Late Quaternary Climate Changes in Central Africa as Inferred from Terrigenous Input to the Niger Fan

Quaternary Research 56, 207–217 (2001) doi:10.1006/qres.2001.2261, available online at http://www.idealibrary.com on

Late Quaternary Climate Changes in Central Africa as Inferred from Terrigenous Input to the Niger Fan Matthias Zabel, Ralph R. Schneider, Thomas Wagner, Adesina T. Adegbie, Uwe de Vries, and Sadat Kolonic Fachbereich Geowissenschaften, Universit¨at Bremen, Postfach 330 440, D-28334 Bremen, Germany E-mail: [email protected] Received October 9, 2000

influence the timing of climate change within the region (e.g., Foley et al., 1994; Kutzbach and Liu, 1997; Ganopolski et al., 1998). For example, the responses of vegetation to gradual shifts in insolation causes changes in moisture advection and precipitation intensity in the subtropics that, in turn, amplify the vegetation response (Claussen et al., 1999). In this context, deMenocal et al. (2000a) discussed a climate-threshold response as being responsible for the timing of climatic transitions. However, continental data sets also reveal that recording of climate change is influenced by a particular response time and by processes of the hydrological system, which complicate the interpretation of these data sets (Gasse, 2000). In contrast to continental records, marine deposits permit examinations of climate variations on a considerably longer time scale. Organic components, such as pollen or other plant material, and the inorganic fraction have been used as proxy indicators of changes in paleoenvironmental conditions. For example, grain-size ratios of inorganic terrigenous particles (e.g., Parkin and Shackleton, 1973; Sarnthein, 1978; Rea and Hovan, 1995), quartz content (Pastouret et al., 1978; Kolla et al., 1979), or bulk sediment ratios (e.g., Ti/Al, K/Al, Zr/Rb, or Zr/Al; Boyle, 1983; Grousset et al., 1989; Matthewson et al., 1995; Schneider et al., 1997; Zabel et al., 1999) are connected to input pathways, weathering conditions in the source area, or the intensity of transport processes such as windstrength. Conversely, these studies clarify that local deposition is controlled by a complex and subtle combination of processes that can exert a strong influence on the paleoclimatic record. This study focuses on three questions: (1) What are the processes that control the supply and composition of the inorganic terrigenous fraction of the late Quaternary deep-sea sediments of the Niger fan? (2) What are the implications of our results for the reconstruction of past climate conditions? (3) What information can be derived concerning climate history of central African?

Time series of terrigenous source elements (Al, K, Ti, Zr) from core GeoB4901-8 recovered from the deep-sea fan of the Niger River record variations in riverine sediment discharge over the past 245,000 yr. Although the flux rates of all the elements depend on physical erosion, which is mainly controlled by the extent of vegetation coverage in central Africa, element/Al ratios reflect conditions for chemical weathering in the river basin. Maximum sediment input to the ocean occurs during cold and arid periods, when precipitation intensity and associated freshwater runoff are reduced. High carbonate contents during the same periods indicate that the sediment supply has a positive effect on river-induced marine productivity. In general, variations in the terrestrial signals contain a strong precessional component in tune with changes in low-latitude solar radiation. However, the terrestrial signal lags the insolation signal by several thousand years. K/Al, Ti/Al, and Zr/Al records reveal that African monsoonal precipitation depends on high-latitude forcing. We attribute the shift between insolation cycle and river discharge to the frequently reported nonlinear response of African climate to primary orbital configurations, which may be caused by a complex interaction of the secondary control parameters, such as surface albedo and/or thermohaline circulation. ° 2001 University of Washington. Key Words: African climate change; Niger fan; terrigenous fraction; elemental ratio. C

INTRODUCTION

Numerous studies of marine and lacustrine sediments have improved knowledge about the extreme complexity of the interacting processes that influence the African monsoon system. In spite of the growing numbers of paleorecords and the availability of computer simulations using fully coupled circulation models, uncertainties remain concerning the course and the timing of tropical climate changes. Clearly, orbital insolation forcing alone cannot explain the abruptness of the transitions between arid and humid conditions (Prell and Kutzbach, 1987), which are best documented by lake-level fluctuations in Africa over the last 30,000 yr (e.g., Street and Grove, 1976, 1979). Climate model simulations suggest that variations in the Earth’s orbit are a primary drivingforce. However, the African monsoon system is sensitive to changes in sea-surface temperatures and vegetation cover (as it affects surface albedo). These fundamental controls

