Evidence of a prolonged drought ca. 4200 yr BP correlated with prehistoric settlement ...

14 Pages·2016·2.51 MB·English

Clim.Past,12,1–14,2016
www.clim-past.net/12/1/2016/
doi:10.5194/cp-12-1-2016
©Author(s)2016.CCAttribution3.0License.
Evidence of a prolonged drought ca. 4200yrBP correlated with
prehistoric settlement abandonment from the Gueldaman
GLD1 Cave, Northern Algeria
J.Ruan1,5,F.Kherbouche2,D.Genty1,D.Blamart1,H.Cheng3,4,F.Dewilde1,S.Hachi2,R.L.Edwards4,E.Régnier1,
andJ.-L.Michelot5
1LaboratoiredesSciencesduClimatetdel’Environnement,Gif-sur-Yvette,France
2CentreNationaldeRecherchesPréhistoriques,AnthropologiqesetHistoriques,Algiers,Algeria
3InstituteofGlobalEnvironmentalChange,Xi’anJiaotongUniversity,Xi’an,China
4DepartmentofGeologicalSciences,UniversityofMinnesota,Minnesota,USA
5LaboratoireGéosciencesParisSud,UMR8148,UniversitéParis-Sud,Orsay,France
Correspondenceto:J.Ruan([email protected])
Received:12June2015–PublishedinClim.PastDiscuss.:3July2015
Revised:3December2015–Accepted:14December2015–Published:15January2016
Abstract.MiddleHolocenecultureshavebeenwidelystud- betweeninsituevidenceofhumanoccupationandofclimate
ied around the Eastern-Mediterranean basin in the last change.Thisapproachshowsthatthetimingofaprolonged
30years and past cultural activities have been commonly drought at ca. 4400–3800yrBP blankets the onset of cave
linked with regional climate changes. However, in many abandonmentshortlyafterca.4403calyrBP,supportingthe
cases such linkage is equivocal, in part due to existing cli- hypothesisthataclimateanomalymayhaveplayedarolein
matic evidence that has been derived from areas outside thisculturaldisruption.
the distribution of ancient settlements, leading to uncer-
tainty from complex spatial heterogeneity in both climate
anddemography.Afewhigh-resolutionwell-datedpaleocli- 1 Introduction
materecordswererecentlyestablishedusingspeleothemsin
the Central and Eastern-Mediterranean basin, however, the As drought in Northwestern Africa is a recurring phe-
scarcity of such records in the western part of the Mediter- nomenon and prolonged dry conditions exert a significant
ranean prevents us from correlating past climate evolutions impact on local social systems, it becomes important to ac-
across the basin and deciphering climate–culture relation at curatelydocumenttheroleofdroughtconditionsonthearea.
finetimescales. For instance, the most recent drought in Algeria began in
Here we report the first decadal-resolved Mid-Holocene 1998,aspartofawidespreadpatternofdryingintheNorth-
climateproxyrecordsfromtheWestern-Mediterraneanbasin ern Hemisphere, and brought considerable loss in regards
based on the stable carbon and oxygen isotopes analyses to water resource and agricultural yields (Hoerling and Ku-
of two U/Th dated stalagmites from the Gueldaman GLD1 mar,2003).Increasinglydrysub-tropicalconditionsarepre-
Cave in Northern Algeria. Comparison of our records with dictedasonepotentialconsequenceofanthropogenicclimate
thosefromItalyandIsraelrevealssynchronous(multi)cen- change, but current general circulation models do not com-
tennial dry phases centered at ca. 5600, ca. 5200 and ca. pletelycapturethemagnitudeandspatialextentofobserved
4200yrBP across the Mediterranean basin. New calibrated droughtconditions(Seageretal.,2007).Tohelpunderstand
radiocarbondatingconstrainsreasonablywelltheageofrich recentclimateanomalies,paleoclimatestudiesarecrucialto
anthropogenic deposits (e.g., faunal remains, pottery, char- characterize the range of potential natural variability in the
coal)excavatedinsidethecave,whichallowsthecomparison past and to improve our understanding of the links between
regional drought and large-scale forcing. Instrumental data
PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion.
2 J.Ruanetal.:Evidenceofaprolongeddroughtca.4200yrBP
◦ (cid:48) ◦ (cid:48)
Figure1.TheGueldamanGLD1Cave(36 26 N,4 34 E,507ma.s.l.).ThetopleftshowsthelocationoftheGueldamanGLD1Cave,in
theNorthern-AlgerianpartoftheWestern-Mediterraneanbasin;thebottomleftshowsaphotoofcaveentranceandlocalvegetationcover;
therightpanelshowsmapsofinnercavewherestalagmitesandarchaeologicaldepositsarecollected.
fromweatherstationsinNorthwesternAfricareportlessthan having access to the correlation of past climate anomalies
100years.TreeringbaseddroughtreconstructionsinAlgeria acrossthebasin.
andTunisiahavebeenextendedbacktothelast9centuries, Asidefromitsclimaticinterest,suchanepisodelikelyin-
whichrevealslargespatialheterogeneityofpastclimateevo- fluenced numerous human cultures. Major societal changes
lutionsinNorthwesternAfricaandconcludesthattheclimate have been observed across the Mediterranean basin during
anomaly1998–2002appearstobethemostsevereinthelast theMid-Holocene,andinparticular,acatastrophicdesicca-
millennium(Touchanetal.,2008,2011).Holocenepaleocli- tion ca. 4200yrBP has been suggested to be the trigger of
matestudiesinotherregions,however,havesuggestedlarger the collapse of the Akkadian Empire in Mesopotamia, the
oscillations at centennial to millennial timescales highlight- Old Kingdom in Egypt and the Early Bronze Age civiliza-
ingtheneedfornewrecordsfromthisarea(Mayewskietal., tions of Greece and Crete (Weiss and Bradley, 2000; Weiss
2004;Wanneretal.,2008). et al., 1993; Wiener, 2014). These studies have been stim-
A significant climate excursion ca. 4200yrBP has been ulating an increasing number of debates on climate–culture
widely reported and is considered as an ideal case to study relationship (e.g., Coombes and Barber, 2005). Uncertainty
the causes and effects of a large-scale climate anomaly that regarding the societal impact of such an event is still large,
occurred against background conditions similar to those of due in part that climatic evidence, in many cases, has been
today (Berkelhammer et al., 2013; Booth et al., 2005; Dixit derivedfromregionsfarfromthedistributionofancientset-
et al., 2014; Roland, 2012). The climatic expression of the tlements(e.g.,Cullenetal.,2000).Althoughthe4200yrBP
4200yrBP event differs around the world. For example, dryeventhasbeenobservedinseveralmid-latitudesites,the
it has been documented as droughts in much of mid-to- databaseremainsincompleteandconflictingobservationsof
low latitudes, across Africa, Asia and North America, wet climatic conditions between seemingly adjacent regions ex-
and stormy in Northern Europe and cooler in North At- ist(Magnyetal.,2013;StaubwasserandWeiss,2006).Addi-
lantic (Booth et al., 2005; Roland, 2012). More recently, tionally,arecentstudydemonstratedthattheclimaticimpact
thisclimaticanomalywascharacterizedbyextremedrycon- on many agricultural settlements in ancient Near East was
ditions on high-resolved speleothem isotope records from diverseevenwithinspatiallylimitedculturalunits(Riehlaet
the Central (Drysdale et al., 2006; Zanchetta et al., 2014) al.,2014).
