Abrupt Shifts of the Sahara-Sahel Boundary During Heinrich Stadials J

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Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | N ◦ , Climate 3 of the Past Discussions N) during Heinrich ◦ , J. Hartmann 1 , M. Zabel 2 120 119 N in the modern-day African Sahel are tes- ◦ , D. Heslop 1 , S. Mulitza 1 1 , A. Govin culties of dating dune formation mean that abrupt millennial-scale climate 1 ffi , and G. Wefer 1 ¨ ohl This discussion paper is/has been under review for the journal Climate of the Past (CP). Please refer to the corresponding final paper in CP if available. MARUM – Center for Marine Environmental Sciences and Faculty ofResearch Geosciences, School of Earth Sciences, The Australian National University, Institute for Biogeochemistry and Marine Chemistry, University of Hamburg, 1 University of Bremen,2 28359 Bremen, Germany Canberra ACT 0200,3 Australia Bundesstraße 55, 20146 Hamburg, Germany Received: 14 December 2012 – Accepted: 20Correspondence December to: 2012 J. – A. Published: Collins 4 ([email protected]) JanuaryPublished 2013 by Copernicus Publications on behalf of the European Geosciences Union. back (e.g. Rosenfeld et al., 2001). U. R global atmospheric dust loading. 1 Introduction Understanding the extent and causesSahara of desert abrupt millennial-timescale is expansions importantcroachment of the causes for major the problems populationsthe (FAO, 2010). Saharan of dust In the plume addition, is Sahel constraining important region, the for size where determining the of dune role en- of dust as a climate feed- on the continent, ratherwas than closely at linked the to LastStadials Glacial North triggered Maximum. Atlantic abrupt We sea increases find of surfacenew that aridity temperatures, dust SSB and which sources. position wind during This strengthturning result Heinrich in circulation the illustrates on Sahel, the the southerly exposing influence extent of of the the Sahara Atlantic desert and meridional has over- implications for have identified Heinrich Stadials asstudies the dustiest have periods mapped of the theWe last spatio-temporal use glacial, evolution the although of major-element no the dust composition spatial export extent of of four from Saharan-dust marine versus Westfrom river-sediment West sediment input Africa. Africa to cores over the the to continental lastcomposition margin reconstruct 60 corresponding ka. This to allows us thethe to Sahara-Sahel Sahara-Sahel map boundary the boundary. reached position Our its ofStadials, most records the suggesting southerly sediment indicate that position (13 these that were the periods when the sand dunes formed at 14 Relict dune fieldstament that to are equatorward found expansions atHowever, di of 14 the Sahara desertevents are during not the always late resolved Pleistocene. in these records. High-resolution marine core studies Abstract Abrupt shifts of the Sahara-Sahel boundary during Heinrich Stadials J. A. Collins Clim. Past Discuss., 9, 119–142, 2013 www.clim-past-discuss.net/9/119/2013/ doi:10.5194/cpd-9-119-2013 © Author(s) 2013. CC Attribution 3.0 License. 5 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | N ◦ south N and N–5 ◦ ◦ ◦ 5 ∼ latitude at the Last Glacial N in the Western Sahara), ◦ ◦ 5 ´ ezine, 1989). However, these 19 ∼ ∼ N (Fig. 1b). As such, this results in ◦ 20 ∼ 122 121 erent methods prevents a direct comparison of ff N) we are able to quantify the relative contribution N (Grove, 1958; Michel, 1973). This is ◦ ◦ 14 N–9 N, centred at erence between river sediment load and dust in terms ∼ ◦ ff ◦ West Africa receives terrigenous material in the form of ff N–15 ◦ 5calka BP; Swezey, 2001). 