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Highlights

� We perform a multiproxy analysis of a sediment core from Lake Raraku (Easter Island).� Shorter gaps than in earlier studies strengthen evidence on environmental changes.� Both climate and humans play a major role in ecological change in the last millennia.� Long, gradual and stepped palm-forest demise, and herb spread occurred from c. 450 BC.� The expansion of the weed Verbena litoralis suggests early human colonisation.

Contents lists available at SciVerse ScienceDirect

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0277-3791/$ e see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.quascirev.2013.04.004

Quaternary Science Reviews xxx (2013) 1

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Please cite this article in press as: Cañellas-Boltà, N., et al., Vegetation changes and human settlement of Easter Island during the lastmillennia: amultiproxy study of the Lake Raraku sediments, Quaternary Science Reviews (2013), http://dx.doi.org/10.1016/j.quascirev.2013.04.004

Vegetation changes and human settlement of Easter Island during the lastmillennia: a multiproxy study of the Lake Raraku sediments

Q4 Núria Cañellas-Boltà a,b,*, Valentí Rull a,1, Alberto Sáez b,2, Olga Margalef c, Roberto Bao d,Sergi Pla-Rabes e,f, Maarten Blaauwg, Blas Valero-Garcés h, Santiago Giralt c

a Laboratory of Palynology and Paleocology, Botanic Institute of Barcelona (IBB-CSIC-ICUB), Palynology & Paleoecology Group, Passeig del Migdia s/n, E-08038 Barcelona, SpainbDepartment of Stratigraphy, Paleontology and Marine Geosciences, Universitat de Barcelona, Marti Franques s/n, E-08028 Barcelona, Spainc Institute of Earth Sciences Jaume Almera (ICTJA-CSIC), Lluís Solé Sabarís s/n, E-08028 Barcelona, Spaind Faculty of Sciences, University of A Coruña, Campus da Zapateira s/n, 15071 A Coruña, SpaineBiogeodynamics and Biodiversity Group, Center for Advanced Studies of Blanes (CEAB-CSIC), Cala St. Francesc 14, E-17300 Blanes, SpainfCentre for Ecological Research and Forestry Applications (CREAF), Cerdanyola del Vallès, E-08193 Barcelona, Spaing School of Geography, Archaeology and Palaeoecology (GAP), Queen’s University Belfast, Belfast BT7 1NN, UKh Pyrenean Institute of Ecology (IPE-CSIC), Apdo. 13034, E-50080 Zaragoza, Spain

a r t i c l e i n f o

Article history:Received 2 February 2013Accepted 8 April 2013Available online xxx

Keywords:Easter IslandPaleoecologyLate HoloceneHuman settlementDeforestationVerbena litoralis

a b s t r a c t

Earlier palynological studies of lake sediments from Easter Island suggest that the island underwent arecent and abrupt replacement of palm-dominated forests by grasslands, interpreted as a deforestationby indigenous people. However, the available evidence is inconclusive due to the existence of extendedhiatuses and ambiguous chronological frameworks in most of the sedimentary sequence studied. Thishas given rise to an ongoing debate about the timing and causes of the assumed ecological degradationand cultural breakdown. Our multiproxy study of a core recovered from Lake Raraku highlights thevegetation dynamics and environmental shifts in the catchment and its surroundings during the lateHolocene. The sequence contains shorter hiatuses than in previously recovered cores and provides amore continuous history of environmental changes. The results show a long, gradual and steppedlandscape shift from palm-dominated forests to grasslands. This change started c. 450 BC and lastedabout two thousand years. The presence of Verbena litoralis, a common weed, which is associated withhuman activities in the pollen record, the significant correlation between shifts in charcoal influx, andthe dominant pollen types suggest human disturbance of the vegetation. Therefore, human settlementon the island occurred c. 450 BC, some 1500 years earlier than is assumed. Climate variability alsoexerted a major influence on environmental changes. Two sedimentary gaps in the record are interpretedas periods of droughts that could have prevented peat growth and favoured its erosion during theMedieval Climate Anomaly and the Little Ice Age, respectively. At c. AD 1200, the water table rose and theformer Raraku mire turned into a shallow lake, suggesting higher precipitation/evaporation rates coevalwith a cooler and wetter Pan-Pacific AD 1300 event. Pollen and diatom records show large vegetationchanges due to human activities c. AD 1200. Other recent vegetation changes also due to human ac-tivities entail the introduction of taxa (e.g. Psidium guajava, Eucalyptus sp.) and the disappearance ofindigenous plants such as Sophora toromiro during the two last centuries. Although the evidence is notconclusive, the American origin of V. litoralis re-opens the debate about the possible role of Amerindiansin the human colonisation of Easter Island.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Easter Island is a tiny and remote island in the South PacificOcean (Fig. 1). The island has achieved global fame as the home ofan ancient and complex society that erected megalithic statues,known as moai, and as an example of dramatic environmentaldegradation wrought by humans. This latter view is largely basedon the palynological studies of lake sediments which have been

* Corresponding author. Laboratory of Palynology and Paleocology, BotanicInstitute of Barcelona (IBB-CSIC-ICUB), Palynology & Paleoecology Group, Passeigdel Migdia s/n, E-08038 Barcelona, Spain. Tel.: +34 934034489, +34 93 2890611;fax: þ34 93 4021340, þ34 93 2890614.Q1

E-mail address: nuriacatcb@gmail.com (N. Cañellas-Boltà).1 Tel.: þ34 93 2890611; fax: þ34 93 2890614.2 Tel.: þ34 93 4034489; fax: þ34 93 4021340.

