flood frequencies reveal holocene rapid climate changes (lower

JOURNAL OF QUATERNARY SCIENCE (2010) 25(5) 700–714Copyright � 2009 John Wiley & Sons, Ltd.Published online 29 December 2009 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/jqs.1347
Flood frequencies reveal Holocene rapid climatechanges (Lower Moulouya River, northeasternMorocco)CHRISTOPH ZIELHOFER,1,2* JENS BUSSMANN,2 HANOUN IBOUHOUTEN2,3 and KATRIN FENECH4

1 Department of Geography, Dresden University of Technology, Dresden, Germany2 Department of Geography, Osnabruck University, Osnabruck, Germany3 Department of Geology, Moulay Ismail University, Meknes, Morocco4 Environmental Archaeology Laboratory, Bonn, Germany

Zielhofer, C., Bussmann, J., Ibouhouten, H. and Fenech, K. 2010. Flood frequencies reveal Holocene rapid climate changes (Lower Moulouya River, northeasternMorocco). J. Quaternary Sci., Vol. 25 pp. 700–714. ISSN 0267-8179.

Received 27 February 2009; Revised 17 July 2009; Accepted 18 August 2009

ABSTRACT: Current high-resolution palaeoenvironmental records reveal short-term Holocene cool-ings. One of these major Holocene rapid climate changes occurred between 3.2 and 2.7 cal. ka BP.The sensitivity of river systems vis-a-vis slight and short-term Holocene climatic variations is a subjectof controversy in the scientific community. In this paper, we present a 4.0 to 1.4 cal ka BP palaeofloodrecord from the Lower Moulouya River (northeastern Morocco) to demonstrate the high sensitivity ofsemiarid rivers in the southwestern Mediterranean towards Holocene environmental changes. TheLower Moulouya flood deposits are characterised by thick, well-stratified, predominantly clayey to
silty overbank fine sediments. These cohesive sediments show evidence of excellent preservationconditions against fluvial erosion and contain a continuous record of mid to late Holocene floodsequences. The Moulouya palaeoflood record can be interpreted in the context of regional and globalhigh-resolution proxy data, revealing a strong coupling with Holocene rapid climate changes. Thecentennial-scale Moulouya palaeohydrological history will be discussed with palaeoenvironmentaldata from the same record (palaeomagnetics, sedimentary charcoal record, anthracological analyses,snail analyses) to generate new ideas about the mid to late Holocene hydrological cycle in thesouthwestern Mediterranean. The deduced features of pronounced Lower Moulouya flooding and thedecreased fire recurrence during Holocene cooling remain somewhat inconsistent with the interpret-ation of other palaeohydrological and paleaoecological records from the southwestern Mediterranean.However, enhanced Lower Moulouya flood frequencies between 3.2 and 2.7 cal. ka BP agree withincreased floodplain aggradation in other major river systems of Mediterranean North Africa.Copyright # 2009 John Wiley & Sons, Ltd.
KEYWORDS: palaeohydrology; fluvial geomorphology; flood frequencies; Holocene rapid climate changes; Morocco.

Introduction

Recently, moderate- to high-resolution records of Holocenearchives have become available for the Mediterranean region.In particular, lake (Gasse et al., 1990; Lamb et al., 1995),speleothem (Bar-Matthews et al., 2003), offshore (Rohlinget al., 2002) and pollen (Fletcher et al., 2007) records reveal ashort-term and abrupt Holocene climatic variability. TheseHolocene rapid climate changes (RCCs) (cf. Mayewski et al.,2004) can also be observed in other parts of the world (e.g.Denton and Karlen, 1973; Haug et al., 2001; Staubwasser andWeiss, 2006) and seem to be of global extent. In this paper, we

* Correspondence to: C. Zielhofer, Department of Geography, Osnabruck Uni-versity, Seminarstr. 19ab, 49069 Osnabruck, Germany.E-mail: [email protected]

discuss the potential of a Mediterranean alluvial archive fromnortheastern Morocco for high-resolution Holocene palaeoen-vironmental reconstruction. The Moulouya catchment islocated in the western Mediterranean of North Africa, a regionwhich is currently (Chbouki et al., 1995; Trigo et al., 2004) andover late Quaternary (Allen et al., 1999; Cacho et al., 2000;Combourieu-Nebout et al., 2002; Moreno et al., 2005) andHolocene (Yll et al., 1997; Fletcher et al., 2007; Reed et al.,2001) timescales influenced by westerly-driven North Atlanticclimatic variability.