40ka; Servant, 1983), a more extensive “Ogolian” phase (24–12 ka; < > N (e.g. Jullien et al., 2007; Mulitza et al., 2008; Tjallingii et al., 2008; Itambi et al., ◦ The main source of windblown dust is the Sahara, with a small contribution from the Using major-element composition of sediments from four marine cores spanning the Marine sediment cores can provide high-resolution and continuous records of mate- Continental pollen records have also been used to reconstruct the continental SSB Relict dune fields in the modern-day Sahel indicate that during the late Pleistocene, of their proportion of Ti. trained into the Saharansome Air reaching Layer the by convective Caribbeangions, which, summer (Prospero for storms the et major and source al.,the travels regions, Pleistocene 1981). have further, (deMenocal, remained 1995). Dust relatively Therefore, arid compared originatesatively during to enriched most from river-sediment, in of dust Si arid is and rel- spans re- K approximately (Table S1 25 in thea Supplement). Si-rich At dust the source coast, located thethese to dust samples, the plume there north is of little the Al- di and Fe-rich fluvial source region. In relatively enriched in Fe and Al (Table S1 in theSahel Supplement). (Goudie and Middleton,margin 2001). is Most blown oftrade from the winds the dust (Sarnthein delivered Mali-Mauritania et to region al., the (Fig. 1981; West Bory 1b) African and by Newton, the 2000). winter Some North-East dust is also en- 2 Setting The continental margintwo o sources: river-suspended sediment andhumid, windblown dust. high-altitude Rivers Fouta originate(Fig. from Djallon 1a). the The region soils and ofposed the enter Fouta mainly Djallon the of are deeply clays ocean chemically (Driessen weathered between and et 16 are al., com- 2001). As such, river-suspended sediment is Sahara to the Guinea coast (21 of Saharan dust versus river-sediment(North–South) to evolution the of continental dust shelf. versuscontinental We river mapped records input the of over spatial dune the last formationrecords 60 supports reflect ka. our the Comparison interpretation position with that of the the sedimentary continental SSB over the last 60 ka. tial extent of the Sahara desert during these millennial-timescale events. 2006). These studies suggested abut stronger a dust stable plume latitudinal at position the of LGM the compared desert to belt. today rial exported from the continent.12 Cores from the tropical NE2009; Atlantic between Zarriess 21 et al.,during 2011) Dansgaard-Oeschger indicate (D-O) strong stadials increases associatedStadials; in with HS). dust Heinrich However, export the Events (Heinrich from usethe relative West of magnitude Africa di of variations at each site and thus prevents mapping of the spa- evolution and infer anMaximum equatorward (LGM; displacement Dupont of records and have a Hooghiemstra, relatively low 1989;scale temporal L climate resolution and changes. do Incussed not on addition, resolve the abrupt LGM marine (Sarnthein, millennial- et sediment al., mapping 1981; Grousset studies et al., have 1998; only Hooghiemstra et fo- al., which we define as theDiester-Haass Sahara-Sahel boundary (1977) (SSB; and Fig. Kocurek 1a)determine et after the e.g. al. Sarnthein timing (1991). and of It dune1991). has formation Nonetheless, been due to certain challenging, re-mobilisation however, phases of to “Early” have dunes phase been (Kocurek ( et identified al., (Talbot,Michel, 1980), 1973; including Servant, an 1983; Lancastermid-Holocene et ( al., 2002), and a “Late” phase following the the Sahara desert during was1980). extended These further relicts south had than previouslyby today migrated (Grove, vegetation equatorwards 1958; (Sarnthein and Talbot, areSahel and now (Fig. Diester-Haass, held 1a) in as 1977). place farof They south the as are modern-day found boundary throughout of the dune formation (at 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | er ff culty of con- ffi West Africa (Fig. 1) ff N (Govin et al., 2012) ◦ N o ◦ dust). Dust% is the median + N and 3 ◦ N to 9 ◦ (river / dust × 100 124 123 = ) and downcore variations, between major climate 2 N), we performed a robust linear regression between lati- O correlation. ◦ 18 δ 3.5wt% SiO < To determine the modern-day dust% value in the sediment at the latitude of the To determine the position of the SSB over time, we linearly interpolated each of Dust% is closely correlated to the Al/Si ratios (Fig. 1c) because these elements di Commonly, major-element data are presented as elemental ratios (e.g. Mulitza et al., Sediment major-element composition was measured using the Avaatech X-Ray Flu- The North–South gradient between the two sources of material is reflected in the tude and dust% for theand surface the sediments late between 26 Holocenecertainty sections is (mean represented of as core-top 68 % to confidence 3 intervals