Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Climate ¨ opelin pro- of the Past ` Discussions eres cedex, E) in the hyperarid desert 3 ◦ N, 20.31 ◦ , and G. Krinner ` ere, BP 96, 38402 St Martin d’H 1 2414 2413 ´ eophysique de l’Environnement, UMR5183 CNRS-UJF , P. Braconnot ´ ezine ([email protected]) 2 ˆ atiment 701, 91191 Gif-sur-Yvette cedex, France , W. Zheng 1 ´ ezine ected the vegetation composition and were at the origin of the retreat of tropical ff This discussion paper is/has beenPlease under refer review to for the the journal corresponding final Climate paper of in the CP Past (CP). if available. LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences, P.O. BoxLaboratoire 9804, de Glaciologie et G Laboratoire des Sciences du Climat et de l’Environnement, UMR8212, CNRS-CEA-UVSQ – gering the long-term southwardis shift widely admitted, of following the Charney boreal (1977), summer that limit changes of in monsoon plant cover flow. and It related cally a plant communities from the Lake Yoa. 1 Introduction The timing and amplitude of theday transition hyper between the arid “green desert Sahara” and atdebate. the the present- end Several of authors the have African discussed Humid the Period (AHP) role is of still the a vegetation matter of feedback in trig- regime to one dominated byBP. The northern direct Mediterranean consequence fluxes of occurredronment this after at was Yoa 4000 at the cal 2700 cal establishment yr yramount of BP. and (2) the dry Changes modern in spell desert climate length)However, changes parameters between envi- (simulated in 6 rainfall the and seasonal 4000 cal distribution yr of BP precipitation were during comparatively this minor. time dramati- atmospheric model with aand hydrological module processes occurring representing above and theument beneath past climatologically inland environmental water relevant and surfaces thermal to climateattention doc- changes is during paid the to last windson, 6000 strength cal and yr and BP. direction, on Special lengthdistribution dry and and amplitude spell pollen of occurrence, transport. thethe rainy all natural sea- In of environment, addition anthropogenic whichtures to changes can are climate be are of highlighted: changes also (1) and primary discussed. the their shift importance from impact Two an for main on earlier plant fea- predominantly monsoonal climate The discovery of groundwater fed Lakeof Yoa (19.03 northern Chad byvides the a German unique, research continuous team sedimentarySaharan ACACIA desert. sequence headed Here of by we late S. present Holocene pollen Kr age data for and the climate entire simulations using the LMDZ Abstract France Received: 27 June 2011 – Accepted:Correspondence 7 to: July A.-M. 2011 L – Published: 19Published July by 2011 Copernicus Publications on behalf of the European Geosciences Union. Grenoble, Domaine Universitaire, 54 Rue Moli 2 100029, Beijing, China 3 A late Holocene pollen andfrom climate Lake record Yoa, northern Chad A.-M. L Clim. Past Discuss., 7, 2413–2444,www.clim-past-discuss.net/7/2413/2011/ 2011 doi:10.5194/cpd-7-2413-2011 © Author(s) 2011. CC Attribution 3.0 License. 1 Orme des Merisiers, B 5 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | C in ◦ ´ ezel (1964) in- (Capot-Rey, 1961; ff ¨ opelin, 2006) or water E, 380 m above sea level, ◦ N, 20.31 ¨ opelin et al., 2008). As a result of ◦ ects on monsoon fluctuations in the past ff C isotherm in the hyper-arid sector of north- ◦ 2416 2415 C in December–February. Precipitation is very low, oc- ◦ ¨ opelin et al., 2008) of a groundwater fed lake in the hyperarid desert ´ ezine, 1988), probably accounted for specific environmental (hydro- N provide a continuous record of the last few thousand years, i.e, from the ◦ ´ ezine et al., 2011), suggest that the transition occurred gradually at the sub- ´ ezel, 1965; Gillet, 1968) (Fig. 1). erent key periods in the last 6000 yr from the data. Section 3 presents the results of The paper is organized as follows. After a presentation of the modern distribution We analyse here pollen samples from Core OUNIK03/04 located in the northeastern The discovery (Kr De Menocal et al. (2000) suggested that this transition occurred abruptly, as recorded ff focus is put on windreflect regimes the and changes dry related spells thatin to are West long the Africa. term two reduction climate of variables that boreal best summer monsoonof activity plants in thedi Lake Yoa area, Sect.model 2 output. presents Discussion the and pollen concluding analyses remarks are and provided isolates in the Sect. 4. sector of Lake Yoa incuss order adaptation to of describe plants the to evolutionyr of the and the shift the major from establishment ecosystems moist ofing and to the a dis- arid global modern conditions climate Saharan during model desert.are the and last regional used Climate 6000 simulations simulations to with combin- a complement zoomed climate atmospheric reconstructions model based on pollen data. A particular sand spit formation acrossInfrequent, Lake gentle Yoa winds and in all theannual the