and Eastern-Mediterranean basin (Bar-Matthews and Ay- In Northern Algeria, the extinction of large mammal
alon,2011),but,untilnow,suchrecordshavenotbeenavail- species (e.g., Syncerus antiquus) during the Mid-Holocene
able in the Western Mediterranean which prevents us from was correlated with regional climate aridity, likely due to
Clim.Past,12,1–14,2016 www.clim-past.net/12/1/2016/
J.Ruanetal.:Evidenceofaprolongeddroughtca.4200yrBP 3
the competition with pastoralists and livestock for increas-
ingly scarce water (Faith, 2014). Similarly, the evidence of
thearidity(i.e.,theterminationoftheAfricanHumidPeriod)
that provoked this extinction has been derived from the Sa-
haraanditssurroundings(deMenocaletal.,2000),whichis
several hundred kilometers away, leaving this assertion am-
biguous and stimulating the search for new high-resolution
paleoclimaterecordsinthearea.
Inthisstudy,wedocumenttheMid-Holoceneclimatehis-
toryintheWesternMediterraneanbydecadal-resolvedstable
carbonandoxygenisotopesanalysesoftwoU/Thdatedsta-
lagmitesfromtheGueldamanGLD1CaveofNorthernAlge-
ria.Wecomparetherecordswiththoseestablishedearlierin
theCentralandEastern-Mediterraneanbasin.Inaddition,we
describearchaeologicaldepositslayersinsidethecavewhose
ages have been reasonably well constrained due to new ra-
diocarbon dating. Finally we test the links between cultural
Figure2.U/ThdatingofstalagmitesGLD1-stm2andGLD1-stm4
changesandclimateanomalieswithaparticularemphasison
from the Gueldaman GLD1 Cave. U/Th dates and 2σ errors are
the4200yrBPevent.
shownnexttosamplingpositions.
2 Samplesandmethods
ber 2013 to April 2015 shows significant variations in cave
2.1 Studysite temperature ranging from 13.7 to 19.5◦C. The relative hu-
midityvariesfrom56to94%.Carbondioxidehasnotbeen
Gueldaman GLD1 Cave is one of a series of karstic caves measured,butitislikelytobeclosetotheatmosphericvalue.
formed within the southeastward slope of the Adrar Guel-
daman ridge, western part of Babor mountains in North-
2.2 StalagmitesAnalyses
ern Algeria (Kherbouche et al., 2014). It is located close to
the large Soummam River, 5–6km from the Akbou town, Two stalagmites and three modern calcites samples were
andapproximately65kmsoutherninlandfromtheWestern- collected in 2012 and 2013 from the main chamber of
Mediterranean Sea (36◦26(cid:48)N, 4◦34(cid:48)E, 507ma.s.l.; Fig. 1). Gueldaman GLD1 Cave. Both stalagmites were laid on
Gueldaman GLD1 is a relatively short cave (total extension the cave floor during collection. Stalagmite GLD1-stm2 is
of ∼80m) that developed in Jurassic limestone. The en- 350mmlongand100–200mmwide;stalagmiteGLD1-stm4
trance,facingtotheSE,isasemi-circular∼6mlargearch, is203mmlongwithadiameterof50–120mm(Fig.2).They
leading to a dome-shaped ∼10m high and 6m wide corri- werehalvedandpolishedalongthelongitudinalaxis.Itwas
dor which ends with the main chamber “Grande Salle” at noticed from the sectioned sample that stalagmite GLD1-
adepthof30–40m.Previousarchaeologicalexcavationand stm2 was broken at the depth of 5mm from the top and
field investigation suggest that Gueldaman GLD1 Cave has covered by the calcite deposited at a certain time after that
probably been open since at least ca. 7ka ago (Kherbouche point. The top 5mm was not analyzed in this study. Both
et al., 2014). The area is covered by a thin layer (<10cm) stalagmitesshowwell-markedlaminaewithseveralshiftsin
ofsoilderivedfromthelimestonebedrock,wind-blownsil- the drip apex of the lower parts (Fig. 2). Black bandings,
icate dust,and organic matterfrom local vegetationsuch as with visible incorporations of charcoal particles, are found
Pistacia lentiscus, Quercus ilex, Buxus sempervirens, typi- throughoutbothstalagmiteprofiles.