ka) on of thethe our regression. four cores (Fig. sediment-core 1c).formed dust% a Un- robust timeseries linear regression everyto between 250 dust% calculate yr. and the latitude For and latitudegated each solved corresponding uncertainties the timestep, equation to include: we uncertainty theon on per- modern-day the modern-day the SSB SSB end-member dust% dust% and composition, uncertainty value. uncertainty on Propa- the regression for the past timesteps most strongly between the dustDowncore and Al/Si river ratios end-members are (Table also S1our given in dust% in the data. Fig. Supplement). S2 in the Supplement for comparisonSahara-Sahel with boundary (19 composition (Table S1 in theing Supplement) analysis was and nonetheless propagated integrated intoresent into the the the final data uncertainty unmix- as on dust%, where thevalue dust% dust% of estimates. the We 500 rep- bootstrap68 iterations % and confidence uncertainty intervals is (16th representedstraining and the as 84th sedimentation non-parametric percentiles). rate Because of ofnot each the estimate millennial-scale di event dust (Just accumulation et rates. al., 2012) we do the marine component (Ca andon Si; biogenic Table S1 silica in contentand the was (Collins Supplement) held et was constant estimated downcore al., (after based biogenic 2011) Mulitza opal et and is al., low the 2010). ( Theregimes measured contribution (e.g. Ca of modern-day, Si LGM concentration, from and HS1) are small. Error on the marine end-member Senegal River suspension samples (Table S1 in the Supplement). The composition of using a bootstrap withportions replacement of routine Al, (500 Si, iterations) Fe, K, based Ti on and the Ca from relative 28 pro- Sahara-Sahel dust and soil samples and 10 calibration is indicated as the fitsamples of (Fig. the S1 calibrated in scanner the data Supplement). with the discrete powder 2008; Zarriess et al., 2011). Rathertify than the interpreting elemental major ratios, element wesuspended directly data quan- material in and terms marine-derivedanalysis of material using. the Details by relative of using proportion thissentative an of analysis “dust” end-member are Saharan and given unmixing dust, “river” in river- end-member Mulitza major-element et al. compositions (2010). were Briefly, estimated repre- Mackensen, 2010; Zarriess et al.,ibrated 2011). to XRF elemental scanner concentrations elementalleast (Weltje intensities and 50 were Tjallingii, discrete cal- 2008)Core powder by 2; samples regression Mulitza from with et at eachEDP-XRF al., spectroscopy core 2008; at (Core Cores the 1; 3 MARUM, University and Bloemsma of 4; et Bremen. this al., The study), 2012; robustness which of were the quantified using 3 Methods We use 4 sediment corescovering (Table 1) the spanning gradient from 21 betweenbased dust on and published age riverand models sources. benthic (see foraminiferal Sediment Table core 1 chronology for references) is usingorescence both Core radiocarbon Scanner at MARUM,et University of al., Bremen 2012 (Cores 1 and and Core 2; Bloemsma 4; this study) and at AWI, Bremerhaven (Core 3; Zarriess and major-element composition of2012; continental Fig. 1c), margin withof surface our the sediments study lowest region. Al/Si (Govin ratios et in al., the northernmost, dust-dominated part 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ) during ◦ 9% dust 3 ± ± N 51ka (Fig. 3c). ◦ ∼ N) to 3 % at Core 4 ◦ during HS3. It reaches ◦ 3 ± N), the regression between ◦ N ◦ 9% (Fig. 1c). ± 126 125 N for the surface sediments and late Holocene at the LGM and 16 ◦ ◦ ) during the mid-Holocene and at 3 ◦ 5 ± ± N N to 3 ◦ N ◦ ◦ erences between the four records (Fig. 2b–e). However, ff O values during HS from Core 2 (Mulitza et al., 2008), sug- 18 δ N; Fig. 1c). At the latitude of the modern-day SSB (19 Over time, however, changes in North–South ocean advection of fine river material The interpolated latitudinal position of the sedimentary SSB (the 43% Dust% values in Cores 1–4 exhibit a similar evolution to each other over the last ◦ shallower and warmer water-masses. Thus, the increases in dust-like material during major control on the dust%imentation evolution processes. and The on uncertainty SSBscatter position on in rather the than the modern-day local-scale