Itambi Et Al., 2010a

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Millennial-scale precipitation changes over

Central Africa during the late Quaternaryand Holocene: evidence in sediments fromthe Gulf of GuineaACHAKIE C. ITAMBI,1 * TILO VON DOBENECK1,2 and ADESINA T. ADEGBIE31 Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany2 Geosciences Department, University of Bremen, Bremen, Germany3 Nigerian Institute for Oceanography and Marine Research (NIOMR), Victoria Island, Lagos, Nigeria

Itambi, A. C., von Dobeneck, T. and Adegbie, A. T. 2010. Millennial-scale precipitation changes over Central Africa during the late Quaternary and Holocene: evidencein sediments from the Gulf of Guinea. J. Quaternary Sci ., Vol. 25 pp. 267–279. ISSN 0267-8179.

Received 12 July 2008; Revised 27 March 2009; Accepted 8 May 2009

ABSTRACT: We combine environmental magnetism, geochemical measurements and colour reec-tance to study two late Quaternary sediment cores: GeoB 4905-4 at 2 8 30 0 N off Cameroon and GeoB4906-3 at 0 8 44 0 N off Gabon. This area is suitable for investigating precipitation changes over Centraland West Africa because of its potential to record input of aeolian and uvial sediments. Threemagnetozones representing low and high degree of alteration of the primary rock magnetic signalswere identied. The magnetic signature is dominated by ne-grained magnetite, while residualhaematite prevails in the reduced intervals, showing increase in concentration and ne grain sizeat wet intervals. Our records also show millennial-scale changes in climate during the last glacial and

interglacial cycles. At the northern location, the past 5.5 ka are marked by high-frequency oscillationsof Ti and colour reectance, which suggests aeolian input and hence aridity. The southern locationremains under the inuence of the Intertropical Convergence Zone and thus did not register aeoliansignals. The millennial-scale climatic signals indicate that drier and/or colder conditions persistedduring the late Holocene and are synchronous with the 900 a climatic cycles observed in NorthernHemisphere ice core records. Copyright # 2009 John Wiley & Sons, Ltd.

KEYWORDS: environmental magnetism; colour reectance; Gulf of Guinea; aridity; Holocene; diagenesis.

Introduction

Many studies have demonstrated that Quaternary climatevariations in equatorial and subtropical Africa are primarilycontrolled by 19 and 23 ka orbital precession cycles whichregulate summer insolation and thus the strength of the WestAfrican monsoon, which controls the amount of precipitationon the West African continent and subsequently the inux of terrigenous materials into the Atlantic Ocean (Sarnthein, 1978;Kutzbach and Street-Perrott, 1985; Ruddiman and Janecek,1989; DeMenocal et al ., 2000a,b). Quite recently attention hasbeen focused on the climatic and environmental change duringthe Holocene following observations of rapid climatic eventsduring the last glacial in high-latitude climate archives (O’Brienet al ., 1995; Bond et al ., 1997). Ice core records from MtKilimanjaro have provided some evidence for Holoceneclimate variability in tropical East Africa (Thompson et al .,

2002). In addition, numerous studies on lake sedimentsrevealed signicant variability in West and Central Africanclimate during the Holocene (e.g. Talbot et al ., 1984; Street-Perrott and Perrot, 1990, Gasse, 2000; Giresse et al ., 2005;Shanahan etal ., 2006). These studies link changes in lake levelsto uctuation in precipitation.

Recent studies such as Weldeab et al . (2007) and Schefußet al . (2005) have reported millennial-scale climatic changesfrom marine sediments during the Holocene in tropical Africaand linked these to sea surface temperature (SST) uctuations.Although these studies show a close similarity in the CentralAfrican climate record to ice core records at the Holocene,knowledge about the cyclicity of climatic signals at the laterpart of the Holocene (0–5.5 ka) and possible forcings remainlacking.

We therefore try to address these questions in this study byapplying a multi-proxy approach combining rock magnetic,elemental abundances and colour reectance proxies onmarine sediments recovered from the Gulf of Guinea offshorefrom Cameroon and Gabon. Most of the previous studies weremostly based on geochemical and palynological proxies (e.g.Gasse and van Campo, 1994; Barker etal ., 2001; Schefuß etal .,2005; Weldeab et al ., 2007).

