Tracking atmospheric and riverine terrigenous supplies variability during the last glacial and the Holocene in central Mediterranean

. A multiproxy study – coupling mineralogical, grain size and geochemical approaches – was used to ten-tatively retrace eolian and ﬂuvial contributions to sedimentation in the Sicilian–Tunisian Strait since the last glacial. The eolian supply is dominant over the whole interval, excepted during the sapropel S1 when riverine contribution ap-parently became signiﬁcant. Saharan contribution increased during the Bølling–Allerød, evidencing the persistence of aridity over North Africa although the northern Mediterranean already experienced moister and warmer conditions. The Younger Dryas is marked by proximal dust inputs, high-lighting intense regional eolian activity. A southward migration of dust provenance toward Sahel occurred at the onset of the Holocene, likely resulting from a southward position of the Inter Tropical Convergence Zone that was probably associated with a large-scale atmospheric reorganization. Finally, a peculiar high terrigenous ﬂux associated with drastic modiﬁcations of the mineralogical and geochemical sediment signature occurred during the sapropel S1, suggesting the propagation of ﬁne particles derived from major ﬂoodings of the Nile River – resulting from enhanced rainfall on northeastern Africa – and their transportation across the Sicilian–Tunisian Strait by intermediate water masses.


Introduction
The Mediterranean is a transitional area where northern and southern climatic influences tightly interact (e.g.Magny et al., 2009).Previous studies revealed that moist conditions developed in the Mediterranean during the early Holocene while a progressive orbitally-driven trend to aridification characterized the mid-and late Holocene.This climatic evolution displays a contrasting regional pattern, with an abrupt transition at 5.7 kyr BP on the western Mediterranean and a more gradual transition in the eastern Mediterranean (Cheddadi et al., 1991;Ariztegui et al., 2000;Magny et al., 2002Magny et al., , 2007;;Frigola et al., 2007;Tzedakis, 2007;Sadori et al., 2008;Roberts et al., 2008Roberts et al., , 2011;;Jalut et al., 2009;Peyron et al., 2011).Precipitation estimations based on lake levels, fire and pollen association, and on speleothem and isotope records also provide evidences of contrasting seasonality across the Mediterranean during the Holocene (Magny et al., 2003(Magny et al., , 2007(Magny et al., , 2009;;Zanchetta et al., 2007;Tzedakis, 2007;Roberts et al., 2008;Peyron et al., 2011;Vannière et al., 2011), but estimating the respective atmospheric and oceanic control on Mediterranean climatic evolution through their impact on eolian and fluviatile systems is still complex.

V. Bout-Roumazeilles et al.: Atmospheric and riverine supplies since last glacial in central Mediterranean
A multiproxy study of the terrigenous supply would help in retracing the variability of both eolian and fluviatile systems.
The nature and provenance of fine-grained terrigenous particles in the Mediterranean is mainly controlled by the balance between riverine supplies driven by precipitation regime of the surrounding continents and eolian supplies from the Sahara (Loÿe-Pilot et al., 1986;Martin et al., 1989;Bergametti et al., 1989b;Matthewson et al., 1995;Guerzoni and Chester, 1996;Foucault et Mélières, 2000;Goudie et Middleton, 2001).Several studies used deep-sea clay mineral associations as tracers of source regions and as indicators of water mass fluctuations (Chamley, 1975;Tomadin and Lenaz, 1989;Petschick et al., 1996;Fagel et al., 1997;Gingele et al., 2001;Liu et al., 2003;Boulay et al., 2005;Fagel and Hillaire-Marcel, 2006;Colin et al., 2010).Indeed, the mineralogical nature of sediments, which depends on the petrographic characteristics of their source areas (e.g.Bout-Roumazeilles et al., 1999;Sionneau et al., 2008), has been used to retrace detrital particles provenance in the Mediterranean and thus to assess the respective eolian and riverine contributions to deep-sea sedimentation and provide valuable information on the sediment propagation pathways (Caquineau et al., 1998(Caquineau et al., , 2002;;Bout-Roumazeilles et al., 2007;Ehrmann et al., 2007a;Hamann et al., 2009;Kandler et al., 2009;Formenti et al., 2011a,b).The clay mineral fraction also provides information on climatic conditions, such as precipitation and runoff patterns over the adjacent continents (Chamley, 1989;Montero-Serrano et al., 2009, 2010), as well as on the dynamics of river inputs (Pinsak et Murray, 1960).The geochemical signature of clays would complementary help in characterizing the main sources and evidencing specific transportation patterns or transfer processes (Haug et al., 2001;Kandler et al., 2009;Formenti et al., 2011a,b).Grain size distribution, which is primarily driven by physical processes (erosion, deflation, transportation, settling), will help in constraining the main transportation modes (Ehrmann et al., 2007b;Montero-Serrano et al., 2009;Sionneau et al., 2010).Within this framework, combining clay mineralogy with grain size analyses and geochemical tracers would allow for retracing significant variations of detrital supply in the Siculian-Tunisian Strait and inferring any major modifications of both atmospheric and oceanic terrigenous transfers patterns since the last deglaciation.

Oceanic circulation
The Mediterranean Sea is a concentration basin where evaporation exceeds precipitation plus freshwater discharge.The surface Atlantic water entering from the Gibraltar Strait transforms into Modified Atlantic Water (MAW) as it flows eastward (Fig. 1).The Levantine Intermediate Water (LIW) flows westward towards the western Mediterranean (Fig. 1) via the relatively shallow Siculo-Tunisian Strait.The dense Eastern Mediterranean Deep Water (EMDW) fills the deep basin (> 800 m depth) (Wüst, 1961;Pickard et Emery, 1982;Malanotte-Rizzoli and Hecht, 1988;Klein et al., 1999).This general pattern is highly dependent on environmental conditions, including eolian activity and precipitations distribution, and recent alteration of the ocean/atmosphere coupling has resulted in enhanced deep-water formation in the Aegean Sea (Klein et al., 1999;Malanotte-Rizzoli et al., 1999).