calMediterraneanGarriguetypeplantassemblage(C3dom- U/Th dating – Seventeen powder samples were drilled
inated). from the two stalagmites and dated by a multi-collector in-
LocalclimateisMediterraneansemi-aridtype,character- ductivelycoupledplasmamassspectrometer.Theprocedure
izedbyhot-drysummersandmild-wetterwinters.Fromthe toseparateuraniumandthoriumwasreferredtoinEdwards
ERA-interimreanalysisdatabetween1979and2013(http:// et al. (1987) and Cheng et al. (2013). The dating work was
apps.ecmwf.int/datasets/)theannualtotalrainfallis516mm, carried out at the University of Minnesota (USA) and the
and the annual mean temperature is 17.2◦C. Rainfall oc- Xi’anJiaotongUniversity(China).Onedatingfromthebase
curs rarely in the summer (37mm) but relatively evenly part of stalagmite GLD1-stm2, for the exploration of pre-
through the autumn (155mm), winter (178mm) and spring liminary age frame, was done at the Laboratoire des Sci-
(147mm).GueldamanGLD1Caveiswellventilatedwiththe encesduClimatetdel’Environnement(LSCE,France).The
outsideatmosphereduetoitslargeropeningandshorterex- U/Thdateswerereportedinyearsbefore2000AD.(Fig.2,
tension.Hobologgerdataat10minresolutionfromNovem- Table 1). The age model for both stalagmites was devel-
www.clim-past.net/12/1/2016/ Clim.Past,12,1–14,2016
4 J.Ruanetal.:Evidenceofaprolongeddroughtca.4200yrBP
Table1.U/ThdatesfromMC-ICP-MSanalysesofstalagmitesGLD1-stm2andGLD1-stm4fromtheGueldamanGLD1Cave.Analytical
errorsare2σ ofthemean.Udecayconstants:λ238=1.55125×10−10(Jaffeyetal.,1971)andλ234=2.82206×10−6(Chengetal.,2013).
Thdecayconstant:λ230=9.1705×10−6 (Chengetal.,2013).∗ δ234U=([234U/238U]activity –1)×1000.∗∗ δ234Uinitial wascalculated
basedon230Thage(T),i.e.,δ234Uinitial=δ234Umeasured×eλ234×T.Corrected230Thagesassumetheinitial230Th/232Thatomicratio
of4.4±2.2×10−6.Thosearethevaluesforamaterialatsecularequilibrium,withthebulkearth232Th/238Uvalueof3.8.Theerrorsare
∗∗∗
arbitrarilyassumedtobe50%. BPstandsfor“BeforePresent”wherethe“Present”isdefinedastheyear2000AD.
Sample 238U 232Th 230Th/232Th d234U* 230Th/238U 230ThAge(yr) 230ThAge(yr) d234UInitial** 230ThAge(yrBP)*** Laboratory
Number (ppb) (ppt) (atomicx10−6) (measured) (activity) (uncorrected) (corrected) (corrected) (corrected)
GLD1-stm2-7 169±0.1 396±0 136±1 863±2.3 0.105±0.001 6297±40 6228±72 863±2.3 6228±72 LSCE
GLD1-stm2-36 154±0.2 1922±39 137±3 910±2.2 0.103±0.000 6036±29 5848±136 925±2.3 5835±136 UM
GLD1-stm2-98 150±0.2 69±2 3077±73 776±2.4 0.086±0.000 5374±29 5366±29 788±2.5 5353±29 UM
GLD1-stm2-180 152±0.2 161±3 1157±25 644±2.0 0.074±0.000 5036±30 5018±33 653±2.1 5005±33 UM
GLD1-stm2-192 162±0.1 182±4 1158±24 807±1.7 0.079±0.000 4858±16 4840±20 818±1.7 4827±20 UM
GLD1-stm2-213 175±0.2 547±11 390±8 697±2.0 0.074±0.000 4841±33 4788±50 707±2.1 4775±50 UM
GLD1-stm2-286 169±0.2 207±4 980±21 756±1.8 0.073±0.000 4601±25 4581±28 765±1.9 4568±28 UM
GLD1-stm2-320 195±0.3 384±8 575±12 759±2.2 0.069±0.000 4321±26 4288±34 769±2.2 4276±34 Xi’anU
GLD1-stm2-340 194±0.3 354±7 612±13 800±2.9 0.068±0.000 4172±20 4142±29 809±2.9 4129±29 UM
GLD1-stm4-10 105±0.1 283±6 415±11 322±1.5 0.068±0.001 5734±90 5675±99 327±1.6 5662±99 UM
GLD1-stm4-24 126±0.1 983±20 135±3 347±1.9 0.064±0.001 5311±49 5143±128 352±1.9 5131±128 Xi’anU
GLD1-stm4-30 106±0.1 1362±27 80±2 298±1.9 0.062±0.001 5321±57 5035±210 302±1.9 5023±210 Xi’anU
GLD1-stm4-47 96±0.1 1104±22 88±2 290±1.9 0.062±0.001 5341±63 5082±194 294±1.9 5070±194 Xi’anU
GLD1-stm4-70 224±0.4 749±15 254±5 334±2.5 0.051±0.000 4282±27 4210±58 338±2.6 4197±58 UM
GLD1-stm4-113 155±0.2 130±3 910±20 363±2.1 0.046±0.000 3761±29 3743±32 367±2.1 3730±32 UM
GLD1-stm4-152 180±0.3 582±12 226±5 373±2.0 0.045±0.000 3590±25 3521±54 376±2.0 3508±54 UM
GLD1-stm4-195 175±0.1 929±19 131±4 369±1.7 0.042±0.001 3403±74 3290±108 372±1.8 3277±108 UM
Stable isotopes – Four hundred and thirty samples were
drilled every 1 to 2mm along the stalagmite central growth
axis (Fig. 2). Stable carbon and oxygen isotopes composi-
tionsofbothstalagmitesandmoderncalcitesweremeasured
using a VG-OPTIMA mass spectrometer at the LSCE. For
each analysis, 60 to 80µg calcite powder is reacted with
phosphoricacidat90◦C,andtheresultantCO ismeasured
2
relative to a reference gas that has been calibrated against a
seriesofisotopicstandards.Duplicateswererunevery10to
20samplestocheckreplicability.Allvaluesarereportedin
‰ relative to the V-PDB (Fig. 4). The error is 0.08‰ for
δ18Oand0.05‰forδ13C.