BACKGROUND

Geochemical Ratios We present ratios of K/Al, Ti/Al, and Zr/Al from bulk sediment. The relatively conservative elements Zr and Ti are

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considered to be contributed by corresponding heavy minerals, such as zircon (Zr[SiO4 ]), rutile, and anatase (both TiO2 ; e.g., Delany et al., 1967; Schutz and Rahn, 1982). A similarly precise affiliation of Al and K to specific mineral phases does not exist. Al concentrations in marine sediments generally, however, can be assigned to the fine-grained aluminosilicate detrital fraction (Calvert, 1976). K is mostly associated with potassium feldspar (K[AlSi3 O8 ]) (Shimmield and Mowbray, 1991; Schneider et al., 1997; Martinez et al., 1999), although illite ((K,H3 O)Al2 [(OH)2 Si3 AlO10 ]) has also been considered as a primary source of K (Yarincik et al., 2000). Regional differences in the geology of the source area are assumed to differentiate between these affiliations. However, both feldspar and illite are representative of low rates of chemical weathering and arid conditions, where physical weathering is dominant. Consequently, the orthoclase-illite differentiation seems to be unimportant in this research. A depletion of K is typically used as an index for the chemical maturity of sediments. Hence, we use K/Al as a proxy for the intensity of chemical weathering. The ratios of Ti/Al and Zr/Al should mainly reflect grain size and would therefore be directly linked to the strength of the transport process (Boyle, 1983; Zabel et al., 1999). The Modern Niger River The modern drainage area of the Niger River covers ∼1.55 × 106 km2 (Boeglin and Probst, 1998). The high rates of annual rainfall (∼1250 mm) result in a water discharge of ∼154 km3 /yr, but the average amount of total suspended material transported

by the Niger River is remarkably low (26 g/m3 ; Konta, 1985, and references therein). This small volume has been attributed to the low-relief gradient of the lateritic river basin that prevents intensive erosion. The suspension load consists mainly of highly weathered solids, with the average composition of suspended crystalline solids being 51% kaolinite, 32% illite, 8% montmorillonite/smectite, 5% quartz, 2% K feldspar, 1% acid plagioclase, and 1% chlorite (Konta, 1985). Additionally, gibbsite, an indicator of the strongest chemical weathering in the tropics and subtropics, was found in trace concentrations. Correspondingly, the chemical maturity (ChM = %Al : %(Na + Mg + Ca)) of modern suspended solids in the Niger River is high (5.5–7.5), also indicative of intense weathering (Konta, 1985; Gaillardet et al., 1999). MATERIAL AND ANALYTICAL METHODS

Gravity core GeoB4901-8 was recovered from the southern flank of the Niger Fan (02◦ 40.70 N, 06◦ 43.20 E, water depth 2184 m) during RV METEOR cruise M41/1 in February 1998 (Fig. 1). The sediments lack any indication of perturbations. The core has a total length of 20.3 m. Samples for element analysis were taken at 5-cm intervals. Digestion of sediments was carried out with a microwave system (MLS, MEGA II). For this purpose, 2 ml concentrated HNO3 , 2 ml HF (37%), and 2 ml concentrated HCl were added to about 50 mg freeze-dried and ground sediment previously placed into Teflon liners. All acids were of suprapure quality. After heating

FIG. 1. Location of GeoB site 4901-8. Also shown are positions of sediment records mentioned or discussed in the text: (A) CH22KW31 (Pastouret et al., 1978); (B) RC13-205 (Pokras and Mix, 1985); (C) GeoB1008 (Schneider et al., 1997); Lake Bosumtwi (Maley, 1991; Talbot et al., 1984); and Lake Barombi Mbo (Maley and Brenac, 1998).