Contents lists available at SciVerse ScienceDirect

Quaternary Science Reviews

journal homepage: www.elsevier .com/locate/quascirev

0277-3791/$ e see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.quascirev.2013.04.004

Quaternary Science Reviews xxx (2013) 1e14

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interpreted as palm-dominated forests that were recently andsuddenly replaced by grasslands. This apparently abrupt landscapechange has been attributed to deforestation by the indigenouspopulation (Flenley and King, 1984; Flenley et al., 1991; Flenley andBahn, 2003; Diamond, 2005; Mann et al., 2008; Mieth and Bork,2010). As a consequence, Easter Island has traditionally beenregarded as an illustrative example of how humans can destroytheir own habitat and cause a societal breakdown (Flenley andBahn, 2003; Diamond, 2005), and it is often used as a model forthe possible consequences of the over-exploitation of natural re-sources. Nevertheless, the environmental history of the island ispoorly understood, and controversies about the timing and causesof the assumed deforestation, its consequences, and the role ofhumans are still ongoing (Mann et al., 2008; Rull et al., 2010).

According to the prevailing view of the history of Easter Island, asmall group of Polynesian settlers reached the island from thewest,most likely from the Marquesas, Tuamotu, Gambier or AustralIslands (Stefan, 2001; Flenley and Bahn, 2003) around AD 800e1000 (Martinsson-Wallin and Crockford, 2002; Vargas et al., 2006)(Fig. 1). As the islanders grew in number, the need for open spaces

for agriculture and dwellings as well as the demand for firewoodand timber would have increased rapidly. This would have led torapid deforestation and soil degradation, thereby initiating a posi-tive feedback of catastrophic consequences. According to this view,before the arrival of Europeans in the 18th century, the inhabitantshad completely deforested the island, which would have led towars, famine and, finally, to cultural collapse (Flenley et al., 1991;Flenley and Bahn, 2003; Diamond, 2005).

Nevertheless this proposed sequence of historical events is notshared by all scholars. The settlement date is controversial anddifferent chronologies ranging from AD 100 to AD 1200e1290 havebeen suggested (e.g. Heyerdahl and Ferdon, 1961; Hunt and Lipo,2006; Butler and Flenley, 2010; Wilmshurst et al., 2011). More-over, dating uncertainties have complicated the assignment of theprecise age of the alleged deforestation, and its causal relationshipwith human activities has not been wholly accepted. Other addi-tional or alternative causes for the vegetation clearance, such asclimatic changes (McCall, 1993; Hunter-Anderson, 1998; Nunn,2000) or the action of introduced fruit-eating rats preventingpalm regeneration (Hunt, 2007; Hunt and Lipo, 2009) have also

Fig. 1. Map of Easter Island. Above: map of Easter Island showing the location of the three permanent water resources on the island (Rano Raraku and Rano Kau crater lakes andRano Aroi fen). Below: the location of the island in the SE Pacific Ocean (map modified from Elix and McCarthy, 2008).

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been proposed. Even the scenario of a completely forested islandremains to be demonstrated (Rull et al., 2010). Likewise, the linkbetween deforestation and cultural collapse has not been provedconclusively. Climate changes (Nunn, 2000; Stenseth and Voje,2009), the European contact (Rainbird, 2002; Hunt and Lipo,2009), a possible contact with the Amerindians (Dumont et al.,1998) or the isolation itself (van Tilburg, 1994) are also regardedas potential drivers of the cultural demise.

One of the main reasons for the ongoing debate about theecological history of the island is the occurrence of extended hia-tuses in most of the sedimentary sequences studied to date, whichprevents us from understanding the events that occurred duringthe time when the great vegetation change took place (Flenley andKing, 1984; Flenley et al., 1991; Dumont et al., 1998; Mann et al.,2008; Sáez et al., 2009; Rull et al., 2010).

Here we present the results of a multi-proxy study (sedimen-tology, geochemistry, diatoms, charcoal and pollen) of aradiocarbon-dated core recovered from Lake Raraku that seek toaccount for the tempo and mode of the vegetation changes in thelast millennia, and to provide insights into how and when climateand/or human activities caused the paleoecological changes. Thesedimentary record studied here has the advantage of having muchshorter hiatuses than the cores analysed to date, thus providing amore complete palaeoecological sequence including new evidencefrom the period previously masked by sedimentary gaps. In addi-tion, the identification of Verbena litoralis, a new pollen typeobserved within the island’s sediments provides new and impor-tant information about potential causes.

2. Regional setting

Easter Island, which is a tiny (164 km2) volcanic island located inthe South Pacific Ocean (27�0701600S, 109�2105900W), constitutes theeasternmost Polynesian island (Fig. 1). It is one of the remotestplaces on Earth, about 3600 km from the Chilean coast and2030 km from the nearest inhabited island (Pitcairn). The island hasa triangular shape resulting from lava flows from three main vol-canoes located at each of the three corners, and around 70 satellitecones. The Terevaka volcano (511 m) is the highest peak on theisland. Hydrologically, the island is characterized by the absence ofpermanent surface streams due to the high permeability of thevolcanic rocks (Herrera and Custodio, 2008). Two lakes (RanoRaraku and Rano Kau) and a fen (Rano Aroi) are the only permanentareas of surface freshwater. The climate is subtropical, with anaverage annual temperature of 21 �C and a range of averagemonthly temperatures between 18 �C in August and 24 �C inJanuary (Mann et al., 2008). The total annual precipitation is highlyvariable, ranging between 500 and 2000 mm, with long alternatingdry and humid periods (Horrocks and Wozniak, 2008). The currentflora contains more than 200 vascular plants (Flenley, 1993), mostof which (w85%) have been recently introduced (Flenley et al.,1991; Zizka, 1991; Flenley, 1993). Today the island is almostentirely covered by grassland (90%), with some forests, shrublandsand urban pioneer vegetation patches, mainly of introduced plants.Little is known about the native flora and fauna but they aredescribed as very poor owing to the island’s isolation and small area(Skottsberg, 1956; Zizka, 1991).

Raraku Lake is a small (0.11 km2), shallow (w3 m), freshwaterlake, situated at 75 m altitude a.s.l., inside a volcanic crater olderthan 300,000 yr (Baker et al., 1974; Sáez et al., 2009). The lake has aflat-bottom morphology and the margins of the small watershedare relatively steep. It is hydrologically closed, disconnected fromthe island’s main groundwater aquifers and fed solely by precipi-tation (Herrera and Custodio, 2008). The catchment area is about0.35 km2 and it is composed of volcanic tuff rich in glass, feldspar,

and ilmenite (González-Ferran et al., 2004). Lake waters are wellmixed, acidic (pH around 6.3), dilute (average conductivity is640 mS cm�1, Geller, 1992) and of CleHCO3eNa type (Sáez et al.,2009). A dense littoral belt, mainly of Scirpus californicus,currently surrounds the lake and forms some floating mat patches.