opposite mean other direction value Ounianga blow of lakes duringthese (Arkell, evaporation summer. hyper is 1964). arid The environmental 6100 estimated mm conditions,tracts plant (Kr of communities plantless are ergs, rare hammadas throughoutdirectly and vast on regs, and water their availability occurrenceQu from appears the to water depend table or surficial runo 2007) and ancient observationsdicate reported that in temperatures Capot-Rey record (1961)April–May high to and a diurnal Qu minimum amplitude, ofcurring from 13 between a April maximum andNE of October winds, 49 blowing with from a October to maximum April, in are responsible August for (7 intense mm). sand transport Strong and N and this site derived from the nearby station of Faya Largeau (World Climate Database, ern Chad, located between the Tibesti and the Ennedi massifs. Meterological data for to the discontinuityconditions. of Furthermore, most none of sedimentary the18 pollen archives sites and from under 25 Northern predominantly Africaend located arid between of the climate AHP tothe timing the and present amplitude day of (Watrin the et paleoenvironmental al., change 2009), at preventing thatof any time. northern discussion Chad on yields aage for unique, the continuous entire sedimentary Saharan desert. sequence24.3 m Lake of water Yoa (19.03 late depth) Holocene lies roughly on the 28 continental scale. Local recordswestern of Sahel abrupt environmental (L changes,geological) as settings, observed as in observed the elsewherethere in is the a tropics need (Vincens toUnfortunately, et refine late al., the Holocene 1999). interpretation pollen of data Thus in proxy this records region in are the extremely Sahel-Sahara rare region. on land owing linked to increased variabilitythe involving vegetation changes to a in stochastic soil force. moisture and theby response the of dramatic increase inat terrigenous 5.5 (eolian) ka sediment BP. depositionnearby This in continent was ODP core interpreted and 658 past as the indicators, reflecting drying such the out asbodies decrease of archaeological (L in the records grass (Kuper Saharan cover and desert. on Kr the However, analyses of and that a positivetion of feedback orbital between forcing. climatearound and This the has vegetation AHP been has usingRenssen intensely lead et intermediate investigated to al., with complexity 2006; amplifica- transient modelsof Liu simulations (e.g., such et Claussen a al., vegetation et 2007). feedback al., Liu and 1999; et suggested al. that (2007) the however questioned drying the out role of the Sahara was land-surface albedo have had considerable e 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 | ) 3 Aca- Ruellia . and Acacia mel- ) in associa- (Bruneau de in wadis and Maerua crassi- A. obtus ´ ezel, 1965; Gillet, per year) (Fig. 4). and and occur. 2 Erica arborea Typha Acacia raddiana, Cenchrus nity sporadically grow in re- Parkia biglobosa, Isoberlinia , A. seyal ffi ) and published pollen atlases and ) prior to processing made possi- expand along the western flank of Albizzia sercocephala raddiana Alnus with E, 380 m a.s.l., 24.3 m water depth). Field Aristida coerelescens ◦ 2418 2417 sp., on temporarily inundated places. Relict species N, 20.31 Eragostis, Imperata ◦ Panicum turgidum on rocky places while an upper belt is characterized by ) and and ´ ezel (1965), Gillet (1968), Wickens (1976), White (1986), www://medias.obs-mip.fr/apd ¨ opelin et al., 2008) (Fig. 2). The 7.49 m long sediment core Helianthemum A. scorpioides Capparis decidua, Acacia Acacia flava, Ziziphus mauritania, Capparis decidua and Acacia stenocarpa, A. laeta, Erhetia obtusifolia, Rhus incanum, and Aristida, Indigofera, Cornulaca, Aerva, Fagonia, Neurada T. articulata ´ and ezel, 1959). Above, the summit is characterized by grasslands of Mediter- nity with e.g., Indigofera suaveolens ffi . Then, between roughly 1800 and 2500 m alt, the montane belt is highly di- spp. to grow. Upward, trees mostly disappear and the upper limit of the tree line and ). When the water table reaches the surface, trees become more abundant ( ´ ´ e and Gillet (1956), Qu e and Qu Pollen taxa have been assigned to several ecological and phytogeographical groups On the Tibesti massif, which reaches up to 3415 m alt. at the Emi Koussi summit, A steep gradient of increasing plant occurrence and diversity occurs to the south- Mir Ozenda (2004) and the on line African Plant database (2009) developed by the (Maley, 1970; Bonnefille and(1991–1997) Riollet, (Vincens 1980). et al., Nomenclaturecluding follows 2007). all Lebrun the Percentages and determinable wereknown Stork pollen concentration calculated grains of against and an a exotic fernble pollen sum estimates spores taxon of in- (Fig. ( pollen 3). and spore The fluxes addition (number of of grainsaccording a per to cm Maire and Monod (1950), Hutchinson and Dalziel (1954–1972), Bruno de African Pollen Database ( has been sampled at 8 cmwere interval processed for according pollen studies. to Ninety-oneHF; standard samples sieving procedures (each at (chemical 1 5 cm