21,000 Years of Ethiopian African Monsoon Variability Recorded in Sediments of the Western Nile Deep-Sea Fan

Reg Environ Change DOI 10.1007/s10113-014-0588-x

ORIGINAL ARTICLE

21,000 Years of Ethiopian African monsoon variability recorded in sediments of the western Nile deep-sea fan

Marie Revel • Christophe Colin • Stefano Bernasconi • Nathalie Combourieu-Nebout • Emmanuelle Ducassou • Francis E. Grousset • Yann Rolland • Sebastien Migeon • Delphine Bosch • Pierre Brunet • Yulong Zhao • Jean Mascle

Received: 14 May 2012 / Accepted: 19 January 2014 Ó Springer-Verlag Berlin Heidelberg 2014

Abstract The Nile delta sedimentation constitutes a highlighted: (1) The rapid shift from the LGM arid con- continuous high-resolution record of Ethiopian African ditions to the African Humid Period (AHP) started at monsoon (EAM) regime intensity. Multi-proxy analyses about 15,000 years. The AHP extends until 8,000 years, performed on hemipelagic sediments deposited on the and we suggest that the EAM maximum between 15,000 Nile deep-sea fan allow the quantification of the Saharan and 8,000 years is responsible for a larger Blue/Atbara aeolian dust and the Blue/White Nile River suspended Nile sediment load and freshwater input into the eastern matter frequency fluctuations during the last 21,000 years. Mediterranean Sea. (2) The transition between the AHP The radiogenic strontium and neodymium isotopes, clay and the arid late Holocene is gradual and occurs in two mineralogy, elemental composition and preliminary paly- main phases between 8,400–6,500 years and nological analyses reveal large changes in source com- 6,500–3,200 years. We suggest that the main rain belt ponents, oscillating between a dominant aeolian Saharan shifted southward from 8,000 to *4,000 years and was contribution during the Last Glacial Maximum (LGM) responsible for progressively reduced sediment load and and the late Holocene (*4,000–2,000 years), a dominant freshwater input into the eastern Mediterranean Sea. (3) Blue/Atbara Nile River contribution during the early The aridification along the Nile catchments occurred from Holocene (15,000–8,000 years) and a probable White Nile *4,000 to 2,000 years. This dry period, which culminates River contribution during the middle Holocene at 3,200 year, seems to coincide with a re-establishment (8,000–4,000 years). The following main features are of increased oceanic primary productivity in the western Mediterranean Sea. Such a pattern imposes a large and rapid northward shift of the rain belt over the Ethiopian Electronic supplementary material The online version of this highlands (5–15°N) since 15,000 years. Precipitation over article (doi:10.1007/s10113-014-0588-x) contains supplementary material, which is available to authorized users.

M. Revel (&) E. Ducassou F. E. Grousset Nice Sophia Antipolis University, CNRS, IRD, Observatoire de UMR-CNRS 5805-EPOC, Universite´ Bordeaux 1, Avenue des la Coˆte d’Azur, Geóazur UMR 7329, 250 rue A. Einstein, faculte´s, 33405 Talence Cedex, France Sophia Antipolis 06560 Valbonne, France e-mail: [email protected] Y. Rolland S. Migeon J. Mascle Geóazur, Universite´ de Nice Sophia-Antipolis, 250 rue A. C. Colin Y. Zhao Einstein, 06560 Sophia Antipolis, France IDES UMR 8148 CNRS, Universite´ de Paris-Sud XI, Baˆt 504, 91405 Orsay Cedex, France D. Bosch Laboratoire de Tectonophysique, Universite´ de Montpellier II, S. Bernasconi 34095 Montpellier, France Geologisches Institut, ETH Zurich, 8092 Zurich, Switzerland P. Brunet N. Combourieu-Nebout Geósciences Environnement Toulouse, UMR5563, 14 Avenue LSCE, UMR 8212, Orme des merisier, Edouard Belin, 31400 Toulouse, France 91191 Gif sur Yvette Cedex, France

