contrasting patterns of precipitation seasonality during the holocene in the south- and...

Contrasting patterns of precipitation seasonalityduring the Holocene in the south- andnorth-central Mediterranean

MICHEL MAGNY,1* ODILE PEYRON,1 LAURA SADORI,2 ELENA ORTU,1 GIOVANNI ZANCHETTA,3

BORIS VANNIERE1 and WILLY TINNER41Laboratoire de Chrono-Environnement, UMR 6249 du CNRS, UFR des Sciences et Techniques, 16 route de Gray,25 030 Besancon, France2Dipartimento di Bilogia Ambientale, Universita ‘‘La Sapienza’’, Roma, Italy3Dipartimento di Scienze della Terra, and IGC-CNR, Pisa, Italy4Paleoecology, Institute of Plant Sciences and Oeschger Center for Climate Change Research, University of Bern, Bern, Switzerland

Received 30 May 2011; Revised 26 August 2011; Accepted 28 August 2011

ABSTRACT: Pollen-based quantitative estimates of seasonal precipitation from Lake Pergusa and lake-level data fromLake Preola in Sicily (southern Italy) allow three successive periods to be distinguished within the Holocene: an earlyHolocene period before ca. 9800cal a BP with rather dry climate conditions in winter and summer, a mid-Holoceneperiod between ca. 9800 and 4500cal a BPwithmaximumwinter and summerwetness, and a lateHolocene period after4500 cal a BP with declining winter and summer wetness. This evolution observed in the south-central Mediterraneanshows strong similarities to that recognized in the eastern Mediterranean. But, it contrasts with that reconstructed innorth-central Italy, where the mid-Holocene appears to be characterized by a winter (summer) precipitation maximum(minimum), while the late Holocene coincided with a decrease (increase) in winter (summer) precipitation. Maximumprecipitation at ca. 10 000–4500 cal a BP may have resulted from (i) increased local convection in response to aHolocene insolation maximum at 10 000cal a BP and then (ii) the gradual weakening of the Hadley cell activity, whichallowed the winter rainy westerlies to reach the Mediterranean area more frequently. After 4500 cal a BP, changes inprecipitation seasonality may reflect non-linear responses to orbitally driven insolation decrease in addition to seasonaland inter-hemispheric changes of insolation. Copyright # 2011 John Wiley & Sons, Ltd.

KEYWORDS: Mediterranean; Holocene; modern analogues technique; precipitation seasonality; sapropel 1.

Introduction

The Mediterranean area lies in a transitional zone that isinfluenced by both the dynamics of (i) the tropical circulationcells with the subtropical anticyclone belt and associatedaridity, and (ii) the mid-latitude westerlies and cyclogenesis(Tzedakis et al., 2009). This results in a marked precipitationseasonality that is crucial for both Mediterranean ecosystemsand societies.Recent studies have pointed to the role of seasonality in

abrupt climate change and in the Mediterranean region(Denton et al., 2005; Kotthoff et al., 2008; Davis and Brewer,2009; Dormoy et al., 2009; Pross et al., 2009). They have alsohighlighted that seasonality can explain apparent discrepanciesbetween palaeoclimatic data derived from pollen, firefrequency, lake levels and isotopes for the mid-Holoceneclimate in the central Mediterranean (Magny et al., 2007;Zanchetta et al., 2007b; Vanniere et al., 2011; Roberts et al.,2008; Peyron et al., 2011; Giraudi et al., 2011; Sadori et al.,2011). However, given the relative scarcity of reliablepalaeoclimatic records for the Holocene in the south-centralMediterranean, additional data are needed to test thishypothesis and provide a more precise picture of variationsin the seasonality in the Mediterranean area.Using the modern analogues technique (MAT; Guiot, 1990),

this study aims to provide a record of quantitative estimates ofthe Holocene climate based on the pollen sequence of LakePergusa in Sicily (Sadori and Narcisi, 2001), with particularattention to the reconstruction of precipitation seasonality. Incombination with the lake-level record established at LakePreola in south-western Sicily (Magny et al., 2011b), thesequantitative estimates provide new insight into the seasonal

precipitation patterns and climatic trends which have charac-terized the Holocene in the south-central Mediterranean.Finally, a comparison with other lake-level data and quanti-tative estimates of climatic parameters obtained at Lake Accesain central Italy (Magny et al., 2007; Peyron et al., 2011) allowscontrasting seasonality patterns to be distinguished betweensouthern and northern border regions of the central Mediterra-nean (Fig. 1).