JOURNAL OF QUATERNARY SCIENCE (2010)25(3) 267–279Copyright ß 2009 John Wiley & Sons, Ltd.Published online 30 September 2009 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/jqs.1306

* Correspondence to: A. C. Itambi, Center for Marine Environmental Sciences,University of Bremen, Leobener Strasse, D-28359 Bremen, Germany.E-mail: [email protected]

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Rock magnetic properties can trace contents and character-istics of environmentally sensitive magnetic minerals in rocksand sediments even at very low concentrations and haveevolved into recognised tools in palaeoclimatic and environ-mental studies since their inception over two decades ago(Thompson et al ., 1980; Thompson and Oldeld, 1986).Amongst many others, studies by Bloemendal et al . (1992),Verosub and Roberts (1995), Frederichs et al . (1999) and Peters

and Dekkers (2003) have demonstrated the potential of environmental magnetism to discriminate sediments bylithology, origin, depositional and post-depositional processesfrom their magnetic mineralogy and grain size. Vigliotti et al .(1999)and Funk etal . (2004) showed that a combined magneticand geochemical analysis can identify diagenetic alterations.Elemental abundances from high-resolution X-ray uorescence(XRF) scanning provide ratios representing climatically drivenvariations of terrigenous and biogenic sources (Arz et al ., 1998; Jansen et al ., 1998). High-resolution colour reectance datahave been used successfully in palaeoclimatic studies tounravel millennial-scale changes, e.g. in the Cariaco basin(Peterson et al ., 2000). In this palaeoclimatic study, wetherefore combine these three methods and their potential todraw conclusions on provenance and transport issues.

Study area

Our study area is the easternmost Equatorial Atlantic within theconnes of the Gulf of Guinea, which runs from the west coastof Ivory Coast to the Gabon estuary (Fig. 1). Seasonal insolationchanges result in two prominent seasons, i.e. dry and rainyseasons.Variation in the WestAfrican monsoon over thisregionis known to produce signicant changes in the atmosphericcirculation controlling annual rainfall, moisture, temperature

and wind (Baldi et al ., 2003). Strong solar radiation in borealsummer heats the landmass, creating a region of low pressure – the Intertropical Convergence Zone (ITCZ) – where air rich inwater vapour ows in from the surroundingocean, contributingto the monsoon rainfall (Ruddiman, 2001). Its location variesseasonally and determines the latitudinal distribution of moisture and rainfall. Strong SE trade winds during borealsummer move the ITCZ to about 18–20 8 N and strong NE trades

push the northern limit close to the equator (3–58

N) in winter.Over land the ITCZ extends farther north or south than over theoceans owing to the seasonal variation in land temperatures.Thestrength of themonsoon has been related to periodicorbitalchanges in summer insolation (Kutzbach, 1981) as well asvegetation cover (Kutzbach, 1996; Brovkin et al ., 1998).

The Gulf of Guinea deposits are composed of a 4000 m thicksequence of Cretaceous, Tertiary and Quaternary sedimentsdeposited via aeolian and riverine pathways. Strong harmattansurface winds (NE trades) transport Saharan dust rich in organicand clastic sediments into the ocean (Westerhausen et al .,1993; Romero et al ., 1999). A recent study by Stuut et al . (2005)traced aeolian dust deposition rich in iron, aluminium, smectiteand titanium from the Sahara Desert to the eastern equatorialAtlantic. These dust particles range in size between 10 and40 mm and are washed out by enhanced precipitation withinand slightly south of the ITCZ boundary.

Numerous rivers such as the Volta, Niger and Congodischarge into the gulf, bringing along vast amounts of terrigenous sediments. The Congo River is the second mostvoluminous river in the world, discharging about 1.5 millioncubic feet of water per second and 40 Mt a À1 of terrigenousmaterials into the ocean (Gaillardet et al ., 1999). The NigerRiver also has a large watershed ( $ 2.3 Â 10 6 km2 ), drainingregions with vastly different climatic conditions. Westerhausenet al . (1993) estimated that > 60% of the total organic carbonaccumulated on the shelf off eastern Liberia, Ivory Coast and

Figure 1 Location map showing the position of the two investigated cores (large dots) and the northern winter boundary of the ITCZ (dash line). Thearrows indicate surface and subsurface currents. Guinea Current (GC), Southern Equatorial current (SEC) and Equatorial Undercurrent (EUC). Thesmaller dots indicate two African lakes whose records are presented in this study. This gure is available in colour online at www.interscience.wiley.com/journal/jqs

Copyright ß 2009 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 25(3) 267–279 (2010)DOI: 10.1002/jqs

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Gabon is of terrigenous origin. Upwelling regions along thewest coast of Africa and Equatorial Atlantic as well as inux of nutrient-rich fresh water boost productivity (Binet and Marchal,1993), increasing the deposition of biogenic matter.

The surface and subsurfacehydrographyof the South Atlantichas been comprehensively described by Peterson and Stramma(1991). The hydrographic regime of the Gulf of Guinea isdominated by the westward owing North and South Equatorial

Currents (NEC and SEC, respectively) and the South EquatorialCountercurrent (SECC) owing eastward between 3 8 Nand2 8 Sat a depth of 100 m. Also owing into the Gulf of Guinea is theeastward-owing shallow water mass known as the GuineaCurrents.

