the main dust pathway, such as cores GIK16876, 16776, 16415 and 16772, show that pollen and n-alkane input were higher during the dry and stormy glacial periods. Thus, outside the main dust plumes, the amounts of the dust input determine the pollen and n-alkane content of the sediments.

Acknowledgements We thank J. Carter, G. Read and J. Mercer for technical help, the Organic Geochemistry Mass Spectrometry Facility for access, the Natural Environment Research Council (GST/02/553), the European Community (EV5V-CT-92-0117), and the Deutsche Forschungsgemeinschaft (We992/26) for financial support, Dr. Alan Mix for the Core V30-49 data, Dr. L. Labeyrie for the U-channel tubes, and the ODP for samples. The data sets will be available from the Pangaea database. We thank Drs Joan Grimalt and Maria Fernanda Sanchez Gon˜i and an anonymous referee for helpful comments. G.E. and M.Z. thank the Hanse Wissenschafts Kolleg for a Fellowship and visitor support, respectively. M.Z. also thanks Dartmouth College for a Burke Award. Associate Editor—J. Grimalt

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