Site, data and methods

Lake Pergusa (378310N, 148180E; 667m a.s.l.) is located incentral Sicily. The catchment area of the lake is about 7.5 km2

and reaches its maximum elevation at Monte Carangiano(911m a.s.l.). The lake’s surface area was ca. 1.4 km2 in 1968.Mean annual precipitation in the area is between 500 and700mm. At the weather station of Enna (about 5 km NNW ofthe lake), mean annual temperature is 13.4 8C, with 37 8C forthe warmest month and –3 8C for the coldest (Zampino et al.,1997).

According to Sadori and Narcisi (2001), Lake Pergusa islocated in the meso-Mediterranean bio-climate belt of theMediterranean region, in the subhumid ombrotype and inferiorthermotype (Brullo et al., 1995). The potential vegetation isformed by mesophilous Mediterranean evergreen woodsdominated by Quercus ilex L.

The Holocene pollen record of Lake Pergusa has beenestablished by L. Sadori from a 4.5-m-long core taken in thelake basin, and extensively published in a previous study(Sadori and Narcisi, 2001). The chronology is based on eightradiocarbon dates in addition to a tephra layer probably relatedto the Sicans event (late Holocene explosion from the Etnavolcano). A calendar chronological scale was recently assessed(Sadori et al., 2011). Seven successive pollen zones have been

JOURNAL OF QUATERNARY SCIENCE (2012) 27(3) 290–296 ISSN 0267-8179. DOI: 10.1002/jqs.1543

Copyright � 2011 John Wiley & Sons, Ltd.

*Correspondence: M. Magny, as above.E-mail: [email protected]

distinguished within the pollen sequence as follows (Sadori andNarcisi, 2001).

� Before ca. 11 000 cal a BP, herbaceous plants are dominant[non-arboreal pollen (NAP) between 56 and 83%] and totalpollen concentration values are low. Both Quercus roburtype and Q. ilex type show continuous curves.

� During the period around 11 000–10000 cal a BP, herbs arestill dominant (56–74%) although decreasing, and total pol-len concentration values increase slightly. Deciduous andevergreen oaks show an increase both in percentage (21–33%) and in concentration.

� The period around 10 000–8000 cal a BP began with anabrupt increase of arboreal pollen (AP) percentages (expan-sion of the forest) and concentrations. The proportion of AP ismainly around 80%. The main taxon is Quercus robur type(40–60%) followed by Q. ilex type (12–25%). As inferredfrom pollen percentages, other important arboreal taxa areQ. suber/cerris, Ericaceae, Ulmus, Corylus and Fagus.

� During the period around 8000–5000 cal a BP, the APpercentages reach the highest value (94%) while AP andNAP concentrations decrease. Quercus robur type andQ. ilex type are always present at high percentages, whilepercentages of Corylus, Fagus and Ericaceae decrease, andthose of Olea show an increasing trend.

� During the period around 5000–3000 cal a BP, AP remaindominant (ca. 70%) and total pollen concentrations are low.Ulmus percentages reach their maximum values.

� The period around 3000–2300 cal a BP is characterized bythe highest expansion of Olea (10–19%).

� Finally, the period from ca. 2300 cal a BP to the presentshows alternate dominant AP and NAP percentages, whiletotal pollen concentrations are very low (values comparablewith those reached before 11 000 cal a BP). Oaks are stilldominant in the AP percentages.