Materials

Two gravity cores recovered along an N–S transect on theeastern Gulf of Guinea during RV Meteor cruise M41/1 (Schulzet al ., 1998) are investigated here: GeoB 4905-4 off Cameroon(28 30.0 0 N, 9 8 23.4 0 E, water depth 1328 m, length 12.18 m)and GeoB 4906-3 off Gabon (0 8 44.4 0 S, 088 22.6 0 E, waterdepth 1274 m, length 12.36 m) (Fig. 1). These cores wereselected based on their latitudinal positions. The rstcore lies inclose proximity to the winter northern position of the ITCZ andhence can potentially record changes in aeolian input, whereasthe southern core is constantly under the inuence of the WestAfrican monsoon and therefore expected to be dominated byuvial input. By studying these strategically placed cores,

possible shifts in the ITCZ position and the southern extent of thearid condition can be identied. The cores consist of diatomand nannofossil oozes with silica-bearing nannofossil oozepredominant in the northern core, while diatom oozedominates the southern location. Bioturbation of varyingdegree is indicated by worm borrows lled with faecal pelletsvisible in some layers.

Methods

Chronostratigraphy

Adegbie et al . (2003) established an age model for the coreGeoB 4905-4 using 14 C ages and correlation of d 18 O record toGISP2 record. This chronology was used to construct an age– depth relationship for GeoB 4906-3 by direct correlation of both Ca records (Fig. 2). Intermediate ages were generated bylinear interpolation between tie-points. Other independentcorrelating parameters (anhysteretic remanent magnetisation(ARM), isothermal remanent magnetisation (IRM) and porosity)were used as checks on these points by going through eachparameter after assigning a tie-point.

The chronology of our sediment cores provides a 45 karecord (GeoB4906–3) and 52 ka (GeoB4905–4, Adegbie etal .,2003) spanning the last glacial and interglacial events, i.e.Marine Isotope Stage (MIS) 1, 2 and late 3. The averagesedimentation rate at this location was 28 cm ka À1 .

Figure 2 Correlation tie-points of Ca records used to transfer the age model of core GeoB 4905–5 established by Adegbie et al . (2003) to core GeoB4906–3. This gure is available in colour online at www.interscience.wiley.com/journal/jqs


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Rock magnetism

Low-eld magnetic susceptibility ( k ) was measured on thearchive halves of the sediment cores at a resolution of 1 cmusing an automatic core scanner with a Bartington InstrumentsMS2F spot sensor, which allowed us to measure backgroundvalues after each point measurement to correct for instrumentaldrifts. This parameter measures primarily magnetite concen-

tration (Thompson et al ., 1980; Verosub and Roberts, 1995).A range of more specialised rock magnetic measurementswere performed on discrete 6.2cm 3 samples taken at 5 cmintervals for GeoB 4906-3 and 10 cm for GeoB 4905-4. Dual-frequency (470 Hz and 4700 Hz) susceptibility measurementswith a Bartington Instruments MS2B unit was used to infer thecontribution of ultra-ne superparamagnetic (SP) particles tothe susceptibility signal using the diagnostic frequency-dependent susceptibility k fd% (Dearing et al ., 1996).

IRM was imparted at 23 incremental steps up to a 700 mTeld in an automated pass-through 2G Enterprises cryogenicmagnetometer 755 R at the University of Bremen. The IRM atthis maximum eld was considered as the saturation remanentmagnetisation (SIRM) and together with 300mT IRM wasused in calculating the S ratio given by the equation (Kingand Channel, 1991; Bloemendal et al ., 1992; Maher andThompson, 1999)

S 0: 3T ¼IRM0: 3T

SIRMThis index reects the relative proportion of the high-coercivityminerals (antiferromagnets haematite and goethite) mainly tolow-coercive (Ti-)magnetite. However, only the haematite andmagnetite fraction could be inferred in this study owing to thelimitation of the saturation eld used. Ferrimagnetic mineralshave S À0.3T close to 1; the ratio decreases with an increasingantiferromagnetic content.

Using the same instrument, ARM was imparted in thelaboratory by a 100 mT alternating peak eld and a 40 mT DCbiasing eld. The ARM was subsequently alternating eld (AF)demagnetised at increments of 5 mT from 5 to 50 mT, andincrements of 10mT from 60 to 100 mT. Anhystereticsusceptibility (k arm ) was generated by normalising the ARMwith the DC biasing eld. ARM is used as grain size andconcentration indicator of submicron magnetite (King et al .,1982; Thompson and Oldeld, 1986; Oldeld and Yu, 1994).

X-ray uorescence

Relative elemental abundances for elements from potassium tostrontium were obtained at 2 cm resolution using a CORTEXXRF scannerat theODP Core Repository Center in Bremen.Theacquired data were processed using Kevex software, givingelement concentrations as counts per second (cps). Jansen et al .(1998) and Rohl and Abrams (2000) provide a detaileddescription of the instrument and measurement. Only theelements Fe, Ti and Ca were used in this study.