Holocene alluvial archives reveal variations in fluvialprocesses due to environmental changes in the catchment.Fluvial archives are by no means unequivocal climatic archiveslike ice or marine cores, but they may mirror geomorphologicalresponses to climatic oscillations and cycles (Arnaud-Fassetta,2002; Benito et al., 2003a,b; Alvarez et al., 2005; Benvenutiet al., 2006; Macklin et al., 2006; Schulte et al., 2008; Vita-

Figure 1 Bioclimatic zones (following Sauvage, 1963) and the Mou-louya catchment in arid to semiarid northeastern Morocco. The whitesquare shows the position of the Djamila alluvial sequence at the LowerMoulouya presented in this paper

MOULOUYA FLOOD FREQUENCIES 701

Finzi, 2008; Zielhofer and Faust, 2008) and/or human impact(Van Andel et al., 1990; Oldfield et al., 2003; Fenech, 2007;Benito et al., 2008).

Generally, fluvial changes in arid to semiarid Mediterraneanriver systems reflect more the hydric than the thermalcomponent of the climate (Rohdenburg, 1989; Giessner,1990), but the palaeoenvironmental interpretation of alluvialchanges remains difficult. Increased flooding has beeninterpreted as the result of either more arid (e.g. MedjerdaRiver in northern Tunisia; Faust et al., 2004), or less aridconditions (e.g. Tagus River in Spain; Benito et al., 2003a) dueto different flood-producing mechanisms. In this study,we correlate flood proxies with environmental proxies fromthe same alluvial record to clarify the environmental causes ofenhanced or reduced flooding.

The potential of Mediterranean fluvial archives for thereconstruction of the Holocene palaeohydrology varies due toheterogeneous fluvial architectures. These correspond tovariations in alluvial strata, sediment textures and structures,and resulting differences in preservation conditions. Especiallyin arid to semiarid regions of the western Mediterranean, slackwater deposits are reported to document Holocene flood events(Benito et al., 2003a; Thorndycraft and Benito, 2006a,b).According to Baker et al. (1983, 1987) and Baker (2008), slackwater deposits represent the most accurate alluvial archive forreconstructing the magnitude and recurrence frequency ofpalaeofloods that are hundreds to thousands of years old.However, non-cohesive, sandy slack water deposits mirrorperiods of enhanced flooding, but a thousands of years longhiatus in the flood record may also be explained by the erosionof former flood layers due to floods (Benito et al., 2003a). Thus,poor preservation conditions will induce a chronologicallynon-continuous palaeoflood record. On the other hand, whenMediterranean fluvial archives are characterised by good pre-servation conditions, as may be the case in western Mediterra-nean slack water archives (Thorndycraft et al., 2005), they maydocument a continuous record of flood events with increasingaggradation levels. The Holocene flood record presented herefrom the Lower Moulouya River mainly consists of low-energy,cohesive overbank sequences. The clayey to silty sediments arewell stratified, characterised by excellent preservation con-ditions and, consequently, markedly suitable for palaeofloodreconstructions.

Site description: Lower Moulouya River andDjamila sequence

The Oued Moulouya (Fig. 1) is the main watercourse innortheastern Morocco, with a catchment area that covers�53 500 km2 (Zarki and Macaire, 1999). It originates in theAtlas Mountains at an altitude of 1770 m and flows intothe Mediterranean Sea. Within the Moulouya catchment theclimate is Mediterranean arid to semiarid, with an averageprecipitation ranging from 200 mm in the lowlands and up to500 mm in the Atlas Mountains (Ngadi, 1995). Most of therainfall is concentrated in only a few days, leading to highfluctuations in water discharge. For example, during the 1963flood the maximum water discharge of the Moulouya Riverreached 5200 m3 s�1, which is nearly 240 times greater than themean discharge (Snoussi et al., 2002).

The Upper Moulouya catchment includes the arid tosemiarid south-facing slopes of the Middle Atlas Mountains,and the northwest-facing slopes of the arid to semiarid EasternHigh Atlas Mountains. Within the upper catchment, the Middle

Copyright � 2009 John Wiley & Sons, Ltd.

and High Atlas Mountains reach a height of over 2500 m abovesea level.

The northern part of the arid Mid Moulouya catchment isformed by the main tributaries Oued Mouloulu and OuedMsoun. Owing to the leeward position of the Mid MoulouyaRiver, the region has a very low annual precipitation (Gersif:192 mm a�1). After crossing the Beni Snassen mountain range,the Moulouya River follows its course into the semiarid LowerMoulouya Basin, a tectonic depression filled with Early to MidPleistocene sediments.

Following the bioclimatic classification of Sauvage (1963),the Mid to Lower Moulouya region indicates the transition fromMediterranean arid to semiarid conditions. This transition isecologically evidenced by the occurrence of Stipa tenacissimaand Artemisia herbaalba (grassland) in the more arid regions,and open woodlands inshore and on the windward slopepositions. These woodlands of the Lower Moulouya littoralzone are characterised by Pistacia atlantica, Olea oleaster,Juniperus phoenicea (semiarid) and Tetraclinis articulata(semiarid to sub-humid). Due to overgrazing in the region,xerophytic woodlands have been repressed. The lowerMoulouya riverine vegetation consists of Populus alba, Salixand Tamarix (Benabid, 2000).