Present-day river supplies
At present day, a major part of detrital clays is supplied to the Mediterranean via rivers (Fig. 1), the most important being the Nile River (from 120 to 230 × 10 6 t yr −1 ) discharging in the eastern Mediterranean.The Po River, discharging into the Adriatic Sea (17 × 10 6 t yr −1 ), and southeastern European rivers associated with Turkish rivers provide about, respectively, 30 × 10 6 t yr −1 and 17 × 10 6 t yr −1 to the Aegean Sea and are major contributors to sedimentation into the central Mediterranean (Holeman, 1968;Milliman and Syvitski, 1992;Stanley et al., 1992;Ehrmann et al., 2007b;Garzanti et al., 2006;Hamann et al., 2009).The detrital supply through the Dardanelles Strait is reduced (0.9 × 10 6 t yr −1 ) because most sediment is trapped within the Black Sea and Marmara Sea (Erhmann et al., 2007a).Central Mediterranean is not affected by detrital supply from the Rhône River nor from the Ebro River, which discharge into the western Mediterranean.Finally, riverine contribution to the central Mediterranean from areas bordering the southern shores is reduced due to the narrow drainage basin and sparse rainfall (Martin and Milliman, 1997).Nevertheless, the Medjerda and Miliane oueds, discharging into the Gulf of Tunis (Fig. 1), may influence the sedimentation in the study area.

Eolian supply
The estimation of the present-day eolian contribution to deep-sea sedimentation varies from 3.9 × 10 6 t yr −1 to 120 × 10 6 t yr −1 (Bergametti et al., 1989c;Matthewson et al., 1995;Guernozi and Chester, 1996;Goudie and Middleton, 2001).This wide range of estimation reflects intermittent and seasonal variations of dust outbreaks toward Europe (D'Almeida, 1986;Guerzoni et al., 1997).Satellite imagery, back trajectories and observations indicate that fine particles originating from North Africa (Fig. 1) are transported by southerly/southwesterly winds (Scirocco, Ghibbli) toward the central Mediterranean (Ganor et al., 2000;Goudie and Middleton, 2001;Washington and Todd, 2005;Engelstaeder et al., 2006;Israelevich et al., 2012).Arid and semi-arid regions of North Africa (Fig. 1) are the main areas of production of dust all year long (Coudé-Gaussen, 1982;Pye, 1987): Tunisia and northern Algeria, Morocco and western Sahara, the South Algeria-Mali region, the Bodélé depression and the southern Egypt-northern Sudan (Brooks and   (Saliot, 2005).The inset shows the detailed bathymetry and oceanic circulation of the study area.Legrand, 2000;Caquineau et al., 2002;Israelevich et al., 2002;Prospero et al., 2002;Goudie, 2003;Formenti et al., 2011a).Most of the northward aerosols transportation across the Mediterranean is linked to the seasonal displacement of cyclones over the Mediterranean (Folger, 1970;Guerzoni et al., 1997;Moulin et al., 1997;Rodriguez et al., 2001).Maximum aerosol is observed over central and eastern Mediterranean in spring (Luck and Ben Othman, 2002;O'Hara et al., 2006) and summer when anticyclonic conditions initiate drought over the area (Prospero et al., 2002;Koren et al., 2006;Roberts et al., 2008;Israelevich et al., 2012).Although Saharan fine particles are mainly produced during warm and hydrolysis period, it is transported from paleosols and small consolidated formations during a dry intervals when vegetation is reduced and eolian erosion becomes more efficient (Brooks and Legrand, 2000).During summer, the Saharan depression favors the transport of dust over the western basin, progressively moving toward central Mediterranean (toward Corsica and Italy) at the end of summer (Bergametti et al., 1989a;Moulin et al., 1997;Barry and Chorley, 1998).

Materials
The core MD04-2797CQ (36 • 57 N-11 • 40 E) was taken during the PRIVILEGE-PRIMAROSA cruise in 2004 at 771 m water depth in the Sicilian-Tunisian Strait (Fig. 1).The Siculo-Tunisian Strait separates the Tunisian and Sicilian continental shelves.Quaternary sedimentation on the southern part of the strait is controlled by marine and eolian supplies.The morphology of the Strait is characterized by two troughs separated by a relatively shallow sill, which likely constrains the westward flow of the deeper part of the saline Levantine Intermediate Water (Astraldi et al., 2002).
The upper 6 m of the core are composed of homogenous greenish-grey clays, with some silty clay intervals between 440 and 430 cm and between 155 and 140 cm.The uppermost part of the core is composed of orange-brown clay (Rouis-Zargouni, 2010;Rouis-Zargouni et al., 2010).A complete lithological description of the core is given in Rouis-Zargouni (2010).Smear slides of sediments were taken on 0.5 cm thick layers every 10 cm all along the 6 upper meters of the core.Microscopic observations did not reveal any evidence of volcanic material, but the occurrence of tephras cannot be ruled out in the absence of a detailed highresolution investigation.According to its geographical setting, the sedimentation of the core is likely influenced by both marine and eolian supplies.The scarcity of rivers along the North African margin and their non-permanent characteristic prevents the area from any major and perennial fluviatile contribution (O'Hara et al., 2006).By contrast, the area is subjected to an intense wind-driven supply of dust all year long, carried along by the dominant south-southwestern low-altitude dusty winds (Scirocco) (Barry and Chorley, 1998) which reach the Adriatic Sea by crossing the Mediterranean (Fig. 1).According to the main wind system, the proximal source of dust for the Sicilian-Tunisian Strait is likely the Tunisia/northern Algeria area (Coudé-Gaussen, 1987).The Mali/Algeria frontier or the Moroccan Atlas (Brooks and Legrand, 2000;Caquineau et al., 2002;Prospero et al., 2002;Goudie, 2003) could also contribute to sedimentation either during intense dust episodes or when a specific and/or seasonal atmospheric configuration affect the main wind directions (Bergametti et al., 1989a,b;Ganor et al., 1991;Pye, 1992;Avila et al., 1997;Goudie and Middleton, 2001;Prospero et al., 2002;Washington and Todd, 2005).We compare our data with previously published δ 18 O and δ 13 C records and with the reconstructed salinity ( δ W ) from core MD04-2797 (Essallami et al., 2007).These data will help in constraining the hydrologic properties of the overlying water masses and evidencing alterations of freshwater input/evaporation budget in the central Mediterranean.The reliability of the δ W reconstruction is high when sea surface temperature (SST) changes are small (Essallami et al., 2007).