2.3 Archaeologicalanalyses
Figure3.AgemodelsofstalagmitesGLD1-stm2andGLD1-stm4 Archaeological excavations were carried out at two sectors
fromtheGueldamanGLD1Cave.Theagemodelswerecalculated S2 and S3 inside the Gueldaman GLD1 Cave during the
usingtheStalAgeprogram(ScholzandHoffmann,2011).Notethat
2010–2012 campaign (Fig. 1). This work consisted mainly
the U/Th date of sample GLD1-stm4-47 was detected as a major
incollecting,identifying,andreferencingthearchaeological
outlier and not used in the final age model of stalagmite GLD1-
materials found in stratigraphic layers (refer to Kherbouche
stm4.The2σ analyticaluncertaintyofeachU/Thdate(dot)isrep-
et al., 2014 for details). More than 7000 anthropogenic re-
resented by the error bars, whereas the 95% uncertainty assessed
mains were collected, consisting mainly of faunal remains,
fromthemodelsimulationisrepresentedbythincurves.
ceramic, and lithic and bone tools. Besides, all sediments
were water screened through 1.5 and 4mm mesh and sub-
jectedsystematicallytoflotationwithcollectionina250µm
oped using the StalAge program (Scholz and Hoffmann, mesh yielding a huge amount of charcoals. Initial radiocar-
2011)wherealinearinterpolationbetweendepthandageis bondatingofupperstratigraphicsequencesfromS2andS3
madethrougheachprogressivetripletofadjacentU/Thdates gavethemedianagesrangingfromca.6800to1500calyrBP
(Fig. 3). This procedure provides a quantitative estimate of (Kherboucheetal.,2014).Inordertorefinethechronology
ageuncertaintycontinuouslyalongtherecorddespitehaving of these deposits, in this study, six new charcoal samples
analyticalconstraintsonlyatlocationswheretheU/Thdates werecollectedfromthekeyarchaeologicallayersinexcava-
exist. Stalagmite growth rates were calculated based on the tion area MN 47/48 of S2. These samples were dated using
StalAgeagemodel(Fig.4). the AMS radiocarbon method at the CEA Saclay (France).
Clim.Past,12,1–14,2016 www.clim-past.net/12/1/2016/
J.Ruanetal.:Evidenceofaprolongeddroughtca.4200yrBP 5
∗
Table2.RadiocarbondatesfromAMSanalysesofcharcoalsfromexcavationsectorS2insidetheGueldamanGLD1Cave. Kherbouche
etal.(2014).Agesarereportedinyearsbefore2000AD.
DepthZ Square Labno. Material 14CAge Medianage Cal.interval Note
(cm) (SacA#) (±2σ;yr) (yr) (2σ;yr)
60 M48 39408 Charcoal 1600±30 1482 1385–1604 Thisstudy
65 N48 29731 Charcoal 1610±25 1484 1415–1547 ∗
84 N48 39410 Charcoal 4020±30 4495 4411–4785 Thisstudy
86 N48 39411 Charcoal 3975±30 4416 4290–4569 Thisstudy
91 M48 39409 Charcoal 3945±30 4403 4244–4522 Thisstudy
108 N47 36982 Charcoal 4355±30 4918 4851–5032 Thisstudy
124 L48 23883 Charcoal 5250±35 6003 5924–6178 ∗
132 L48 23884 Charcoal 4260±30 6025 5933–6178 ∗
147 M47 36981 Charcoal 6120±35 7002 6907–7157 Thisstudy
Detailedproceduresofthechemicalpreparationandthedat-
ing in the lab were referred to Cottereau et al. (2007). The
dateswerecalibratedusingtheIntCal13dataset(Reimeret
al.,2013)andreportedinyearsbefore2000AD(Table2).
3 Results
3.1 StalagmitesU/Thdatesandgrowthrates
The uranium contents of measured stalagmites samples are
relatively high ranging from 95 to 225ppb (Table 1). The 2
sigma U/Th errors vary from 20 to 210years with an aver-
age of 77years (1.6%). The U/Th date (5070±194yrBP)
of sample GLD1-stm4-47 was detected as a major outlier
by the StalAge program, thus, it was not used to calculate
the final age model. Calculated StalAge age model for sta-
lagmiteGLD1-stm4showslargeerrorsupto500yearsdur-
ing ca. 4900–4200yrBP (Fig. 3). This may partly be due
to relatively large errors of adjacent U-series dates and/or
impropriate hypotheses applied in the algorithm. Based on
individual StalAge age model, stalagmite GLD1-stm2 grew
fromca.6200to4000yrBP(4100yrBPifexcludingthetop
5mm),whereasstalagmiteGLD1-stm4grewfromca.5800
to3200yrBP(Fig.3).
Stalagmite GLD1-stm2 shows high and variable Figure 4.Theδ18O,δ13CandgrowthrateofstalagmitesGLD1-
growth rates (mean=180µmyr−1) with higher values stm2andGLD1-stm4fromtheGueldamanGLD1Cave.U/Thdates
∼400µmyr−1 at ca. 4800–4600yrBP; whereas stalag- with2σ errorsarepresentedatthetop.Theisotopicrangesofmod-
ern calcites are also shown on the left (rectangles). Growth rates
mite GLD1-stm4 shows relatively lower and less variable
arecalculatedfromtheStalAgeagemodel.Notethattheextraordi-
growth rates (mean=120µmyr−1) with higher values
narilyhighgrowthatca.4600yrBPinstalagmiteGLD1-stm2and
∼200µmyr−1atca.3800–3200yrBP(Fig.4).
ca.3800yrBPinstalagmiteGLD1-stm4arelikelyattributedtoar-
tificial simulations by the StalAge program and thus are not fully
discussedintermsofclimateinthetext.
3.2 Stablecarbonandoxygenisotopes
The isotopic compositions of modern calcite vary from
−5.40 to −5.56‰ for the δ18O and from −8.43 to respectively. The δ18O and δ13C significantly correlate in
−10.34‰ for the δ13C. The δ18O values from stalagmites both stalagmites: R=0.87, P <0.01 for GLD1-stm2 and
GLD1-stm2 and GLD1-stm4 range from −7.8 to −2.8‰ R=0.92,P<0.01forGLD1-stm4.Albeitthedifferentam-
and from −7.3 to −0.6‰, respectively; the δ13C values plitudes, the isotopic profiles of the two stalagmites show
range from −10.6 to −3.3‰ and from −11.9 to −0.6‰, similarities during their common development of ca. 5800–
www.clim-past.net/12/1/2016/ Clim.Past,12,1–14,2016
6 J.Ruanetal.:Evidenceofaprolongeddroughtca.4200yrBP
4000yrBP:relativelyelevatedisotopevaluesarefoundatca.
