Contrasting patterns of precipitation seasonality during the Holocene in the south- and north-central Mediterranean

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<ul><li><p>Contrasting patterns of precipitation seasonalityduring the Holocene in the south- andnorth-central Mediterranean</p><p>MICHEL MAGNY,1* ODILE PEYRON,1 LAURA SADORI,2 ELENA ORTU,1 GIOVANNI ZANCHETTA,3</p><p>BORIS VANNIE`RE1 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</p><p>Received 30 May 2011; Revised 26 August 2011; Accepted 28 August 2011</p><p>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 0004500 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 reect non-linear responses to orbitally driven insolation decrease in addition to seasonaland inter-hemispheric changes of insolation. Copyright # 2011 John Wiley &amp; Sons, Ltd.</p><p>KEYWORDS: Mediterranean; Holocene; modern analogues technique; precipitation seasonality; sapropel 1.</p><p>Introduction</p><p>The Mediterranean area lies in a transitional zone that isinuenced 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</p><p>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, refrequency, lake levels and isotopes for the mid-Holoceneclimate in the central Mediterranean (Magny et al., 2007;Zanchetta et al., 2007b; Vannie`re 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),</p><p>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</p><p>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).</p><p>Site, data and methods</p><p>Lake Pergusa (378310N, 148180E; 667m a.s.l.) is located incentral Sicily. The catchment area of the lake is about 7.5 km2</p><p>and reaches its maximum elevation at Monte Carangiano(911m a.s.l.). The lakes 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 is</p><p>located 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 been</p><p>established 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</p><p>JOURNAL OF QUATERNARY SCIENCE (2012) 27(3) 290296 ISSN 0267-8179. DOI: 10.1002/jqs.1543</p><p>Copyright 2011 John Wiley &amp; Sons, Ltd.</p><p>*Correspondence: M. Magny, as above.E-mail:</p></li><li><p>distinguished within the pollen sequence as follows (Sadori andNarcisi, 2001).</p><p> 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.</p><p> During the period around 11 00010000 cal a BP, herbs arestill dominant (5674%) although decreasing, and total pol-len concentration values increase slightly. Deciduous andevergreen oaks show an increase both in percentage (2133%) and in concentration.</p><p> The period around 10 0008000 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(4060%) followed by Q. ilex type (1225%). As inferredfrom pollen percentages, other important arboreal taxa areQ. suber/cerris, Ericaceae, Ulmus, Corylus and Fagus.</p><p> During the period around 80005000 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.</p><p> During the period around 50003000 cal a BP, AP remaindominant (ca. 70%) and total pollen concentrations are low.Ulmus percentages reach their maximum values.</p><p> The period around 30002300 cal a BP is characterized bythe highest expansion of Olea (1019%).</p><p> 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.</p><p>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 res 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</p><p>parameters is the MAT, which was rst developed by Overpeck</p><p>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 gure is availablein colour online at</p><p>SL 152LAKEPERGUSA</p><p>LC</p><p>LA TPNF</p><p>MD95-2043</p><p>LMe</p><p>LAl</p><p>SC</p><p>LAKEPREOLA</p><p>30 E0</p><p> 45 N</p><p> 40 N</p><p> 35 N</p><p>30 N</p><p>0 500 km</p><p>AFRICA</p><p>EUROPENORTHATLANTIC</p><p>MEDITERRANEAN SEA</p><p>BLACK SEA</p><p>0</p><p>50</p><p>100</p><p>150</p><p>200</p><p>250</p><p>300</p><p>350</p><p>Pwinte(mm)</p><p>Psummer(mm)</p><p>18O minimum</p><p>Pergusalake-level</p><p>LakeLevel</p><p>PANN(mm)</p><p>400</p><p>500</p><p>600</p><p>700</p><p>800</p><p>AGE (ka cal BP)11 10 9 8 7 6 5 4 3 2 1 0</p><p>-1</p><p>0</p><p>1</p><p>2</p><p>CA sc</p><p>ores</p><p>higher</p><p>lowerSAPROPEL 1</p><p>&gt; 17 x 104 &gt; 10 x 104</p><p>Quercusdeciduous</p><p>0</p><p>40%</p><p>40%</p><p>0</p><p>AP percentage</p><p>Total Pollen conc.(grains/g)</p><p>LAKE PERGUSA</p><p>LAKE PREOLA</p><p>moisturedecrease</p><p>increasingaridification</p><p>marke</p><p>d dryn</p><p>ess</p><p>mode</p><p>rate</p><p>dryne</p><p>ssincreasingmoisture we</p><p>ttest</p><p>cond</p><p>itions</p><p>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 uctuations around 105009000 and 65004500cal 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.</p><p>Copyright 2011 John Wiley &amp; Sons, Ltd. J. Quaternary Sci., Vol. 27(3) 290296 (2012)</p><p>HOLOCENE PRECIPITATION SEASONALITY IN THE CENTRAL MEDITERRANEAN 291</p></li><li><p>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 nd, 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</p><p>(Overpeck et al., 1985):</p><p>d2ik Xm</p><p>j1pfij pfkj2 (1)</p><p>where fij anf ftj are the relative frequencies of pollen taxon j (outof m 104 taxa) in modern pollen spectrum i and fossil pollenspectrum k, respectively. Equation (1) is used to nd 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:</p><p>0Rt XS</p><p>i1Ci=d</p><p>2it =XS</p><p>i1d2it (2)</p><p>Instead of a unique standard deviation around 0Rt, the lowerand upper limits of this mean estimate were computed(condence intervals). The lower limit LLt is given by thedistance-weighted mean of the analogues (out of the total s 8)with Ci</p><p>0Rt. Thesecondence 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 inuence ofnon-climatic factors.In this study, the MAT is based on a new modern pollen-</p><p>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</p><p>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</p><p>assemblages were available for quantitative climatic recon-struction, i.e. a mean temporal resolution of ca. 145 years persample over the Holocene period.</p><p>Results and discussion</p><p>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</p><p>Ps (summer precipitation: June, July and August). The threecurves show similar evolutions and allow three distinct periodsof precipitation changes to be distinguished: a rst 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).</p><p>Before ca. 9800 cal a BP: a dry early Holocene</p><p>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 precipita...</p></li></ul>


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