Chronostratigraphy
The age model is based on 13 AMS 14C age datings (Rouis-Zargouni, 2010;Rouis-Zargouni et al., 2012).The ages were corrected using reservoir ages of 400 yr during the Holocene, the Younger Dryas and the late glacial, 560 yr during the Bølling-Allerød (B-A) and 800 yr during the Heinrich event 1 and Older Dryas (H1-OD), following recommendations of Siani et al. (2001) (Table 1).The top of the core is dated at 668 yr cal BP, suggesting that the sea-sediment interface was not preserved during coring.The terrigenous mass accumulation rates (MART) were calculated as follows: MART (g kyr −1 cm −2 = linear sedimentation rates (cm kyr −1 ) • dry density (g cm −3 ) • (100 − (% opal + % CaCO 3 )).The total CaCO 3 percentage was determined using a Bernard calcimeter and following standard procedures with a precision < 5 %.Total carbonate corresponds to the sum of biogenic carbonate plus detrital carbonates provided by eolian and riverine supply from Northern Africa.A recent review of the chemical composition of the main African mineral dust (Formenti et al., 2011b) indicates that dust originating from Tunisia and Algeria, and from the Mali-Algerian border, containing up to 50 % carbonates (Paquet et al., 1984;Coudé-Gaussen, 1991;Alastuey et al., 2005).A maximum carbonate content (5-70 %) is evidenced in western Saharan dust (Avila et al., 1997;Khiri et al., 2004;Kandler et al., 2009).By contrast, dust from central Libya (O'Hara et al., 2006) is characterized by intermediate carbonate content (1-25 %).Considering an eolian contribution toward the Mediterranean ranging from 10 to 50 % (Loÿe-Pilot et al., 1986;Loÿe-Pilot and Martin, 1996;Guerzoni et al., 1997), carbonate of eolian origin may account for 2 to 35 % of the sediment, whereas total carbonate ranges from 30 to 50 % (Fig. 4).The importance of the eolian carbonate may thus lead to underestimation of MART, especially during the B-A when CaCO 3 represents 50 % of total sediment, but should be less significant over the Holocene as the carbonate content is rather stable.

Clay minerals
The analyses were performed according to the protocol described in Bout-Roumazeilles et al. (1999).XRD (Xray diffraction) determinations were performed using a Bruker D4 Endeavor coupled with a Lynxeye detector.Each clay mineral was then characterized by its layer plus interlayer interval as revealed by XRD analysis (Brindley and Brown, 1980).The illite crystallinity, or Kübler Index, is based on the expression of the width of the illite peak at 10 Å and allows for identifying the anchizone, the limit of diagenesis and the onset of the epizone.The presence of palygorskite has been confirmed by MET observations.Semi-quantitative estimation of clay mineral abundances, based on the pseudo-voigt deconvolution for the doublets illite-palygorskite (10-10.34Å) and kaolinite-chlorite (3.57-3.53Å), was performed using the software MacDiff developed by Petschick (2001).Quartz abundance is based on the intensity of diffraction measured at 4.25 Å weighted by the clay-size fraction deposited on the oriented glass.

XRF scanning
A series of element abundances -including K, Ti, Zr, Al -were measured on U-channels using the Avaatech core scanner from the EPOC laboratory at the University of Bordeaux.The sediment was protected with Ultralene during Xray transmission foil in order to avoid contamination.The data were acquired at a 30 s count time, using 10 kV voltage and 400 mA intensity.Richter et al. (2006) give technical details on the XRF scanning technique.The results are expressed in counts per second, with a 2 % precision according to standard samples.The Zr/Al, Ti/Al and K/Al ratios as used in the text refer to Zr XRF /Al XRF , Ti XRF /Al XRF , and K XRF /Al XRF ratios respectively.Trace elements are normalized to Al (Tribovillard et al., 2008), in order to avoid dilution effect by carbonates.
palygorskite are secondary components with 10 and 6 % of the clay content, respectively (Table S1 in the Supplement).The composition of the clay mineral assemblage displays some variations over the studied interval (Fig. 2).The kaolinite content remains slightly below 45 % between 18.5 and 11.7 ka.Afterward it increases and represents 50 % between 10.5 and 8.5 ka.The percentage of kaolinite decreases at 8.5 ka and remains low between 8 and 5.8 ka.It then increases progressively between 5.8 and 2 ka and reaches its maximum (> 50 %) around 2 ka.The smectite remains stable between 18.5 and 8.5 ka, excepted two maxima around 12 and 11 ka (Fig. 2).The smectite content increases up to 38 % of the clay mineral fraction between 8.5 and 5.8 ka, as the kaolinite content is minimum.This smectite-rich interval is interrupted by a slightly depleted level around 7.2 ka (Fig. 2).The smectite content decreases progressively starting at 5.8 ka down to 15 % at the top of the core.The illite content is maximum between 18.5 and 11.7 ka, then it drops down and remains low (< 15 %) between 11.7 and 5 ka.Then illite increases slightly and remains stable around 15 % over the more recent part of the record (Fig. 2).The Kübler Index is low (< 0.4 • 2θ), indicating high structural order when the illite content is maximum.By contrast, the Kübler Index is higher (< 0.5 • 2θ) -low ordering between 11.7 and 5 ka when the percentage of illite is reduced.The illite to kaolinite ratio (I/K) is characterized by a drastic change at 11.7 ka from values around 0.35 to values as low as 0.25.The chlorite content does not correlate with other clay species and shows slight variations, being higher than average between 18.5 and 15.5 ka.The palygorskite is less abundant between 8.5 and 5.8 ka, while smectite is maximum (Fig. 2).