5700–5400,ca.5200,andca.4500yrBP(Fig.4).Twoother
isotopically enriched periods in stalagmite GLD-stm2 are
foundatca.6200andca.4900yrBP(Fig.4).Thereisacom-
mon isotopic enrichment trend since ca. 4800–4600yrBP
(depending on individual age model; abrupt in stalagmite
GLD1-stm4 whereas more gradual in GLD1-stm2). Toward
theendofthistrend,themostprominentanomalyoccursin
stalagmiteGLD1-stm4atca.4400–3800yrBPduringwhich
the δ18O values are enriched by approximately 3.5‰ rel-
ative to the background values of that time as well as the
modern calcite values for a period of ∼500years (Fig. 4).
Specificallywithinthisanomalousperiod,thereisamildiso-
topicdepletionatca.4200–4000yrBP,blanketedbytwosig-
nificant enrichments at ca. 4400–4200yrBP and ca. 4000–
3800yrBP (Fig. 4). The last part, ca. 3800–3200yrBP, of
GLD1-stm4, is characterized by a δ18O recovery of about
−3‰,synchronouswithincreasedgrowthrates(Fig.4).
3.3 Anthropogenicdepositsand14Cdates
ExcavationsinsideGueldamanGLD1Caverevealedalarge
variety of archeological remains and, among them, are nu-
merouspreciousmacrocharcoalsthathavebeenusedfores-
tablishing the chronology of the deposits. In the ∼7m2 to- Figure5.Comparisonofhigh-resolutionMid-Holocenestalagmite
talexcavatedareaofS2,morethan7000archaeologicalob- δ18OrecordsacrosstheMediterraneanbasin.Fromthetoptobot-
jects were identified and consisted of faunal remains, lithic tom are stalagmite records from the Gueldaman GLD1 Cave in
artifacts and grinding equipment, potteries, bone tools, or- NorthernAlgeriaoftheWestern-Mediterraneanbasin(thisstudy),
naments, and ochre. In addition, a fragment of a human the Corchia Cave (Zanchetta et al., 2014) and the Renella Cave
mandibleandtwoisolatedteethwerefoundduringtheexca- (Drysdaleetal.,2006)inCentralItalyoftheCentralMediterranean
basin,andtheSoreqCave(Bar-MatthewsandAyalon,2011;Kauf-
vation 2010–2012 in Gueldaman GLD1 Cave (Kherbouche
manetal.,1998)inIsraelofEastern-Mediterraneanbasin.Different
etal.,2014).ThesedepositsbelongmainlytotheNeolithic;
stalagmitesfromeachareaarerepresentedindistinctcolors.U/Th
only the top level of the sequence contains potsherds of the
dateswith2σ errorsareshownatthetopofeachcurve.Agesare
historic period. In the lower Neolithic levels, identified do-
reportedinyearsbefore2000AD.
mestic species (i.e. sheep and goats) represented ∼25% of
totalfaunalassemblages(N=2378)suggestingapartlypas-
toralbasedeconomy.Thepotteries(N=825)aremostlyre- therearetwolevelsclearlymarkedbytheirpovertyinchar-
latedtocookingvesselsof25-40cmrimdiameter.Hundreds coalandpotteryduringtheperiodsofca.6003–4918andca.
ofblackcharcoals(>1cm)werefoundandalwaysassociated 4403–1484calyrBP(Fig.6).
withceramicconcentrationssuggestingevidenceofcooking
activities.
4 Discussions
Determined radiocarbon dates give the median ages of
the sequence between 7002 and 1482calyrBP, with their
4.1 Climaticsignificanceofstalagmitesproxies
2σ-error intervals varying from 132 to 374years (Table 2).
These dates provide a first chronology for the archaeo- Under isotopic equilibrium precipitation, stalagmite calcite
logical deposits excavated from sector S2 (Fig. 6; Kher- δ18O depends mainly on the temperature of calcite-water
bouche et al., 2014): anthropogenic remains (i.e. charcoals, fractionationandontheδ18Oofdripwaterthatiscontrolled
bones, teeth and potteries) are numerous during ca. 7002– by local rainfall δ18O (Genty et al., 2014; Lachniet, 2009).
6003calyrBP (depths of ∼150-120cm), decreased at ca. Observations from the IAEA network show that the rainfall
6003–4918calyrBP (depths of ∼120–105cm), most abun- δ18O at many Mediterranean stations (including one in Al-
dantintheperiodofca.4918–4403calyrBP(depthsof105– giers, Algeria) are partly controlled by the amount of rain-
75cm), significantly diminished during the long interval of fall (IAEA, 2005), which is coherent with previous stud-
ca. 4403–1484calyrBP (depths of 75–60cm), and finally, iesthat stalagmiteδ18Orecords fromthe Mediterranean re-
numerous again from ca. 1484calyrBP (depths of ∼60– gions were interpreted to primarily reflect changes in rain-
50cm).Withanoveralldecreaseinarcheologicalmaterials, fallamount(inverselycorrelated;e.g.,Bar-MatthewsandAy-
Clim.Past,12,1–14,2016 www.clim-past.net/12/1/2016/
J.Ruanetal.:Evidenceofaprolongeddroughtca.4200yrBP 7
Figure6.RadiocarbondatingofanthropogenicdepositslayersinexcavationsectorS2insidetheGueldamanGLD1Cave.Fromtheleftto
rightare14Cdatesofcharcoalsamples,anthropogenicdepositdistribution,andaphotoatdepthacross∼75–88cmshowingatransition
oflayerfromrichtorareanthropogenicdeposits.Thegraycolorhighlightstwophaseswithdiminishedanthropogenicremains(especially
potteryandcharcoal)atca.4403–1484calyrBP(depthsof∼75–60cm)andca.6003–4918calyrBP(depthsof∼120–105cm).