According to the genetic classification of floodplains byNanson and Croke (1992), Holocene alluvial archives as foundat the Lower Moulouya River represent cohesive floodsediments, characterised by fine-grained clastic overbankaccretion (Figs. 2–4). The overbank fines are well stratified.Due to the high preservation potential of cohesive overbankfines against lateral erosion, the floodplain reveals a clearspatial dichotomy. Besides cohesive flood deposits, the braidedLower Moulouya River features non-cohesive coarse channelfills with strong lateral migration dynamics (Fig. 3). However,lateral migration and erosion are more or less limited on non-cohesive channel fills, point and braid bars. Consequently, thesevere transition from non-cohesive to cohesive alluvial fills ischaracterised by thick exposures of Holocene overbank fines,which are easily accessible for stratigraphic correlation.

This paper focuses on the Djamila exposure (Figs. 2, 3 and 5)at the Lower Moulouya River, which reveals around 15 m ofHolocene overbank deposition. Although the Holocene over-

J. Quaternary Sci., Vol. 25(5) 700–714 (2010)

Figure 2 Holocene overbank deposits at the Lower Moulouya River (Djamila exposure, northeastern Morocco): the fluvial exposures exhibit fine-grained cohesive flood deposits with excellent preservation conditions. The distance between Djamila L and Djamila �A amounts to 420 m. Thestratigraphic correlation of the Djamila exposure is shown in Fig. 5. This figure is available in colour online at www.interscience.wiley.com/journal/jqs

702 JOURNAL OF QUATERNARY SCIENCE

bank fines are very close to the former and current channel bed,the profile shows well-stratified sedimentation sequences.Greyish, ash-rich layers permit a clear stratigraphic correlationof the flood deposits, traceable over hundreds of metresalongside the exposure. Therefore, it is considered that theDjamila exposure embodies a representative archive for theHolocene flood history of the lower river.

Methods

Lithostratigraphic parameters

For stratigraphic correlations and a preliminary separation ofthe Moulouya flood units, the sediment structure and sedimentcolour were determined by field survey. The Moulouyaoverbank fines can be differentiated between clayey to siltysediments with a predominantly laminated structure, clay-richsediments with a weakly subangular structure, and greyishlayers rich in charcoal and snail remains. The sediment colourswere determined using a Munsell soil colour chart.

For the grain size analyses, 10 g of sediment sample was leftovernight in 25 mL dispersing solution (sodium hexametapho-sphate: (NaPO3)6–39 g L�1 H2O). After the addition of200 mL H2O the sample was shaken for at least 2 h. The grain

Figure 3 Geomorphological position of the Djamila sequence within the L


size measurements of the sand fraction were carried out bymeans of the classical wet sieve technique (2.0–0.63 mm: coarsesand; 0.63–0.2 mm: medium sand; 0.2–0.063 mm: fine sand).

Environmental parameters

Macro-charcoal remains were sampled from charcoal-richlayers and analysed at the Laboratory for Quaternary Woods,Langnau, Switzerland. The snail remains were separated from2 kg bulk samples with a 2.0 mm wet sieve procedure duringthe field campaign.

Findings from quantitative charcoal analyses have been usedin Quaternary science as a proxy for palaeo-fire recurrences(e.g. Rhodes, 1998; Sadori and Giardini, 2007). We wet-sieved0.2 g of each Moulouya sediment sample (dry weight) with afine-sand sieve (0.063 mm) and counted macro-charcoalremains with the aid of a binocular microscope. Unlike swampor lake sediments, the Moulouya deposits do not containunburned plant macro-remains, therefore standardised H2O2

bleaching methods (cf. Rhodes, 1998) were not required.Owing to high variations, the counted charcoal particles arepresented on a logarithmic scale.

Magnetic susceptibility was measured in SI units using aBartington MS2 susceptibility meter and MS2F probe. Gener-ally, three measurements were averaged (Dearing, 1999).

ower Moulouya floodplain


Figure 4 At the Lower Moulouya River, late Holocene palaeoflooddeposits reveal very often an enrichment of charcoal fragments at thetop of a sequence. These reddish to dark-greyish layers are easilyrecognisable in the exposure and are most suitable for stratigraphiccorrelations. The richness of macro-charcoal and snail remains is mostsuitable for anthracological and palaeoecological analyses. On top ofthe z3 ash-greyish layer, a fine-laminated pale-grey silt layer indicatesthe onset of a new flood sequence. This figure is available in colouronline at www.interscience.wiley.com/journal/jqs

Figure 5 Stratigraphic correlation of the individual Djamila exposures (Low



Chronological controls

Macro-charcoal remains from charcoal-rich Moulouya over-bank fines have been processed by the conventional14C laboratory at Cologne University (KN) and the acceleratormass spectrometry 14C laboratory at Kiel (KIA). All radiocarbonmeasurements were converted into calendar years BP (cal. aBP, 1s) using the Calpal calibration program of CologneUniversity (www.calpal.de) (Table 1).