Grain size
The grain size main mode varies between 4 and 10 µm, indicating the detrital fraction is mainly composed of cohesive silt-size particles.Cohesive particles indeed represent 50 to 70 % of the total terrigenous fraction, being slightly less abundant between 18.5 and 11.7 ka and between 7.5 and 5.5 ka (Fig. 3).By contrast, the clay-size fraction is generally low (around 11 %), excepted between 8.5 and 5.5 ka when it represents up to 25 % of the detrital material.The sortable silt (10 to 63 µm) composes 25 % of the detrital fraction, being more abundant between 13 and 12 ka where it represents 45 %.The sand-size fraction is rare, but shows two maxima between 14 and 12 ka and between 8.5 and 7.5 ka (Fig. 3).

Elemental geochemistry
Zr/Al and Ti/Al are used as eolian proxies (Boyle, 1983;Grousset et al., 1989;Martinez et al., 1999) because Ti and Zr are conservative.Ti oxides are present in the silt and finesand fractions from highly weathered bedrocks.Due to their density and size, Ti oxides are the main components of fine eolian dust together with quartz (e.g.Calvert and Pedersen, 2007).Due to its resistance to chemical weathering, and to its high specific gravity, zircon mainly belongs to the fine and medium sand fraction together with quartz.As a consequence, the Zr/Al ratio can be used as a grain size proxy, and may be useful to retrace coarse eolian input (Calvert and Fontugne, 2001).K/Al is used to reflect the balance between aluminium-rich supply (kaolinite) and potassium-rich supply (illite and feldspars) (Schneider et al., 1997;Yarincik et al., 2000).In the Mediterranean, these ratios have been successfully used in estimating the respective river and eolian contribution to sedimentation (Jiménez-Espejo et al., 2007;Frigola et al., 2008;Rodrigo-Gamiz et al., 2011).The Al content displays a decreasing trend from 18.5 to 14 ka (Fig. 4).Some intervals are characterized by higher than average K/Al ratio, between 14 and 11.7, between 5.8 and 5.5 ka and between 2.5 and 1 ka.The two latter intervals are also characterized by the highest values of the Ti/Al ration (Fig. 4).The Zr/Al ratio displays a peculiar behavior, peaking at 13.4, 7, 6.4, 1.3 ka and being high between 12.8 and 12.3 ka.

Clim
The illite to kaolinite ratio (I/K) is a useful tracer of Saharan versus Sahelian dust provenance (Schütz and Sebert, 1987;Caquineau et al., 1998Caquineau et al., , 2002;;Formenti et al., 2011a), which is thought to be conservative after long-range transport (Chester et al., 1972;Caquineau et al., 1998Caquineau et al., , 2002;;Kandler et al., 2009).Highest ratio characterizes the northwestern areas (Fig. 1), whereas low ratio retraces the Sahelian belt: the lowest ratios are observed in dust collected in Sahel (0.1 to 0.3), intermediate values characterize south and central Sahara (0.3 to 0.7), moderate to high values are observed in Tunisia and northern Algeria (1 to 2), whereas highest ratios (1.3 to 2.6 up to 6) occur in northwestern Sahara (Fig. 1) and in Morocco (Chester et al., 1972;Glaccum and Prospero, 1980;Paquet et al., 1984;Coudé-Gaussen, 1991;Kiefert et al., 1996;Avila et al., 1997;Caquineau et al., 1998Caquineau et al., , 2002;;Kandler et al., 2009).The I/K ratio also displays a longitudinal gradient (Fig. 1) with ratio decreasing from northwestern Africa (I/K = 2 to 1.1) toward northeastern Africa (I/K = 0.2 to 0.7) (Gomes et al., 1990;Ganor and Forer, 1996;Caquineau et al., 1998).The relation between enhanced supply of eolian particles and climate is complex.Indeed, Saharan dust is produced through deflation/erosion V. Bout-Roumazeilles et al.: Atmospheric and riverine supplies since last glacial in central Mediterranean processes that prevail during arid periods and/or associated with intense eolian activity, when scarce vegetation cover favors soils erosion.Production of Saharan dust particles is favored by the presence of small consolidated soils that mainly formed during a former humid period.Moreover, while arid conditions favor the erosion and transportation of dust, major deposition occurs as wet deposit with precipitation in the western Mediterranean (Bergametti et al., 1989c;Guerzoni et al., 1997;Loÿe-Pilot et al., 1986;Loÿe-Pilot and Martin, 1996).In that sense, enhanced eolian dust input is mainly related to wind activity, and indirectly related to aridity, which limits vegetation cover, promotes soils erosion and increases dust availability.The studied area is characterized by strong seasonal contrast, with warm and dry summers, and cool and wet winters when moisture being advected from the Atlantic Ocean.Increased aridity/summer-like conditions in the Mediterranean thus favor dust production and export toward the Mediterranean.Dust washing from the atmosphere is thus controlled by moisture advection from the Atlantic during winter.However, the relative contribution of wet fallout versus dry deposits resulting from gravitational settling processes whose importance has been recently evidenced in the northeastern tropical Atlantic Ocean (Skonieczny et al., 2011(Skonieczny et al., , 2013) ) is still a matter of debate in the central Mediterranean.
The grain size of dust displays strong seasonal variations (Guerzoni et al., 1997), with a fine population (2 to 10 µm, main mode around 2.7 µm) characterizing long-distance and high latitude transport and a coarser population (5 to 50 µm, main mode around 20 µm) typical of dust storms carried over few km, being restricted to adjacent continental and marine areas (Prospero, 1981;Torres-Padron et al., 2002).Comparison between Ti/Al and Zr/Al ratios and grain size distribution are thus used to identify specific eolian vs. riverine transport processes, associated with the development of sand dunes during the Holocene, or with remote fine-grained eolian outbreaks or coarse-grained fluvial supply (Haug et al., 2001;Martinez-Ruiz et al., 2003;Ehrmann et al., 2007b;Hamann et al., 2009;Montero-Serrano et al., 2009).