modern-day SSB sed- tion composition surface of from samples the (Fig. past 1c)contributed SSB is dune position. transferred or Finally, to dust-like increasedDiester-Haass, our material 1977; turbidite reconstruc- from Hanebuth the activity and shelfhave during modified and Henrich, the HS upper composition 2009; could of slope Henrich Westbenthic (Sarnthein have African et foraminiferal sediments. and However, al., typical deep 2010)gest water and autochthonous calcification could and deposition rather than downslope transport from the river-source regions display in-phasesediments responses. The (Fig. scatter between 1c) modern-day as may sediment be reworking, associated partitioning andplain with winnowing. some local-scale Over of time, controls such thethe on changes overall small grain may similarity ex- di size of the such 4 records suggests that a large-scale process must be the ocean advection and aggradation ofsuggests the that river sediments end-member (Tjallingii are etthe slightly al., material biased 2008). exported towards This from the the river adjacent end-member, continent. relative to are unlikely to be thethe main sediment control and on on the theanti-phase position past response of changes at the in sites SSB. dust located Ifsource. versus this within river Instead, were and material the our to in case, the cores we north positioned would of expect within the an (Cores main 2–4) river-sediment and to the north (Core 1) of 1). Nonetheless, this value isdata comparable (Tjallingii with et estimates al., of 2008), dust%and suggesting based river it on end-members is grain-size not (Tablematerial S1 due is to in generally mis-characterisation finer the of than Supplement). our dustsize (Gac Rather, dust sorting and because processes. Kane, river-derived 1986), The it low is likely dust% to values be are due to hence grain- thought to be due to lateral sediments adjacent to the dust-dominated region appear rather low (e.g. 51 % at Core section of thethe cores adjacent (Fig. continent 1c) suggests (Figs. that 1a, the b). dust However, proportion the reflects absolute input values from of dust% from HS 1, 2, 4 andits 5. northernmost It reaches position 15 (21 5 Discussion 5.1 Controls on past sediment composition The gradient in dust from 26 Bølling-Allerød (B-A; 14.7–12.9 ka) and increase11.6 during ka). the In Younger-Dryas Cores (Y-D; 12.9– 2 to 4, dust% values arevalue; lowest Fig. during the 3c) Holocene followsOver (Fig. a 2c–e). time, similar the pattern SSB-dust to value dust% reaches values its of southernmost the position individual (13 Cores 1–4. For the surface sedimentsvalues decrease and from late north Holocene to(9 south, section e.g. of from 51 the %latitude at sediment and Core dust cores, 1 yields (21 dust% a dust% value of 43% 60 ka (Fig. 2).are The strongest main in features Cores aredust 2 and increases and river in 3, dominated dust%from which regions. 60 during ka are There to HS positioned the is 1–5, LGM in also in and the Cores a these 1–3. transition gradual In zone trend Cores 1 between of and increasing 2, dust dust% values decrease during the sion for timesteps from a range of climate states are given (Fig. S3 in4 the Supplement). Results (based on 68 % confidence intervals on the regression). Examples of the linear regres- 5 5 25 15 20 10 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 40ka > latitude from the ◦ 6 N (Fig. 1a) in Sene- N, was the Ogolian ◦ ± ◦ at the LGM, (which is ◦ ◦ 14 3 ∼ ± N ◦ , i.e. at HS1 rather than the LGM. ◦ 3 ± N ◦ 128 127 at HS1. Considering that the glacial dust plume N, the Early phase, has been dated at ◦ ◦ 3 ± N ◦ rather than at 15 ◦ 3 N were likely formed during HS4 and HS5. N while the sedimentary SSB was located further to the north ◦ ◦ ± N ◦ N during HS: i.e. the entire latitudinal desert belt did not shift to the (Fig. 3c). This represents a shift of at least 7 ◦ ◦ ect of sea-level changes (i.e. the core to shore distance) over time 3 ff ± ´ ezine, 1989), and 13 N ◦ N). As such, we suggest that the southernmost dunes formed when the sedimen- ◦ Another dune building phase at 14 What caused the equatorward shift of dunes and resultant increase in width of the Another important factor controlling both dust export and dune formation is wind Changes in wind