123 M. Revel et al.

Ethiopia increased from 15,000 to 8,000 years. It was authors conclude that regional climate variations over the followed by a gradual southward shift of the rain belt over Nile catchment have a stronger impact on deep-sea fan the equator from 8,000 to 4,000 years and finally a large geometry than sea-level variations (Ducassou et al. 2009). shift of the rain belt south the equator between 4,000 and Increased terrigenous supply on the Nile delta reflects the 2,000 years inducing North African aridification. We Ethiopian African monsoon (EAM) intensity. Continuous postulate that the decrease in thermohaline water Medi- sequences with high temporal resolution recording Nile terranean circulation could be part of a response to huge hydrological history are found in the western Nile deep- volumes of freshwater delivered principally by the Nile sea fan and allow us to establish the timing of Nile River from 15,000 to 8,000 years in the eastern hydrological change and the climate connection between Mediterranean. the eastern Mediterranean and the subtropical African monsoon during the Quaternary (Revel et al. 2010; Zhao Keywords Nile deep-sea fan Ethiopian African et al. 2011; Caley et al. 2011). monsoon Nile River palaeohydrology Nile freshwater Here, we present a 21,000-year multi-proxy record outflow Saharan aeolian dust Sr and Nd isotopes from sediment core MS27PT. This site is located close to Holocene Last deglaciation the Nile fluvial discharge (90 km outward of the Rosetta mouth of the Nile), close enough to monitor past variations of the Nile flood discharge on the slope (Fig. 1b). Introduction In this study, bulk elemental composition, clay mineralogy, strontium and neodymium isotopes and terrigenous The African Humid Period (AHP) was characterized by fluxes are used to reconstruct the variations in Saharan the development of extensive lakes and of savannah-type aeolian dust and the Blue/Atbara and the White Nile vegetation in the Sahara (Kro¨pelin et al. 2008). The terrigenous inputs for the last 21,000 years. In parallel, increased amounts of freshwater and riverine sediments pollen data are used to document the vegetation changes transported to the eastern Mediterranean Sea (EMS) by along the Nile catchments. The aims of this study are (1) the Nile River, during the AHP, drastically changed the to determine the precise timing of the change in the Nile Mediterranean thermohaline circulation (Voelker et al. hydrological regime and reconstruct the evolution of the 2006, Fig. 1a). This pattern has led to reduce deep-water EAM regime during the onset and termination of the ventilation, which, in turn, contributed to the formation of AHP and (2) to investigate the influence of the EAM on the organic carbon-rich dark layer called sapropel 1 (De Nile fluvial inputs to the EMS. Lange et al. 2008; Kholeif 2009). After *6,000 years, a weakening of the monsoon system in North Africa induced the Saharan desertification. The progressive southward shift of the Inter Tropical Convergence Zone Methods (ITCZ) during the late Holocene caused the desiccation of the vast system of lakes and rivers of the Sahara. The The great sensitivity of core MS27PT to record the Nile decrease in African monsoon intensity and possibly an flood frequency has been demonstrated in previous studies increasingly negative North Atlantic oscillation are likely in which sedimentological characteristics have been to have changed the Mediterranean precipitation regime described (Ducassou et al. 2009; Revel et al. 2010). during the mid-Holocene (Wanner et al. 2008; Magny Chronology of core MS27PT for the last 21,000 years is 14 et al. 2013). However, the timing and amplitude of the based on 19 AMS C measurements performed on transitions between the Last Glacial Maximum (LGM) monospecific planktonic foraminifera (Supplementary period, the AHP and the present day arid desert are still Material 1 & 2). In this study, hereafter, all ages are dis- matter of debate (Menviel et al. 2011;Le´zine et al. cussed, as cal. ka BP. Bulk elemental compositions were 2011a). Detrital sediments and water discharge in deltaic measured millimetre-by-millimetre on core surface using margins can be used as proxies to quantify these climate an Avaatech core scanner. Clay minerals were identified by changes. Deltaic sediment is highly sensitive to changes X-ray diffraction using a PANalytical diffractometer. The 143 144 87 86 in the rainfall regime and vegetal cover in their water- radiogenic Nd/ Nd and Sr/ Sr isotope compositions shed. The sedimentological analysis of more than 70 of the terrigenous fraction (\63 lm) were measured, after sediment cores from the Nile margin allowed researchers removal of the biogenic components, using a multi-col- to define different sedimentary facies (hemipelagites, lector mass spectrometer Finnigan MAT 261. A detailed turbidites, slumps) and their spatiotemporal distribution description of methods for the different analyses performed on the entire Nile margin over the last 200,000 years on core MS27PT is provided in the Supplementary (Ducassou et al. 2007, 2009; Migeon et al. 2010). These Material 1. 123 The western Nile deep-sea fan