alon,2011;Bar-Matthewsetal.,2003,1997;Drysdaleaetal., two other processes – evaporation and disequilibrium iso-
2004,2006;Zanchettaetal.,2014).Therefore,higherstalag- topicfractionation(Mickleretal.,2006)thatareverylikely
miteδ18Ovaluesareexpectedduringperiodsofdecreasein to have occurred at the Gueldaman GLD1 Cave due to its
rainfall(drier).Thetemperatureeffectoncalcite-waterfrac- large entrance. Longer residence time during drier (lower
tionation, on the other hand, is partly counteracted by the rainfall)periodswouldallowextendedevaporativeandnon-
condensation temperature effect on rainfall δ18O (Drysdale equilibrium fractionations, which drives stalagmite isotopes
et al., 2006). We notice that this interpretation may partic- furtherhigher(Mickleretal.,2004).Ithasrecentlybeenob-
ularly hold true for the present study because the regional servedthatevaporationinsemi-aridcavescouldcause4–5‰
temperatureseemstohavebeenrelativelyconstantsincethe δ18Oenrichmentsofawiderangeofdripwaters(Cuthbertet
Mid-Holocene(Martratetal.,2004). al.,2014).TheHendytest(i.e.studyingtheisotopicvariation
Stalagmite δ13C variations have several potential causes, in contemporaneous laminae; Hendy, 1971) carried out at
the most likely, considering the studied location and time threedifferentdepthsinstalagmiteGLD1-stm2showthatthe
interval, being variations in soil CO input and water flow δ18Oandδ13Csignificantlycorrelateandsimultaneouslyin-
2
rate (Genty et al., 2001a; McDermott, 2004). Despite the creasebyupto∼1‰fromtheapextotheedgeprobablydue
fact that soil biogenic CO production varies according to to the presence of progressive kinetic fractionations when
2
bothtemperatureandmoisturelevel,moistureislikelytobe feedingwatersflowoutwardsfromtheapex,suggestingthat
amajorcontrollingfactorduetolowtemperaturevariability the stalagmite formed out of isotopic equilibrium (see the
of the considered time interval and limited water availabil- Supplement).Thesetwoprocessesmaypartlyaccountforthe
ity under semi-arid climates. Moisture also influences wa- significantcorrelationoftheδ18Oandδ13Cprofilesinboth
ter flow rate and thus the CO loss during the prior calcite stalagmites(R=0.87forGLD1-stm2;R=0.92forGLD1-
2
precipitation (Fairchild et al., 2000). Longer residence time stm4).Comparisonsoftwostalagmitesshowdifferentampli-
duetolowerflowrate,underdiminishedmoisturecondition, tudesintheirisotopicprofiles.Thismightbeduetothefol-
enhances CO loss and preferential 12C removal from solu- lowing:(1)havingbeenfedbyreservoirsthatsmoothrainfall
2
tioncausingenrichmentsinstalagmiteδ13C(Johnsonetal., signaldifferently,and(2)havingbeensufferedfromvariable
2006;Mickleretal.,2004).Eventually,atmosphericrainfall evaporativeandkineticenrichmentsassociatedwithdifferent
largely determines the moisture level and controls the δ13C rechargefeatures.Despitethisdiscrepancytheisotopicpro-
variations.Therefore,thesignificantcorrelationofstalagmite filesoftwostalagmitesbroadlyshowsimilarpatterns.Con-
δ13Candδ18Omaysuggestacommoncontrolofrainfall. sequently,synchronousδ18Oandδ13Cvariationsinthetwo
The rainfall signal imprinted in the Gueldaman stalag- stalagmites can be interpreted in terms of humidity change.
miteisotopes(inverselycorrelated)isprobablyamplifiedby A prolonged severe drought is inferred from the most ele-
www.clim-past.net/12/1/2016/ Clim.Past,12,1–14,2016
8 J.Ruanetal.:Evidenceofaprolongeddroughtca.4200yrBP
vated isotopes values during ca. 4400–3800yrBP together arid Mid-Holocene condition at ca. 4800–4000calyrBP
withdrierperiodsfromhighervaluesatca.6200,ca.5700– was interpreted using maximum salinity values, more
5400,ca.5200,andca.4500yrBP(Fig.4),whilewetterpe- positive organic carbon isotope values, and decreased algal
riodsaresuggestedfromdepletedisotopesatca.4600–5100 productivityinEstanyaLake(Morrellonetal.,2009).Inthe
andca.5300yrBP. Mediterranean,SouthernSpain,desertificationphasesatca.
Moreover,stalagmitegrowthphasehaslongbeenregarded 5200 and ca. 4100calyrBP were inferred using multiple
asanenvironmentalindicator(e.g.,Gentyetal.,2001b;Stoll paleoecological indicators including pollen, microcharcoal,
et al., 2013) and could be used to complement and/or test spores of terrestrial plants, fungi, non-siliceous algae, and
theclimaticinterpretationfromisotoperecords.Wateravail- other microfossils in Siles Lake (Carrión, 2002). Similar
ability is an essential factor for stalagmite growth in arid- environmentchangeshavealsobeenobservedintheCentral
to-semi-aridareas,asshownbyVaksetal.(2013)whocor- Mediterranean basin, as shown by a synthesis study of
related growth periods well with periods of effective rain- lacustrine palynological data which suggested a dryness
fallregimes.ThegrowthcessationofstalagmiteGLD1-stm2 peakingatca.4000calyrBP(Sadorietal.,2011).Evaluated
at ca. 4000yrBP may suggest a phase of increased aridity, carbonate oxygen isotopes from Lake Shkodra were argued
which is consistent with the dryness inferred from elevated to be an indicator of dryness at ca. 4100–4000calyrBP
isotopes(Fig.4).However,thecontinuousgrowthofstalag- (Zanchetta et al., 2012b). Increasing aridity at ca. 5000–
miteGLD1-stm4atthattimesuggeststhatthegrowthcessa- 4000calyrBP was suggested to explain the increases in
tionmightalsohavebeencausedbyshiftsindrippingposi- non-treepollenpercentageandmicrocharcoalcontentinthe
tionunderdeterioratingclimates(FairchildandBaker,2012). Lago di Pergusa Lake, Sicily (Roberts et al., 2011; Sadori
Moreover,faststalagmitegrowthstogetherwithwidediam- andGiardini,2007;SadoriandNarcisi,2001).Closertoour
eters usually are associated with high drip rates suggesting site, a study at Preola Lake, Eastern Sicily, documented a
humid conditions. A wet period ca. 4800–4600yrBP is in- significant low stand lake level at ca. 4500–4000calyrBP
dicated by the high growth rates of stalagmite GLD1-stm2, suggesting extreme aridity (Magny et al., 2011). Moreover,
whichisbroadlyinlinewiththehumidperiodinferredfrom six tephra layers were carefully studied to correlate climate
depleted isotopes (Fig. 4). Another wetter period ca. 3800– anomaliesatca.4500–3800calyrBPfromdifferentarchives
3200yrBPissuggestedbyfastergrowthsandrelativelyde- in the central Mediterranean (Zanchetta et al., 2012a). In
pleted isotopes of stalagmite GLD1-stm4 (Fig. 4). The dis- the east, Finné et al. (2011) reviewed the climate history
crepancyinthegrowthrateprofilesofthetwostalagmitesis of the Eastern Mediterranean over the last 6000years and
possibly due in part to (1) the lack of substantial U/Th dat- concluded with much evidence of drying conditions at ca.