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(∼200◦ C) under a pressure of 30 × 103 bar, the acids were evaporated to near dryness, and the residue was dissolved in 1 ml concentrated HNO3 and filled up to 100 ml with double-deionized water. Inductively coupled plasma atomic emission spectrometry (Perkin Elmer, Optima 3300RL) was used to measure Al, Fe, K, Ti, and Zr concentrations in the bulk sediment (precision 2%). To check the results of the digestion procedure for possible residual oxides or evaporation losses, some samples were combusted at 550◦ C and treated with a second acid mixture (HCl, HF, and H2 SO4 ). Concentrations of solutes from both procedures showed a maximum deviation of 1%. Marine reference sediment MAG-1 (U.S. Geological Survey) was repeatedly digested. Measured values were within 4% of the accepted value. The standard deviation of three replicates was less than 3%. Inorganic carbon was measured in homogenized samples using a Leco CS-300 elemental analyzer (precision of measurement ±3%). All data can be found in the German paleoclimate data archive PANGAEA (http://www.pangaea.de). Our age model is based on the stable oxygen isotope (δ 18 O) record of the benthic foraminifera, Cibicidoides wuellerstorfi (Adegbie et al., in prep.), and its visual comparison with the SPECMAP oxygen isotope stack (Imbrie et al., 1984). The dry bulk density for the calculations of accumulation rates (AR) were determined by volume measurements. Spectral and crossspectral analyses were performed using Spectrum (V2.1; Schulz and Stattegger, 1997) and AnalySeries 1.1 (Paillard et al., 1996), respectively. RESULTS

Sediments of core GeoB4901-8 mainly consist of claybearing diatomaceous nannofossil oozes (Adegbie et al., 1998). Based on the present δ 18 O stratigraphy, the core record represents the consecutive sequence of the last seven marine oxygen isotope stages (MIS) and starts at the MIS 7/8 boundary (termination III) at 245,000 yr. The sampling interval of 5 cm is equivalent to ∼620 yr temporal resolution (SR: 8.8 cm/103 yr average). Although information on the portion of biogenic opal is not yet available, the low content of carbonate (0.1–36.1 wt%) indicates the dominance of the terrigenous fraction at this site. In contrast to other studies on tropical Atlantic sediments, which describe carbonate maxima during warm periods (Crowley, 1983; deMenocal et al., 1993; Verado and McIntyre, 1994), the pronounced high concentrations and accumulation rates of CaCO3 in the Niger Fan indicate a higher biological productivity during cold periods (Fig. 2). Bertrand et al. (1996) have explained this apparent contradiction in marine productivity between glacial (M¨uller et al., 1983; Sarnthein et al., 1988; this study) and interglacial maxima in marine productivity by local differences in wind stress and/or sea-level change. However, the glacial– interglacial contrast in carbonate deposition at site GeoB4901 is modulated by higher frequency fluctuations. Apart from the clear indication of a dependence on high-latitude forcing (100,000-yr

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FIG. 2. Downcore variations in calcium carbonate concentrations (weight percent) and accumulation rates (AR; g/cm2 /103 yr) versus age (×10−3 yr). The relatively low carbonate content indicates that the Niger Fan sediments are dominated by terrigenous material. The CaCO3 concentrations and AR reveal intense marine productivity during cold periods. The typical asymmetric “sawtooth” pattern of both records reflects changes in sea level.

period) clearly has an influence, but effects of other orbital cycles are uncertain. Element flux rates were calculated on a carbonate-free basis to preclude the influence of carbonates. There is no indication of any carbonate dissolution. Variations in the accumulation of Al, K, Ti, and Zr are regular and parallel each other (Fig. 3). This concerns contemporaneous periods of high and low flux rates as well as relative amplitudes of the variations expressed as percentages of the standard deviation from the mean (20.2– 23.7%; Table 1). This similarity implies that these elements are subject to the same fluctuations of sediment supply and may be derived from the same soil types. Statistical correlation also reveals differences in the flux rates. A nearly perfect correlation only exists between the accumulation rates (AR) of Al and Fe (r 2 = 0.94, p < 0.01), which is further confirmed by spectral analysis. No phase shift occurs between Al and Fe implying that relative Al and Fe contents are constant over time (Shimmield and Mowbray, 1991). In the Niger Fan drainage the strong link between Al and Fe may originate in feldspar weathering, which results in the concurrent formation of kaolinite and Al and Fe hydroxides and depletion of K. Although Al AR, K AR, Ti AR, and Zr AR records are similar, the scatter of correlation