Colour spectroscopy

A Geotek multi-sensor core logger (MSCL) mounted with adigital Geoscan II camera was used for acquiring colourreectance for the split cores. The cores were cleaned tosmooth the surface and remove any oxidised layer beforemeasurement. Line scanning was performed at a high

resolution (0.01 mm) and averaged over 1 cm intervals.Sediment colour reectance is closely correlated with thepigmented antiferromagnetic minerals haematite and goethitewhich are typically contained in dust and give sediments areddish-brown appearance (Balsam et al ., 1995). The reec-tance is also affected by carbonate and organicmatter contents.High carbonate content results in higher reectance, whereasorganic contents lower the reectance (e.g. Mix et al ., 1995;

Balsam et al ., 1999).

Results

Environmental magnetism

Primary and secondary signals

Secondary signals resulting from post-depositional diagenesisand authigenesis can seriously compromise the palaeoenvir-onmental interpretation of rock magnetic signals (e.g. Karlin

et al ., 1987). In order to discriminate between pristine andoverprinted records, biplots of the magnetic parameters k arm vs.k (King et al ., 1982) and S À0.3T vs. k arm / k were examined.

k arm vs. k (Fig. 3(a) and (b)) shows the dependence of magnetite concentration and grain size. Grain size variationsare indicated by changes in slope, while a change along a lineof constant slope is indicative of varying magnetite concen-trations.The two plots show that all thesamples can be groupedinto three well-dened magnetozones (A, B and C, Fig. 3). Theconcentration of ne-grained single-domain (SD) magnetite ishighest in the rst group of samples (A), with k and k arm valuesgreater than 80 Â 10À6 SI and 4000 Â mSI, respectively. Thesusceptibility values are more constant in group B, while thek arm varies. Group C is characterised by very low susceptibilityand remanence approaching zero.

At site GeoB 4905–4 (Fig. 3(a)), magnetozone A ischaracterised by a broad dispersal of the data points withoutany preferential alignment along a gradient line. In GeoB4906– 3 (Fig. 3(b)), there is a wide variation in concentration but alongthe same gradient. The two cores exhibit similar trends in bothgroups B and C.

The S À0.3T vs. k arm / k plots combining haematite/magnetiteratio and magnetite grain size (Fig. 3(c) and (d)) also separateinto three similar groups. Magnetozone A represents areaswhere ne-grained magnetite strongly contributes to thesignals. In core GeoB 4905-4 this zone is less extensive inthe grain size parameter. The wider distribution in GeoB 4906-

3 (k arm / k between 50 and 150 units (Fig. 3(d)) indicates largermagnetite grain size variations.

Down-core proles of magnetic parameters

Variations in the concentration, mineralogy and grain sizes of the magnetic minerals are shown in Fig. 4. At the northernlocation (GeoB 4905-4) the magnetic mineral concentrationsare higher, with magnetic susceptibility ranging between 40and 200 Â 10À6 SI. The ARM which represents the concen-tration of ne-grain magnetite also varies signicantly down-core, with a higher frequency variation than observed in thesusceptibility record. Because of the strong correlation in theARM and IRM records, only the ARM is shown here, andtherefore also represents the concentration and variation of remanence-carrying magnetic particles. Highest values in the



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magnetic susceptibility occur at the top of the core (0–2 ka).This, however, is not reected in the ARM records. Anexplanation for this discrepancy would be that the top of thiscore contains superparamagnetic minerals which have thepotential to increase the magnetic susceptibility but not beingable to acquire remanence. This reasoning was supported bythe frequency dependence susceptibility, which is not shownhere because of the high noise levels in much of the core. Itdisplayed a high value of up to 8% at this interval.

An interval (40–43 ka) showing minima in magnetic mineralconcentrations occur and corresponds to B and C clusters onthe bivariate plots of Fig. 3. Apart from the interval withmagnetite dissolution, the most signicant changes in themagnetic mineral concentrations are observed during the last18 ka. At this age the concentration increases sharply, butstarted decreasing soon after, at an interval corresponding tothe Bølling–Allerød warm phase. This warming, interrupted bythe cold Younger Dryas (YD) (12.5ka), continued at 11 ka andinto the middle Holocene, where concentration remained verylow. However, since the middle Holocene input of magneticparticles has steadily increased.

GeoB 4906-3, on theother hand, haslower input of magneticminerals ( k ranging between 40 and 120 Â 10À6 SI) (Fig. 4). Thesusceptibility and ARM show a good down-core correlation forthis core. There has been a gradual increase in theconcentration since ca. 6 ka, with a minor decrease at 5 ka.Unlike in the northern core, GeoB 4906-3 hastwo intervals thatappear to have been affected by diagenesis, occurring between28–37 ka and 40–43 ka.