treatment µm)and with and spore analyzed. HCL taxa and They for pollen yielded counts(STable an ranging 1). abundant between Pollen microflora 88 identification and of 848 was 167of according performed pollen to the using the Museum the samples extensive National reference d’Histoire material Naturelle (Paris), electronic photographs from the 2.2 Core OUNIK03/04 Pollen data Core OUNIK03/04 is composed ofeastern two sector overlapping of sections Lake recovered Yoa (19.03 frommethods, the sediment north- composition, andgermont chronology et have al., been 2008;is detailed Kr composed elsewhere of (Eg- finelythe laminated present with clayey a and sediment sandy rate muds averaging 0.10–0.15 deposited cm per from year. 6000 The BP sediment to core Ficus (between 2500 and 3000 m)Mir is marked by theranean presence a of 150 mm allowing patula verse with numerous plants of southern Sahelian origin. Here annual rainfall averages stricted areas along wadis andical temporary humid ponds environmental and confirm conditions the with,doka, former among extent Anogeissus others, of leiocarpus, trop- populifolia, Syzygium Stereospermum guineense, Vitex kunsthianum, doniana, Piliostigma Rauvolfiamespiliformis. reticulatum, and cafra, Diospyros Ficus five specific vegetation associations haveis been characterized distinguished: by the lower montane belt biflorus, Aristida funiculata the Ennedi. A “lower vegetationlifera belt” has been identified in thisAchyroclime massif luzuloides, with Vernonia aschersonii Acacia stenocarpa of Sudanian and Sudano-Guinean phytogeographical a tions with rare trees ( folia cia raddiana, A. seyal, A.brachystilis adstringens, Faidherbia albida, Hyphaene thebaica, Tamarix east (the Ennedi plateau) andfrom to approximately the 10 mm west to (the1968). 200 Tibesti mm Sahelian massif) (Maire grasslands along and and a Monod, rainfall wooded 1950; gradient grasslands Qu with 2.1 Modern plant distribution The Lake Yoa regionalscattered environment herbs is ( dominated by steppic plant communities with 2 Modern and past vegetation around Lake Yoa 5 5 25 20 15 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ´ e er- ff , Aster- pollen have also ´ ezel, 1954), domi- Olea, Pinus dominate the upper Erica 1 %), the arboreal taxa erentiated pollen types ff < ´ ezel and Martinez, 1958) ´ ezel, 1965). However an Ephedra and pollen deposited after 2500 cal yr are found today in the Emi Koussi crater Olea ` eve and the South African National Bio- N (Van Campo, 1975; Schulz, 1980). The Artemisia ◦ also grows in the Mediterranean region today 2420 2419 , endemic to the Ahaggar (Qu recovered from Yoa is most likely the Tibesti massif. ´ ezel, 1958) which record abundant pollen grains of Erica arborea species are present in the upper belts of the Tibesti mas- forests. Its pollen is however badly dispersed and never Erica arborea nity that can be subdivided into five groups: tropical undi Olea laperrini ffi 30 N (Schulz, 1980). and may reach percentages as high as 45 and 20 % respectively ◦ Erica arborea Ephedra between 5330 and 5660 cal yr BP. Scattered and ´ ezel, 1959). This agrees with earlier studies carried out in the Ahaggar Plantago ¨ opelin et al., 2008a). Quercus suber species is not reported in the Tibesti massif (Qu 5330 cal yr BP thatduring confirm the that Holocene. this species expandedwithin in all the Saharan massifs exceeds more than 2modern grains surface even samples when south nearsource of the area 22 source of area. It is not found in levels of the Lake Yoa pollenpollen diagram, taxa. in association Their with source all is the most above probably mentioned The located nearest in populations the of Mediterranean region. where they are considered aand relict Qu of former widerat populations 2000 (Bruneau m de alt Mir Erica (Pons arborea and Qu been found in lowland Holoceneand sites Sudan from (Ritchie Borkou et (Qu al., 1985; Ritchie and Haynes, 1987) between 8100 and haran (including Saharo-Mediterranean plant types) (28tains taxa), (4 and taxa). Tropical Moun- Taxa of northern(5 taxa). origin: Mediterranean (6 taxa) andOlea European-Temperate olive tree species, nated mid-Holocene vegetation at roughly 2000 m alt (Thinon et al., 1996). Given Artemisia sif. They also widely spreadies at of the Saharan-Mediterranean modern transition. pollen Here,aceae, rain stud- show that they(Van are Campo, associated 1975). with They alsotween 22 record and high 29 percentages in the Murzuk basin be- Taxa from local lake-shore and(6 river bank taxa) communities: and reed-swamp populations because ferns of (6 their wider taxa). ecological range Grasses including drierTaxa and of areas. sedges tropical a wereentiated considered with separately a broadcluding modern “tropical humid” phytogeographical plant range types (20taxa), (19 Saharo-Sahelian taxa), taxa)), (39 Sahelian Sudanian taxa) s.l. and (in- (including Sudano-Sahelian Sahelian plant (9 types (23 taxa)), Sa- its complete absencespecies in is the unlikely contemporaneous toBP core (Kr be interval the source at of Lake Yoa, this – – – – – – ´ ezine et al., 