In summary, maximal total pollen concentrations areobserved between ca. 9800 and 4700 cal a BP with valuespeaking between ca. 9300 and 6600 cal a BP (Fig. 2). Fromabout 5000 cal a BP, the forest has become more open. Per-centages of deciduous oaks decrease while those of evergreenelements become relatively more important and dominate afteraround 2800/2700 cal a BP. Important fires occurred at the endof the Lateglacial and since the end of the Bronze Age, i.e. in thelast 3200 years (Sadori and Giardini, 2007). On the basis of thepollen data, Sadori and Naricisi (2001) come to the generalconclusion that, at Pergusa, human impact on the vegetationduring the late Holocene overlapped a drying trend that startedaround 8000 cal a BP and did not produce important effects onthe already open landscape.The method used for the reconstruction of climatic

parameters is the MAT, which was first developed by Overpeck

Figure 1. Geographical location of LakesPergusa and Preola and reference sites in theMediterranean and in west-central Europe. LA,Lake Accesa; LAl, Lake Albano; LC, LakeCerin; LMe, Lake Mezzano; NF, Nisi Fen;SC, Soreq Cave; TP, Tenaghi Philippon; SL152 is a marine core. The dotted line showsthe limit of the Mediterranean sensu strictobiogeographical zone. This figure is availablein colour online at wileyonlinelibrary.com.

SL 152LAKEPERGUSA

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Figure 2. Comparison of the pollen data (curves of arboreal pollenand Quercus deciduous) from Lake Pergusa (central Sicily; Sadori andNarcisi, 2001; Sadori et al., 2011) with (i) quantitatives estimates (witherror bars) of annual precipitation (PANN), summer precipitation (Ps)and winter precipitation (Pw) reconstructed using MAT from Pergusapollen data (this study), and (ii) the lake-level record of Lake Preola(south-western Sicily; Magny et al., 2011b). Rectangles with dottedlines mark transition intervals of large lake-level fluctuations around 10500–9000 and 6500–4500cal a BP. Additional oxygen-isotope dataand pollen- and sediment-inferred lake-level data (Zanchetta et al.,2007a, b; Sadori and Narcisi, 2001; Sadori et al., 2008) are presented forLake Pergusa. Grey-shaded vertical bandsmark periods ofmajor changes.The vertical dotted line around 6500cal a BP points to decreasing(summer) humidity. The chronology of Sapropel 1 is that given by Emeiset al. (2000). Arrows correspond to present-day levels of precipitation.

Copyright � 2011 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 27(3) 290–296 (2012)

HOLOCENE PRECIPITATION SEASONALITY IN THE CENTRAL MEDITERRANEAN 291

et al. (1985) and extended by Guiot (1990). It is a commonlyused and accepted method for the reconstruction of Lateglacialand Holocene climate oscillations from both continental andmarine sequences (Guiot et al., 1993, 2009; Cheddadi et al.,1997; Davis et al., 2003; Davis and Brewer, 2009; Kotthoffet al., 2008; Pross et al., 2009; Dormoy et al., 2009). Thegeneral principle is to find, for each fossil pollen assemblage,several closest modern spectra (or modern analogues) on thebasis of an appropriate distance index or chord distance. Theclimate of these analogues is averaged to provide an estimate ofthe fossil assemblage climate.The search for analogues is based on the chord distance

(Overpeck et al., 1985):

d2ik ¼

Xm

j¼1

ðpfij �pfkjÞ2 (1)

where fij anf ftj are the relative frequencies of pollen taxon j (outof m¼ 104 taxa) in modern pollen spectrum i and fossil pollenspectrum k, respectively. Equation (1) is used to find a set of sclosest modern analogues of the fossil spectra. The quality ofthe reconstruction is expressed by the climate homogeneity ofthe s analogues. The reconstructed climate value 0Rt for eachfossil spectrum k is the distance-weighted mean (by the inverseof the distance in eqn 1) of the climate values Ci associated withthe s best analogues:

0Rt ¼ ðXS

i¼1

Ci=d2it Þ=ð

XS

i¼1

d�2it Þ (2)

Instead of a unique standard deviation around 0Rt, the lowerand upper limits of this mean estimate were computed(confidence intervals). The lower limit LLt is given by thedistance-weighted mean of the analogues (out of the total s¼ 8)with Ci<

0Rt and the upper limit ULt is given by the distance-weighted mean of the analogues with Ci>

0Rt. Theseconfidence limits implicitly include errors in the modernclimate observations (usually small, a few tenths of a 8C or afew mm per month), the natural variability of the assemblagesfor a given value of the climatic variable, and the influence ofnon-climatic factors.In this study, the MAT is based on a new modern pollen-

climate dataset, which includesmore than 3500 pollen samplesfrom moss polsters, top cores and soil samples from throughoutEurasia, i.e. from the British isles to the Kamtchatka peninsula,and from Scandinavia to the Mediterranean (Bordon et al.,2009; Dormoy et al., 2009).As discussed by Peyron et al. (2011), despite possible human

impact on present-day vegetation, statistical tests based on theupdated modern pollen dataset have shown that the observedclimate is well reconstructed from the modern pollenassemblages (root mean square error of prediction values forthe seasonal parameters). Moreover, recent studies found theMAT to have the lowest error for all reconstructed climatevariables in comparison with three other methods of quanti-tative reconstruction applied to Mediterranean pollensequences (see extensive discussion in Peyron et al., 2011).For the present study, 81 samples with fossil pollen

assemblages were available for quantitative climatic recon-struction, i.e. a mean temporal resolution of ca. 145 years persample over the Holocene period.

Results and discussion

Figure 2 presents the results obtained for the three climaticparameters related to precipitation level and seasonality asreconstructed by MAT: PANN (annual precipitation), Pw(winter precipitation: December, January and February) and

Ps (summer precipitation: June, July and August). The threecurves show similar evolutions and allow three distinct periodsof precipitation changes to be distinguished: a first periodbefore ca. 9800 cal a BP with relatively dry conditions (drywinters and summers), a second period between ca. 9800 and4500 cal a BP with maximal wetness (wetter winters andsummers), and a third period since ca. 4500 cal a BP withdecreasing wetness (drier winters and summers).

Before ca. 9800 cal a BP: a dry early Holocene

Considered as a whole, relatively dry conditions with drywinters and summers characterize the Pergusa area during theearly Holocene. However, this period also shows a generaltrend towards increasing moisture availability following dryconditions at the beginning of the Holocene. Around 11 500 cala BP, at the transition between the Lateglacial and theHolocene, climate conditions appear to have been drier thantoday, with PANN and Ps values below 500 and 20mm,respectively. The following increase in precipitation ispunctuated by large fluctuations which suggest unstableclimatic conditions.

The period between ca. 9800 and 4500 cal a BP:a humid mid-Holocene

Conditions that were wetter than today generally prevailedduring the mid-Holocene. PANN, Pw and Ps indicate maximalwetness with PANN and Ps peaking around 9000 cal a BP at ca.800 and 100mm, respectively. Two phases may be distin-guished within this mid-Holocene period. The first before ca.7500/7000 cal a BP is characterized by a precipitationmaximum, and broadly coincides with the highest total pollenconcentrations observed in the Pergusa pollen sequence(Sadori and Narcisi, 2001; Perez-Obiol and Sadori, 2007)and lower d18O values of bulk carbonates (Zanchetta et al.,2007b; Sadori et al., 2008). The second corresponds to aprogressive decrease in precipitation.

The period from ca. 4500 cal a BP to the present:a drier late Holocene

The PANN, Pw and Ps curves show a continuation of thegeneral trend towards declining wetness observed since ca.7500/7000 cal a BP. However, examination of the confidenceinterval (error range) of the three climatic parameters providesstronger evidence of the limit around 4500 cal a BP between awet mid-Holocene and a slightly drier late Holocene. More-over, the curves suggest that Ps was more affected by the dryingthan PANN and Pw. Thus, the mid- to late Holocene transitioncorresponds to a mean lowering by ca. 10–8% for PANN andPw whereas the decrease in wetness reaches more than 30% insummer. This progressively leads to a stronger seasonal contrastin precipitation, typical of the current Mediterranean climate ofthe region.