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FIG. 3. Flux rates of Al, K, Ti, and Zr over the last ∼ 240,000 yr in core GeoB4901-8. The pattern for each element is similar. Higher input rates occur during cold periods. Accumulation rates(cf) were calculated on a carbonate-free basis.

coefficients indicates distinct dissimilarities in the distribution of respective elements (Table 1). The striking glacial–interglacial difference observed in the carbonate record is not seen in the elemental flux rates, although the AR pattern suggests a contrast between warm (wet) and cold (dry) conditions, with relatively larger inputs of terrigenous TABLE 1 Statistics on Downcore Accumulation Rates of Terrigenous Source Elements

max min mean SD SD (%) Statistical results Fe AR(cf) K AR(cf) Ti AR(cf) Zr AR(cf)

Al AR(cf)

Fe AR(cf)

K AR(cf)

Ti AR(cf)

464.4 160.0 331.7 67.1 20.2

257.2 80.4 175.7 37.1 21.1

81.7 24.7 53.6 11.4 21.2

26.4 7.8 17.4 3.7 21.4

0.94∗ 0.81 0.70∗ 0.41

— 0.87 0.76∗ 0.61

— — 0.90 0.66

— — — 0.76

Zr AR(cf) 0.63 0.17 0.40 0.09 23.7 — — — —

Note. AR values are in (mg/cm2 /103 yr) and were calculated on a carbonatefree basis. SD (%) indicates standard deviation as percent of mean. Statistical results show Person coefficients of determination (r 2 ; p < 0.01, ∗n = 404, otherwise n = 200).

particles during the latter. There is no indication of a significant influence by intense shelf erosion and redeposition as a result of sea-level changes. Considerable oscillations occur in the ratios of K/Al, Ti/Al, and Zr/Al (Fig. 4), and the relationship to climate changes is more pronounced than for the individual element AR. In particular, the K/Al and Ti/Al records indicate a pronounced relative decrease in Al concentration during the cold phases of both glacial and interglacial stages (e.g., MIS 2 and 4, or substages 5.2 and 5.4). Feldspar and heavy minerals are relatively reduced in sediments from warm periods. If the formation of kaolinite (Al) is favored by intense chemical weathering under humid conditions, the Al ratios document moisture responses to changes in the interglacial–glacial boundary conditions, as was described for northwestern Africa in dust deposition studies (e.g., Sarnthein, 1978; Pokras and Mix, 1985; Gasse et al., 1990; deMenocal et al., 1993). In Congo Fan sediments, which are dominated by fluvial input, Schneider et al. (1997) attributed oscillations in elemental ratios to changes in the mineral composition and postulated a larger input of kaolinite as compared with feldspar during warm and humid periods. Martinez et al. (1999) showed that in sediments off Cape Blanc (West Africa), where dust dominates the terrigenous fraction, the K/Al and Zr/Al ratios generally reflect the content of the sand and coarser silt-size fraction. Oscillations in these ratios likely reflect changes in

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FIG. 4. Elemental ratios of the terrigenous fraction from Niger Fan sediments (GeoB4901-8). Arrows denote the modern composition of suspended matter in the Niger River (Gaillardet et al., 1999). Variations in the elemental composition reveal intense chemical weathering in the Niger Basin during warm periods. Note the weaker response during MIS 1 and MIS 5e compared with other warm stages and substages.

FIG. 5. Spectra of harmonic time series analysis on elemental ratios and respective element flux rates (AR). The spectra were calculated at the 95% confidence level using a 6 dB bandwidth of 4.95 ×10−3 (1/103 yr).

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atmospheric circulation (Boyle, 1983; Zabel et al., 1999). Because enhanced wind stress and aridity are closely connected in tropical Africa (e.g., Parkin and Shackleton, 1973; Sarnthein, 1978; deMenocal et al., 1993; Matthewson et al., 1995), both interpretations of low ratios (i.e., relatively high kaolinite content or weak transport energy) are equivalent. Visual inspection of the GeoB4901-8 data suggests that the discharge of river-borne particles strongly depends on orbital forcing. Changes in the terrigenous composition and flux rates of K, Ti, and Zr (Fig. 5) show a clear relationship to precessional variations, especially in the 19,000- to 23,000-yr frequency band. Except for Al AR, oscillations in all other records reveal a strong precessional (23,000-yr) power. The low response of Al flux rates may be caused by the nonspecific mineral assignment (see above), which slightly distorts the sensitivity of Al to climate conditions. Ti/Al, K/Al, and Zr/Al show a lowfrequency response (122,000 or 140,000 yr), but the temporal resolution of the core indicates a questionable reliability of the low-frequency response. In AR records, an additional 51,000yr power is evident, which is reduced in favor of an extremely weak obliquity signal (41,000-yr period) as far as weight percent records are concerned. DISCUSSION