3. Methods

A multiproxy analysis was carried out on the uppermost sectionof the core RAR-08 recovered from Lake Raraku using a UWITECcorer installed on a UWITEC platform raft (see Sáez et al., 2009 forcoring details).

3.1. Radiocarbon dating and age-depth model

Twelve AMS radiocarbon dates (Table 1) from the uppermost41 cm of the core RAR 08 were used to construct the age-depthmodel. Samples were processed following the standard pollenprotocol excluding acetolysis, alcohol dehydration and silicone oilstorage. Dating was performed at Poznan Radiocarbon Laboratory(Poland) and Beta-Analytic (USA).

Radiocarbon dating of the lake Raraku sediments has beensurrounded by controversy. Besides the discontinuity of the sedi-mentary record (hiatuses) observed in the Lake Raraku sequences,another difficulty in establishing a sound chronological frameworkis the frequent existence of many chronological inversions in theradiocarbon dates (Flenley and King, 1984; Flenley et al., 1991;Flenley, 1996; Butler et al., 2004; Horrocks et al., 2012), which hasbeen attributed to different sedimentary processes (Butler et al.,2004; Mann et al., 2008). In order to improve the chronologicalframework of the sequence studied, an age-depth model was builtwith a Bayesian approach using the Bacon software (Blaauw andChristen, 2011). This approach was chosen since it allows theincorporation of previously known stratigraphic information (i.e.presence of one or more gaps or hiatuses) as well as constraints onaccumulation rate and variability in model building, which iscrucial for a realistic chronological interpretation. Earlier workscarried out on the Lake Raraku sediments have demonstrated thepresence of hiatuses in its sedimentary record (Flenley et al., 1991;Dumont et al., 1998; Mann et al., 2008; Sáez et al., 2009). In the corestudied in the present work (RAR 08), two hiatuses were inferred at19.5 and 10 cm depth, taking into consideration the raw radio-carbon dates, X-ray Fluorescence (XRF) core scanner images (notshown), and the lithological and sedimentological features of thecore. See Appendix 1 for the detailed specification of settings usedfor the building of the age-depth model with Bacon.

Table 1AMS 14C radiocarbon dates. Radiocarbon dates obtained from pollen-enrichedextract from Raraku Lake core used for age-depth modelling.

Sample name Laboratoryname

Depth(cm)

Fraction dated 14C yr BP �

RAR 08-01-4 Poz-42955 4 Pollen-extract 106.94 � 0.39 pMCa

RAR 08-01-09 Poz-42957 9 Pollen-extract 101.03 � 0.35 pMCa

RAR 08-01-10-11 Beta-316585 11 Pollen-extract 380 30RAR 08-01-12-13 Poz-32007 13 Pollen-extract 505 30RAR 08-01-17 Poz-42958 17 Pollen-extract 840 30RAR 08-01-17-18 Beta-316586 18 Pollen-extract 780 30RAR 08-01-18-19 Beta-316587 19 Pollen-extract 1180 30RAR 08-01-21 Poz-42959 21 Pollen-extract 2120 35RAR 08-01-22-23 Beta-316588 23 Pollen-extract 2970 30RAR 08-01-25 Poz-19940 25 Pollen-extract 2160 30RAR 08-01-29 Poz-42960 29 Pollen-extract 4800 35RAR 08-01-40-41 Poz-32120 41 Pollen-extract 4530 40

a Radiocarbon date post-bomb.

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3.2. Multiproxy analysis

Magnetic susceptibility and density were measured every cen-timetre using a GEOTEK� Multi-sensor Core Logger and total ni-trogen (TN), total carbon (TC) and d13C were determined by aFinnigan delta Plus EA-CF-IRMS spectrometer. Twenty-four sam-ples were extracted from the upper 26 cm of the core and processedfor pollen analysis according to slightly modified standard labora-tory procedures (Rull et al., 2010b), which include sieving, KOH, HCland HF digestions, and acetolysis. Lycopodium tablets were added toeach sample before chemical processing as an exotic marker.Microscopic slides were mounted in silicone oil and pollen count-ing was performed until at least 200 pollen grains had beenreached, excluding aquatic and semi-aquatic taxa (Cyperaceae andPolygonum). Charcoal particles larger than 5 mm of twenty-fiverandomly distributed fields at �20 magnification were counted,and the total concentration was inferred using the exotic markers(Lycopodium). The charcoal counts were carried out using the samepollen slides. The charcoal influx was calculated from total con-centration, using accumulation rates and sediment density. Theirvalues for the uppermost centimetres of the sequence were notcalculated owing to the lack of suitable density measurements.Samples for diatom analysis were taken every 2.5 cm and wereprocessed using standard techniques (Renberg, 1990). The cleanedsubsamples were dried onto coverslips and mounted on micro-scope slides with Naphrax� high refractive medium (R. I.¼1.73). Atleast 400 valves were counted per sample. Diagrams for both pollenand diatom data were plotted with psimpoll 4.26 (Bennett, 2009).Statistically significant pollen and diatom zones were obtainedwith the same software, using the method of optimal splitting byinformation content (OSIC) (Bennett, 1996).

4. Results and interpretation

4.1. Sedimentological, geochemical and chronostratigraphic data

According to the age-depthmodel (Fig. 2), the section of the corerecords the time spanning 1790 yr BC (w2570ew1270 BC at 95%uncertainty intervals). The sequence is interrupted by two sedi-mentary hiatuses that span from c. AD 500 to c. AD 1165 and from c.AD 1570 to c. AD 1720.