ingespeciallyforstalagmiteGLD1-stm4between5023and 4600–3800yrBP.Althoughthesamplinganddatingresolu-
4197yrBP,and/or(2)site-specificprocessesduetodifferent tions in most of the above studies are low, they are in good
reservoirsplayakeyroleincontrollingstalagmitegrowth. agreement with the present study regarding the 5200yrBP
Finally,themoderncalciteisotopicvaluesfallintherange dry event, the drying trend from 4800–4600yrBP onward,
of two stalagmite records (Fig. 4), which suggests that the andthe4400–3800yrBPdrought.
currenthumiditycondition inNorthernAlgeriaseems tobe Recently, high frequency Mid-Holocene climate change
withintherangeofitsMid-Holocenevariability.Theaverage has been well documented by multiple detailed speleothem
δ18Ovalueduringca.4400–3800yrBPinstalagmiteGLD1- studies from the Central and Eastern-Mediterranean basin
stm4 is more enriched by about 3.5‰ relative to the mod- (Fig.5).Drysdaleetal.(2006)demonstratedaseveredrought
ern values (Fig. 4), therefore, the 4400–3800yrBP climate peakedat 4400–3800yrBPthroughmultiproxy analyseson
anomalymaybeconsideredanalogoustoendnumbersofthe a U/Th-dated flowstone from Renella Cave, Central Italy.
mostrecentandongoingdrying. This finding is supported by following work at nearby
Corchia Cave. Zanchetta et al. (2007) established a high-
resolutionspeleothemisotoperecordfortheentireHolocene
4.2 Mid-Holoceneclimateanomaliesacrossthe
and interpreted it as an indicator of rainfall change, in
Mediterraneanbasinandtheirdynamicimplications
which a drier period persisting fromca. 4800 to 3800yrBP
High-resolution absolute-dated Mid-Holocene climate could be inferred from relatively elevated δ18O values. El-
records are rare in the Western-Mediterranean basin, how- emental analysis on the same stalagmite by Regattieri et
ever, there are number of paleoenvironment studies using al. (2014) presented more clear signals of dryness peaked
sedimentcoresthatdocumentlargeoscillationsinvegetation atca.4700–4200yrBP.IntheEastern-Mediterraneanbasin,
ecology and provide clues of climate change during the Bar-Matthews and Ayalon (2011) explicitly discussed the
Mid-Holocene. A drying trend from ca. 4600calyrBP Mid-Holoceneclimatechangebyhigh-resolutiondatingand
onwards was inferred, based on the decreasing pollen ratio isotopic analyses on speleothems from Soreq Cave in Is-
ofdeciduousbroad-leafvs.evergreensclerophylloustaxaat rael;theyidentifiedmultipledryeventsat6250–6180,5700–
Capestang in the Mediterranean, Southern France (Jalut et 5600 and 5250–5170yrBP as well as a long drying trend
al., 2000). At a nearby site in Northeastern Spain, the most since ca. 4700yrBP peaked at 4200–4050yrBP. Zanchetta
Clim.Past,12,1–14,2016 www.clim-past.net/12/1/2016/
J.Ruanetal.:Evidenceofaprolongeddroughtca.4200yrBP 9
Table3.Summaryofthefeaturesofclimateandhumanactivityindifferentclimateperiodsandoccupationphases.
Climate Age(230Thyr) Climatecondition Occupation Age(14Ccalyr) Humanactivity
period phase
– – – 0 ∼7002–6003 Permanentandintensive
occupation
1 ∼6200–5100 Wet&oscillatory 1 ∼6003–4918 Permanentbutlessintensive
occupation
2 ∼5100–4400 Wettest,endingwithadramatic 2 ∼4918–4403 Permanentandmostintensive
shiftinthelast∼200years occupation
3 ∼4400–3800 Extremelydry 3 ∼4403–1484 Abandonmentofthe
cave/occasionalvisit
4 ∼3800–3200 Relativelywetter 4 ∼1484– Re-occupationofthecave
et al. (2014) carefully compared the speleothem isotope ing widespread synchronous drought (Hoerling and Kumar,
records from Corchia Cave and Soreq Cave in the Central 2003).However,thechallengeinapplyingthedynamicsun-
andEastern-Mediterraneanbasin,andfoundtwocoevaldry derthe1988–2002droughttowardsanunderstandingofthe
events at ca. 5600 and ca. 5200yrBP from the comparable 4400–3800yrBP climate anomaly is that while the mech-
δ18Oenrichmentsintwospeleothems.Detailedcomparisons anism operates effectively on short timescales, it has never
ofthesespeleothemrecordswithournewrecordsfromGuel- been tested as to whether they could produce an anoma-
damanGLD1Caverevealmanyconsistencies.Inparticular, lous climate mode for several centuries (Berkelhammer et
periods with elevated δ18O values at ca. 5700–5400, 5200 al., 2013). General circulation model simulations that begin
and4400–3800yrBPobservedinGueldamanstalagmitesare with realistic boundary conditions and are perturbed with a
allidentifiableinthespeleothemsfromCorchia,Renellaand varietyofforcingshavebeensuccessfullyundertakentoun-
Soreq Cave (Fig. 5), suggesting that anomalous dryness at derstand potential mechanisms that lead to the 8200yrBP
these periods synchronously developed across the Mediter- event(TindallandValdes,2011).Similareffortswouldbea
raneanbasin.Theseobservationsindicatethatclimatesinthe usefulstartingpointtoproducehypothesesforthedynamical
Mediterraneanbasinmighthavebeenunderanidenticalre- underpinningsofthe4200yrBPevent.
gionalscaleatmosphericregimeduringtheMid-Holocene.