As illustrated by Fig. 2, these results are broadly consistentwith previous climatic interpretations inferred from oxygen-isotope, sediment and pollen data at Pergusa (Sadori andNarcisi, 2001; Zanchetta et al., 2007b; Sadori et al., 2008,2011). Furthermore, as shown by Frisia et al. (2006), it isnoteworthy that the curves of AP percentages and total pollenconcentration at Pergusa are in agreement with the climaticsignal inferred from carbon isotope composition of onespelothem collected in the Carburangeli cave in north-westernSicily.

The precipitation curves reconstructed by the MAT frompollen data of Pergusa (Fig. 2) show a general agreement withthe lake-level record of Lake Preola in south-western Sicily ca.


292 JOURNAL OF QUATERNARY SCIENCE

120 km west of Lake Pergusa (Magny et al., 2011b). The Preolalake-level record has been established from a sedimentologicalapproach (Magny, 2006). Given the warm-season developmentof the sedimentological indicators used to reconstruct past lake-level changes at Preola, the lake-level record may beconsidered, in a first approximation, to reflect changes insummer wetness. Like the Pergusa precipitation curves, thePreola lake-level record gives evidence of three successiveperiods within the Holocene, with a very dry early Holocenebefore ca. 10 500 cal a BP (absence of a lake), a wet mid-Holocene until 4500 cal BP, and a drier late Holocene after4500 cal BP. Two transition intervals around 10 500–9000 and6400–4500 cal a BP are characterized by relatively unstableclimatic conditions with large lake-level fluctuations. Thus,these two independent records based on two different types ofproxies similarly point to the key change in wetness around4500 cal a BP at the mid- to late Holocene transition. Inaddition, the transition intervals observed at ca. 10 500–9000and 6400–4500 cal a BP in the Preola record may be equivalentto the periods in the Pergusa Ps curve marked by largeoscillations before 9000 cal a BP and a decreasing wetness afterca. 6500 cal a BP.Recent studies developed at Lake Accesa in Tuscany (Italy;

Fig. 1) provided pollen and lake-level data which offer theopportunity to compare both (i) quantitative estimates ofprecipitation (Peyron et al., 2011; Finsinger et al., 2010), and(ii) a lake-level record (Magny et al., 2007) from the north-central Mediterranean with those established from the Siciliansites of Pergusa and Preola in the south-central Mediterranean.Figure 3 (panel 4A) presents the Pw and Ps curves and the lake-level record obtained at Lake Accesa. According to Peyron et al.(2011), on amillennial scale, they show three distinct periods ofprecipitation changes as follows.

� A first period before ca. 10 500–10 000 cal a BP correspondsto a weak precipitation seasonality with Ps nearly as abun-dant as Pw. This period coincided with high lake level.

� A second period between 10 500/10 000 and 7800 cal a BPwas characterized by strongMediterranean conditions with aprecipitation maximum in winter and a marked dryness insummer responsible for a lake-level minimum.

� Finally, a third period after ca. 7800 cal a BP corresponds to ageneral trend towards a decrease in Pw and a slight increasein Ps (weakening of the seasonal contrast), while the lake-level record shows a general rise of the water table punc-tuated by a major rise event around 4500 cal BP. The generalrise of lake level may at least partly reflect the influence of (i)the long-term cooling of the sea surface in the NortheastAtlantic and western Mediterranean observed by Marchalet al. (2002), and (ii) increasing westerly flow releasing moreprecipitation on the site.