This stratigraphic interval, which is 26 cm thick, is composed ofdiatom-bearing, peaty sediments. Two different facies were iden-tified (Fig. 3). The lowest part of the core, below 19.5 cm depth, iscomposed of reddish-brown massive or banded muddy peat,formed mainly by macroremains of sedges. It has been interpretedas deposited in a mire environment (Sáez et al., 2009; Cañellas-Boltà et al., 2012). Above this depth, the sediments are composedof silicate-rich silty peat, interpreted as deposited in a shallow lakesurrounded by floating mats as currently shown by the Rarakucrater (Sáez et al., 2009; Cañellas-Boltà et al., 2012). See Sáez et al.(2009) for a more detailed lithostratigraphic description. Accordingto the age model, the mire episode recorded in this short corecomprises the last 2290 years (from c. 1790 BC until c. AD 500) of along Holocene mire sedimentation phase initiated approximately8900 years ago. The upper lacustrine interval covers the remainingw800 years until the present (Sáez et al., 2009).

Slight changes in density, magnetic susceptibility (MS), C/N ra-tio, total carbon (TC), d13C and total nitrogen (TN) are observedalong the core (Fig. 3). After an initial increase of up to 24 cm, TCand C/N values tend to decrease up to 17 cm depth. In contrast,density, d13C, and TN remain fairly constant up to this depth. A largepeak of magnetic susceptibility is observed at the bottom. From 17

Fig. 2. Age-depth model for Lake Raraku sediment core developed by Bacon for the interval studied here. A) Age-depth model and age distributions of calibrated 14C dates. Linecurve shows the best model, grey-scales indicate chronological uncertainties. The date at 25 cm depth appears too young when compared with the other dates and the model; thissample may have been contaminated by ingrowths of roots from peat above it. B) From top to bottom: (1) over 4000 stored iterations show a stable MCMC run; Prior information(green curves) and posterior estimates (grey histograms) of (2) accumulation rates, (3) variability/memory of accumulation rate, and (4) hiatus sizes. (For interpretation of thereferences to colour in this figure legend, the reader is referred to the web version of this article.)

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to 10 cm depth, TC, TN, C/N, d13C and MS values remain essentiallyconstant except for a peak of magnetic susceptibility detectedaround 16 cm depth, which coincides with a decrease in TC, C/N anddensity. From 10 to 6 cm depth, a marked decrease in sedimentdensity, TC, TN and in d13C is observed. The C/N peak at 8 cm depthcoincides with a TN fall. From 6 cm depth to the top, the values ofthe parameters remain constant with minor fluctuations.

4.2. Pollen and charcoal data

The pollen results (percentage) and charcoal data (influx) aredepicted in Fig. 4. The palynological record is characterised by theoccurrence of few pollen types, with palm, grasses and V. litoralisKunth as the predominant elements. Besides palms, Triumfettasemitriloba Jacquin, Sophora toromiro (Philippi) Skottsb., andAsteraceae are the other main woody taxa identified through therecord. Several ferns, Cyperaceae, Polygonum acuminatum Kunthand cf. Solanum are also present in moderate to relatively highpercentages. The main change identified is the replacement of thewoody plant pollen (mainly palms) by herbaceous plant pollen(primarily Poaceae and V. litoralis). Two zones are statistically sig-nificant, highlighting this main shift around 18.5 cm depth. Somechanges inside these zones allow us to distinguish different sub-zones (Fig. 4).

4.2.1. Pollen zone RAP-1 (26e18.5 cm, c. 1790 BCec. AD 1200)This zone is characterised by a high dominance of tree and shrub

pollen, particularly from palms, with smaller amounts of Triumfettaand Sophora. Fern spores show lowabundances in this zone, as doesAsteraceae pollen, which is absent from several samples. Twosubzones can be identified. The subzone RAP-1a (26e22.5 cmdepth; 1790 BCec. 450 BC) is clearly dominated by palm pollen,with values around 90% of the total pollen sum. Triumfetta, Sophora,

and few Asteraceae complete the tree and shrub pollen record inthis subzone. A few Cyperaceae and herb pollen are also observed.In the subzone RAP-1b (22.5e18.5 cm depth; c. 450 BCec. AD 1200),palm pollen declines markedly, with percentage values reachingonly up 60e70% of the pollen sum. Poaceae, V. litoralis and Cyper-aceae pollen percentages show a marked increase. Solanum andpsilate monolete fern spores undergo a slight increase and psilatetriletes appear in the record for the first time. Triumfetta andSophora are still present and charcoal influx shows a slight increase.Asteraceae are absent.

4.2.2. Pollen zone RAP-2 (18.5 cm depth to top) (c. AD 1200 topresent)

Palm pollen undergoes a second significant drop at 18.5 cm andherb pollen becomes dominant, with alternating dominance ofPoaceae and V. litoralis. Ferns spores (primarily psilate monoletes)and Asteraceae pollen are also more abundant in this zone than inRAP-1. The considerable increase in charcoal influx throughout thiszone is also noteworthy. Three subzones can be distinguished ac-cording to changes in the record of palms, herbs and charcoal. Thesubzone RAP-2a (from 18.5 to 12.5 cm depth; c. AD 1200ec. AD1475) is characterized by co-dominance of Verbena and Poaceae,intermediate abundances of palm pollen (20e37%), and highabundances of Cyperaceae. Charcoal influx is remarkably higherthan in the previous zone. In the subzone RAP-2b (from 12.5 to6.5 cm; c. AD 1475ec. AD 1875), palm pollen falls and shows thelowest percentages in the record (<10%), Poaceae shows a slightdecrease and V. litoralis becomes dominant. Other distinctive fea-tures of this subzone are the decrease in Cyperaceae and thehighest charcoal influx values. The uppermost section of the core(the subzone RAP-2c, 6.5 cm depth to the top; from c. 1875 topresent) is characterised by a marked increase in Poaceae, whichbecome dominant at the expense of Verbena. New pollen taxa

Fig. 3. Lithology, physical and geochemical data. Physical and geochemical parameters from the Lake Raraku sequence. Lithostratigraphy and AMS 14C radiocarbon dates areindicated on the left of the diagram and calibrated dates according to the age model used on the right. Dashed horizontal lines indicate depth of the discontinuity of sedimentaryhiatuses.