Reconstruction of Moulouya palaeofloods

In order to separate the Lower Moulouya sediments into single-like flood events, the sediment structure and grain sizedistribution were investigated. With regard to the sedimentstructure, boundaries between flood units should be abrupt,distinct and clearly detectable in the alluvial profiles (Ely,1997). At the Lower Moulouya, variations in structural featuresare indicated by strong changes in the sediment colour andabrupt shifts in lamination characteristics. Additionally, the topof an isolated flood event is very often characterised by anenrichment of charcoal particles (Fig. 4).

er Moulouya, northeastern Morocco)


Table 1 Radiocarbon samples (new data; Linstadter, 2008; Zielhofer et al., 2008) from the Lower Moulouya River

Profile Type Lab no. Material 14C (BP) 14C (cal. a BP) (1s) Sedimentationseries

d13C Above riverlevel

DJ H z5 level Charcoal layer KN 5836 Charcoal 1519�52 1362–1492 Series III �22.79 13.76 mDJ J 12.34 m Ex situ KN 5820 Charcoal 2040�30 2011–2089 Series III �22.35 12.34 mDJ H z3 level Charcoal layer KIA 30748 Charcoal 2170�20 2148–2289 Series III �22.42� 0.15 11.80 mDJ H 11.44 m Ex situ KN 5837 Charcoal 2355�36 2350–2444 Series III �22.60 11.44 me level (MOL) Bulk sample KIA 30744 Humic acid 2580�30 2714–2749 Series III �20.98� 0.12 9.94 mDJ A 7.85–8.10 m Ex situ KN 5819 Charcoal 3110�35 3089–3284 Series III �22.55 8.00 mDJ C 6.01 Charcoal layer KN 5835 Charcoal 3570�35 3858–4011 Series II �23.14 6.01 m5.45 m (BOU) Ex situ KIA 30742 Charcoal 3765�30 4096–4202 Series II �24.98� 0.30 5.45 mDJ �A 4.47 m Ex situ KIA 30745 Charcoal 5805�35 6562–6654 Series I �22.79� 0.44 4.47 mDJ �A 4.47 m Bulk sample KIA 30745 Humic acid 5925�35 6701–6789 Series I �21.58� 0.09 4.47 mDJ MTL5(3) Neolithic site KIA 31001 Humic acid 6000�35 6740–6810 Series I �24.46� 0.24 3.95 mDJ MTL5(3) Neolithic site KIA 31001 Charcoal 6020�40 6759–6894 Series I �25.54� 0.13 3.95 mDJ �A 3.88 m Ex situ KIA 30740 Charcoal 5920�35 6697–6784 Series I �20.73� 0.25 3.88 mDJ �A 3.88 m Bulk sample KIA 30740 Humic acid 6065�30 6891–6964 Series I �21.30� 0.15 3.88 mDJ MTL5(1) Neolithic site KIA 31002 Charcoal 6110�35 6905–7139 Series I �25.17� 0.23 3.50 mDJ MTL6 Neolithic site KIA 31003 Charcoal 5840�35 6570–6723 Series I �23.01� 0.40 3.10 mDJ �A 2.93 m Neolithic site KN 5834 Charcoal 6388�46 7283–7395 Series I �23.14 2.93 m


According to Ely (1997), top positions of fining andcoarsening ups imply boundaries between isolated floodevents within the alluvial record. Here, we used coarse,medium and fine sand contents of the Moulouya overbankfines to gain supplemental information about the deposi-tion of single flood events. For abrupt shifts in grain sizes,especially fine-sand contents are used for the separation ofisolated fining and coarsening-up sequences.

Results

Chronostratigraphic record at the Djamilaexposure (Lower Moulouya River)

At the Djamila exposure the Lower Moulouya chronostrati-graphic record may be divided into three characteristicsedimentation series (Fig. 5). At the bottom margin of theDjamila exposure Series I (250–480 cm above current riverlevel) reveals an alternation of sandy layers with humic rich,sandy to loamy overbank fines. The slightly aggregated, humic-rich sediments resemble buried A horizons, indicating formerphases of low flood activity with corresponding soil formationand landscape stability. Several Neolithic open-air sites overliethe buried A horizons. Series I shows hydromorphic featureswith small-sized mottling of bleaching and reddening. Twobulk samples of humic acid (6964–6891 and 6789–6701 cal. aBP) and two ex situ charcoal samples (6784–6697 and 6654–6562 cal. a BP) from humic-rich horizons within the DjamilaSeries I indicate a mid Holocene age (Table 1). An in situ fire sitesituated 2.93 m above channel bottom reveals an age of 7395–7283 cal. a BP.