Deposition of Terrigenous Material at Site GeoB4901 Knowing the transport mechanism that dominates the particle supply is essential for understanding the relationship of terrigenous input and changes in environmental conditions. Terrigenous material reaches the Niger Fan as river-suspended matter (RSM) via the Niger River and as eolian dust carried mainly by the northeasterly trade winds. The mineral or elemental compositions of the particle load are compared with estimate the respective importance of the two mechanisms. The elemental concentrations in the dust particles and the RSM have been documented previously (Wilke et al., 1984; Gaillardet et al., 1999). Nigerian dust contains a larger relative proportion of Al and lower concentrations of Ti and K than the suspension load of the Niger River (Table 2). However, the composition of the terrigenous fraction at site GeoB4901 cannot be explained by applying dust and RSM data in a simple end-member approach, and some other alteration processes must be considered. For example, in contrast to Al, which may be slightly affected by scavenging (e.g., Orians and Bruland, 1985), we can eliminate the enrichment of Ti or Zr at site GeoB4901, because most of the heavy minerals are already trapped in the littoral sediments (Komar and Wang, 1984). Consequently, only a narrow range of possible mixing rates between dust and RSM can occur. Using the concentrations given by Gaillardet et al. (1999) for RSM and by Wilke et al. (1984) for dust particles, the proportion of eolian input presently amounts to a maximum of 7–15%. This value depends on the behavior of K, which is still not well constrained. However, the terrigenous fraction of the deep-sea fan is clearly dominated by material which is derived from the Niger River.

TABLE 2 Elemental Ratios

UCCa Dust (Nigeria)b RSM (Niger)c RSM (Niger)d Niger Fan (avg) Niger Fan (min) Niger Fan (max) Niger Fan (surface) Deep-sea claye

Ti/Al

K/Al

Zr/Al

0.045 0.087 0.053 0.063 0.052 0.066 0.040 0.054 0.060

0.33 0.43 0.07 0.14 0.16 0.13 0.19 0.14 0.30

0.0031 — — 0.0030 0.0012 0.0007 0.0018 0.0013 —

Note. UCC, upper continental crust; RSM, river suspended matter. Mean (avg), minimum (min), and maximum (max) values are given for the Niger Fan sediments. a Wedepohl (1995). b Wilke et al. (1984). c Porrenga (1966). d Gaillardet et al. (1999). e Martin and Meybeck (1979).

This observation strictly applies only to the modern situation, but the site’s relatively short distance from the river mouth supports the assumption that this relationship was not significantly different in the past. A comparison between surface values from core GeoB4901-8 and the composition of particles suspended in the modern Niger River shows that K/Al and Ti/Al ratios match the fluvial transport of terrigenous particles (Fig. 4; Table 2). This is not the case for Zr/Al. RSM contains more Zr than is found in the deep-sea sediments. The real difference probably is greater, considering that Zr is transported mainly in the bottom sands of rivers (Dupr´e et al., 1996), and data presented for RSM have been determined in samples collected from the middle of the main river channel (Gaillardet et al., 1999). We attribute the decrease in Zr/Al in the deep-sea Niger Fan to a more efficient Zr filtration in the estuary environment, as compared with Ti-bearing minerals. Considering the more than twofold higher molar weight of zircon relative to rutile or anatase, this assumption seems highly plausible and is confirmed by our unpublished data from 18 shelf locations close to the Niger mouth (Zr/Al = 0.014). The disproportionate loss of Zr may also account for the poorer correlations among the elemental flux rates (Table 1). Nevertheless, Zr/Al fluctuation is still significant, and the record closely parallels profiles of K/Al and Ti/Al. Temporal Variations in Supply of Terrigenous Material Spectral results suggest that variations in the terrigenous fraction of the Niger Fan are controlled by precession-modulated insolation changes (Fig. 5), mechanisms which are also key for governing the climates of tropical central Africa (Pokras and Mix, 1985; McIntyre et al., 1989). In contrast, changes in Al ratios parallel climate shifts at northern high latitudes, which are dominated by fluctuations of the continental ice sheets (Fig. 4).