direction might be expected to control the trajectory of the dust Therefore, we suggest that shifts in the sedimentary SSB are mainly due to shifts in The four cores are from a similar water depth (2278–3223 m) and so any grain size dust plume? Sand dune formation and dust mobilisation can both be triggered by tary SSB was atMorphological 13 studies have suggestedby the shifting path dunes during of the the Ogolianplace, (Michel, Senegal or 1973). was River Again, most was it severe, seems obstructed during likely HS1. that this took (Servant, 1983). During HS4 anditude HS5, of the 14 sedimentary SSB wasbackground also glacial located state at a atconditions, lat- this dunes at time. 14 As such, rather than under background glacial also in agreement withstra, estimates 1989; L from pollen-based studies;was stronger Dupont than and today (Sarnthein Hooghiem- etcould al., have 1981), reached it 14 is unlikely(15 that the continental dunes 5.2 Dune formation and dust mobilisation The main episodephase, of which dune has formation, beenslightly when dated later at dunes 20–12 ka 15–24 reached ka inlian in 14 Senegal Mauritania phase (Michel, 1973) (Lancaster is and et normallyindicate Chad al., associated (Servant, that 2002), 1983). the with and The sedimentary LGM Ogo- SSB conditions was (e.g. positioned Talbot, at 1980). 15 Our data South during HS but ratheralso the suggested southern boundary for shiftedhave the equatorwards, controlled LGM as the has (Sarnthein been sedimentarybe et SSB the al., separately major control from 1981). oncontinental the Although the dunes continental sedimentary (see wind SSB, SSB above). strength because it of could cannot the latitudinal position of the to the North of 21 increased wind strength. This latter point also emphasises that the Sahara remained gal and Chad during glacial timeslatitude (Grove, to 1958; the Michel, southernmost 1973; Servant, position 1983), of a the similar sedimentary SSBstrength (Fig. 3c). (Sarnthein andcreased Diester-Haass, at 1977; the LGM Talbot, (Sarntheinequatorward 1980). et shifts al., Wind 1981) of and strength the wasest was very SSB. dust likely in- For end-member increased during example, of other of during the much sediment HS, stronger was the winds. increased grainthe In (Mulitza size LGM addition, et of even Core al., the though 1 2008), coars- exhibits indicative it increased is dust% positioned during north HS of and the SSB, and this is probably due to during the glacial relativewind to direction. today (Fig. 3c) cannot be attributed toposition a of more the meridional continentalwhich SSB. indicate Strongest that support sand dunes for were this forming comes as from far south dune as records, a control of sea levelsee (i.e. no increased such proportion control of of sea river level material on during dust% the inplume glacial), our we and cores (Fig. thus 2). However, could dune also trends control inwind the trajectory Mauritania (NE–SW) dated latitudinal compared to between position todayrelict (N–S; 15–25 dunes of Lancaster ka from et the indicate al., other 2002). regions a sedimentary1968; In also Talbot, fact, more 1980). SSB. most display As zonal a such, (NE–SW) the trajectory observed equatorward (Grove shifts and of Warren, the sedimentary SSB slope, but rather as wind-blown dust. fractionation associated with transportaddition, distance the should e beshould similar be between the cores. same In between cores. Moreover, although Just et al., (2012) suggest HS cannot have been delivered to the core site as turbidites originating from the upper 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ciently ffi N; Watrin et al., 2009). ◦ ) is in broad agreement with ◦ 3 ± N ◦ 130 129 The abrupt changes in SSB position appear to be the result of a sharp decrease in Increased precipitation in arid regions could have potentially acted to increase input appears to be closely(HS1, HS4 coupled and HS5; to Fig. shiftsfront 3b) and of (Eynaud thus the the et most SSB. al.,HS2 intense equatorward is For 2009) shifts slightly example, resulted of anomalous the the in inalthough polar this coolest the there regard SSTs largest is (most records equatorward afrom show SSB larger Martrat small magnitude cooling shifts et cooling) at (Fig. al.coupling HS2 (2007), 