It has been suggested that climate change in mid-latitude
4.3 Possiblerelationsbetweenclimateanomalyand
Europe and the Mediterranean might arise from a perturba-
culturalchange
tion of the Westerlies from a high-latitude trigger (i.e. the
North Atlantic; Bond et al., 2001; Drysdale et al., 2006; A regional drought ca. 4200yrBP has been widely linked
Zanchetta et al., 2014) or from dynamics within the trop- toancientculturalchangesintheEasternMediterraneanand
ics (Booth et al., 2005; Hoerling and Kumar, 2003). The Asia(StaubwasserandWeiss,2006),though,inmanycases,
three dry periods in the Mediterranean are broadly in phase climatic inferences have been derived from sites that are
with the ice rafting events in the subpolar North Atlantic distant to these human settlements. For instance, evidence
(Bond et al., 2001), which suggests some links with the of reduced precipitation from elevated δ18O of Soreq sta-
North Atlantic circulation. Based on the coincidence with lagmites (Israel) and increased dust input into the Gulf of
theelevated windstrengthin Iceland(Jacksonet al.,2005), Oman sediment core has been suggested to contribute to
Zanchetta et al. (2014) argued that the dry events at ca. the collapse of the Akkadian empire in Mesopotamia (Bar-
5700–5400 and ca. 5200yrBP might be caused by reduc- Matthews and Ayalon, 2011; Cullen et al., 2000; Weiss et
tion of vapor advection into the Mediterranean, due to the al.,1993).Similarly,adryperiodinferredfromreduceddis-
intensificationandnorthwarddisplacementoftheNorthAt- charge of the Indus river and elevated δ18O of a Northeast-
lantic Westerlies. However, lacking evidence of strength- Indian stalagmite has been linked with the Indus Valley de-
ened wind in the fourth millennium BP argues for a differ- urbanization(Berkelhammeretal.,2013;Staubwasseretal.,
ent forcing of the 4400–3800yrBP drought. The consider- 2003; Staubwasser and Weiss, 2006). A recent study in the
ably lower amplitude of the Bond ice rafting event at ca. ancient Near East, however, revealed that the regional im-
4200yrBP than at the fifth millennium BP also indicates a pactofthedroughtonancientcivilizations,beinginfluenced
variedocean-atmospherecirculationstate.Themodernmid- by geographic factors and human technology, were highly
latitude droughts (1998–2002) have been linked to the in- diverseevenwithinspatiallylimitedculturalunits(Riehlaet
creased warmth in equatorial oceans (Booth et al., 2005). al., 2014). This highlights the need for caution when link-
Duringthisevent,SSTchangesleadtopersistenthighpres- ing evidence of human activities from a site to evidence of
sure over the Northern Hemisphere’s mid-latitudes, caus- climateoscillationsfromanotherone.
www.clim-past.net/12/1/2016/ Clim.Past,12,1–14,2016
10 J.Ruanetal.:Evidenceofaprolongeddroughtca.4200yrBP
Figure7.ComparisonbetweenevidenceofancienthumanoccupationandofpastclimatechangefromtheGueldamanGLD1Cave.The
definitionofoccupationphases0–4andclimateperiods1–4arereferredtothetext.
ThepresentstudyintheGueldamanGLD1Caveprovides – Phase 2 (∼4918–4403calyrBP): permanent and most
anopportunitytotestclimate–culturerelationsbycomparing intensiveoccupation;
insituarcheologicalsequencesandhigh-resolutionpaleocli-
mate records, thereby avoiding the uncertainty of inter-site – Phase 3 (∼4403–1484calyrBP): abandonment of the
correlationarisingfromcomplexspatialheterogeneityincli- cave/occasionalvisits;
mateanddemography.
To facilitate the comparison, stalagmite-inferred climate – Phase4(∼1484calyrBP-):re-occupationofthecave.
changesatthecavesiteduringca.6200–3200yrBParesep-
aratedintofourperiods1–4(Table3): Correlations can be identified when comparing the climatic
and archaeological records, though this does not necessar-
– Period 1 (∼6200–5100yrBP): wet, superimposed by ilymeanthatoccupationofthecavedependsmerelyoncli-
severalcentennial-scaledrierevents; mate (Table 3; Fig. 7). When the climate was wet and vari-
ableca.6200–5100yrBP(Period1),theGueldamanGLD1
– Period 2 (∼5100–4400yrBP): wettest, ending with a Cavepreservedafewbonesandrarecharcoalsandpotteries
∼200-year long shift from the wettest to extreme dry (Phase 1; Fig. 7). When the climate was wettest ca. 5100–
conditions; 4400yrBP (Period 2), the most abundance of bones, char-
coalsandpotteriessuggestsapermanentandmoreintensive
– Period 3 (∼4400–3800yrBP): a drought-like climatic
occupationofthecave(Phase2;Fig.7).Morestrikingisthe
anomaly;
drought-like climate anomaly that has been establishing ca.
4400–3800yrBP(Period3),fromwhichthecavewasaban-
– Period4(∼3800–3200yrBP):relativelywetter.
doned for ca. 3000years (indicated by a dramatic decrease
in anthropogenic remains, especially charcoal and pottery;
In parallel, from the abundance of archaeological remains
Phase 3; Fig. 7). The rarer bones seen in this period imply
(especially bone, charcoal and pottery; Fig. 6), the tempo-
that the cave might have been occasionally visited until its
ral evolutionof past caveoccupations can beseparated into
re-occupation at ca. 1484calyrBP (Fig. 7). These observa-
fivephases0–4(Table3):
tions argue for links between climate and settlement activ-
– Phase0(∼7002–6003calyrBP):permanentandinten- ityespeciallyduringthe4200yrBPclimateanomaly.Water
siveoccupation; availabilitywaslikelycrucialtomaintaintheNeolithiccom-
munity at Gueldaman, Northern Algeria and the prolonged
– Phase1(∼6003–4918calyrBP):permanentbutlessin- severedroughtca.4400–3800yrBPmighthaveplayedarole
tensiveoccupation; intriggeringthesettlementabandonment,indicatingthatthe
Clim.Past,12,1–14,2016 www.clim-past.net/12/1/2016/