Fig. 1 a Map of North East A Africa and location of sites or regions referred to in the text and ﬁgures. b Bathymetry of the Nile margin showing the location of the gravity core MS27PT (N31°47090; E29°27070, 1,389 m water depth). Locations of other climate records are shown for comparison

Results and interpretation mud beds interpreted by Ducassou et al. (2008)asrepre- senting ﬂood events of the Nile River. Among these clastic Age model, lithology and sedimentation rate mud beds, six have speciﬁcally been studied in this paper because their thickness is [0.5 cm. The sedimentation rate The 315-cm MS27PT core covers the last 25,800 years with (SR) is highly variable between 1.8 and 108 cm/ka (Fig. 2). bioturbated sediment for the last deglacial period and the The highest SR values ([50 cm/ka) are observed for the late Holocene and laminated sediment from 10 to *6ka early Holocene (10–8.4 ka). The lowest SR values are (Fig. 2). The main feature of the laminated interval is the observed for the LGM (1.8 cm/ka) and between 4 ka presence of several individual dark, millimetre-scale clastic (2.8 cm/ka) and 3 ka. A gradual increase in SR from

123 M. Revel et al.

Fig. 2 Age/depth relation versus sedimentation rate for core are [0.5 cm, are highlighted by grey bands. A zoom of the carbonate- MS27PT. Linear interpolation was used between the nineteen 14C rich facies–clastic mud rich-facies transition is presented based on the calendar ages (red dots). Inferred sedimentation rates are given in cm/ sediment thin section. The 14C calendar ages are listed in the ka (right axis). Six clastic mud beds, the thicknesses of which Supplementary Material 2 (color ﬁgure online)

1.8 cm/ka to 100 cm/ka is documented during the last de- Globigerinoides ruber var. alba are documented from *15 glacial period (15–10 ka). A gradual decrease in two steps to *9.6 ka corresponding to the progressive transition (8.4–6.5 and 6.5–3.2 ka) is recorded at the end of the AHP. between bioturbated and laminated facies (Fig. 3a). Further up section, the highest sulphur intensities, highest TOC Oxygen isotope of planktonic foraminifera and redox- contents ([1.3 %) and the most negative d18O values are sensitive elements as a proxies for Nile freshwater observed between 9.6 and 6.5 ka. Several X-ray diffraction outﬂow analyses (not shown) indicate high amounts of pyrite (*3 %) associated with the laminated facies. As such, Progressive increase in sulphur, total organic carbon (TOC) the high concentration of sulphur is related to the forma- and less positive d18O values of planktonic foraminifera tion of authigenic pyrite under anoxic conditions. This

123 The western Nile deep-sea fan

123 M. Revel et al. b Fig. 3 a Down core characteristic of MS27PT: X-ray images, individual clastic mud beds display sharp increases in Fe/ geochemical records (TOC from Revel et al. (2010), Mn, S wt%, Al, Ti/Al ratios and grain size, whereas the CaCO3 is less log Fe/Ca ratio), log smectite/illite ratio, 87Sr/86Sr ratio and eNd(0), terrigenous and carbonaceous fluxes, Pediastrum algae and Podo- than 1 % (Fig. 3b). Ti and Fe are covariant indicating that carpus pollen (concentration/concentration of sediment) and d18O changes in Fe content are mainly related to grain size (and (Revel et al. 2010) along depth (cm). Relative Fe/Ca ratios are so to mineral fractionation) rather than to oxidation– indicated by grey line. Absolute Fe contents in wt% (red dots). The reduction processes. The grain size distribution frequency nineteen 14C calendar ages are plotted. The main information deduced from the different proxies (highlighted by different colours) has been curve indicates two dominant modes at 2 lm and 20 lm added to the right of the figure. The major changes are underlined by (Fig. 3c). The grain size distribution curve of the individual dotted lines at 15, 8, 6.5 and 3.2 ka. All 87Sr/86Sr and eNd(0), pollen laminae shows an increase (1) in modal size from 2 to data are listed in the Supplementary Material 2. b Focus on laminated 6 lm and (2) in % of the mode at 20 lm indicating the facies characterized by several clastic mud beds: Fe/Al, Ti/Al ratios and grain size along depth (cm). c The grain size distribution presence of higher current speeds able to transport larger frequency curve (color figure online) particles such as Ti-rich heavy minerals. An increase in the magnitude of the Nile floods could explain such a pattern.