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appear in the record such as P. guajava L., Plantago sp. and Euca-lyptus sp.. The absence of Sophora and the slight increase in palmpollen are also significant.

4.3. Diatom data

Pseudostaurosira trainorii E.A. Morales, Pseudostaurosira neo-elliptica (Witkowski) E.A. Morales and Pinnularia joculata (Man-guin) K. Krammer are the most abundant diatoms in the record(Fig. 5). The main shift is the replacement of benthic by tycho-planktonic taxa around 19.5 cm depth, which marks the boundarybetween two statistically significant zones. This boundary roughlycoincides with the boundary between the previous pollen zones.

4.3.1. Diatom zone RAD-1 (26e19.5 cm depth) (c. 1790 BCe c. AD500)

This zone is dominated by benthic diatoms, notably P. joculata.The presence of numerous aerophilic benthic taxa (e.g. Eolimnaminima (Grunow) H. Lange-Bertalot, Hantzschia amphioxys(Ehrenberg) Grunow, Luticola sp. aff. mutica (Kützing) Mann, Nitz-schia debilis (Arnott) Grunow, Pinnularia sp. aff. divergentissima) isalso noteworthy. Above 22.5 cm (c. 450 BC) slight changes in thediatom assemblages are observed, with an increase in non aero-philous P. joculata up to values of 80% of the total assemblage. Mostof the diatoms in this zone have an acidophilic character. Thediatom frustules/chrysophycean stomatocysts ratio shows lowvalues.

4.3.2. Diatom zone RAD-2 (19.5 depth to top) (c. AD 1165 to thepresent)

This zone is characterised by the predominance of tychoplank-tonic and alkaliphilic diatoms belonging to the Fragilaria s. l. group,basically P. neoelliptica and P. trainorii, which replace the benthicdiatoms of the former zone. Furthermore, two subzones can bedifferentiated. The subzone RAD-2a (19.5e10 cm depth, c. AD1165ec. AD 1570) is dominated by P. neoelliptica and P. trainorii(absent in the previous zone), and displays a diverse assemblage ofbenthic diatoms, with P. joculata as the most abundant one but withlower values than in RAD-1. By contrast, the subzone RAD-2b(10 cm depth to the top, c. AD 1570 to the present) is almostentirely dominated by tychoplanktonic diatoms, and benthic di-atoms are practically absent with a minor presence of Pinnularia.

An overall upward increase in the frustules/chrysophyceanstomatocysts ratio is observed in this zone. After an isolated peakaround 16 cm depth, a gradual rising trend occurs from 14 to 4 cm.This ratio shows a marked decrease in the four top centimetres.

4.4. Vegetation and palaeoenvironment reconstruction

The reconstruction of the environmental and vegetation dy-namics in the Raraku catchment and surroundings through time isbased on pollen and diatom zoning, taking into considerationchanges in the sedimentary facies, the charcoal influx and thephysical and geochemical proxies. Five main phases can be recog-nised (Fig. 6).

Fig. 5. Diatom diagram of the sequence from Lake Raraku. AMS 14C radiocarbon dates are indicated on the left of the diagram and diatom zones on the right. Calibrated datesaccording to the age model used are indicated on the right.

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4.4.1. Phase 1: c. w1790 to c. 450 BCBoth the sedimentary composition (peaty deposits) and the

diatom data suggest that the bottom of the Lake Raraku crater wascovered by a mire during this period. As in earlier studies, the LakeRaraku pollen record provides evidence of a wooded landscapedominated mainly by palm trees in the Lake Raraku surroundingsaround the start of the late Holocene. T. semitriloba and S. toromiroshrubs would have dominated the understorey. High total carbon(TC around 40%), C/N values exceeding 10 and heavy d13C valuesindicate the prevalence of terrestrial organic matter in the sedi-ment, mainly derived from C3 plants (Meyers and Terranes, 2001).The predominance of benthic diatoms, many of which show anaerophilic affinity, points to a mire environment with ephemeraland shallow water ponds. Furthermore, the presence among theaerophilic taxa of subaerial species such as H. amphioxys, Luticolamutica, N. debilis and Pinnularia borealis Ehrenberg (Fig. 5) couldindicate periods with higher aridity and drying out conditions(Denys, 1991; Johansen, 2010).

4.4.2. Phase 2: c. 450 BCec. AD 1200At the start of this period, the characteristics of the sediments

point to the continuity of the mire conditions of Phase 1. However,the decrease in benthic diatoms with aerophilic affinity and theprogressive diminution of the C/N ratio during this phase suggest apeaty environment that gradually became waterlogged. This envi-ronment could have favoured the growth of semiaquatic plantssuch as Cyperaceae.

Sedimentary and chronological evidence indicate a period ofsubaerial exposure of the depositional surface after AD 500 andbefore AD 1200 and the subsequent erosion resulting in a hiatus inthe sequence. This hiatus was probably the result of drought phasesbetween AD 500 and AD 1200 (Mann et al., 2008; Sáez et al., 2009).The Raraku basin was flooded again c. AD 1200, as evidenced by

changes in the composition of the sediment, and by the onset of atrend towards tychoplanktonic dominance in diatom assemblages.

The first significant vegetation change in the sequence occurredc. 450 BC (790 BCe90 BC) when a trend of replacement of woodedlandscape by herb-dominant communities starts. In this regard, amarked increase in herbaceous taxa (notably grasses, V. litoralis andSolanum sp.) coupled with a fall in palm pollen indicates a reduc-tion in the arboreal cover, and an openness of the landscape (Fig. 6).The expansion of herbs, especially the spread of V. litoralis, a weed/ruderal plant that is associated with human-disturbed sites (Zizka,1991; Wagner et al., 1999), suggests the possible onset of humanpresence at the beginning of this phase. A slight increase in charcoalinflux during this period, pointing to greater fire incidence, couldalso support human presence. The small rise in fern spores (mainlypsilate monoletes) could be due to the increase in local burning.Secondary fern communities are rapidly established in areasaffected by fire (McGlone and Wilmshurst, 1999; Rull, 1999).