3c). between in HS4 dust Cortijo is andHS anomalously et SST (Bouimetarhan cool is al. et compared also (1997). to al., seendust% Also, within other (Fig. 2012) in 2b) HS: records. appears the during the The to record HS1 three-phase and be evolution HS2. visible of in both the SST (Fig. 2a) and 1997; de Abreu et al.,2009; 2003; Salgueiro Martrat et et al., al., 2007; 2010).the Eynaud This AMOC et cooling and al., is reduced 2009; thought deepwater KageyamaThe to et production have al., cooling during been HS due is (e.g. tothe McManus manifested slowdown Iberian et of as al., Margin 2004). equatorward (Eynaud shifts et al., of 2009). the The polar magnitude front of SST as changes far during South HS as that the Late phase ofnor sand mobilisation dune of formation sand (Fig.increase (Talbot, 3) 1980). in was Lastly, dust associated none with fromrecord of relatively (deMenocal our the mi- et records mid al., display totion 2000). an forcing. Our late abrupt data Holocene thus (Figs. suggest 2b–e, a gradual 3c) response asNorth to seen Atlantic insola- sea in surface temperatures another (SSTs).Iberian The Margin North (Figs. Atlantic 2a, and particularly 3b) the underwent extreme cooling during HS (e.g. Cortijo et al., Sahel and Sahara (e.g. Salzmanntimate and of Waller, SSB 1998; position Watrin during etestimates the al., mid-Holocene determined 2009). (21 from Our dust plantThe es- increase community in distribution dust% (23 minor from compared the to mid the to rest of late our Holocene record. (Figs. This 2b–e, is 3c), in was line relatively with dune records suggesting West African monsoon (Kutzbach and Liu, 1997), increasing vegetation coverage in the increases in aridity and windJAS strength. Conversely, cross-equatorial during insolation the mid-Holocene, gradient increased induced greater northward penetration of the The SSB exhibits acan gradual be explained equatorward as shift ation from product gradient of 60 (Fig. ka the 3a), gradual to asThis reduction the would has in probably LGM JAS been have cross-equatorial decreased (Fig. proposed insola- thepenetration for 3c). SW other of wind This studies the strength (Zarriess summer and reducedand monsoon, et the thus al., the northward 2011). causing SSB. an Thethe equatorward increasing gradual shift severity of equatorward of vegetation SSB glacial conditions shift from may 60 ka also to be the partly LGM and attributable the to associated terial would reach the corewind. site: Moreover, the hydrological proxies coarser suggest fraction drieret is conditions al., normally during 2010), only HS when be (Niedermeyer our transported data by display high dust% values. 5.3 Global climate teleconnections chemically weathered than those in theweathered Sahara than (Stuut the et river-derived end-member al., sourced 2005)Kane, from this the 1986; is Fouta still see Djallon much also (Gac less and soilprecipitation maps amount of also controls Driessen input et ofet al., river al., 2001). sediment 2005). As (Gac and well As Kane, asof such, 1986; dust the reduced Coynel mobilisation, river precipitation end-member would and simultaneously increase act input to of reduce theof dust input “unweathered” end-member. material via river transport. However, it is unlikely that this coarse ma- Diester-Haass, 1977; ProsperoSSB and probably Lamb, reflects 2003). aposed As equatorward new such, shift dust an sources ofas equatorward in dust the sources, shifted the vegetation since modern-day belts theyarid are Sahel. and conditions too Sand this for coarse, dunes but the likely they mobilisation do ex- indicate of not persistence dust. of normally Although su act Sahelian dust sources are more reduced vegetation cover, which is in turn due to reduced precipitation (Sarnthein and 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | N ◦ . south ◦ (Fig. 3c). This 1 N), were present − ◦ C during HS (Fig. 3d), ◦ N at this time (Lancaster at the Y-D (Fig. 3c) which ◦ during HS1, 2, 4 and 5 rel- ◦ ◦ 19 2 ∼ ± N ◦ 132 131 n et al., 2005; Bristow et al., 2005; Muckersie ı ´ during HS1, 2, 4 and 5. This represents an equatorward erent mechanism of transmission compared to HS, per- West Africa. Our data indicate that sediments composed ◦ ff 3 ff ± N ◦ This work was supported by ESF-EUROMARC project “RETRO”, the