The magnetic minerals dominant in our cores are determinedby the S ratio, in which values closer to 1 indicate low-

coercivity mineral (e.g., magnetite) and values much less than 1indicate high-coercivity minerals such as haematite. The S ratioranges between 0.92 and $ 0.99 at the northern location (GeoB4905-4), with most of the prole $ 0.99, implying that thesesediments are mostly enriched in magnetite. However, thepresence of residual haematite is conrmed at the intervals withS À0.3T < 0.98 due to the preferential depletion of magnetite. InGeoB 4906-3, the S ratio is a bit lower, ranging between 0.88and $ 0.98. The entire core, apart from the two reduced layers,hasvalues of $ 0.98and thereforealsodominated by magnetite.Diagenesis seems to have been more severe at this location,especially at the interval between 40 and 43 ka, where the S ratio is $ 0.88.

With S ratio indicating dominance of magnetite to theconcentration, the grain size of this mineral falls within the SDrange. This is supported by the ARM, which correlates to the S ratio and is considered a recorder of the concentration of SD orne–grained magnetite. Another grain size parameter ( k arm / k )follows a similar trend to S 0.3T . Its down-hole variation is morepronounced, showing a coarser assemblage at the northernlocation especially during the last glacial (MIS 2). The reducedlayers are coarser because of preferential dissolution of nerparticles during reductive diagenesis.

Elemental abundances

Proles of elemental abundances (Fig. 5) show signicant varia-tions in sediment input. Three elemental records representing

Figure 3 Diagnostic biplots of magnetic parameters indicating three magnetozones. Cluster A represents pristine conditions, cluster B a transitionzone with partially reduced magnetite and cluster C pervasive magnetite dissolution. This gure is available in colour online at www.interscience.wiley.com/journal/jqs



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terrigenous input (Fe and Ti) and marine productivity (Ca – carbonate content) are considered in this study.

The terrigenous signals differ at the two locations. In GeoB4905–4, Fe and Ti show little variation from the bottom of thecore until ca. 30 ka. This represents MIS 3, a relatively warmperiod. However, in MIS 2 (ca. 11–30 ka) we observe verysignicant variations in the records, with difference inamplitude between the two elements. Ti show high amplitudevariation compared to Fe. The interval between 0 and 11 ka(MIS 1) also shows some variation in the records. It is worthnoting, however, that there are no data between 6 and 9 ka.

At thesouthern location (GeoB4906–3), the patterns are verydifferent. There is a slight decrease in the Fe and Ti intensitiesfrom the late part of MIS 3 (i.e. from the core bottom) to 30 ka.MIS 2 remains fairly constant, with little variation in theintensities of Fe and Ti. This value remains fairly constant for Tiup to the present except for a signicant drop (also observed inFe) occurring at 12.5 ka. For Fe, its intensity increases steadilyafter 20 ka until 12.5 ka, where it drops and again rapidlyincreases at about 11.5 ka, dropping in intensity only at 5.5 ka.Since then, there has been a steady increase, with a tiny drop at2.5ka. In GeoB 4905–4, theintensity of themarine productivitysignal Ca is much less than in GeoB 4906–3, with valuesranging from 200 to1000 cps and from 400 to 2000 cps,respectively. GeoB 4905–4 shows high amplitude and

frequency variation. At 16 ka, both cores show an abrupt dropin Ca intensity. The decreasing trend is reversed at YD. After theYD, Ca drops to its lowest values between 11 and 5 ka, a periodcorresponding to the African Humid Period (AHP). Since thelate Holocene until present, Ca has remained very low.

The terrigenous and biogenic recordsat the northern locationappear to correlate positively with each other more especiallyduring theMIS 2 whereas, they anti-correlate at MIS 3. Fe andTiin thesouthern core, on the otherhand, both anti-correlate withCa throughout the core.

Colour reectance

As has been observed in the magnetic and elemental records,the colour reectance (Fig. 6) also shows down-core variationsthat differ at the two locations. The northern core (GeoB 4905-4) portrays a higher degree of variability, with the top 15 kaindicating climate variation at millennial timescales. Thereectance is also higher at this location, with up to 60% totalreectance. A broad peak of high reectance occurs between25 and 30 ka, while there is a signicant minimum ( $ 35%reectance) at 7.5 ka.

Figure 4 Compilation of environmental magnetic records of cores GeoB 4905–5 (lighter) and GeoB 4906–3 (darker). The grey band represents theAHP, while the dark grey bands shows intervals severely affected by reductive diagenesis of iron oxides. This gure is available in colour online atwww.interscience.wiley.com/journal/jqs



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Figure 5 Compilation of elementand colour reectance records of cores GeoB4905–5(lighter)and GeoB4906–3(darker). Fe/ k nd is also a diagenesisproxy.The grey band is theAHP,whilethe shading indicatestransition from glacial to interglacial conditions.This gureis available in colour onlineatwww.interscience.wiley.com/journal/jqs

Figure 6 Colour reectance for the two cores, showing a higher variability at the northern location. This gure is available in colour online atwww.interscience.wiley.com/journal/jqs



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Core GeoB 4906–3, on the other hand, has little variationand with much lower reectance values that range between25% and 45%. A prominent minimum also corresponding tominima in the Fe and Ti records occurs at 12.5 ka.