geochemical pattern is ascribed to decreased bottom-water Clay mineralogy as a proxy for Nile River-borne ventilation as has also been demonstrated in the detailed and Saharan dust contributions geochemical studies (e.g. Thomson et al. 2006). The Mn peak following the Fe peak along with the decrease in The smectite/illite ratios in core MS27PT are illustrated in sulphur and TOC % is interpreted as a result of the re- Fig. 3a. The clay assemblage is mainly composed of establishment of oxic conditions. The d18O variations are smectite (65–94 %), kaolinite (4–22 %) and illite due to a combination of changes in sea surface tempera- (1–11 %). Palygorskite, a characteristic mineral in dust ture, salinity and of the d18O of seawater. However, several from the northwestern African deserts (Caquineau et al. works have shown that enhanced freshwater influx into the 2002), was not detected in core MS27PT in agreement with EMS at sapropel time can be demonstrated by negative results obtained in the distal part of the Nile deep-sea fan anomalies of d18O measured on surface-dwelling plank- (Zhao et al. 2012). In detail, the depth intervals tonic foraminifera (e.g. Emeis et al. 2000) and pore water 330–294 cm (corresponding to the LGM) and 10–9 cm analyses (Paul et al. 2001). As core MS27PT is very close (dated at about 3.2 ka) are characterized by a decrease in to the Nile mouth, we suggest that the negative d18O smectite/illite ratio. The depth interval 294–25 cm (dated associated with higher terrigenous fluxes is probably from 15 to 6.5 ka) is characterized by high values of the reflecting the increased delivery of freshwater originating smectite/illite ratio. In the modern Nile Rivers sediments, from the Nile River. smectite represents up to 70–80 % of the clay fraction and is mainly a weathering product of basaltic rocks in the Elemental composition and terrigenous flux as proxies Ethiopian highlands, whereas illite constitutes only 5–10 % of detrital contribution (Stanley and Wingerath 1996). Illite and kaolinite in the Levantine Basin are mainly transported by winds (Hamann The Nile margin is suitable for geochemical studies to trace et al. 2009). Illite constitutes 42 % of the clays in Saharan source sediment because of the contrasting mineralogical dust (Scheuvens et al. 2013). Therefore, we consider signatures between detrital Blue Nile material derived from smectite to be a proxy for Nile input, whereas high pro- Ethiopian Tertiary volcanic rocks rich in Fe, and the Sah- portions of kaolinite and illite are taken as a tracer of aran/Libyan dusts derived from crystalline basements of Saharan aeolian input. Neoproterozoic to Archaean ages (Weldeab et al. 2002). The Fe/Ca ratios and Fe % in core MS27PT associated with Strontium and neodymium isotopic compositions: terrigenous fluxes (Fig. 3a) can be used as proxies for the proxy for the Saharan dusts, and White and Blue Nile Blue/Atbara Nile discharges intensity and frequency (see River sediments the plot of Fe, Ti and Ca % in Supplementary Material 1 for developments about the interpretation of Fe data). In The Sr and Nd isotopic compositions versus depth of the detail, the depth intervals 330–300 cm (corresponding to core MS27PT are presented in Fig. 3a. The clastic Nile the LGM) and 10–9 cm (dated at about 3.2 ka) are char- margin sediment is a three-component system: Saharan/ acterized by a decrease in the Fe/Ca ratio, the lowest Fe % Lybian dusts, Blue/Atbara Nile and White Nile inputs. The (\3 %) and terrigenous fluxes (Fig. 3a). The depth interval Sr and Nd isotopic compositions of the present-day Blue 294–56 cm (dated from 15 to 8 ka) is characterized by high Nile sediment are characterized by eNd(0) [-4 and Fe/Ca ratio, terrigenous fluxes and Fe % ([6 %), sug- 87Sr/86Sr ratios \ 0.707, whereas those of the Saharan dust gesting an increase in the Nile flood magnitude. The are characterized by 87Sr/86Sr ratios [0.715 and 123 The western Nile deep-sea fan eNd(0) \-11 (Padoan et al. 2011). The plot of the Cyperaceae pollen increase indicates change in terrestrial 87Sr/86Sr versus eNd(0) isotopic signature of core MS27PT vegetation probably in response to increased Nile flow. and potential source area samples, which allow identifying Podocarpus occurrences are documented through signifi- and quantifying the source contribution variations, are cant concentration from 260 cm (12 ka) to 38 cm (7.2 ka). provided in Supplementary Material 3. The abundance of Podocarpus pollen in core MS27PT In Fig. 3a, a rapid change in Sr and Nd isotopes is mark long-distant transport from the Ethiopian tropical recorded at 15 ka, whereas the mid-Holocene transition region since this taxon is at present only observed in these occurred in two main steps (8.4–6.5 and 6.5–3.2 ka). areas and has never been recorded in the Nile delta zone Between 15 and 8.4 ka strong Blue/Atbara Nile, terrige- nor in the Nile River valley (Rucina et al. 2009; McGlynn nous material inputs are indicated by low Sr radiogenic et al. 2013). values and less negative eNd(0) ratios. From 8.4 to *4 ka, we suggest that sediments consist of a mixture of Blue and White Nile materials and we hypothesize a differential Discussion progressive weakening of Blue Nile material as a source with respect to White Nile material. While water temperature changes over the EMS are quite The highest contribution of Saharan dusts is documented well constrained for the Holocene (Essalami et al. 2007; from 4 to 2 ka and from 25 to 15 ka indicating an aridifi- Castaneda et al. 2010), changes in the hydrological regime cation along the Nile catchments. In detail, a quantification are more difficult to quantify due to the lack of direct (see Supplementary Material 3) indicates a Saharan dust precipitation proxies. Marine sediments close to the river contribution of 88 % at 20 ka; 80 % at 16 and 3.2 ka and mouths have the advantage of recording flooding events. 77 % at 630 years suggesting that the 3.2 ka dry period Thus, variation in terrigenous input can potentially be used could have generated more windblown dust than at present. to reconstruct the palaeohydrological regime of the drain- age basin. Nevertheless, because changes in vegetation Palynology and/or geomorphology can modify soil stability, the relationship between precipitation and terrigenous input may This paper only presents the concentration curves of three not always be direct (Macklin et al. 2012). To overcome taxa: (1) Pediastrum (Fig. 3a), a fresh water algae that is these ambiguities, we use a combination of independent well recognized in palynological slides as cell colonies and proxies measured on the same core depth. On the basis of the occurrence and repartition of which is typically linked Sr/Nd isotopes and clay mineralogy, we trace geographical to river discharges (Matthiessen et al. 2000) and which provenance (i.e. Nile material and Saharan dusts). The therefore may throw light on the Nile River activity; (2) terrigenous flux is used to quantify the lithogenic input. Cyperaceae pollen (Supplementary Material 2), a sensitive The palynological analyses are used to document change in marker of precipitation over the Nile headwaters (Bern- fluvial transport potential and/or onland vegetation. The hardt et al. 2012) and (3) Podocarpus (Fig. 3a), a tree that combination of these proxies provides detailed information today grows between 1,200 and 1,600 m in the tropical on the timing and magnitude of changes in the Nile mountains (Schu¨ler et al. 2012) and the abundance of hydrological regime for the last 21,000 years. which among the pollen in core MS27PT can be taken to reflect distant inputs from the Ethiopia Highlands through Timing of Nile hydrological changes and their relation river transport. The interpretation of the pollen assem- with the East African monsoon during the last blages is based on the modern climatic plant relationships 21,000 years (Woodward 1987). These pollen and algae colonies are only identified from 260 cm (12 ka) to 36 cm (7 ka). At Arid Last Glacial Maximum and the last deglaciation the beginning of the Holocene, following the climate amelioration, an increase in total pollen concentration Palaeoclimatic records of the LGM in the EMS suggest that indicates that vegetation began to re-expand in the Nile the region was cooler, dryer and dustier than at present catchment. We suggest that Pediastrum cell colonies are a (Calvert and Fontugne 2001; Castaneda et al. 2010). In sensitive marker of freshwater in the Nile delta. Several core MS27PT, we observe low terrigenous flux, with the peaks should, therefore, reflect pulses of fresh water in the most negative eNd(0) values between 26 and 15 ka Nile delta area that may represent recurrent floods. Recent (Fig. 3a), suggesting a drastic decrease in Nile suspended works from Burullus Lagoon in the Nile delta documented matter discharge. This is supported by the absence of decrease in Cyperaceae pollen at 6–5.5, 4.2 and 3 ka, and pollen and algae indicating either low vegetation cover on they interpreted the decrease as a marker for diminished the Nile delta and catchment area or a low Nile River Nile flow (Bernhardt et al. 2012). We suggest that discharge. The low eNd(0) as well as low smectite/illite 123 M. Revel et al.