A similar agreement between minima of pollen-inferredestimates of summer precipitation and marked lowstands oflake level have been recognized for the mid-Holocene at LakeAccesa from a profundal core using a method of reconstructionbased on a weighted averaging/partial least squares regressionmethod (Finsinger et al., 2010).Thus, comparison of the lake-level and seasonal precipi-

tation records from Sicily (from Lakes Pergusa and Preola) andTuscany (from Lake Accesa) suggest contrasting palaeohydro-logical patterns for the Holocene between the south- and thenorth-central Mediterranean. As illustrated by Fig. 3, this issupported by a further comparison of Preola and Accesa lake-level records with the lake-level record of Lake Cerin in west-central Europe (Magny et al., 2011a) and the oxygen-isotoperecord of Soreq Cave in the eastern Mediterranean (Bar-

Matthews et al., 1998). Clearly, two opposite patterns appearwith (i) the Accesa and Cerin records to the north characterizedby a mid-Holocene minimum in summer wetness, and (ii) thePreola and Soreq Cave records south of 408N marked bymaximal wetness during the mid-Holocene. However, ashighlighted by recent studies (Kolodny et al., 2005; Marinoet al., 2009; Develle et al., 2010), the Soreq isotopic recordshows strong similarities with other regional lake and caveisotopic records, but it also reflects a combination of effects oflow seawater isotope composition (source effect) reinforced byenhanced inland rainfall (amount effect). Nevertheless,comparison of the Preola lake-level and the Soreq isotopicrecords points to marked synchronous rainfall-regime tran-sitions at ca. 10 500 and 4500 cal a BP.Focusing on the Mediterranean, the data presented in Figs. 2

and 3 suggest that both the south- and the north-central

AGE (ka cal BP)

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δ18O ‰ minimumat Corchia cave

(Italy, 43°N)

Figure 3. Quantitative estimates of summer and winter precipitationreconstructed using MAT from pollen data of Lake Accesa, central Italy(panel 4A; Peyron et al., 2011) and comparison of the Preola lake-levelrecord (panel 5;Magny et al., 2011b) with the annual insolation at 408N(panel 1; Berger and Loutre, 1991), and with other lake-level/palaeo-hydrological records from the Mediterranean and west–central Europe,i.e. Corchia Cave (panel 3; Zanchetta et al., 2007a), Lake Accesa (panel4B; Magny et al., 2007), Lake Cerin (panel 2; Magny et al., 2011a) andSoreq Cave (panel 6; Bar-Matthews et al., 1998). Note that the lake-level records from Preola, Accesa and Cerin have been established fromthe same sedimentological approach. The insolation curves at 408N(Berger and Loutre, 1991) provide evidence that the general late-Holocene decline of the annual insolation also coincided with animportant reorganization in seasonality around 4500–4000 cal a BP.Rectangles in dotted lines indicate phases of rapid changes linked to (i)orbitally driven changes in insolation around 10 500–10 000 and 4500–4000 cal a BP, and (ii) the disappearance of the Fennoscandian icesheet. This figure is available in colour online at wileyonlinelibrary.com.



Mediterranean underwent maximal precipitation in winterduring the mid-Holocene, and more particularly during theinterval around 10 000/9500–7500/7000 cal a BP, i.e. broadlyduring the deposition of sapropel 1 (Figs. 2 and 3). This is ingeneral agreement with the quantitative estimates of seasonalprecipitation reconstructed by Kotthoff et al. (2008), Dormoyet al. (2009) and Peyron et al. (2011) from pollen data of marinecore SL 152 in the northern Aegean sea and of TenaghiPhilippon (Fig. 1), which reflects climatic conditions in thenorthern Mediterranean borderlands.The mid-Holocene summer precipitation regimes appear to