Above Series I, overbank sediments of Series II become muchfiner (480–730 cm above channel bottom). The sand contentseldom exceeds 1–2%. Like Series I, flood deposits of Series IIreveal hydromorphic features. Some silty to clayey fining-upsequences end with consolidated narrow bands rich incharcoal. Ash-greyish-brown (10YR) layers and strong reddish(5YR) spots point to the impact of fires on the former flood plainsurface. Two 14C samples in Series II reveal calibrated agesbetween 4202–4096 and 4011–3858 cal. a BP.


The final sedimentation Series III (730–1540 cm abovechannel bottom) reaches the present-day floodplain surface.Series III is characterised by extremely fine laminated fining andcoarsening-up sequences of silty to clayey deposits. Theircolouring varies from greyish-brown (10 YR) to reddish (7.5 YR)laminae. As in Series II, the sedimentation processes have beeninterrupted by several charcoal-rich sediments, indicating theimpact of fire. Six 14C samples from Series III reveal calibratedages between 3284–3089 and 1492–1362 cal. a BP.

Palaeoecological proxies

Figure 6(B) shows findings from the Lower Moulouyaanthracological analyses. Apart from the results from theDjamila exposure itself, some findings from adjacent alluvialprofiles have also been integrated into the figure. From thelowermost part to the top of the Moulouya record Olea spp. andPistacia spp. are dominant and ubiquitously present. Charcoalremains of Juniperus spp. can sometimes be found in the LowerMoulouya archaeological sites, but they are absent in thewidespread charcoal-rich flood layers. This might be anindicator for anthropogenic selection practices. Owing to itshigh fuel value, Juniperus is currently used in the region as fuelwood.

Quantitative macro-charcoal analyses show an increase ofcharcoal remains from the lower part to the upper part of theDjamila profile. However, the least amount is located in thelower sequence of Series III, between 3284–3089 and 2749–2714 cal. a BP (Fig. 6(C)).

Semi-quantitative snail analyses reveal three terrestrial snailclusters. The first cluster features taxa which are predominantlycommon in grassland and open woodland ecosystems(Sphincterochila maroccana, Rumina decollata, Otala spp.).The second cluster indicates a preference for living under rocksand stones in arid environments (e.g. Caracollina lenticula).The third cluster is characterised by snails living on shrubs togain more distance from the hot soil surface (Helicellaverignoni, Helix aspersa maxima). Although all samples weretaken from the Moulouya overbank fines within a short distanceto the current riverbed (<10 m), the analysed individuals revealonly terrestrial and no aquatic species.


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Copyright � 2009 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 25(5


) 700–714 (2010)

Figure 7 Alluvial stratigraphy of the Djamila (DJ) exposure at the Lower Moulouya River. The figure is subdivided into several profiles from thelowermost DJ �A sequence (a) to the DJ H sequence (e) at the top. The stratigraphical context of the individual profiles is illustrated in Fig. 5


Single flood events in the Moulouyapalaeohydrological record

Figure 7(a)–(e) shows a detailed stratigraphic record of theDjamila (DJ) site at the Lower Moulouya River. Stratigraphicalboundaries deduced from field findings (sediment structure andcolour) are marked with an ‘S’. They represent 26 major breaksin flood deposition and clearly show the transition from one


flood event to another. Especially within the sedimentationSeries II and III, the upper part of the flood deposits often showincreased charcoal amounts (Fig. 4).

Figure 7 shows the contents of coarse, medium and fine sand.Connections with solid lines indicate fining and coarsening upswithin the alluvial exposure. Major breaks (S) in the flood recordcoincide with top positions of coarsening or fining ups (e.g.b andg atDJA; eatDJD; z2, z3, z4 and z5 atDJH); hence the changes in


Figure 7 (Continued)


granulometric data support the previous field findings. However,grain size analyses indicate additional breaks in the sedimen-tation process (G). Regarding the interruptions of fining andcoarsening up-sequences within the whole DJ profile, a totalnumber of 50 single flood events can be considered.

Flood magnitude and frequency proxies of theMoulouya palaeohydrological record

Between 0.00 and 2.50 m and between 4.60 and 6.00 m abovecurrent channel bottom, no flood events are presented owing tosampling gaps in the DJ exposure (Fig. 7). From 2.50 to 4.60 m,flood events of Series I are documented. Concerning itslithostratigraphic structures and related depositional processes,these flood layers cannot be compared with the laminated flooddeposits of Series II and III. Thus the following results for thecalculation of the Moulouya flood frequency and magnitudeproxies are concentrated on the palaeoflood record withinSeries II and III. As a general comment it should be pointed outthat the Moulouya sedimentary flood units are accumulatedunder an increasing flood stage elevation threshold and,therefore, should be interpreted with this limitation.