4.4.3. Phase 3: c. AD 1200ec. AD 1475The sharp increase in tychoplanktonic diatoms and the

compositional and geochemical data (lower values of TC, C/N andd13C) demonstrate that the Raraku site was transformed into ashallow lake with abundant littoral vegetation not unlike condi-tions at present. Indeed C/N and d13C values strongly suggest thatthe organic matter is mainly terrestrial in origin. Geochemical datashow a higher increase in sediment delivery to the lake (highersilicate content). The dominance of small diatoms of the Fragilaria(s.l.) group, which includes the genus Pseudostaurosira, also cor-roborates the increased erosion in the catchment. Indeed, thisgroup of diatoms is typical of shallow but open waters and littoralzones, and their growth is favoured by alkalinizing base cationsreleased from the catchment (Reed et al., 1999; Axford et al., 2009).The dominance of these diatoms would indicate enhanced runoff

Fig. 6. Summary multiproxy diagram of the sequence from Lake Raraku. AMS 14C radiocarbon dates are indicated on the left of the diagram, and calibrated dates according to theage model used on the right. The arrows in the Arecaceae curve indicate palm decline events discussed in the text. Dashed horizontal lines indicate phases identified in this study.The two sedimentary hiatuses in the sequence are depicted by horizontal lines. The sedimentary hiatuses observed in other studies are indicated on the right. Note that early humansettlement (450 BC) and the gradual nature of ecological change fall within the hiatuses of earlier studies.

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and increased cation input to lake waters, probably as a result ofvegetation clearance. The increase in the frustules/chrysophyceanstomatocysts ratio could also point to enhanced eutrophicationconditions (Smol, 1985).

A second notable vegetation shift took place c. AD 1200 andentailed a drastic landscape transformation. The pollen record wasno longer dominated by palms but by herbaceous taxa, indicatingthe increasing prevalence of open herbaceous communities (Fig. 6).Poaceae and V. litoralis continued their expansion initiated in theprevious phase, but this time with a pronounced acceleration.Although the palm pollen decreased gradually during all thisperiod, palms remained as an important element of the vegetation,together with Triumfetta, Sophora, and Asteraceae, which increasedin abundance during this phase. The large rise in charcoal influxsuggests higher fire frequency, which is probably associated withcontinued human disturbance.

4.4.4. Phase 4: c. AD 1475ec. AD 1875The sedimentary, geochemical and diatom records indicate

persistent shallow-lake conditions during this period. However, asedimentary hiatus between c. AD 1570 and AD 1720 indicatesother period of drought. During this phase the palm pollen attainsits lowest values (<10%), suggesting a landscape almost entirelydominated by herbaceous plants, with abundant Verbena, and a fewsparse palms. A notable increase in Verbena percentage and char-coal influx values probably indicate the continued intensification ofdisturbance by humans.

4.4.5. Phase 5: c. AD 1875 to presentSignificant changes in vegetation took place during this last

phase. The Raraku landscape became dominated by Poaceae after asharp fall in V. litoralis. However, Verbena continued to be animportant element of the vegetation as shownby themarked pollenvalues still observed in the sediment. Another striking change is thepresence of pollen of plants known to be introduced by humansduring the 20th century, such as P. guajava and Eucalyptus, showingan increase in allochthonous elements. The endemic shrub S. tor-omiro, which survives today only as a cultivated plant on the islandand in botanical gardens in other parts of the world (Zizka, 1991),disappears from the Raraku area during this phase. The highestvalues of the frustules/chrysophycean stomatocysts ratio also sug-gest the maximum eutrophic conditions in this record.

Unfortunately, as stated in the Methods section (Section 3), itwas not possible to calculate charcoal influx in the uppermostcentimetres of the sequence, which prevents the interpretation offire dynamics in this period.

5. Discussion

The multiproxy study of a Lake Raraku sedimentary core pro-vides relevant information about the late Holocene vegetation dy-namics and environmental changes in the Lake Raraku catchmentand surroundings. Despite the presence of two sedimentary hia-tuses, the unrecovered time span in this study is considerablyshorter (around 650 and 200 yr, respectively) than in earlier studiesof the Lake Raraku sediments (around 3000e6000 yr in e.g. Flenleyet al., 1991; Mann et al., 2008) (Fig. 6). Our record therefore pro-vides a more comprehensive view of the late-Holocene history ofthe lake and furnishes empirical evidence for vegetation changesand their potential causes that have remained hidden to date.

5.1. Climate changes

Earlier studies of the island have suggested significant climaticchanges during the Last Glacial Maximum and the early-mid

Holocene (Flenley et al., 1991; Azizi and Flenley, 2008; Sáez et al.,2009) but there are very few data on more recent times. Ourstudy shows that both climate and human activities have largelymodified the landscape during the last millennia. Besides changesin the aquatic organisms and chemical proxies (Section 4.4), theoccurrence of sedimentary gaps in the record could be interpretedas a result of intensified erosion during dry climatic phases (Mannet al., 2008; Sáez et al., 2009). These dry phases would haveresulted in a fall in the water level or in a total drying out, thusfavouring peat erosion and/or preventing its growth. The older gap(c. AD 500ec. AD 1200) is roughly coeval with theMedieval ClimateAnomaly (ca 9th�14th centuries) when warm and dry conditionsprevailed in the South Pacific Basin ( Q2Nunn and Britton, 2001; Nunn,2007). After this dry period, the rise in the water level at Raraku ledto the formation of a lake. This climate shift could be attributed tothe AD 1300 event, a phase of rapid cooling and wetter conditions,especially in the eastern Pacific Basin, which has been linked to anincrease in the frequency of El Niño events (Nunn, 2000, 2007;Nunn and Britton, 2001). The younger hiatus (wAD 1570eAD 1720)could be associated with the occurrence of several drought phasesin the Pacific Basin during the Little Ice Age (Nunn, 2000, 2007). Thecontinued dominance of tychoplanktonic diatoms and the sedi-mentary composition suggest higher lake levels and a prevailingwet climate from c. AD 1200. These climatic changes would havebeen coupled to human activities, resulting in synergistic effectsthat could have favoured or slowed down population growth, palmgrowing and regeneration, or fire incidence.