1990; Vincens, 1982), pollen grains of local herbaceous plant commu- sponding plants. In consequence, the pollenon zones the described fluctuations below of are the not major only pollen based types but also on the appearance/disappearance are taxa with lowAs taxonomical is range common corresponding in arid toL palynology trees, (e.g., shrubs, Ritchie herbs etnities or al., (Poaceae, 1985; Cyperaceae, lianas. Amaranthaceae-Chenopodiaceae) Ritchie are andmagnitude several Haynes, orders more 1987; of numerous than thosezoophilous of taxa tropical with trees, low because pollen the productiondemonstrated and later that dispersal. are even primarily However, when Watrin found etcan in al. very (2007) be low considered percentages ( with confidence as indicators of the local presence of the corre- The physiognomy of the sourcebeen plants distinguished: has the been arboreal determined, pollenand and group two shrubs including categories and trees, have the palms, non-arboreal epiphytes, lianas group including herbs. Undi Some pollen taxa may have several possible origins: diversity Institute (Table 1): Conservatoire et Jardin botaniques de Gen 5 5 25 15 20 10 25 10 15 20 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | . 8 %). Olea nities. 41 %), = , which ffi = Acacia , Meliaceae, and , Amaranthaceae- (maximum ), started to expand . significantly increase. Celtis , disappears and is replaced Cornulaca / Cornulaca Artemisia 0.4 %), increases from the base with Sahelo-Saharan a / Boehravia = Aerva a drier, Sahelian (Sahelo-Sudanian) and Aerva Salvadora persica and Commiphora and the progressive increase in Saharan 2422 2421 Tribulus Piliostigma reticulatum percentages allows distinction of sub-zone 3b from nearby montane plant communities, occurs Faidherbia albida Commiphora, , reach their maximum values (3 %). Boehravia and , remains present in high abundance. Amaranthaceae- Pinus Phoenix Erica arborea and Erica arborea, Tribulus and the appearance of Mediterranean pollen types such as er greatly from the lowest ones, exhibiting the dramatic increase in ff Erica arborea Hyphaene Ephedra , Balanites aegyptiaca are noticeable. Sub-zone 2cdisappearance (3250–2700 cal of BP):and Saharo-Mediterranean This elements last like sub-zone is defined by the Sub-zone 2b (4000–3250by cal BP): Amaranthaceae-Chenopodiaceae, Chenopodiaceae, Brassicaceae, Asteraceae and at the end of the previous zone and is continuously present. Sub-zone 2a (4750–4000 cal BP):and All the ferns) humid indicators disappearrence (Sudanian in plant at types significant this values zonetaxon. of which isChenopodiaceae defined associated by with theplant characteristic types continuous (e.g., Saharan occur- and Sahelo-Saharan – – – a module representing the climatologically relevantoccurring thermal and above hydrological and processes beneath inland water surfaces has been included (Krinner et al., 3 Snap shot experiments for 63.1 ka, 4 ka and Climate 0 k simulations In order to discuss linksof between mid-Holocene vegetation climate and discussed climate,changes in we in insolation consider Krinner forcing the and et simulations feedbackswas from run al. vegetation using and (submitted) a lakes. version that of This the simulation account LMDZ for atmospheric model the (Hourdin et al., 2006) in which Increase in (1500-850 cal BP) andpermost sub-zone levels 3c di (850 calMediterranean BP–present and day), temperate pollen respectively. grains. The up- Pollen zone 3 (2700 cal BP–present day) This zoneand is Saharo-Mediterranean elements,Artemisia typified mainly by the dominance of Amaranthaceae-Chenopodiaceae Zone 2 can be further sub-divided in three sub-zones: This transition zone recordsfrom the northern progressive origin, increase mainly occurs of only dry sporadically at indicators. low valuesof in Pollen this zone types 3 zone (mea to the top of the sedimentary sequence, averaging 2 % (maximum humid (Sudanian) elements, ofGrewia which continuously at significant values throughout thisshore zone lake along and with fern river spores bankthe from communities. near base Poaceae of reaches this a zonesequence maximum to and value 17 then (90 % %) regularly (zone at decreases 3c). toward the top of the sedimentary Pollen zone 2 (4750–2700 cal BP) Pollen zone 1 (6000–4750 cal BP) This zone exhibits the predominance among the regional arboreal taxa of tropical of the most characteristic taxa. Three main zones can be distinguished. 5 5 20 15 10 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ected ff ) than in winter. As 1 − ected the geographical ff E. The size of the box is a com- ◦ ) north-easterly (mainly east-north east 1 − W to 22.5 2424 2423 ◦ N and 17.5 ◦ N). Orbital parameters were prescribed to those of 6 ka BP, ◦ (30 ◦ N to 22 ◦ ects of lakes in this simulation. ff erences between 6 ka BP and pre-industrial SSTs were added to the modern SST Figures 4 and 5 show, respectively, the simulated precipitation as a function of month During the second half of the Holocene the changes in incoming solar radiation at the ff precipitation less than a pre-definedperiod threshold, of and dry a days. dry This spell later is metric defined as is extended widely used as indicator of drought conditions located in a subsidence zone (Fig.events. 