Seven 14C dates from Series II and III between 6.00 and13.76 m reveal a chronological sequence of mid to lateHolocene flood events at the lower Moulouya River. Figures 7and 8(A) document the stratigraphic positions of sampledcharcoals and their calibrated ages (BP, 1s). Through thechronological interpolation of flood layers between the


calibrated 14C dates, the Moulouya palaeoflood history maybe reconstructed. Figure 8(A) reveals single flood eventsbetween 4.0 and 1.4 cal. ka BP. The graph features thesedimentation magnitude of the isolated floods. Generally,high sedimentation magnitudes occur with coarser grain sizes,whereas thin flood layers feature high clay contents. In Fig. 9(D)the total number of flood events per 100 a is computed,indicating a proxy for the Moulouya palaeoflood frequency.

Discussion

Mid to late Holocene palaeoenvironmentalchanges at the Lower Moulouya

Regarding anthracological and snail analyses from the LowerMoulouya flood deposits (Fig. 6), the findings indicatehomogeneous plant and terrestrial molluscan diversities fromthe mid Holocene until present. No significant shifts in plantand snail assemblages can be deduced. The dominance ofPistacia spp. and Olea spp. corresponds to the presentbioclimatic conditions, with a presence of Pistacia atlanticaand Olea oleaster close to the Lower Moulouya littoral zone (cf.Benabid, 2000). Mid to late Holocene molluscan remainsfeature species, which are widespread in present aridenvironments of the western Mediterranean. Typical Moulouyariverside vegetation (e.g. Salix, Tamarix) and aquatic snailspecies are absent in the overbank archives, indicating


Figure 7 (Continued)


continuous terrestrial conditions on the former mid to lateHolocene floodplain surface.

However, lithostratigraphic features and further palaeoenvir-onmental parameters imply evidence for palaeoenvironmentalchanges at the Lower Moulouya in the course of the mid to lateHolocene period: at the bottom margin of the Djamila exposure(Fig. 6), Series I features humic-rich, sandy to loamy overbankfines. The widespread presence of buried Neolithic open-air sites(Table 1; Zielhofer et al., 2008) at the top of the humic rich layersreveals a brief human occupation of the floodplain between7395–7283 and 6723–6570 cal. a BP. Initial soil formationindicates a period of minor flooding (cf. Zielhofer et al., 2009),although the hydromorphic colouring of the sediment matrixpoints to a persistent groundwater level and, therefore, to a morehumid climate. These findings correspond to several otherpalaeoecological studies from adjacent regions in the westernMediterranean, which show a maximum in temperature andhumidity at that time (e.g. Lamb et al., 1995; Zeroual, 1995; Reedet al., 2001; Carrion et al., 2003; Zielhofer and Linstadter, 2006;Fenech, 2007; Fletcher et al., 2007; Mercuri, 2008).

The following Djamila Series II becomes much finer andindicates a clear change in the alluvial sedimentation. Whereashydromorphic features in the sediment matrix show a morepersistent groundwater level than today, the presence of ash-rich greyish charcoal layers (Fig. 7: a, 4011–3838 cal. a BP)indicates an increasing impact of fire at the Lower Moulouya.


The final Series III reaches the present-day floodplain surface.Clayey to silty fining and coarsening up laminae arecharacterised by overlying ash-rich charcoal layers. Theincreased impact of fire is not only shown by the presenceof charcoal layers, but also by a high number of macro-charcoalremains and high values in magnetic susceptibility (Fig. 8).Series II and III indicate more seasonal environmental condi-tions and/or an increased impact of humans on the ecosystems.The beginning of enhanced flood dynamics at the LowerMoulouya with the onset of Series II somewhat before4.1 cal. ka BP corresponds to the observations at the adjacentLower Kert River, where an increase in ‘sedimentary accretion’and an enhanced impact of fires are also deduced (El Amraniet al., 2008). These findings support global (Steig, 1999) andregional (Wengler and Vernet, 1992; Roberts et al., 2004;Zielhofer et al., 2004) indications for a brief mid Holoceneclimatic shift during the fifth millennium BP. However,increased human impact cannot be excluded. Surprisingly,the sedimentation of the thick Series III ends abruptly somewhatafter 1.3 cal. ka BP. This phenomenon is also observed at theLower Kert River (El Amrani et al., 2008). A climaticexplanation has been inferred from marine records (Holzet al., 2007), which would imply more humid conditions in theNorth African Atlantic region from about 1.3 cal. ka BPonwards. These may correspond to changes in flood thresholdsand lower flood amplitudes.