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TABLE 3 Cross-Spectral Analysis Phase angle Standard for comparison Insolation 20◦ N

δ 18 O (23,000-yr period)

Phase

Parameter

Coherency

(◦ )

±

(103 yr)

±

Al AR(cf) K AR(cf) Ti AR(cf) Zr AR(cf) CaCO3 AR K/Al Ti/Al Zr/Al Al AR(cf) K AR(cf) Ti AR(cf) Zr AR(cf) K/Al K/Al Ti/Al Zr/Al Inso 20◦ N

0.99 0.96 0.97 0.96 0.97 0.96 0.95 0.96 0.96 0.98 0.98 0.97 0.97 0.96 0.99 0.94 0.95

32.7 56.2 56.8 66.0 52.2 63.7 76.3 79.2 −53.3 −26.9 −26.9 −16.6 −31.5 −23.5 −6.9 −5.8 −84.9

4.0 7.7 6.6 7.2 6.6 8.0 8.6 7.2 7.4 5.2 5.2 6.3 6.3 7.5 4.3 9.2 8.0

2.1 3.6 3.6 4.2 3.3 4.1 4.9 5.1 −3.4 −1.7 −1.7 −1.1 −2.0 −1.5 −0.4 −0.4 −5.4

0.3 0.5 0.4 0.5 0.4 0.5 0.5 0.5 0.5 0.3 0.3 0.4 0.4 0.5 0.3 0.6 0.5

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This trend applies to both the ages and the magnitudes of the amplitude and confirms the considerable influence of high-latitude forcing on tropical African climate. This conclusion is further supported by the minima of relative Al (kaolinite) contents during MIS 2 and 6 and the slight but characteristically asymmetric “sawtooth” pattern of the Al ratios (particularly Ti/Al and Zr/Al; Fig. 4). The terrestrial records indicate that chemical weathering is least during maximum icecoverage. This interpretation of glacial aridity agrees with a huge number of mineralogical, geochemical, faunal, and palynological studies on deep-sea cores (e.g., Sarnthein, 1978; Leroy and Dupont, 1994; Matthewson

Note. Cross-spectral analysis between 23,000-yr filters of downcore parameters AR Al, AR Ti, and Ti/Al and standards (oxygen isotopes in the precession frequency band (23,000-yr period) for insolation at latitude 20◦ N on June 21, Northern Hemisphere solstice). Filteration of records was conducted at a frequency of 0.0435 ± 0.005. Negative phase angles indicate a lead of the parameter relative to the respective standard.

We used cross-spectral analysis of our data to find evidence for a link between orbital climate forcing and the terrestrial proxies in the Niger Fan sediments. The first comparison was with maximum boreal summer insolation (20◦ N) because it is the most likely mechanism controlling the variability of the monsoon intensity in central equatorial Africa (Pokras and Mix, 1985; Dupont and Leroy, 1995; deMenocal et al., 2000a). All coherencies exceeded 0.9 with a high statistical significance (Table 3). Fluctuations in the suspension load of the Niger River showed a significant lag behind variations of solar radiation, implying that the terrigenous supply of sediments does not respond to the insolation cycle directly. Oscillations of solar radiation led the variations of the terrigenous composition by ∼4100–5100 yr. Elemental flux rates responded somewhat earlier (2100–4200 yr; Fig. 6). Lags are not constant and reflect the modulation of the precessional period by the 100,000-yr cycle, depending on the respective amplitude (Fig. 6e). If the response time of terrestrial signals to the intensity of chemical weathering in the Niger basin is considerably shorter than the observed temporal shift at site GeoB4901, then insolation cycle does not bear a strong effect on monsoonal precipitation. Oscillations in the terrigenous fraction were compared with the variations in the δ 18 O record, which represents global climate change. Given the well-known time lag of the global isotopic (ice-volume) response to insolation forcing (Imbrie et al., 1984), terrestrial signals fluctuate more in tune with the 23,000-yr orbital signal of the δ 18 O record (Table 3, Figs. 6b, 6d, and 6e).