Bølling-Allerød
The Bølling-Allerød (B-A) is marked by enhanced terrigenous supply starting at 14.6 ka, associated with increased contributions of both illite and palygorskite while kaolinite and chlorite decrease (Fig. 2).The relationship between illite content and sortable silt-size fraction (14 µm) indicates that illite is mostly transported through eolian processes.The increase of the I/K ratio suggests that the increased eolian supply is associated with a modification of the main detrital source (Fig. 5a).The increased proportion of illite may indicate a northward migration of the main clay provenance over Libya, as illite has been shown to be more abundant in dust collected over northern Libya (O'Hara et al., 2006).Nevertheless, the I/K ratio range -varying between 0.35 and 0.4 -during the B-A is rather low compared with observed values for Libya and does not further support this hypothesis (Fig. 5a).The observed sedimentological characteristics tend to indicate Tunisia/northern Algeria as the main dust provenance (Formenti et al., 2011a).According to previously published data, the mineralogical composition is indeed consistent with a source from northern or central Algeria (Fig. 7a), but the low content in palygorskite ruled out any major contribution from Tunisian loess (Bout-Roumazeilles et al., 2007).The development of humid conditions during the B-A is evidenced by an increased contribution of river vs. eolian supply in the Aegean Sea (core SL128, Fig. 1) (Hamann et al., 2008), by pollen association from the Alboran (ODP976, Fig. 1) and Adriatic Sea (Combourieu-Nebout et al., 1998, 2002;Fletcher and Sanchez-Goñi, 2008) and by a speleothem record from the eastern Mediterranean (Bar-Matthew et al., 2003), contrasting with the clay mineral record from the Sicilian-Tunisian Strait.The sea surface temperatures (SST, Fig. 3) -as reconstructed from planktonic foraminifera at site MD04-2797 -are high during the Bølling-Allerød (Ellassami et al., 2007), consistently with general warm oceanic conditions, relatively high humidity and temperatures compared with present-day in Sicily and central Mediterranean (Ramrath et al., 2000;Allen et al., 2002;Zielhofer et al., 2008).The dominance of semi-desert plants at site MD04-2797(Desprat et al., 2013) confirms the persistence of dry continental conditions over Africa during the B-A.These results tend to confirm contrasting climatic evolution (e.g.Roberts et al., 2008), with humid conditions in the northeastern and eastern Mediterranean but aridity still persistent in the southern Mediterranean.

Younger Dryas
A major peak in the Zr/Al ratio is observed around 12.8 ka, associated with an increase in the abundance of both sortablesilt and sand-size particles (Fig. 5b).The clay association does not exhibit major modification but smectite is more abundant at the expense of kaolinite, displaying a peak at 12 ka (Fig. 2).These characteristics indicate the supply of coarser particles that may either be associated with a volcanic supply, a river supply or an intense dust episode (Martinez-Ruiz et al., 2003;Ehrmann et al., 2007b;Hamann et al., 2009;Skonieczny et al., 2011).Smectite peaking at 12 ka supports the hypothesis of a tephra (Fig. 5d).Moreover, when compared with the chronology of tephras (e.g.Zanchetta et al., 2011), the increase of Zr may be associated with the Agrano Pomici Principali (12.8-12.2ka) tephra (DiVito et al., 1999;Siani et al., 2004).Nevertheless, optical observations of the sediment do not give any support to this hypothesis.The hypothesis of enhanced river supply is also unlikely, considering the general arid climatic conditions that prevailed during the Younger Dryas (YD).Fluvial archives evidence the development of arid conditions in Tunisia (Zielhofer et al., 2002(Zielhofer et al., , 2004(Zielhofer et al., , 2008;;Faust et (Essallami et al., 2007).and MART (g kyr −1 cm −2 ) from Fig. 2.
2004) between 12.4 and 11.8 ka BP, attributed to the Younger Dryas.The Medjerda fluvial system was particularly active, characterized by aggradation, with sand deposits and high sedimentation rates in the mid-floodplain (Zielhofer et al., 2008).Such fluvial dynamic may also be responsible for the major increase in the Zr/Al ratio observed around 12.5 ka BP in the Sicilian-Tunisian Strait (Fig. 4).However, increase fluvial dynamic within the floodplain is not likely associated with enhanced terrigenous supply through the northern outlet, since the runoff was probably severely altered due to increased aridity and overall reduced precipitations.This specific level could thus alternatively reflect intense Sahara eolian activity that has previously been reported in sediment cores from the northeastern tropical Atlantic (deMenocal et al., 2000;Cole et al., 2009;Skonieczny et al., 2013).
Cold intervals are generally associated with intense dust deposition/eolian activity resulting from reduced vegetation, promoting the erosion of soils, low moisture availability and enhanced atmospheric circulation.The North Atlantic Oscillation (NAO) is likely responsible for some part of the Mediterranean climate variability, since its impact on the wind activity and precipitation balance (Pittalwala and Hameed, 1991;Moulin et al., 1997;Sanchez-Goñi et al., 2002) has been shown to be positive during the YD (Kim et al., 2007).A positive phase of the NAO is thought to be responsible for the development of dry conditions over southern Europe and North Africa and for promoting northward intrusions of Saharan air masses toward the western Mediterranean (ODP976, Fig. 1) during cold climatic events of the last climatic cycle (Combourieu-Nebout et al., 2002;Bout-Roumazeilles et al., 2007).Salinity is low during the YD in central Mediterranean (Fig. 3), as during cold events like H1, suggesting enhanced contribution of less saline waters originating from the Atlantic Ocean (Essallami et al., 2007).These cold events were characterized by increased palygorskite supply associated with the dominance of arid steppe (Artemisia) and the occurrence of endemic pollen (Argania), pinpointing a dust source from western Morocco being transported along the North African coast (Bout-Roumazeilles et al., 2007).The absence of specific enrichment in palygorskite during YD in the Sicilian-Tunisian Strait indicates that the main long-range transportation pattern of fine Saharan dust is not responsible for the peculiar characteristic of sediments deposited in the Sicilian-Tunisian Strait.This is in agreement with grain size and geochemical data, which rather point to medium-to short-range transportation of coarse particles from proximal continental areas (Pye and Tsoar, 1987;McTainsh et al., 1997;O'Hara et al., 2006).The occurrence of such a coarse-grain proximal supply during the YD, consistent with the presence of loess deposits and coastal dunes in Tunisia (Coudé-Gaussen et al., 1987;Grousset et al., 1992;Crouvi et al., 2010) and further supported by the model suggesting that Libyan desert and coastal areas are major coarse-grain dust sources for central and eastern Mediterranean (Callot et al., 2000), may thus reflect local eolian activity forced by ocean-atmosphere linkage at global scale (Fig. 7b).