relative to the modern-day. Evidence for relict sand dunes at 14 ◦ ). erence between the Iberian Margin (Martrat et al., 2007) and the Gulf for barchan dunes in NW Sudan (Haynes Jr, 1989) and between 0.17 ff for other regions (Mar 1 1 − − 9% dust, equivalent to those at the modern-day SSB (19 ± north in 4 ka, equivalent to a rate of approximately 400 myr ◦ The response of the position of the SSB to changes in SST was rapid, both equa- Finally, the latitudinal expansion of the dust plume by 6 The SSB position was shifted equatorward to 17 SST is linked to the SSB position via its control on precipitation and wind strength. www.pangaea.de of Bremen, Germany. The( data reported in this paper are archived in the Pangaea database DFG Research Centre/Cluster ofHelmholtz Excellence Climate Initiative MARUM “REKLIM” “The andfor Ocean GLOMAR Marine in – Sciences. Bremen the We International acknowledge Earthtance Graduate Vera during System”, Lukies School XRF and the Karsten scanningGeoB Enneking and Core for powder Repository analytical at analysis. assis- the Sample MARUM material – Center has for been Marine provided Environmental by Sciences, University the Acknowledgements. Supplementary material related to thishttp://www.clim-past-discuss.net/9/119/2013/cpd-9-119-2013-supplement.pdf article is available online at: of the continental SSB.of We the find SSB that positiontemperatures the were during largest likely Heinrich induced Stadials. and by most extremely abrupt cold equatorward North shifts Atlantic sea surface 6 Conclusions We have presented a high-resolution recordary of the (SSB) position over of the the Sahara-Sahelfrom last Bound- a 60 ka latitudinal based transect on o of the 43% dust% in hemipelagicdown to sediments a retrieved latitude ofshift 13 of around 6 latitude on the continent supports our interpretation that the sediments reflect shifts mospheric dust loading requires furtherbution investigation, to in the particular cooling regardingHeinrich its (Miller Stadials. contri- and Tegen, 1998) and aridity (Rosenfeld et al., 2001) of atmospheric dust loading. The magnitude of feedbacks from such an increase in at- ative to today, combined with increased plume strength would have increased global expansion and retreat ofand the 7 SSB. Atis HS5 at for least example, anrate: the 7.5 order myr SSB of migrated magnitudeand 7 greater 30 myr than modernand Shepherd, estimates 1995). of There-activation dune speed and migration of increased dune dustWestern migration Sahel export highlights (Meehl during et the a al., potential 2007). future for precipitation dune decrease in the (Fig. 2a), although coolingThis in therefore the suggests northern a highhaps di latitudes via was an atmospheric relatively teleconnection. large (Fig. 2f). torward and poleward (Fig. 3c). All cores responded coevally indicating a very rapid that the SST di of Guinea (Weldeab et al.,acted 2007), to which increase was the greater strength byplume. of up This the to NE mechanism 4 trade may winds explainstrong and why during thus increases HS. the in “strength” dust of input the and dust grain size areis so in line withet increased al., 2002). dune In activity most records, in there Mauritania is a at relatively small SST decrease in the NE Atlantic The marked cooling in the NorthAfrican Atlantic Easterly is Jet thought to which have led increased toBouimetarhan the reduced strength et precipitation of in al., the the 2012) Sahelshift (Mulitza and et of would al., 2008; have vegetation causedcontrolled zones. by the the Trade-wind aforementioned meridional strength, SST equatorward gradient on (e.g. Kim the et al., other 2003). Therefore, hand, we suggest is thought to be 5 5 20 25 15 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | S): ◦ ´ e, J. S., N–49 ◦ , 2009. ¨ ohl, U.: History of West- N, Earth Planet. Sc. Lett., ◦ , 2005. ´ e, B., and Cacho, I.: Position of and 60 ◦ ´ eol. Bull., 39, 99–130, 1986. doi:10.1029/2009GC002398 ˜ 134 133 ni, M. F., Malaiz ¨ onfeld, J., Salgueiro, E., Turon, J.-L., Penaud, A., ´ egal: I, Bilan hydrologique et flux continentaux de , 2012. ´ en S) during the last 145 ka: orbital- versus millennial-scale ¨ onfeld, J., Hall, M., and Chapman, M.: Millennial-scale ◦ ret, G.: Changes in sea surface hydrology associated with ff doi:10.1029/2004GB002335 ´ anchez Go the Western Iberian margin during the last two glacial periods, N–20 ◦ ff ` a l’embouchure, Sci. G doi:10.1029/2011GC003785 ` eres particulaires ˜ neda, I. 