Fig. 4 The log Fe/Ca, log smectite/illite ratio (S/K), eNd(0) isotopic comparison (Marshall et al. 2011). Sea-level data from Arz et al. composition, Pediastrum algae and Podocarpus pollen, and d18O (2007) is reported. June insolation at 15°N (Berger and Loutre 1991) curve data of the core MS27PT are presented for the last is reported for comparison. S1 sapropel 1, LGM Last Glacial 21,000 years. The 87Sr/86Sr isotopic compositions recorded in the Maximum, BA Bo¨lling/Allerod, YD Younger Dryas, H1 Heinrich Somalian coast are included for comparison (Jung et al. 2004). The Ti event 1. Supplementary Materials (3) (mg/g modelled) recorded in the lake Tana is also reported for ratios (Fig. 4) suggest a dominance of aeolian dust from the 2012) and triggered a *120 m rise in global eustatic sea Sahara. These results are consistent with the increased level (Fairbanks 1990). In the Red Sea, this global sea-level 87Sr/86Sr ratios (0.708) observed in a speleothem from the rise recorded (Arz et al. 2007) is characterized by an initial Jerusalem Cave (Frumkin and Stein 2004). This pattern increase in *20 m from 18 to 14.6 ka followed by a larger was interpreted as an indication of higher dust fluxes dur- increase from 14.6 to 13 ka (Fig. 4). At *15 ka, MS27PT ing the LGM. This increase in Saharan dusts during the eNd(0) values display a rapid shift towards less negative LGM is synchronous with the known global enhanced values and higher proportions of terrigenous material in our atmospheric dust loads (Biscaye et al. 1997). Glacial site (2–25 cm/ka, Fig. 2). Because there is no correlation periods were characterized by a widespread aridity in between sea-level rise and terrigenous flux increase, we Africa and hyperarid conditions in the Sahara. The con- hypothesize that changes in sea level have a minor influ- sequent reduction in savannah-like vegetation, and ence on sediment dispersal and deposition into the studied decrease in soil cohesiveness throughout the northern site. This rapid geochemical change is also documented by Sahara, favoured higher dust production. a rapid change in lithology along the sedimentary During the deglaciation, the increase in summer inso- sequence. From the thin section (Fig. 2), this change is lation (Fig. 4) that followed the LGM favoured the retreat reflected by a continuous change in colour without any of the northern hemisphere ice sheet (e.g. Darnault et al. erosional surface or turbidite layer. This pattern provides