Onset of the Holocene
The end of the Younger Dryas (11.7 ka) is marked by a drastic change in the clay assemblage (Figs.5a, 6a and b) and a decrease of the sortable silt proportion.The kaolinite increases while illite diminishes, resulting in a progressive decrease of the I/K ratio.This shift, associated with a modification of the illite structural composition, evidences a major change of provenance of the clay fraction, interpreted as an increase contribution of the Sahelian area (Fig. 6).Low I/K indicates enhanced contribution from the southern Algeria-Mali border region (Paquet et al., 1984;Bergametti et al., 1989b;Alastuey et al., 2005;Bout-Roumazeilles et al., 2007), but the clay association suggests a provenance from southern Algeria rather than from northern Mali because kaolinite is more abundant southward (e.g.Bout-Roumazeilles et al., 2007), becoming dominant in southern Algeria whereas the Mali border is characterized by smectiterich assemblages (Fig. 7c).The abundance of semi-desert pollen taxa exhibits a sharp decrease at 12.25 ka (Desprat et al., 2013), before the mineralogical transition, suggesting that the modification of vegetation, i.e. a major change in steppe composition and a development of scarce deciduous oak woodlands, might prevent soils from weathering, displacing southward the areas of eolian erosion.The end of the YD is marked by a similar modification of the clay mineral fraction in a core from the northeastern tropical Atlantic off Senegal and interpreted as recording a southward shift of the Inter Tropical Convergence Zone (ITCZ), which strongly affects the transport of Saharan dust toward the Atlantic Ocean.Moreover, the onset of the Holocene is also marked by a decrease of the I/K ratio off Portugal (Stumpf et al., 2011).The synchronicity of the clay modification at these sites, subjected to distinct wind regimes, signifies that the end of the YD is marked by a large-scale atmospheric reorganization over the North African continent, with the development of dry conditions during the early Holocene, affecting both the high-latitude long-range transportation and the regional low-latitude wind regime over the Mediterranean.
Consequently, most studies focused on the precipitation distribution and moisture availability that prevailed on the continent at the time of S1.But investigations of the marine propagation of the sapropelic layer from eastern toward western Mediterranean evidenced the absence of true sapropels and/or synchronous events (Mercone et al., 2000) in the western basin, suggesting lower export of production fluxes into the basin (Martinez-Ruiz et al., 2003) or different hydrographic conditions (Weldeab et al., 2003).Smectite provenance during the sapropel in the Sicilian-Tunisian Strait would provide additional information on the modifications of oceanic environmental conditions.The enhanced supply of smectite may also be eolian, originating from Tunisian loesses, which are characterized by their high content in smectite and palygorskite (Fig. 1) (e.g.Bout-Roumazeilles et al., 2007).However, smectite and palygorskite display opposite variations during the sapropel, suggesting that they are not reworked from the same geological formation.Moreover, the overall evidences of increased moisture availability in the Mediterranean during the sapropel SI rule out an eolian origin for smectite over that time interval.Considering the site location, a contribution from authigenic smectite should be discussed.Indeed, volcanic ashes deposited in deep-sea sediments are easily altered and may transform into smectite (Griffin et al., 1967).Formation of smectite at the expense of volcanic materials was evidenced near the Santorini archipelago in the eastern Mediterranean (Chamley, 1971).However, a volcanic origin of smectite from Sicilia is not supported by optical observations or by the chronology of the main tephra (Siani et al., 2004).The enhanced supply of smectite may originate from Tunisian loess (Fig. 1) (e.g.Bout-Roumazeilles et al., 2007), but the low content of palygorskite during the smectite-rich interval does not further support this interpretation.Moreover, the enhanced moisture availability over that period, associated with pollen association -i.e.open oak forest with heath underbrush or maquis and Asteraceae-Poaceae-Cyperaceae steppe (Desprat et al., 2013) preventing soils from erosion -and the geochemical signal give little support to significant eolian activity.Considering the location of the core, near the Tunisian coast and directly under the MAV, the enhanced supply of smectite may result from an increase contribution of the Medjerda Oued, one of the main fluvial systems draining Algeria and Tunisia, as its main stream is transported southeastward via the Mediterranean surface layer (Astraldi et al., 2002).The hydrological regime of an Oued is typically irregular, being seasonally modulated by precipitation regimes, which may provoke major floods during heavy rainfall in winter and spring (Zahar et al., 2008).Even if the contribution of the Medjerda fluvial system to deep-sea sedimentation of the Siculo-Tunisian Strait is relatively low, such major flood events may transport one-year discharge in a few days (Claude et al., 1977).Moreover, the Medjerda suspended loads are enriched in smectite (up to 50 %) (Claude et al., 1977), which is consistent with the observed mineralogy of the sapropel S1 deposit.In sub-humid to sub-arid environments, enhanced moisture availability likely promotes the development of vegetation that prevents soils from erosion, whereas enhanced fluvial dynamic occurs during dry conditions (Rohdenburg, 1989;Zielhofer et al., 2008).The Oued Medjerda experienced humid conditions between 11 and 7.8 ka cal BP with soil formation around 9.5 ka cal BP (Zielhofer et al., 2002).Such moist conditions generally do not support enhanced fluvial activity, unless rainfall is characterized by strong seasonality.Pollen assemblages show the expansion of temperate trees and shrubs between 10.1 and 6.6 ka -i.e.open oak forest with heath underbrush or maquis and Asteracae, Poacae, Cyperacae steppe -while Mediterranean vegetation only developed after 6.6 ka cal BP, suggesting wetter conditions during sapropel S1 resulting from increased winter precipitation (Desprat et al., 2013).Strong winter precipitations may thus be responsible for moisture availability during the sapropel S1, sustaining the expansion of vegetation and triggering seasonal fluvial discharge, all of which may be responsible for the enhanced detrital supply observed in the Siculo-Tunisian Strait.However, the rather large sediment supply observed in the Siculo-Tunisian Strait makes the long-term contribution of the Medjerda Oued unrealistic over the whole sapropel S1.Indeed, sedimentation rate, smectite concentration, as well as the Zr/Al ratio and grain size distribution suggest that the terrigenous supply during S1 was multiphased (Fig. 5c and d).Especially the smectite contribution is maximum between 6.8 and 6 ka cal BP, with the Zr/Al ratio peaking at 7 and 6.5 ka cal BP, suggesting enhanced river supply.Enhanced fluvial activity has been evidenced in the Medjerda fluvial system between 6.6 and 6 ka BP (Zielhofer et al., 2008), suggesting that the Medjerda Oued may be responsible for the observed terrigenous supply over that Smectite may alternatively be supplied toward the Mediterranean by the Nile River and transported as suspended particles within the Mediterranean surface water (Ventakatarathnam and Ryan, 1971;Stanley and Wingerath, 1996;Foucault and Mélières, 2000;Hamann et al., 2009) and, to a lesser extent, by eolian processes since Saharan dust deposited in the eastern Mediterranean basin is composed of kaolinite associated with smectite (Chester et al., 1977).In that context, the LIW (Levantine Intermediate Water) likely redistributes smectite-rich fine-grain size particles as it flows westward in the sub-surface layers across the Siculo-Tunisian Strait (Fig. 1).This interpretation is consistent with an increased contribution of the Blue Nile draining the Cenozoic volcanic provinces of Ethiopian Highlands (Ukstins et al., 2002) during the African Humid period (Revel et al., 2010).In the Levantine basin (SL112, Fig. 1), the terrigenous fraction, deriving from the Nile River (Krom et al., 2002;Schilman et al., 2001;Hamann et al., 2008), reveals an increase in cohesive fine-grained particles during the sapropel S1 (Hamann et al., 2010), interpreted as enhanced pluvial period in Egypt (Revel et al., 2010) during the African Humid Period (AHP) (Cole et al., 2009;deMenocal et al., 2000).Alternatively, smectite may also be redistributed from the Adriatic Sea by the dense Eastern Mediterranean Deep Water (EMDW) flowing at depth across the Siculo-Tunisian Strait (Fig. 1).However, this hypothesis is not supported by data since the major salinity episode that occurred around 7.7 ka BP likely resulted from a massive discharge of the Po River, which mainly carries illite and chlorite (Combourieu-Nebout et al., 2013;Tomadin, 2000).
Considering uncertainties on the age models, the enhanced contribution of fine particles is synchronous in the Levantine Sea and in the Sicilian-Tunisian Strait, supporting a provenance of the fine smectite-rich particles from the Eastern Mediterranean basin.The abundance of fine particles may thus result from slightly slower oceanic circulation during the sapropel S1, promoting the deposition of the finest fraction through settling processes (Cram and O'Sullivan, 1999).The circulation pattern is consistent with this hypothesis since the Levantine Intermediate Water (200-600 m water depth) overflows toward the western Mediterranean basin across the Siculian-Tunisian Strait.Moreover, coldwater corals and benthic foraminifera assemblages evidenced the impact of sapropelic conditions in shallow water (600 m water depth) from the Ionian Sea (Fink et al., 2012), giving additional support to a westward propagation of sapropelic conditions through intermediate water masses (Fig. 7d).The occurrence of a smectite-rich interval in the Alboran Sea (Bout-Roumazeilles et al., 2007), and on the Atlantic side of the Gibraltar Strait during the AHP, suggest a potential transport of smectite through the Mediterranean Overflow Water (Stumpf et al., 2010).