5.2. Palm forest decline and recent vegetation changes

Palynological evidence from Raraku sediments supports theoccurrence of former palm-dominated vegetation at the start of thelate Holocene and the progressive grassland expansion thatreplaced these palm forests (Fig. 6). This significant change invegetation, which started c. 450 BC, entailed a gradual and steppedreplacement that lasted about two thousand years. Three mainpalm declines are identified in the record. The first decline tookplace c. 450 BC although palms still remained as the dominantelement in the landscape. At c. AD 1200, the second palm declineled to a more open landscape with the probable onset of dominantherbaceous communities. The last palm decrease occurred c. AD1475, but palm pollen did not disappear entirely from the record ashas been reported in other paleoecological studies (Dumont et al.,1998). This is consistent with the presence of some isolated palmtrees as noted by the first Europeans reaching the island during the18th century (Zizka, 1991; Hunt, 2007; Mann et al., 2008). Theprogressive palm decline observed in our results is at variance withearlier studies that have proposed a sudden and rapid vegetationchange, i.e. a catastrophic palm forest demise (Flenley and King,1984; Flenley et al., 1991; Mann et al., 2008). In these studies,which were also performed in the Lake Raraku sediments, palmpollen sharply declines and is replaced abruptly by grass pollen,probably as a result of sedimentary gaps masking the trends shownby our data.

Of the three recorded palm declines, only the second can belinked to a sedimentary gap (wAD 500 to wAD 1200), which isinterpreted as indicative of dry climates (Fig. 6). This would suggestsome influence of climate on the palm demise. However, thedecline occurred immediately after the hiatus, coinciding with thetransition between lower and higher water levels, as indicated bydiatoms, and also by a marked increase in charcoal, which stronglysuggests human disturbance. It may be that the vegetation changewas brought about by a combination of climatic and anthropogenicforcings. Dry climates would have depleted natural resources andfavoured vegetation ignitability, leading to an intensification of

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forest exploitation and local burning. The first and third palm de-clines occurred without evidence of significant climate change. Thefirst decline took place within a lowstand phase and coincides withincreases in charcoal and herbs, suggesting human disturbance.However, there is no evidence of human occupation of Easter Islandat that time. This possibility is discussed in the next Section 5.3 onthe basis of new evidence provided in this paper. The last palmdecline occurred in a lake highstand situation (i.e. wetter climates)and is coeval with a second notable increase in charcoal influx,which strongly suggests intensification of burning. The secondsedimentary gap (c. AD 1570ec. AD 1720) does not appear to haveaffected the vegetation significantly.

In the two last centuries, floristic and vegetation changes con-sisted in the introduction of allochthonous plants, such as Euca-lyptus and P. guajava, and the disappearance of native ones as is thecase of Sophora (Figs. 4 and 6). The last appearance of Sophora in therecord roughly coincides with the onset of sheep farming on theisland (Porteous, 1978; Mieth and Bork, 2005). It is well docu-mented that sheep farming at the end of the 19th century andthroughout 20th century intensified the degradation of the land-scape (Porteous, 1978; Mieth and Bork, 2005), which would havecontributed to the extinction of Sophora. The indigenous vegetationof Easter Island has been relegated to a few isolated locations andhave been mainly replaced by allochthonous grasses and otherintroduced plants (Zizka, 1991).

5.3. V. litoralis colonisation and expansion

The decline of palm forest was accompanied by the expansion ofherbs, mainly Poaceae and V. litoralis. The presence of the pollen ofthe V. litoralis (Fig. 7) in the sedimentary records of Easter Island hasnot been documented to date, although this plant forms part of thepresent vegetation. V. litoralis is a native species of Americacommonly associated with disturbed sites (agricultural or ruderal)(Zizka, 1991; Wagner et al., 1999) and is considered to have beenintroduced to the island during the two last centuries (Zizka, 1991).The continued and predominant occurrence of V. litoralis pollen,and its trends over time, in the Raraku record from 450 BC could bedue a priori to three causes: 1) arrival of the plant in the island bynon-human (natural) dispersal mechanisms, such as birds or raft-ing, and establishment and expansion as a result of favourableenvironmental conditions; 2) arrival by natural dispersal agents butspreading and expansion as a result of human activities and 3)

arrival, establishment and population expansion linked to humanactivities.

In the first case, the onset of herb expansion coinciding with thefirst palm decline would have been due to environmental, probablyclimatic, causes. According to this hypothesis, palm forests werealready decaying when humans colonised the island. A scenario ofthis type was suggested by Hunter-Anderson (1998), who proposeda geo-climatic model to account for the disappearance of the palmsin which mid to late-Holocene climatic instability would haveplayed a major role. According to this author, humans arrived in theisland during “favourable conditions of the Little Climatic Opti-mum” between AD 800 and AD 1400 (1200 and 600 yr BP), whenthe island was already covered by grasses with only few trees andshrubs growing at protected sites. Likewise, droughts and changesin ENSO frequency have been proposed as possible causal agents forvegetation changes by other authors (Orliac and Orliac,1998; Nunn,2000; Orliac, 2000; Nunn and Britton, 2001). However, noconclusive evidence has been obtained to date. This hypothesis hasbeen dismissed using inferences from the climate dynamics of thelast 50 years (MacIntyre, 2001; Genz and Hunt, 2003). There is noindependent evidence in the palynological record to either supportor challenge this hypothesis (Rull et al., 2010). Other possible causalagents of vegetation change could have been short-term pertur-bations such as intense storms. The continuous nature of the trendof change observed in our records does not appear to support theoccurrence of episodic events as potential causes.