123 The western Nile deep-sea fan an argument for increased Nile River clay material inter- the Ethiopian highlands. Based on the timing of Nile fluvial preted as enhanced Nile flood intensity and related to discharge with respect to the EAM period, we demonstrate increased precipitation on the Ethiopian highlands follow- a quasi-direct relationship between maximum North ing the summer insolation. However, meltwater from gla- Hemisphere summer insolation and EAM maximum cier retreat in the Ethiopian highlands could also have (Fig. 4). caused higher Nile discharge as early as 16 ka. Indeed, This AHP has already been documented in several African sedimentological and geochemical studies on Lake Garda lakes from about 12–5 ka (e.g. Marshall et al. 2011;Le´zine Gurach document a progressive retreat of a high-altitude et al. 2011b;Foersteretal.2012). However, when examined (*3,000 m) glacier in the Bale Mountains after 17 ka in detail, a delay is observed between the beginning of ter- (Tiercelin et al. 2008). Thus, a part of the lower eNd(0) rigenous input associated with the change in source at *15 ka signature (Fig. 4) from 16 to 14.5 ka is coupled to a pro- and the gradual change towards more negative d18O. A pos- gressive increase in terrigenous flux (Fig. 3a), which could sible explanation for this is that the Nile discharge at the be explained by the Ethiopian glaciers retreat generating beginning of the AHP was not in itself sufficiently important meltwater and glaciogenic sediments. The onset of higher to cause a freshening that would induce a decrease in venti- Blue/Atbara Nile discharges around 15.6 ka, as recorded in lation (and consequently the sapropel formation at 10.8 ky core MS27PT, is consistent with the record of Lake Tana, BP), which required an additional freshwater sources and/or which is the source of the Blue Nile. Marshall et al. (2011) more intense freshening. This could explain the delay between demonstrated that the lake began to overflow into the Blue the beginning of terrigenous input at *15 ka, the beginning of Nile at 15.3 ka, whereas the Blue Nile discharge must have the d18O change at 12 ka and the beginning of the sapropel been greatly reduced until this time. formation at *10 ka. In other words, the decoupling could be The most negative values in eNd(0), low smectite/illite due to a direct response of the terrigenous input, i.e. smectite and Fe/Ca ratios in core MS27PT between 12.8 and 12.3 ka clays, which are stocked on the Nile deep-sea fan, whereas the are synchronous with the Younger Dryas cold episode freshwater flow from the Nile River is diluted into the eastern observed in the northern hemisphere between ca. 12.8 and Mediterranean surface water mass and the subsequent 11.6 ka (Bard and Kromer 1995) in the EMS (Essalami decrease in water column ventilation occurred gradually over et al. 2007; Castaneda et al. 2010) and in African lakes *5,000 years. These two proxies from a single archive seem (Garcin et al. 2007). to record the same change with a slightly different time resolution. The high-resolution study of Osborne et al. (2008, The African Humid Period 2010) based on planktonic foraminiferal d18OandeNd data seems to indicate a freshwater addition to the surface Medi- The onset of the AHP in core MS27PT is indicated by a terranean before sapropel 5 deposition, in accordance with rapid increase in eNd(0) at 15.6 ka, a more gradual what we found in our study for sediment load/transport pre- increase in terrigenous flux from 15 to 8 ka interrupted by a ceding earlier sapropel formation (Caley et al. 2011). decrease in these parameters between 12.8 and 12 ka The transition between the AHP and the arid conditions (Fig. 4). These parameters, together with higher Podocar- is recorded by gradual decrease in sediment derived from pus pollen (tropical high-altitude taxa), are a strong indi- Ethiopian basalts. Two main steps at 8.4–6.5 and cation of enhanced Nile flood intensity linked to 6.5–3.2 ka are evidenced (Fig. 4). The endpoint of aridi- precipitation on land and, in particular, to high terrestrial fication along the Nile catchments is recorded around run-off from the Ethiopian highlands where these trees 3.2 ka. The Nd isotope results demonstrate (1) a relatively developed. In parallel, progressively less positive d18O constant basaltic Blue Nile source for terrigenous material values in G. ruber are observed from 15 ka and the most from 15 to 8 ka with the highest terrigenous fluxes (except negative values are reached from 9.6 to 6.5 ka, consistent during the YD interruption), (2) a possible shift towards a with a maximum of freshwater flow from the Nile River. White Nile source from 8 to *4 ka and (3) a Saharan dust Higher magnitude centennial-scale fluctuations of Nile source from 4 to *2 ka. discharge are dated at 8.5, 8.7, 8.8, 9 and 9.1 ka (within the Compared with regional African records of continental uncertainties of the AMS 14C dates for marine cores which climatic conditions, our record shows that the onset of the could be a couple of 100 years) from the preserved flood precipitation decrease can be pinpointed in the Nile margin laminae (Fig. 3a). at *8 ka, and the proposed two step transition is con- We suggest that the EAM maximum between 15 and temporaneous with: 8 ka is responsible for a larger sediment load capability of 1. The decrease in the Lake Tana water-level docu- the Nile leading to higher freshwater and sediment inputs mented by Marshall et al. (2011). Two drought into the EMS. This can be attributed to a stronger summer episodes occurred at 8.4 and 7.5 ka (interpreted as a monsoon and/or a northward migration of the rain belt over 123 M. Revel et al.