Late Holocene
The upper part of the record is marked by an increase of the Ti/Al and Zr/Al ratios (Fig. 5c), associated with a high supply in quartz, both suggesting enhanced wind activity.The dominance of silt-size particles and the stable MART (Fig. 5c) suggest long-range transportation rather than enhanced eolian supply.The increases in kaolinite and palygorskite abundances (Fig. 5c) as well as the low I/K ratio indicate a provenance from the Sahelian area and/or the southwest Morocco (Fig. 7e).This event is synchronous with the occurrence of one of the cool/arid events in the Mediterranean (Rapid Climate Coolings-RCC; e.g.∼ 1.2-1.0kyr BP), suggesting relationships with high-latitude climatic phenomena and altered runoff regime (Mayewski et al., 2004).The hypothesis of enhanced eolian activity during this interval is supported by an increase in palygorskite and quartz content, observed between 1.15 and 0.65 ka BP (starting at 1.7 ka BP) in the Alboran Sea (Rodrigo-Gamiz et al., 2011) and by several evidences of arid continental conditions over the Mediterranean North Africa, e.g.vegetation in Tunisia (Brun, 1992;Stevenson et al., 1993), terrestrial archives in Morocco (Zielhofer et al., 2010) and Tunisian fluvial archives (Faust et al., 2004;Zielhofer et al., 2002Zielhofer et al., , 2004Zielhofer et al., , 2008)).
The sedimentation rate displays a sharp decrease at 5.5 ka BP, evidencing a major modification of environmental conditions.The late Holocene is characterized by increased palygorskite and quartz contents, while the kaolinite shows a progressive increase starting at 3 ka BP at the expense of smectite, associated with a high Ti/Al ratio.These observations suggest enhanced eolian supply that may be linked with the development of arid conditions on the North African continent.This interpretation is supported by the occurrence of arid periods (Fig. 4) in Tunisia around 4.7, 3.0, 1.6, 1.1 and 0.4 ka (Faust et al., 2004), evidenced by major modifications of fluvial dynamics in the Medjerda Oued and associated with late Holocene North Atlantic coolings (Faust et al., 2004;Zielhofer et al., 2008).Previous studies evidenced a climate change around 4.5 ka cal BP, with distinct regional late Holocene climatic evolution in the Mediterranean (Roberts et al., 2008;Sadori et al., 2008Sadori et al., , 2011;;Peyron et al., 2011;Vannière et al., 2011;Magny et al., 2012).Southern sites experienced a drastic decrease in summer precipitations (Magny et al., 2012) while northern sites were submitted to increased precipitation.According to presentday atmospheric patterns, the late Holocene mineralogical and geochemical records are consistent with a mean southward position of the ITCZ, which may also explain the lowlatitude aridity associated to the RCC (Haug et al., 2001;Hodell et al., 2001) (Vannière et al., 2011) as did enhanced high-altitude export of dust (Skonieczny et al., 2011).