6) and is subject to only a small fraction of convective 3.3 Simulated dry spells Vegetation is very sensitive to thedry length events of or the dry dry season months.or as This the well cannot as mean be the seasonal directly persistence cycle of inferredof of from dry precipitation. the months. This convection For is regimes the why modern we climate, also a computed dry the day number is defined as a day with an amount of north-easterly winds, but most of thewind events come amplitude from from the this north-north sector eastin is phase. the much The observations, weaker only (Table rare 1, south-westerlyof 1.16 ms wind small events are amplitude simulated and (Fig.showing are a 5, always very Table weak 1). monsoonal flow Only over this 4 region. events The are region found is in therefore the mainly 0ka simulation by dividing the wind directionof into these 8 directions sectors. is Thepresent used average day, the to strength simulations define and show that the angles precipitationa in wind occurs peak from each amplitudes in April June. and to The September dominant peak with amount direction. occurs is slightly For earlier quite than well inunder represented the the observations, (Fig. influence but of the 4). strong rainfall phase) (Table 1, During winds 3.3 winter ms indicating (DJF, that 90in Yoa days) is winter (Fig. mainly the 5). tied region During to is the climate summer in (JJAS, 120 the days), Mediterranean it region is also under the influence of not consider local changespattern in of this vegetation around analysis core that OUNIK03/04. could have a and the frequency and amplitudedirection of indicated winds by during the DJF wind and JJAS compasses. as a These function wind of compasses wind were computed major climate characteristics over Yoa yet remain representative of Yoa climate. We do extending from 17 promise that allows for inferring the large scale climate changes that have driven the model (Marzin and Braconnot,inland water 2009). bodies that can Sincemann be used et there as al. boundary is (1998) conditions no reconstructionthe for this is e available period, used reconstruction the for Hoelz- of 6 ka BP; however, this3.2 may overestimate Simulated climate characteristics All the LMDZ simulationsmatology are and 41 daily values yr for long. temperature, precipitation, We and consider 10 in m wind the over following a the region mean cli- This induced changes inthe latitudinal characteristics thermal of the gradients Africana and monsoon simulation seasonality (Hely of et which 4 ka al., a protocol BP 2009). is climate This the in is same addition why as to weBerger the that consider (1978), 6 used ka and for BP 6 the ka. simulation. changes The Orbital in experimental parameters SSTs are come prescribed from following a simulation of the IPSL climate considered using sea surface temperaturesthe (SSTs) IPSL global from coupled mid-Holocene general simulationsdi circulation of model (Marzin andclimatology. Braconnot, 2009). The The reference isthe thus 1980–2000 a HADISST simulation climatologyadjusted of for (Rayner the each et modern time al., period. climate 2003). forced by Trace with gasestop were of the also atmosphere are driven by changes in the Earth’s obliquity and precession. resolution of 1 to 1.5 and changes in vegetationthe were atmospheric computed model using and an2003). the asynchronous The BIOME4 coupling fraction of equilibrium between the vegetationfollowing land Hoelzmann model surface et occupied (Kaplan al. by et inland (1998). water al., The bodies response was of prescribed the ocean to insolation forcing was 2004). This global atmospheric model was zoomed over Africa to reach a regional 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 | and for 4 ka ¨ opelin et 1 ), which is − 1 − ected at 6 ka ff Boerhavia , 420 mm yr Tribulus ∼ ( 1 /river flows from the Tibesti − ff erences between these periods. From ff pollen grains to the core site. This agrees erences are found for wind amplitude: the ff ¨ opelin et al. (2008). The dry spells did not 2426 2425 over a period lasting longer than 5 months. As and then reduces to 2.96 and 2.81 m s 1 1 − − Erica arborea ´ e and Quezel (1959) and Thinon et al. (1996) who noted an erent periods. Therefore we suggest that changes in boreal sum- ff expansion in the Saharan massifs during the mid-Holocene linked , which are therophytics with a very short life cycle (Gillet, 1968) and . The annual mean rainfall reached 1.16 mm day 1 − to the slopes ofprogressive increase, the from Ennedi 4000 andBlepharis cal yr Tibesti BP massifs onwards, (Fig. of 1), was completed. The Simulations show that winterand characteristics 4 ka are (Fig. only 5, marginallywind Table 1). a speed reaches The 3.35 only m s and di 6 ka, respectively.between During the di boreal summer,mer the conditions situation were is the4750 more most to marked contrasted di 2700 caland yr