southward shift in the monsoon front) and a precip- to 6.3 ka recorded in the South Adriatic Sea (Siani et al. itation decline after 6.8 ka. Furthermore, another dry 2013). Thus, the reduction in intermediate/bottom ventila- episode is indicated at *4.2 ka (Fig. 4). tion, which characterized the Holocene especially during 2. The humidity increase in the AHP with a remarkably insolation maxima, could be due to changes in the eastern abrupt onset and a gradual termination from the Chew Mediterranean net evaporation (Rogerson et al. 2012). The Bahir basin in southern Ethiopia (Foerster et al. 2012). Eastern Mediterranean ventilation changes show an evolu- 3. The Somalian coast aridification (Jung et al. 2004) tion that also coincides with the primary oceanic produc- with a first aridification step at 8.5 ka followed by an tivity off western Iberia and in the Alboran basin (Abrantes unstable transitional period up to 6 ka (Fig. 4). 1990; Barcena et al. 2001). These authors have concluded 4. The precipitation decrease recorded in Holocene from diatom data that the early Holocene is characterized by stalagmites from Qunf and Hoti caves in Oman, which lower productivity followed by a re-establishment of more showed that the mean summer ITCZ continuously productive conditions at about three 14C ka. We postulate migrated southwards from 7.8 ka to Present (Fleit- that this feature of ventilation change in the western Medi- mann et al. 2007). terranean could be part of a response to large volumes of 5. The lake Yoa (Northern Chad) vegetation record freshwater delivered primarily by the Nile River from 15.7 indicating a gradual desertification from 5.6 to 2.7 ka to 8 ka into the EMS. The maximum of Nile freshwater BP and the development of Mediterranean plants at outflow from *10 to 6 ka (Fig. 3) could be responsible for about 2.7 ka BP (Le´zine et al. 2011a, b). a weakening of the surface water mass ventilation (Rhodes gyres), enabling surface water to store more heat which in Considering all this information, we propose that at 8 ka, turn induced more evaporation. This coupling between water the rain belt started migrating southwards resulting first in mass temperature and atmospheric conditions would induce reduced precipitation over the Ethiopian highlands. Then more precipitation to the south of the Mediterranean basin. between *8and*6 ka Nd isotopes seem to indicate a This hypothesis is consistent with the recent work of Magny change in source with a decrease in Blue Nile material and a et al. (2013), which suggests that the Holocene humid period slight increase in White Nile material, whereas the d18O (9–4.5 ka) has been favored by an increase in winter and record still indicates negative values until 6.5 ka, suggesting summer precipitation in the southern Mediterranean area. continued significant White Nile discharge. Another possi- bility is that the freshwater signal could be related not only to the Nile discharge but also to a northern Libya and Egypt Conclusions borderlands contribution related to the local Mediterranean precipitation regime (Scrivner et al. 2004; Osborne et al. The deltaic archives are particularly well suited to inves- 2010; Meijer and Tuenter, 2007; Verschuren et al. 2009). tigate changes that affected both the continental and marine This study also highlights an increase in Saharan aerosols domains. The Nile margin core MS27PT provides a unique blown from the Libya coast between 4 and 2 ka. We show archive from which a several proxies dataset allow for the that this dry period should have generated more windblown reconstruction of fluctuations in the Saharan dust input and dust than at present, as it is also proposed along the Atlantic the Blue/White Nile River palaeohydrology during the last African margin (McGee et al. 2013). 21,000 years. The interpretation of the proxies in combination with previously published marine and lacustrine The interplay between Mediterranean and African data allows us to reconstruct the evolution of the EAM monsoon Holocene climate changes regime during the onset and termination of the AHP.