FIG. 6. Comparison of the oxygen isotope record, the insolation cycle, and Ti/Al, Al AR, and Ti AR (a–d) at a 23,000-yr periodicity. Dashed lines trace changes in the elemental variables. Maximum insolation for latitude 20◦ N is given in W/m2 . The phase and magnitude in amplitude indicate that variations in terrigenous input is more similar to changes in the northern hemispheric ice volume than to solar radiation. Variations in the phase shifts (e) reflect the overlay of the 100,000-yr periodicity.

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FIG. 7. Comparison between indices of chemical weathering and sediment discharge (Niger River), marine productivity, solar radiation, δ 18 O precessional signal, and precipitation index. Dotted lines belong to the lower axis. Precipitation intensity is based on pollen records (Lake Bosumtwi, Maley, 1991; Lake Barombi Mbo, Maley and Brenac, 1998), and sea-level estimates (Talbot et al., 1984; Street-Perrot and Harrison, 1984; Street-Perrot and Perrot, 1990; Talbot and Johannessen, 1992). YD, Younger Dryas.

et al., 1995). For example, highest abundances of freshwater diatoms in central equatorial Atlantic sediments were documented during glaciations, when dry conditions permitted their deflation from the African lake beds (Pokras and Mix, 1985, 1987). Nevertheless, best verification that the terrigenous fraction in core GeoB4901-8 does reflect climate conditions comes from continental data. Terrestrial records from subequatorial West Africa are extremely rare and only span the last ca. 30,000 yr. Nevertheless, they contain the only primary information about the cyclicity of aridity–humidity events and reveal the high complexity of the climate system during the Holocene and late Pleistocene (Gasse, 2000, and references therein). A simplified precipitation index can be made from pollen and sea-level studies (Street-Perrot and Harrison, 1984; Talbot et al., 1984; Street-Perrot and Perrot, 1990; Maley, 1991; Talbot and Johannessen, 1992; Maley and Brenac, 1998; Fig. 7). In spite of some chronological uncertainties, variations in maximum insolation (20◦ N) and the precessional signal from the δ 18 O record clearly indicate that the last African humid period, at least in this area, predominantly parallels high-latitude climate variability. Disregarding the interruptions by shorter dry intervals (centered around 11,000 yr B.P. [Younger Dryas] and 8000 yr B.P.), wet conditions have prevailed between ∼13,000 and 3000 yr B.P. Several hypothesis suggest that relationships among atmospheric processes (solar radiation and wind regime), extent of the vegetation cover, and sea-surface temperature explain the complexity of the African monsoon system (deMenocal and Rind, 1993; Foley et al., 1994; Kutzbach et al., 1996; Coe and Bonan, 1997; Kutzbach and Liu, 1997; Brovkin et al., 1998; Ganopolski et al., 1998; Claussen et al., 1999). However, fau-

nal studies of deep-sea sediments indicate the importance of the oceanic thermohaline circulation (Mulitza and R¨uhlemann, 2000; deMenocal et al., 2000b). Our data can only make a limited contribution to a discussion of causes, but as explained in the following, our results underscore the influence of vegetation on the cyclic character of the terrestrial signals in deep-sea sediments. Factors Controlling the Terrestrial Signals Sediment discharge and runoff are roughly inversely correlated (Fig. 7). Although exceptionally high freshwater inputs coincide with periods of maximum wetness from 13,000 to 4400 yr B.P. (Pastouret et al., 1978), the sediment supply to the ocean was relatively reduced at these times. A maximum terrigenous input is documented during the last glacial maximum, when dry conditions prevailed (Talbot et al., 1984; Maley and Brenac, 1998). Chemical weathering indices generally reflect high rates of precipitation fairly well, which correspond to the described enhancement of kaolinite input to this region (Pastouret et al., 1978; Bonifay and Giresse, 1992). The first onset of the African humid period between about 16,000 and 14,500 yr B.P. (Pastouret et al., 1978; Street-Perrot and Perrot, 1990) is only weakly marked and may refer to a low change of amplitude of the climate forcing conditions at the beginning of the arid–humid transition (deMenocal et al., 2000a). However, such reversals between the amount of RSM, the quantity of freshwater runoff, and the mineral signature of suspended solids agree with the observations of Gaillardet et al. (1999) and are also confirmed for other river systems. Any transfer of soils into the deep sea initially requires physical erosion, with the intensity of deflation and abrasion