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Fouta Djallon 10°W 750 ) for West Africa (Mahowald et al., 2005), highlighting the Saharan dust plume. 4 1 Figure 1 3 2 − MD01-2444 1 250 20°W yr (a) 2 Annual-mean rainfall (mmyr 0 − Sediment core transect. 40°N 30°N 20°N 10°N CoreLabel Core Number Latitude Longitude Depth (m) based on Age model reference Water Age model 1 GeoB7920-2 20 23 GeoB9508-54 GeoB9526-5 15 12 GeoB9528-3 09 Al/Si (blue; axis inverted) and dust% (black and red) of surface sediments (Govin et al., represent 68 % conﬁdence intervals, which equates to on average plus/minus 9 % dust. Fig. 1. (a) cipitation dataset ( sition (gm so that the regression lies in the interval (0, 100). The correlation ( lines highlight the mainwell African as rivers numerous of smalltion the rivers of study in relict Guinea, area: sand Sierra dunes theapproximate in Leone boundaries Senegal the and of River, Sahel, Liberia. Gambia modern-day re-drawnRed Hatching dune after River Grove formation marks dots as (1958). (Sarnthein the mark Black and posi- dotted locationsMD01-2444 Diester-Haass, lines of 1977). (Martrat mark the Cores et 1–4locations al., of (see 2007) surface Table samples and 1). plotted MD03-2707 Blue in Fig. (Weldeab dots 1c et mark (Govin(c) al., et the al., 2007). locations 2012). Black of2012) dots cores and mark late Holocenebetween section latitude of and Cores dust%dust 1–4. and Solid (plot river line material is (26 represents rotated robust 90 linear regression Table 1. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

N), Core 3 SST of the ◦

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18 Dust (%) Dust (%) δ 0 (‰ VSMOW) 60000 0 20 40 60 80 100 0 20 40 60 80 100 -32 -36 -40 -44 -48 O of Greenland ice (NGRIP, 2004, 60000 50000 18 HS5 δ 50000 HS5 Early N), Core 2 (GeoB9508-5; 15 (f) HS4 ◦ 40000 N). Note that y-axes are inverted. Solid line ) HS4 40000 ◦ P ) P B

B s s r r y

HS3 y HS3 ( ( 30000 30000 e 142 141 Core 2 (GeoB9508-5; 15°N) g

e HS2 A g Insolation Core 1 (GeoB7920-2; 21°N) Core 4 (GeoB9528-3; 9°N) HS2 A Δ LGM Core 3 (GeoB9526-5; 12°N) 20000 LGM Iberian Margin SST HS1 (30°N-30°S, Jun) Latitudinal position of the sedimentary SSB (43 % dust 20000 -D SSB position Y Ogolian HS1 10000 (c) -D (f) (c) (e) (b) (d) (a) Y (3°N to 38°N) Figure 2 0 10000

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(d) (b) (a)

20 18 16 14 12 10 20 40 60 80 20 40 60 80

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Figure 3 30 25 20 15 10

132 152 148 144 140 136

(a) (°N) position SSB ) (W/m 2 O. 18 δ

N) and Core 4 (GeoB9528-3; 9 erence (5 point running average) between the Iberian Margin (MD01-2444; ◦ ﬀ

Dust% for Core 1 (GeoB7920-2, 21 Boreal summer (JAS) insolation gradient between 30 SST di Records of dust% from the West African margin compared with NE Atlantic SST and (b)–(e) (d) N; Martrat et al., 2007) and the Gulf of Guinea (MD03-2707: 3 ◦ Iberian Margin (Martrat et al., 2007). Fig. 3. (a) value) over the last 60 ka. Thickdashed line lines) represents includes: the uncertainty five-point running onon average. the Uncertainty modern-day end-member (grey dust% compositions at (as the in SSBon Fig. (based the on 2), 68 uncertainty regression % confidence forSF3). intervals; Fig. past 1c) timesteps and uncertainty (6838 % confidence intervals; examples are given in Fig. Vertical grey bars highlightformation are HS, marked. YD and LGM. The Early, Ogolian and Late phases of dune Greenland Ice Core 2007). (GeoB9526-5; 12 is the median of(black 500 dashed iterations. lines) Thick includes:84th line percentiles uncertainty represents of on a 500in 5 iterations) end-member and opal point composition uncertainty running content). (based on average. Grey on marinein Uncertainty bars component 16th Greenland (based highlight and ice on HeinrichSvensson (Fig. variations Stadials. et 2f; Timing al., Svensson is et 2008;ages. based Open al., 5 triangles on indicate 2008). point the tie-points derived D-O running from benthic stadials average). foraminiferal Filled triangles indicate AMS radiocarbon Fig. 2. Table 1 for references).