Conclusions
The clay mineral, grain size and geochemical studies of sediment deposited on and into the Sicilian-Tunisian Strait have helped retrace atmospheric vs. riverine terrigenous supplies variability since the last glacial in central Mediterranean.Although the eolian supply is dominant at the studied site -excepted during the sapropel S1 -both flux and the main provenance of particles display strong variations, related to aridity/moisture balance and vegetation cover, driven by largescale atmospheric reorganization.
The Bølling-Allerød is marked by increased terrigenous flux while both illite and palygorskite became main components of the clay mineral fraction.The concomitant dominance of silt-size particles indicates an eolian origin for this enhanced detrital supply.The geochemical and mineralogical data indicate a dominant Saharan contribution, pinpointing central and northern Algeria as a main provenance.This increased eolian contribution from the Sahara highlights contrasting regional climatic evolution, with the development of moist conditions over the north and eastern Mediterranean during the Bølling-Allerød, while glacial aridity persisted in the southern Mediterranean.
A short-term coarse-grained and Zr-rich interval characterized the Younger Dryas, as arid and cold climatic conditions promoted soil erosion.Similar events characterize the western Mediterranean and the northeastern tropical Atlantic Ocean, although clay mineralogy clearly indicates different provenances.These results suggest local increase of wind activity driven by wide ocean-atmosphere interactions.
The onset of the Holocene is marked by a major change of clay mineralogy and cristallinity corresponding to a southward migration of the main clay provenance toward the Sahelian belt.This change is associated with a progressive development of the Mediterranean vegetation and the southward shift of the ITCZ.Similar signals are recorded in the northeastern tropical Atlantic Ocean and off Portugal, suggesting a large-scale atmospheric reorganization.
High terrigenous flux associated with the dominance of very fine-grained Pd-rich smectite characterized the earlyto mid-Holocene in central Mediterranean, while the organic content indicates pre-sapropelic conditions.The sedimentological characteristics rule out any eolian supply but rather suggest enhanced riverine contribution from remote areas during the sapropel S1.The peculiar signature of this event in the central Mediterranean and in the Levantine basin suggests the propagation through intermediate water masses of fine-grained smectite originating from the Nile River during the sapropel S1, resulting from enhanced precipitations on northeast Africa, coincident with the late phase of the AHP.
Finally, an increase supply of dust originating from northern Sahel or southern Morocco characterizes the last 3 kyr, indicating intense eolian activity linked to modification of the rainfall regime.

bFig. 6 .
Fig.6.Comparison of the mineralogical and geochemical signatures before and after 11.7 kyr, (a) kaolinite versus illite (XRD counts/s) evidencing contrasting provenance between 18.5 and 11.7 ka (red squares) and between 11.7 ka and top of the core (blue squares); (b) illite to kaolinite ratio as a function of illite crystallinity (Kübler Index), diagenesis, epizone and anchizone limits.
. A southward position of the ITCZ would indeed explain the geographically contrasted Mediterranean climatic evolution by promoting the aridification of North Africa during which increased summer precipitation www.clim-past.net/9etal.: Atmospheric and riverine supplies since last glacial in central Mediterranean occurred in northern Mediterranean Ehrmann et al. (2007a)data from northwest Aegean province and West Turkey province: e.g.Ehrmann et al. (2007a); data from Marmara Sea: Armynot du Châtelet et al. (2013); North and South Libya dust from O'Hara et al. (2006).Position of the sediment cores mentioned in this study.

Table 1 .
Siani et al. (2001)rformed on core MD04-2797.The ages were corrected using reservoir ages of 400 yr during the Holocene, the Younger Dryas and late glacial, 560 yr during the Bølling-Allerød and 800 yr during the Heinrich event H1 and the Older Dryas, following recommendations ofSiani et al. (2001).