be opposite between the south- and north-central Mediterra-nean, with a maximum in Sicily and a minimum in north-central Italy. Such a combination of differences and similaritiesbetween the south- and the north-central Mediterranean duringthe mid-Holocene depending on the season considered mayexplain the apparent conflict between palaeoclimatic recordsdepending on the proxy used for the reconstructions. Thus, thestable isotope record from a speleothem of Corchia Cave innorth-central Italy, which shows enhanced rainfall in thewestern Mediterranean between ca. 8900 and 7300 cal a BP(Zanchetta et al., 2007a), probably reflects an increase inwinterprecipitation originating from the North Atlantic but with areduced recharge during summer, a situation which can favourthe preservation of the winter rainfall isotopic signal (e.g.Longinelli et al., 2006). This period of time appears to beassociated with a maximum of flooding activity between ca. 8and 7 cal ka BP in the area of Corchia cave, as indicated by thestratigraphy of the Renella cave (Zhornyak et al., 2011). Theprogressive increase of the oxygen stable isotopes at ca.4500 cal a BP in Corchia Cave might be also consistent with anincrease in recharge during summermonths, associated with anaverage decrease in annual precipitation. Furthermore, theprecipitation records obtained from Lakes Accesa in Tuscany,and Pergusa and Preola in Sicily are in agreement with drier andwarmer summers inferred from pollen data at Nisi Fen innorthern Greece (ca. 418N), for the period 10 500–7500 cal aBP (Lawson et al., 2005). Finally, these records are alsoconsistent with pollen data from marine core MD95-2043taken in the Alboran Sea in the south-western Mediterranean,where an abrupt increase in forest populations at 10 600 cal aBP suggests a climatic shift towards wetter conditions. Thislong-term mid-Holocene forest development was punctuatedby short-lived forest declines due to brief cooling episodesassociated with drier conditions in the south-western Medi-terranean in contrast to high lake-level intervals in west-centralEurope and in Tuscany (Fletcher et al., 2010; Magny et al.,2003, 2007).A last topic of discussion are the climate processes behind the

changes inwetness observed during theHolocene in the centralMediterranean. As pointed out by Tinner et al. (2009), the mid-Holocene wetness increase observed in coastal Sicily probablyrelates to a decrease in the Hadley circulation (trade winds) andmonsoonal activity after the boreal insolation maximum. Asimilar forcing has been suggested for other areas of theMediterranean (Tzedakis et al., 2009). Gaetani et al. (2007)have shown how an intense African monsoon reinforces theHadley circulation, and consecutively strengthens the NorthAtlantic anticyclone and its blocking effect for the western wetairflow towards theMediterranean. Such an evolution of orbitalforcing may explain the general early Holocene drynessobserved in the Mediterranean area south of ca. 408N andillustrated by the lake-level records in Fig. 3. Nevertheless,recent observations have also highlighted a possible influenceof rainstorms originating from the Tropics over the easternMediterranean (Ziv et al., 2005). Taken together, the new(palaeo)climatic evidence presented in this study suggests that

wetter conditions developed in the southern Mediterraneaneven before the insolation and Hadley circulation maximumstarted to decline (Fig. 3). Earlier studies (Tzedakis, 2007;Tinner et al., 2009) attributed the wetness increase at around10 000 cal a BP to a possible increase in local convectiveprecipitation, in response to the orbitally driven insolationmaximum.

Moreover, the abrupt lake-level lowering observed around9000 cal a BP at both Lake Accesa in the north-westMediterranean and Lake Cerin in west-central Europe appearsto be synchronous with the disappearance of the Fennoscan-dian ice sheet. This suggests a possible influence of deglaciationin the reorganization of the general atmospheric circulation inwestern Europe at that time (Magny et al., 2011a; Shuman andPlank, 2011) with associated impact over the mid-Europeanlatitudes and the Mediterranean northern borderlands.