their BP, replacement the by Sahelian disappearancements (Sudano-Sahelian, of record then the most Sahelo-Saharan) progressive establishment ele- of oflowlands. dry tropical environmental This conditions humid transition in indicators occurred the innian trees two from main 4750 steps to with 4000to yr the BP 3250 retreat cal and of the BP. retreat At the ofmodern Suda- that the phytogeographical Sahelian time, units trees we in from 4000 can Northern consider Chad, with that Sahelian the trees modern restricted distribution of the to climate conditions500 wetter mm. than today with mean annual rainfall around 200– Moutnains which carried with Bruneau de Mir Erica arborea From 6000 to 4750together cal with BP: ferns, Occurrence theby of spores rivers a (Tschudy, of highly 1969), whichduring diverse indicates are this tropical that time. well pollen a known The monsoonal flora tropical low to climatic forest percentages be formations regime of never primarily tropical prevailed existedrestricted trees carried in however to the show wadi that region banks dense andregional where that landscape they tropical remained benefited open, trees characterized were fromcommunities, by dominated fresh extended by water herbaceous grasses availability. plant helian and The containing elements. numerous Saharan Werainfall and suggest at Sa- that that both time, allowing lowal., persistant 2008; and fresh Eggermont high water et altitudes al., conditionsin 2008). at received gallery Rainfall Yoa greater (Kr led forests to andIt the persistence to was of also the tropical responsible widespread trees for expansion increased of surficial grasses runo in the lowlands. – – subsidence (Fig. 6). Thelasting from rainy May season to November was with more3 a mm peak intense d in and July–August where longer precipitationin exceeded than good at present, agreement withexceed 10 estimates months, by meaning that Kr The there was climate only characteristics a simulated limitedrecorded for number 6 of at ka entirely Yoa. thus dry accurately years. monsoon They describe influence. are the similar first period to those found today in areas under predominant Simulations for 6 ka confirm thatwest and more the than monsoonal 75 % flowwere penetrated of as wind characterized far events by north originated deep as from Lake south- convection Yoa. developing Precipitation over events the region at the expense of 4 Discussion and conclusion 4.1 Environmental and climate changes The Lake Yoa pollen recordties which shows were the widely progressive spreadAfrican retreat throughout Humid of North Period tropical (de Africa plant Menocal (Watrindominated communi- et et desert al., al., ecosystems. 2000) 2009) Three and during main their the periods replacement have by been herbaceous- defined: monthly rainfall is belowexpected 1 from mm day the simulatedat precipitation Yoa between is associated the to 3 an climate increase periods, in aridification the length of dry spells (Fig. 7). (e.g., Benniston et al., 2007). Here we call a dry spell a dry period during which the 5 5 25 15 20 10 25 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | and Cornu- recorded Artemisia Artemisia erences in precipitation occur ff pollen during this time shows that Pinus is absent from zone 1 and 2 and only spo- 2428 2427 ´ e (Tibesti), although they were less developed Olea ´ ezine et al., 1990). This can be interpreted as re- ´ pollen grains are present in the lower levels of our e-Aray Artemisia ) (White, 1986) confirms the local expansion of sand dunes around ¨ opelin et al., 2008). The considerable increase in pollen taxa of , indicates a complete reversal of prevailing winds to a predominant NE-SW ¨ akel (1979) who notes that fluvial terraces still formed at 4600 and 3500 cal yr corresponding to conditions usuallythe found Saharan in lowland early- sites, to atal., Selima mid-Holocene 1985) (Richie levels and and of Chemchane Haynes, all flecting (L 1987), moderate Oyo NE (Ritchie trade et in wind Lake fluxes. Yoa pollen zone The 3 percentagesMediterranean-dominated are of considerably climate higher (up regime to couldward 8 %), expansion have suggesting of that also the the facilitated species. radically occurs the in zone south- 3.it A single reaches exception occurs 3 in %, theNorth uppermost indicating Africa. level the where recent reinforcement of the NE trade winds over From 2700 onwards, thelaca, increase Ephedra in psammophilous plantLake types Yoa (mainly (Kr northern Mediterranean (and Saharo-Mediterranean) origin, mainly Olea direction as is the caseduring today. summer This is (Fig. reflected 5). onthe the It corridor simulated is morphology wind probable compasses ofEnnedi that the these massifs. Ounianga winds plateaus wererecord even between (pollen enhanced the by zones Tibesti 1 and and the 2), but only sporadically with values less than 1 % thus well adapted todry long rainless seasons periods, compared confirmsspell to the at the 4 establishment ka of previous comparedreported longer to period 8.8 in (i.e., month the dry a2009). Tassili spell 11 from at