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essentially dependent on the vegetation cover. As increasing vegetation has a reverse effect on physical erosion, the supply of terrigenous sediment consequently decreases during humid periods when vegetation belts are extensive and vegetation covers more dense. Hence, the stream-flow rate is of minor importance for the total amount of RSM. However, a high volume of percolated water, necessary for hydrolytical processes, is one of the prerequisites for the formation of kaolinite. The high and relatively rapid response of the RSM composition to the contrast between aridity and humidity gives evidence that the degree of weathering, or rather the type of soils eroded, is the controlling factor here (Fig. 8). In contrast to the eolian pathway, where the strength of the wind regime controls the frequency of heavy minerals in deep-sea sediments (Boyle, 1983; Ruddiman 1997; Zabel et al., 1999; Martinez et al., 1999), our data reveal that, at least in the Niger River system, fluctuations in Ti/Al and Zr/Al predominately reflect the type of source ma-

terial rather than the total water discharge (Fig. 7). Therefore, both ratios can be used as chemical weathering indices, similar to K/Al. Marine Productivity and Sediment Discharge Niger Fan sediments reveal higher marine productivity during cold periods (Fig. 2). In agreement with the foregoing discussion, we attribute this regional pattern in ocean fertility to a combination of variations in the riverine nutrient supply and the zonal trade-wind intensity. Apart from the close connection between climate conditions and the strength of the atmospheric circulation, our data give additional support to the fluvial-induc-ed productivity. Carbonate concentrations and flux rates basically increase when the total sediment discharge is high (Fig. 7). The positive effect of riverine contribution to marine productivity is difficult to distinguish from the wind-induced upwelling of nutrient-rich subsurface waters (Diester-Haass, 1983). CONCLUSIONS

Late Quaternary Niger Fan sediments are dominated by riverborne material, with eolian input of minor importance (7–15% of the terrigenous fraction). Enhanced sediment discharge and high carbonate content during cold periods indicate river-induced productivity. Records of the terrigenous fraction give evidence that the mineral composition is sensitive to weathering conditions on the continent. Although the total riverine sediment input into the ocean is mainly controlled by physical erosion and thus the extent of vegetation, the composition of river-suspended solids largely reflects the intensity of precipitation. Records document intense chemical weathering processes during warm and humid periods. Hence, the terrigenous fraction of the sediments of the sediments of the Niger Fan reflects the history of Central African climate. Results from core GeoB4901-8 clearly underline the extremely nonlinear response of African climate to orbital configurations. Our data reveal that solar radiation alone cannot trigger the frequent and almost abrupt changes between arid and humid conditions. Based on a compilation of orbital cycles and records of different parameters sensitive to climate cyclicity, variations in monsoonal precipitation reveal a high dependency on highlatitude forcing. In addition, the data presented here substantiate the important role of the expansion of the vegetation cover for recording the terrestrial climate change in deep-sea sediments. On account of the slow delayed response time of vegetation growth on primary forcing factors, the duration of the climate changes may be shorter than is documented in the records of the Niger Fan sediments. ACKNOWLEDGMENTS FIG. 8. Relationships among factors controlling the composition of the terrigenous fraction in Niger Fan sediments during (a) humid and (b) arid conditions. In contrast to the eolian pathway, the composition of RSM depends more on soil type than on the transport energy.

We thank Torsten Bickert, Lydie Dupont, Stefan Mulitza, Carsten R¨uhlemann, and Horst D. Schulz for stimulating discussions and useful comments, and Joseph Hell, Sigrid Hinrichs, and Karsten Enneking for technical assistance.

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We are also indebted to Suzanne Leroy, an anonymous reviewer, and Patricia Anderson for constructive comments that helped to improve the manuscript. This study was funded by the Deutsche Forschungsgemeinschaft (DFG) within the scope of the Collaborative Research Center SFB 261 (The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems) at Bremen University (contribution No. 331).

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