A similar humid period based on pollen records has been Acknowledgments We are grateful to Paul Capiez and Thierry deﬁned for the circum-Mediterranean region. Three intervals Courp for major element and mineralogy analyses, respectively. We are also grateful for the constructive reviews undertaken by Dupont are described Magny et al. (2013): (1) a primarily humid Lydie and Fatima Abrantes and the anonymous reviewer. We period (11.5–7 ka); (2) a transition phase (7–5.5 ka) and (3) acknowledge the ﬁnancial support of the French RELIEFS pro- an arid period (5.5–0 ka). Southern European lake levels gramme. We thank the Laboratoire de Mesure du Carbone 14, UMS 14 also indicate primarily humid conditions in the early Holo- 2572, ARTEMIS in Saclay for C measurements. cene and drier conditions after 5 ka (Harrison and Digerfeldt 1993). The increase in Nile runoff into the Levantine basin for 7,000 years (15–8 ka) coincides with the reduction in References bottom ventilation in the Mediterranean Sea (Rohling et al. 2002; Cacho et al. 2006; Melki et al. 2009; Schmiedl et al. Abrantes F (1990) Increased upwelling off Portugal during the last 2010;Toucanneetal.2012) and a lower salinity from 11.5 glaciation: diatom evidence. Mar Micropaleontol 17:285–310

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