However, the 6 month dendrochronological ka, brief simulated Fig. dataBP return 6). (Cremaschi dry in to This this and wet pattern later conditions Zerboni, site is between is also 3260 not and recorded 1440 at cal Yoa. yr – The 4 ka simulation is representative of this transition period (Figs. 5–6). The JJAS Compared to 4 ka, the meantions. precipitation These is dramatically drier reduced conditions inand reflect the are the control increased simula- characterized subsidence byseveral over consecutive pronounced the very dry region dry (Fig. years. spellsto Thus, 6) that there 4 ka. is persist a 27.1 major climatic months, transition indicating compared variability, low rainfall duringvegetation. certain years did in fact have a notable impact on the The transition between the twothe opposite convection 6 regimes ka (Fig. and 6). presentwhereas day Deep medium patterns convection is is to also not lowrainy found present in convection season in is is the quite 4 dominant ka similaronly in simulation, to during JJAS. the However, the 6 the kaannual second pattern. length mean half of The precipitation di of the onlycontinuous half the period that rainy of of dry season occurringyear, months starting explaining at was why 6 in generally dry ka disrupted spells July, (Fig. did by which 4). not the exceed explains rainy As 11 season an months. for in However 6 each ka, due to the interannual However, the remarkable representationnortherly of wind fluxes had already begun to strengthen. wind directions were quite unstablesubstantial and amount covered a of wide days rangeSimilar with of to wind directions, today, a blowing showing large a from portion the of western events also to came northern from the quadrant. East-Northeast (Table 1). Pollen transport from the Tibesti Mountains3000 cal remained, yr though BP, indicating decreasing, until that roughly ings rains by persisted J at altitude. ThisBP corroborates in earlier the Enneri find- Zoumri-Bardagu than those duringmaschi the and early Zerboni (2009), Holocene whowas note maximum. characterized that the by noticeable period It fluvial between 5990 activity is and in 3520 also the cal wadi yr in Tannezzuft BP (eastern agreement Tassili). with Cre- 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 | ´ ery ¨ opelin Cerealia was also Hyphaene the- , which has a better nu- Hyphaene thebaica could have been introduced into pollen is continuous, and its percent- is edible, its fiber and leaflets are used to Phoenix dactylifera 2430 2429 pollen are also present in Lake Yoa zone 1 and Hyphaene ected the vegetation composition and were at the ff Phoenix dactylifera Hyphaene Hyphaene thebaica This research is part of a French project funded by the National Agency pollen are sporadically found in early to mid-Holocene Saharan sites at palm trees which currently characterize the northern African dry lands and grow . Hyphaene (University of Cologne, Germany) for accessibilityproject to samples of as part the of Deutsche a collaborative Forschungsgemeinschaft research (ACACIA), D. Verschuren (Gent University, origin of the retreat of tropical plant communities fromSupplementary the material Lake related Yoa. to thishttp://www.clim-past-discuss.net/7/2413/2011/cpd-7-2413-2011-supplement. article is available online at: pdf Acknowledgements. for Research in France (projet ANR-vulnerability “SAHELP”). Thanks are due to S. Kr ranean fluxes occurred afterestablishment 4000 of cal the modern yr desert BP.climate The environment parameters at direct (simulated Yoa rainfall at consequence amount 2700 of cal andwere BP. dry this comparatively (2) spell minor. Changes was length) in However, the between changestion 6 in and during the 4ka this seasonal time distribution dramatically of precipita- a culture also included wheatpollen (Capot-Rey, 1961), grain in explaining the the upper presence most of level a of our diagram. 5 Conclusion Comparison between pollen-based paleoenvironmental reconstructionsern at Chad) Yoa (North- and climate simulationslier point to predominantly two monsoonal main climate features: regime (1) the to shift one from an dominated ear- by northern Mediter- gions date palm plantations became(Cleuziou increasingly and common from Costantini, the 1980; 3rdin Baum, millennium Libya, 1988). BC at Fossil levels date datedbetween pits from 2800 2145 were and cal found yr 2400 cal much BP yrstill later at BP exists Uan in at Muhuggiag Fezzan Yoa (Tadrart (van where Acacus), der it and Veen, benefits 1995). from A the date proximity palm of oasis the groundwater. Here, agri- the Nile Valley during the pre-dynastic period (Roubet and Hadidi, 1981). In both re- and Tengberg, 2009). In Egypt, long considered a sacredgoes back tree to by the theLake New Yoa ancient Empire zone between Egyptians, 3, roughly andages/influx the 1552 values its occurrence and increase of first 1080 significantly, BC suggesting representation cultivated, that (Baum, at least doum 1988). intentionally palm favored In trees byThey local were, were inhabitants if replaced, after not after 1500 850 