Discussion

Diagenesis

Studies such as Karlin and Levi (1983) and Leslie et al . (1990)have shown that primary iron oxides can be dissolved throughreductive diagenesis. In a reducing, typically sulphidicenvironment the ne-grained magnetite is preferentiallydissolved, leaving behind the more resistant coarser fraction.In addition, haematite and goethite are more resistant toreductive dissolution and therefore less depleted in diagene-tically affected sediments (e.g. Bloemendal et al ., 1992). Highhaematite and goethite (lower S À0.3T ), low susceptibility,coarse-grain magnetite and low remanence therefore indicatediagenesis. From our cross-plots (Fig. 3) we can identify suchintervals by very low k and k arm inthe k arm vs. k plot. Coincidinglow S À0.3T , k and coarser grain size (low k arm / k ) in the S À0.3T vs.k arm / k plot conrm magnetite dissolution (Bloemendal et al .,1992). We therefore suggest that magnetozone A representsvariation in the concentration of detrital magnetite of primaryorigin. Magnetozone B is a zone of partial depletion, whilezone C is fully depleted.

Further conrmation of post-depositional modication of thesediment composition was derived using the diagenetic proxyFe/ k nd (Funk et al ., 2004) (Fig. 5). Our observations point to asignicant degree of magnetite alteration at the grey shaded

bands identied in the magnetic parameters (Fig. 4) charac-terised by lack of remanence and low susceptibility. Thesebands contain high Fe mineral as depicted in the Fe/ k nd (Fig. 5),but show no corresponding increase in magnetic susceptibility.During reductive diagenesis ne-grained magnetite is reduced,leading to the precipitation of non-magnetic iron phases suchaspyrite. At both locations diagenetic effects are detectedbetween 40 and 44 ka. Two reasons could account for thisoccurrence. The rst is signicant input of organic matter intothe sediments and/or high sedimentation rates. Secondly, thislayer depicting sever diagenesis may represent the sulphatemethane transition (SMT), which is the interval where diffusingsulphur frombottom water meets upward migrating methane inthe sediment column, leading to the oxidation of methane. Ithas been shown (Garming et al ., 2005) that stagnation of theSMT severely alters the magnetic signal by the destruction of magnetite due to anaerobic oxidation of methane. Highproductivity and availability of organic matter provide athriving environment for microphylic bacteria which breakdown methane to H 2 S (Froelic et al ., 1979). This then reactswith (oxyhydr)oxides, resulting in the formation of non-magnetic iron sulphides (Karlin and Levi, 1983).

Most of the samples fall within the ne-grain threshold,suggesting that the sediments were deposited in a low-energyenvironment. Although the study area lies in close proximity tothe volcanicchain of islands along the Cameroon volcanic line,no signicant evidence was seen that suggests accumulation of volcanicmaterial here over the last 52 ka. This is either becausethese islands might have stopped erupting before 52 ka or thevolcano-clastic materials were being deposited in anotherregion.

Provenance of terrigenous and marine sediments

The variation of terrigenous vs. biogenic (carbonate) accumu-lation is depicted by the Fe/Ca and Fe/Ti ratios (Fig. 7), wherehigh values signify enhanced terrigenous sedimentation andenhanced uvial input, respectively. The Fe/Ca ratio shows avery high degree of variability in terrigenous input especially atthe interglacial periods MIS 1 and 3. These signatures are

closely correlated to Northern Hemisphere ice core record(GISP).A dramatic change in the records occurs at ca. 16 ka, where

there is a signicant increase in the terrigenous component aftera relatively stable period dominated by biogenic sedimentationduring MIS 2. Several authors, e.g. Schaefer et al ., 2006, andWeijers et al ., 2007, identied similar climatic transitionsaround this age and linked it to the onset of the lastdeglaciation. The onset of the last AHP has also been estimatedto coincide with this age (Maley and Brenac, 1998; deMenocalet al ., 2000a). Tropical African temperatures are reported tohave signicantly increased during the last deglaciation morethan tropical ocean temperatures, leading to a high-tempera-ture gradient that resulted in enhanced precipitation overCentral Africa (Weijers et al ., 2007). Input of terrigenoussediments was therefore due to precipitation-enhanced weath-ering, erosion and river runoff. This wet period was brieyinterrupted between 13 and 11.5 ka, corresponding to theYD, arelatively dry phase in the Northern Hemisphere.

A return to wetter conditions followed the YD, marking theonset of the Holocene (Garcin et al ., 2007; Weldeab et al .,2007) and also the peak of the humid period. The AHP is theproduct of the orbital precessional cycles controlling tropicalAfrican climate (Kutzbach, 1981; deMenocal et al ., 1993,2000a). Summer insolation intensies at precession maxima,strengthening the African monsoon and resulting in intenseprecipitation and increased vegetation cover over most of

North Africa (Sarnthein, 1978; COHMAP members, 1988).Model studies by Prell and Kutzbach (1987) showed that asummer insolation 8% greater than today results in a 40%increase in precipitation. The last insolation maximumoccurred between 9–10 ka and had its impact well imprintedin the sediments from the eastern equatorial Atlantic, wherehigh terrigenous input and low marine productivity persisted inthe Gulf of Guinea (Fe/Ca, Fig. 7).