Figure 8 (A) Reconstruction of single flood events and thicknesses of the Moulouya flood layers: the X-axis shows the elevations of the flood layersabove the current river base level (data from Djamila exposure). The elevations of the charcoal layers (DJ a to DJ z5) are also shown. The Y-axis showsthe thickness of the flood layers. The metric resolution is limited to a least 10 cm per flood event due to the resolution of the sampling intervals. Henceisolated flood deposits with a thickness<10 cm are presented in condensed form. The 14C data from the Djamila (DJ) exposure indicate two continuouspalaeohydrological records between 7.3 and 6.6 cal. ka BP and between 4.0 and 1.4 cal. ka BP. (B) The X-axis shows the elevations of the flood layersabove the channel bottom. The Y-axis reveals the number of charcoal pieces (>0.063 mm) per 0.2 g sediment sample. (C) X-axis like part B, the Y-axisfeatures magnetic susceptibilities from the DJ exposure in 10�6 SI units

Copyright � 2009 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 25(5) 700–714 (2010)


Figure 9 Mid to late Holocene palaeohydrology of the Lower Moulouya River in relation to other regional proxy records: (A) Lower Moulouyaaverage sedimentation rate (mm a�1) computed by 14C data and sediment thicknesses between 4.0 and 1.4 cal. ka BP. The numbers document14C ranges (1s). (B) Reconstructed Lower Moulouya single flood layers and their thicknesses. The thickness of the isolated flood layers is interpreted as aproxy for the flood magnitude. (C) Number of macro-charcoal pieces in the Moulouya overbank fines (counts per 0.2 g sediment sample). The bold linefeatures a �5 layer smoothing. (D) Reconstructed number of Moulouya single floods per 100 a (flood frequency proxy). Here, the thickness of flooddeposits is not considered. (E) The black line indicates xerophytic pollen taxa (%) from Lower Guadiana peat record in southern Portugal (Fletcher et al.,2007). (F) The graph shows the centennial-scale average sedimentation rate (mm a�1) of the Mid-Medjerda floodplain in northern Tunisia (Faust et al.,2004). (G) The black bars feature abrupt desiccation phases at the Laguna de Medina in southwestern Spain (Reed et al., 2001). (H) The graph showsmicro-charcoal counts (n) from Lake Siles in southern Spain (Carrion, 2002)


Moulouya flooding responses to Holocene rapidclimate changes

The recorded mid and late Holocene flood frequencies at theLower Moulouya (Fig. 9(D)) coincide with high-resolution proxyrecords from the North Atlantic (Fig. 10(C), Bond et al., 2001) andthe Aegean Sea (Fig. 10(D), Rohling et al., 2002). Rapid increasesin North Atlantic ice-rafted haematite content and decreases inwarm foraminifer species in the Aegean Sea imply Holocenecooling. These signals correlate with increased Moulouya flood


frequencies. The high flood frequency periods between 3200 and2700 cal. a BP are synchronous with a major Holocene RCC.RCCs are global cooling phases in the course of the Holocene,which are deduced from glacial (Denton and Karlen, 1973; Hauget al., 2001), lacustrine (Magny et al., 2003; Magny, 2004; Parkeret al., 2006) and marine (Bond et al., 2001; Rohling et al., 2002)palaeotemperature records. According to Mayewski et al. (2004),solar variability is a plausible forcing for RCCs (cf. Plunkett andSwindles, 2008). Following the amplitude of the cosmogenic14C record between 4000 and 1500 cal. a BP (Fig. 10(E), Stuiver


Figure 10 Mid to late Holocene palaeohydrology of the Lower Moulouya River in relation to other regional to global proxy records. (A) Reconstructednumber of Moulouya single floods per 100 a (flood frequency proxy) between 4.0 and 1.4 cal. ka BP. (B) Number of macro-charcoal pieces in the Moulouyaoverbank fines (counts per 0.2 g sediment sample). The bold line features a �5 layer smoothing. (C) The black line indicates the haematite content as aproxy for ice-rafted debris in the North Atlantic (Bond et al., 2001) and features abrupt changes in mean temperatures. (D) The black line shows warmforaminifer species in the Aegean Sea (Rohling et al., 2002). (E) The black line shows atmospheric 14C residuals (Stuiver et al., 1998) and the bold black lineshows the 200 a Gaussian smoothing, which may be interpreted as a proxy for changes in solar variability (Mayewski et al., 2004)


et al., 1998), enhanced signals in solar variability match withincreased flood frequencies at the Lower Moulouya (Fig. 10(A)).

Benito et al. (2008) have compiled Holocene flood recordsfrom Iberian rivers and reveal a coupling of flood signals withlarge-scale North Atlantic coolings. These findings supportresearch on Mediterranean alluvial deposits that show highsensitivities of flood dynamics to short-term Holocene climaticshifts (Vita-Finzi, 2008; Zielhofer and Faust, 2008).


Cold and wet or cold and dry?

As far as the high sensitivity of Mediterranean rivers toHolocene RCCs seems to be clear, the question concerning theflood-producing mechanisms remains uncertain. According toFaust et al. (2004), Fenech (2007), Giessner (1990), Sanchoet al. (2008) and White et al. (1996), arid to semiaridMediterranean catchments exhibit decreased flooding with



soil formation under more humid conditions. These lead to adense vegetation cover with soil formation and low surfacerunoff, while more arid conditions thin out vegetation andresult in alluvial activity and increased flooding (cf. Rohden-burg, 1989).