Finally, all the records presented in Fig. 3 suggest a strongclimatic change around 4500 cal a BP. Marked environmentalchanges were also observed at Lakes Mezzano and Albano insouth-central Italy around 4000–3800 cal a BP in pollen anddiatom assemblages (Sadori et al., 2004, 2011; Ramrath et al.,2000) as well as in sediment markers (Ramrath et al., 1999;Ariztegui et al., 2001). As discussed by Zhao et al. (2010) andillustrated in Fig. 3, the climatic oscillation around 4500–4000 cal a BP may reflect a non-linear response of the climatesystem to the gradual decrease of insolation, in addition to keyseasonal and inter-hemispherical changes in insolation. Thisorbital forcing was associated with a reorganization of thegeneral atmospheric circulation with a further southward shiftof the Intertropical Convergence Zone in the tropics (Hauget al., 2001; Denton et al., 2005), which may have led to asouthward shift of the westerlies bringing more wetness to themid-European latitudes and the north-west Mediterranean. Bycontrast, an orbitally driven reduction in sea surface tempera-ture (Marchal et al., 2002) may have contributed to drierconditions over the south-western and eastern Mediterranean,south of 408N, corresponding to today’s climatic conditions(Weiss et al., 1993; Enzel et al., 2003; Arz et al., 2006; Drysdaleet al., 2006). It is also worth noting that the successive steps inthe summer precipitation decrease observed in Sicily around7500/7000 and 4500 cal a BP broadly coincide with successivesteps around 7000 and 4000 cal a BP in the decline of themonsoon as reconstructed by Revel et al. (2010) from a marinecore off the Nile delta, as well as in the decline of sub-Saharanlake levels (Hoelzmann et al., 1998). This suggests that theymay have resulted from large-scale changes in the atmosphericcirculation affecting the southern Mediterranean area over themid- to late Holocene transition.

Conclusions

Using the MAT, quantitative estimates of seasonal precipitationwere reconstructed from pollen data of Lake Pergusa andcompared with lake-level data from Lake Preola in Sicily. Thisallowed us to distinguish three successive periods within theHolocene: an early Holocene period before ca. 9800 cal a BPwith dry climate conditions in both winter and summer; a mid-Holocene period between ca. 9800 and 4500 cal a BP withmaximal wetness both in winter and in summer, although adecrease in precipitation began as early as 7500–7000 cal a BP;and a late Holocene period after 4500 cal a BP marked by avery moderate drying trend towards modern humid to sub-humid climatic conditions in Sicily (Baldi et al., 2004).

This evolution observed in the south-central Mediterraneanshows strong similarities with that recognized in the easternMediterranean. However, it contrasts with that inferred frompollen-based quantitative estimates of precipitation and lake-



level data in north-central Italy. There, the mid-Holoceneappears to be characterized by a winter precipitationmaximumand a summer precipitation minimum, while the late Holocenecoincided with a decrease in winter and a slight increase insummer precipitation. Thus, the deposition of sapropel 1around 9500–6500 cal a BP coincided with a maximum ofwinter and summer precipitation in south-central Mediterra-nean and of winter precipitation in north-central Italy.These contrasting seasonal patterns of precipitation may

explain apparent conflicts between palaeoclimatic recordsdepending on proxies used for the reconstruction.In the south-central Mediterranean, the dry early Holocene

may reflect the influence of the boreal insolation maximumassociated with a more intense African monsoon, a reinforce-ment of the Hadley circulation and, consecutively, astrengthening of the North Atlantic anticyclone and its blockingeffect for the western wet airflow towards the Mediterranean.Moreover, the climatic change around 4500 cal a BP at themid-to late Holocene transition may reflect a non-linear response ofthe climate system to the gradual decrease of insolation, inaddition to the key seasonal and inter-hemispherical changes ofinsolation.Finally, additional palaeoclimatic records based on multi-

proxy approaches are needed to support the working hypoth-eses presented in this study for the central Mediterranean andtest them for the eastern Mediterranean.

Acknowledgements. Financial support for this study was provided bythe French ANR (project LAMA, M. Magny and N. Combourieu-Neb-out), as well as by the Ecole Francaise de Rome. The authors alsoexpress their sincere thanks to J. Olsen for his help with the English text.The constructive comments of two anonymous reviewers helped toimprove the manuscript.

Abbreviations. AP, arboreal pollen; MAT, modern analoguestechnique; NAP, non-arboreal pollen.

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