In contrast to the Fe content variation that reects terrigenousinput, carbonate content, which indicates primary productivity,dominates the dry periods. This distinction is most conspicuousat the northern location offshore from Cameroon, where Fe andCa show positive correlation at MIS 2. Fe and Ti correlatepositively and are frequently but not systematically opposed toCa intensity in the rest of the cores. Zabel et al . (2001) andAdegbie et al . (2003) have interpreted these anti-correlatedsignals as mutual dilution of marine input by terrigenoussediments. However, the positive correlation of Fe and Ca inthenorthern core is evidence of a third component that controlsthe signal. We therefore suggest the presence of biogenic opal(diatoms) in this sediments and their abundance could berelated to the input of aeolian transported silica during dryperiods. Dissolution of calcite can also mask the productivityinterpretation. However, this seems not the case at our studyarea since the lysocline is located between 4700 and 4900 mwater depth in deep eastern Atlantic (Biscaye et al ., 1976;Thunnel, 1982), much deeper than the $ 1300 m water depthsat which the studied cores were recovered. The intensicationin marine productivity is related to upwelling which is stronglyassociated with the wind system (Voituriez, 1981). Stronger NEtrades persisted during dry periods over North Africa (Verardo



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and McIntyre, 1994), upwelling nutrient-rich waters thatboosted productivity.The terrigenous fraction is transported both via uvial and

aeolian pathways. Sea-level lowstands during glacial periodsexposed the shelves of the West African continental margin,leading to sediment erosion and export to the ocean. At thesame time, stronger winds and drier conditions prevailed,eroding, transporting and depositing aeolian dust from theSahel into the ocean. Increased surface and river runoff due toprecipitation also enhances input.

To distinguish between uvial and aeolian contributions, weuse the ratio Fe/Ti. Fe is mostly transported into this region viathe many rivers existing (e.g. Sanaga and Ougoue). It thereforevaries positively with hydrological changes. Ti is a goodindicator for Saharan dust and has been used by several authorsto trace such dust in marine sediments (Balsam et al ., 1995;Zabel et al ., 2001; Itambi et al ., 2009). Loess tends toconcentrate heavy minerals, and is therefore enriched in higheld-strength elements, such as Ti. From our records, it isevident that uvial input is higher during warm periods whenprecipitation is higher. In contrast, dry periods such as MIS2 and the YD see an increase in aeolian dust. This variabilityis strongly imprinted at the northernmost location, which issituated closer to the southern boundary of the Sahara dustplume and therefore potentially registers both aeolian anduvial input. The wide distribution of samples in A on the k arm

vs. k plot of GeoB 4905-4 (Fig. 3) without any preferentialgradient line strongly conrms that different sources (aeoliandust and uvial sediments) contribute to sediments deposited atthis location, whereas for the southern location only uvialinput in signicant, as indicated by the samples plotting along aline of single gradient.

Late Holocene millennial–scale climatevariability

Our sediments originating from the Central African regiondocument a signicant degree of climate variation during theHolocene, similar in character to those observed in ice corerecords at higher latitudes in the Northern Hemisphere (Schulzand Paul, 2002). The Holocene record is characterised bymillennial- to sub-millennial-scale climate oscillations thatrange from hundreds of years to a few thousands years (Fig. 8).Our sampling resolution corresponds to 40 a for colourintensity and 80 a for element data, resolving sub-millennial– scale climate changes. In core GeoB 4905-4 thelast $ 5.5 kaarecharacterised by high-amplitude decadal- to millennial-scaleclimate changes in reectance and Fe/Ti (Fig. 7). The strongpositive correlation observed between the two parametersfurther conrms that there is a strong aeolian signatureindicating drought or reduced precipitation. Reddened desertsands have previously been used to trace the contribution of aeolian dust in marine sediments (Diester-Hass, 1976;Sarnthein et al ., 1981). Such pigmented sediments have beenreported (Kalu, 1979; Sarnthein and Koopmann, 1979) tooriginate from the zone of ferralitic soils in the Sahel region andsouth of the Sahara.

Short-term changes in hydrology during the last 16 ka similarto those observed in our marine records have been reportedfrom several African lacustrine sediment records: LakeBosumtwi in West Africa (e.g. Talbot et al ., 1984; Street-Perrott and Perrott, 1990; Shanahan etal ., 2006), lakes BarombiMbo, Bambili and Ossa in Cameroon, Central Africa (Giresseet al ., 2005; Stager and Anfang-Sutter, 1999) and East Africa(Barker et al ., 2001; Gasse, 2000).