On the other hand, Benito et al. (2003a) strongly linked short-term Holocene flood increases of the Tagus River to enhancedmoisture influx and winter precipitation in the IberianPeninsula. They support their findings with adjacent proxyrecords such as lake level and vegetation changes (e.g. Van derBrink and Janssen, 1985; Zeroual, 1995; Julia et al., 1998).Furthermore, Bernardez et al. (2008) and Thorndycraft andBenito (2006a,b) present Holocene palaeohydrology recordsfrom off Iberian Peninsula and Spanish catchments that matchabrupt Holocene cooling with increases in flooding due toenhanced annual precipitation. This may be supported byBarriendos and Martın-Vide (1998), who conclude fromhistorical sources that extreme flood events on the easternIberian Peninsula correspond to Little Ice Age cooling andprecipitation maxima. However, the regional and chronologi-cal correlation of Holocene flood records with other westernMediterranean palaeoenvironmental archives remains compli-cated so far due to dating uncertainties, resolution problemsand difficulties in the understanding of process complexity ofproxy signals.

In contrast, results from lake and estuary cores from thesouthern Iberian Peninsula and Mediterranean North Africaprovide evidence of submillennial arid intervals in the course ofNorth Atlantic coolings. Notably arid phases during the majorHolocene RCC event between 3200 and 2700 cal. a BP aredocumented in many of the western Mediterranean palaeoen-vironmental records (Fig. 9(H): Carrion, 2002; Fig. 9(E):Fletcher et al., 2007; Lamb and van der Kaars, 1995;Fig. 9(G): Reed et al., 2001).

In this paper, Holocene fluvial signals are matched withenvironmental proxy data from the same record. We used asedimentary charcoal record (cf. Pierce et al., 2004) andmagnetic susceptibility as proxies for changes in fire densities(Fig. 8(B) and (C)). The higher the magnetic susceptibility andthe number of charcoal remains in the alluvial sediment, thehigher the impact of fire. The Moulouya palaeohydrologyrecord features pronounced flooding in the course of theHolocene RCC event between 3200 and 2700 cal. a BP.Pronounced flooding correlates with decreased numbers ofmacro-charcoal remains (Fig. 9(C)). Here, a positive correlationbetween flooding and low fire recurrences is deduced. Takeninto account that Mediterranean fire ecosystems in arid tosemiarid environments are fuel limited (Turner et al. 2008;Linstadter and Zielhofer, 2009), we may postulate more aridconditions in the Lower Moulouya basin between 3200 and2700 cal. a BP cooling.

Conclusions

The Lower Moulouya floodplain in northeastern Moroccofeatures a continuous record of mid to late Holocene flooding.High-resolution palaeohydrological signals can be deduced byflood frequency proxies. While floodplains in more humid andtemperate regions of Europe are characterised by lateral erosionprocesses and the redeposition of former flood sediments(Macklin et al., 2006), the Lower Moulouya alluvial archivesprove excellent preservation conditions against fluvial erosionand demonstrate continuous archives of distinctive floodevents.


The Moulouya alluvial record shows increased floodfrequencies during Holocene cooling. This can be inferredespecially for the major Holocene RCC between 3.2 and2.7 cal. ka BP. During the same time, the sedimentation rate ofthe Mid Medjerda floodplain in northern Tunisia features ashort-term and significant increase (Fig. 9(F): Faust et al., 2004).This suggests the high sensitivity of major river systems in thesouthwestern Mediterranean against abrupt Holocene globalchanges.

Increased flood frequencies are accompanied by decreasedmacro-charcoal particles in the Lower Moulouya alluvialarchives, indicating a decreased impact of fires in thefloodplain. Reduced fire recurrences in Mediterranean aridenvironments are interpreted as fuel-limited systems; thereforewe postulate more arid conditions at the Lower MoulouyaBasin between 3.2 and 2.7 cal. ka BP.

Acknowledgements We would like to thank the Volkswagen founda-tion (II/81 227) for supporting our research in northeastern Morocco.We also thank Abdeslam Mikdad (Institut National des Sciences del’Archeologie et du Patrimoine, Rabat) and Josef Eiwanger (DeutschesArchaologisches Institut, Bonn, Germany) for their support during theLower Moulouya survey. For AAS analyses at the Geomorphologicallaboratory of Dresden University of Technology (Germany) we are mostthankful to Beate Winkler. We would also like to thank Werner H.Schoch (Langnau, Switzerland) for the anthracological analyses andDominik Faust (Dresden University of Technology, Germany), JohannaLomax (Cologne University, Germany) and Varyl Thorndycraft (RoyalHolloway University of London) for their helpful comments.

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flood frequencies reveal holocene rapid climate changes (lower

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