caltrititional cal yr yr value. BP BP The at by Yoa. establishmentin of the such a Middle plantation Easttlements at where of Yoa the occurs Abu much earliest Dhabi later consumption and than Kuwait of around dates the come late from 6th-early Neolithic 5th set- millennium BC (M 2, confirming the wideHolocene. distribution The of fruit wild of weave baskets doum and palms its inas trunk northern demonstrated is Africa by used as during charredbaniyan building fruits the sites material. fragments in recovered the It at Nile has the valley long (Wendrof been late et collected, Pleistocene al., Kub- 1988). additional resources to ensure thethe increasing food regional security drought. of Saharan populations in the faceBir of Atrun (Ritchie andet Haynes, al., 1987), 2008) Oyo from (Ritchiebaica et 10 000 al., to 1985) 5500 and calin yr Gobero areas BP. (Sereno Here benefiting they from arewadi the assigned beds to (White, proximity 1986). of underground waters, such as in oases and The local environment abruptly changed atto 1500 cal Cremaschi yr BP,the and date onthe Zerboni which, oases according (2009), of due wadiestablishment to Tanezzuft of contracted a to oasis dramatic their agriculture increase present at state. in Yoa regional at It this dryness, is probable date that corresponded the to a search for 4.2 Anthropogenic changes 5 5 25 20 15 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | L.) 81). ◦ r, Journal ı ¨ Erica arborea ´ ebaine d’Ineni (n ´ eographie, Paris 315, , 2009. ´ e Biog ´ et ` ere en arbre ( ¨ oll, R., Semmler, T., and Woth, ´ egionale. 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Lakeside PLoS Cemeteries ONE W Winter 0ka Winter 4ka Winter 6ka Direction AmplitudeSSW NbWSW AngleNNW 0.1026NNE 0.0729 DirectionENE 0.1343 Amplitude 1 Nb 3 1.1664 197.88 247.93 4 3.1047 Angle 98 327.56 SSW 14 WSW Direction 30.1 WNW Amplitude 46.06 0.3345 Nb 0.3568 NNW NNE 10 0.438 Angle 21 211.11 247.25 0.5298 23 1.4011 SSW 19 WSW 294.7 35 340.28 WNW 30.12 0.8976 0.8287 NNW NNE 84 9 0.2251 223.179 0.2421 238.41 5 0.8534 3 286.5 19 352.97 31.02 Direction Amplitude NbENE Angle Direction 3.348 Amplitude Nb 90 Angle 49.5 Direction ENE Amplitude Nb Angle 2.9636 88 48.44 ENE 2.8129 88 48.28 World Climate Database: White, F.: La vegetation deWickens, l’Afrique, G. ORSTOM, E.: UNESCO, The p. flora 384, of 1986. Jebel Marra (Sudan Republic) and its geographical a Wendrof, F., Schild, R., Close, A. E., Hillman, G. C., Gautier, A., van Neer, W., Donahue, D. 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M., Sali Schulz, E.: Zur Vegetation der 5 30 25 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 750 outliers 700 650 600 ´ ezel, 1964 and Capot-Rey, anchor points 550 500 450 bulk dates 400 350 core depth (cm) 2438 2437 300 cal yrs +2s 250 Lake Yoa OUNIK03/04 age model Yoa OUNIK03/04 Lake 200 cal yrs -2s 150 100 50 cal yrs BP 0 0

500

-500 2000 1500 1000 6500 6000 5500 5000 4500 4000 3500 3000 2500 calendar years BP years calendar Calendar age-depth model for the composite OUNIK03/04 core from Lake Yoa (from Synthetic vegetation map of Faya Largeau (redrawn from Qu ¨ opelin et al., 2008). Fig. 2. Kr 1961). Fig. 1. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2440 2439 Precipitation at Yoa simulated for 6 ka, 4 ka and 0 ka. Monthly mean seasonal cycle. Lake Yoa synthetic pollen diagram of percentages versus time. Only the major pollen The error barsestimated represent using the 41 yr standard of deviation simulated representing data. interannual variability and are Fig. 4. types are presented. Fig. 3. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | . 1 − N averaged) from the daily model outputs for ◦ 2442 2441 N–22 ◦ E, 17 ◦ N) for 6 ka (green), 4ka (red) and ctrl (black). Positive values ◦ summer time (JJAS) and the 3 climate periods considered in this E–22.5 ◦ (b) E, 17–22 ◦ Probability density function of 500 hPa (w500, in hPa.s-1) vertical velocity over Lake Number of wind events and mean wind speed as a function of wind direction diagnosed winter time (DJF) and Fig. 6. YOA region (17.5–22.5 represent subsidence regions and negativeas values a convective regions. proxy for Vertical convection. velocity is used over Lake YOA region (17.5 Fig. 5. (a) study: 6 ka, 4 ka and present. Units for the wind amplitude: m s Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2444 2443 ers for Sahelian (s.l.) and Saharan pollen types which largely dominate the ff Lake Yoa main phytogeographical groups. Percentages are indicated in dark grey Mean length of dry spells as simulated at Yoa for 6 ka, 4 ka and 0 ka. Because of the pollen assemblages. Thisclimate figure regimes shows which the occurred at mainMediterranean Yoa dominated pollen from climate a groups regime monsoon corresponding at dominated present. to climate Colors three regime refer at main to 6 Figs. ka 4 to and a 5. and magnification (10 times)that in the light scales di grey. Influxes are shown with dashed black lines. Note Fig. 8. Fig. 7. length of the simulations usedlength to of compute 492 a months. continuous dry climate would be represented by a