Figure 7 Diagnostic ratios of environmental conditions: Fe/Ca indicates the variation in terrigenous vs. biogenic fraction. Fe/Ti is a proxy fordetermining Saharan dust uxes, with lower values (high Ti) being indicative of higher aeolian input. This gure is available in colour online atwww.interscience.wiley.com/journal/jqs



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TheFe/Ti andreectance records show that the northern coreGeoB 4905-4 is most suitable to register short changes inprecipitation over Central Africa. Seasonality changes (dry andrainy seasons) are much stronger to the north of Central Africa.The dry season becomes lengthy northward, whereas the rainyseason becomes more dominant to the south. Because thenorthern boundary of the winter position of the ITCZ and thesouthern limit of aeolian dust deposition are in close proximity,this core is well positioned to register dust and precipitationchanges. The southern core GeoB 4906–3 receives terrigenoussediments from forested regions with signicant rainfallthroughout the year, maintaining more humid conditions evenduring dry periods. This therefore masks the signals of anypossible aoelian dust that might have been deposited at thislocation.

The occurrence of millennial-scale oscillations superim-posed on the AHP and throughout the late Holocene rules outthe 23 ka precessional changes in solar insolation as the soledriving force behind precipitation changes over centralAfrica. Previous studies such as Broecker (2003), Ganopolskiand Rahmstorf (2001) and Hemming (2004) showed thatmillennial-scale climate changes could be related to theshutdown of the deep-water circulation during pulses of freshwater input into the north Atlantic. However, this does notexplain the signicant variations observed in our recordsduringthe last 5.5 ka as sea level has not changed much during thisperiod (Fairbanks, 1989; Perrott and Perrott, 1990). Byanalysing the pacing of D-O events at the 1470 a climaticcycle in the Greenland ice core record, Schulz (2002) alsoargued that these millennial-scale events were not primarily

Figure 8 Correlation of marine records to lacustrine signals from Lake Ossa and Bosumtwi (Giresse et al ., 2005; Shanahan et al ., 2006) and runningmean of GISP2 ice core record (Grootes and Stuiver, 1997). The records show signicant variation in aridity (grey shaded bars), especially during thelate Holocene. This gure is available in colour online at www.interscience.wiley.com/journal/jqs



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controlled by the rate of North Atlantic Deep Water formation.Schulz and Paul (2002) reported a 900 a climatic signal in theNorth Atlantic at the Holocene. They attributed this to internalperturbations in the climate system caused by major volcaniceruptions or changes in major atmospheric circulation modes.Others (e.g. van Geel et al ., 1999) have proposed a high-frequency variation in the climate system to solar outputvariability. In the tropical Atlantic, where changes in summer

insolation control the West African monsoon and henceprecipitation, the later forcing (solar radiation) may be the mostprobable cause of the observed millennial-scale variationsduring the last 5.5 ka.

The frequency of variability in our records during the last 5– 6 ka closely resembles the 900 cycles (Schulz and Paul, 2002)of d 18 O of Greenland ice core, thus suggesting a link betweenCentral African and high-latitude climate. Following thetermination of the AHP at 5.5ka, drier conditions set in andhave persisted until the present. Cold, dry air blows from theNorthern Hemisphere high latitudes towards the Tropics,leading to the southward migration of the ITCZ and hence thesubsequent drying of the Sudano/Sahelian regions. This relativedry period, which has been widely reported (Moeyersons,1997; Zogning et al ., 1997; Vincens et al ., 1999) to havepeaked between 2.8 and 1.1 ka, also appears in our records.The period has, however, been interrupted by wet periods.Short-term changes in solar radiation could have reorganisedthe atmospheric circulation, increasing the land–sea tempera-ture gradient and resulting in the increase in precipitation onthe African continent. This assertion is supported by Weijerset al . (2007), who linked the land–sea temperature gradient toprecipitation changes in Central Africa.

Conclusions

Combining high-resolution rock magnetic, element andreectance measurements has been an effective strategy toreconstruct the palaeoclimate and sedimentary environment of the Gulf of Guinea and to decipher in detail the depositionaland post-depositional processes of the last52 ka. The sedimentsof this region have been affected in two sections of MIS 3 byiron diagenesis at varying intensities, depending on the organiccarbon supply and bottom-water oxygenation. However,most of our record remain unaffected and therefore recordsprimary climate signals of the adjacent continent. Our resultsdemonstrate that the African climate varied during theHolocene with several intervals of aridity, consistent withonshore lake records and demonstrating a close similarity withGreenland ice records, hence conrming the close linkbetween the climate systems of the two regions at suchmillennial timescales. This variability demonstrates andsupports the argument that the Central African climatevariability is controlled not only by precessional changes ininsolation, but also by factors such as solar radiation andreorganisation of atmospheric circulation, possibly driven byinternal changes in the climate system which may occur atshorter timescales.

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