The two other hypotheses attribute the expansion of V. litoralisand the associated landscape transformations to human activities.Both hypotheses suggest human presence on the island to at leastw450 BC, some 1500 years before the commonly accepted date(discussion in Section 5.4). The main difference between these twohypotheses is the dispersal agent responsible for the transportationof the plant to the island, either by non-human agents (hypothesistwo) or by the eventual first colonisers (hypothesis three). As statedabove, V. litoralis, which is commonly associated with human ac-tivities, it is also regarded as a weed. At present, the species is wellestablished on the island, especially alongwaysides, coasts and lakeshores (Zizka, 1991). Moreover, it has been suggested that V. litoralis(known as “puringa” by present-day Easter Islanders) was culti-vated on the island in historical times for its medicinal properties(Zizka, 1991; Rauch et al., 1996), which could have favoured thespread of this herb. Therefore, the relationship between this plantand human activities seems, at present, fairly clear.

Fig. 7. Photographs of a Verbena litoralis growing at present around Lake Raraku (AeD) and pollen identified as Verbena litoralis in the microscopic slides (EeH) Bar ¼ 10 mm. Theslightly smaller size of fossil pollen is probably because of the use of silicone oil during pollen protocol and storage (Rull and Rinaldi, 1988).

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The Raraku record shows a significant correlation betweencharcoal and V. litoralis (r ¼ 0.61, p-value ¼ 0.003), and betweencharcoal and herbs (basically V. litoralis þ Poaceae) (r ¼ 0.70, p-value ¼ 0.0004), which highlights a relevant role of fire in thereplacement of palms by open vegetation and with the expansionof V. litoralis. This indirectly supports the role of humans throughburning in the vegetation changes of the last millennia. Indeed,vegetation clearance, which has been associated with high in-creases in charcoal influx and grasses, sedges and ferns at theexpense of trees, has been commonly attributed to human activ-ities on Easter Island and on many other Pacific islands (e.g. Kirch,1986; McGlone and Wilmshurst, 1999; Anderson, 2002; Kennettet al., 2006). In this regard, the initial spread of the V. litoralis(and Poaceae too), which occurred c. 450 BC, was probably theresult of a low environmental impact caused by a small humanpopulation during the initial settlement. Further V. litoralis andgrass expansions would have been favoured by the continuedpopulation growth and intensification of agriculture. Numerousstudies have reported intensive agricultural practices on EasterIsland c. AD 1200e1600 (e.g. Stevenson, 1997; Stevenson et al.,2002; Mieth and Bork, 2005). Recently, Horrocks et al. (2012)have suggested that the Lake Raraku crater was also intensivelymulti-cropped. Our results support the interpretation of theintensification of human impact c. AD 1200. They confirm a highlymodified landscape surrounding Lake Raraku as evidenced by thestriking abundance of V. litoralis and herbs, the marked decreasein palms and the increase in charcoal influx. Moreover, the pres-ence of diatoms of the genus Fragilaria s.l. indicates enhancedrunoff and cation input to lake waters, also suggesting increasedcatchment erosion at that time.

Finally, the pattern of ecological transformation is continuousand irreversible, which is consistent with the action of a persistentforcing mechanism hampering an eventual vegetation recovery.Overall, these changes strongly support the role of humans inlandscape transformation since c. 450 BC.

5.4. Early human settlement and potential origins

Our results show that vegetation change starts c. 450 BC (790BCe90 BC) according to the age-depth model inferred, which isabout 1500 years before the date generally accepted for palmdecline as well as for the arrival of Polynesian settlers. The firstarchaeological works on Easter Island suggest that the initial hu-man settlement took place around AD 400 (Heyerdahl and Ferdon,1961). Further palynological studies have proposed that humanarrival and initial deforestation occurred c. AD 750e1200 (Flenleyand King, 1984; Flenley et al., 1991; Flenley, 1993), and that islanddeforestation was complete by AD 1500 (Flenley, 1993). However,an earlier date of c. AD 300 for colonisation has been subsequentlyput forward (Flenley, 1996). Increasing archaeological work and thereanalysis of previous radiocarbon dates led to Martinsson-Wallinand Crockford (2002) to propose the interval AD 800e1200 as themost reliable date for the first human settlements, which iscurrently the most commonly accepted date (Vargas et al., 2006).Nevertheless, earlier and later dates for these events have also beenproposed since then. Hunt and Lipo (2006) proposed that thecolonization occurred around AD 1200, a date supported bydeforestation records according to Mann et al. (2008). Recently,even later dates, around AD 1200e1290 have been suggested(Wilmshurst et al., 2011). The earliest date proposed to date hasbeen AD 50e100 (1900 cal yr BP) (Butler and Flenley, 2010). Theseauthors speculated about human presence at this date on the basisof a charcoal peak and a short palm pollen decline at Lake Kau. Theevidence proved to be inconclusive because the palm pollenshowed a rapid recovery and because it was not possible to

ascertain whether the fires were of human or natural origin (Butlerand Flenley, 2010).

These different chronologies of settlement on Easter Island arealso significant in terms of the debate on the timing of the eastwardspread of Polynesians across the Pacific. A definitive consensusremains to be achieved on many islands including Easter Island.Divergent settlement chronologies that vary by more than 1000years coexist (Kirch and Kahn, 2007; Wilmshurst et al., 2008). Thisprecludes the knowledge of the regional pattern of settlement(Wilmshurst et al., 2011). Most researchers accept the existence of a“long pause” of several centuries to a millennium between theinitial Lapita settlement on TongaeSamoa (occurred during the firstor second millennia BC) and the subsequent expansion of thePolynesian population to the east (Kirch and Kahn, 2007;Wilmshurst et al., 2011) (Fig. 1). The most accepted view is that theinitial colonisation of central Polynesia (including Marquesas andSociety Islands) followed by the settlements in remote islandsoccurred c. AD 300e900 (Kirch and Kahn, 2007; Wilmshurst et al.,2011), although a recent work suggests later dates (AD 1025e1120)(Wilmshurst et al., 2011). Our results, which suggest early humanpresence, would give further support to an early age of eastwardspread of the Polynesians without a pause or with only a short timelag between the colonisation of West and East Polynesia.

Another classic controversy concerns the American or Polyne-sian origin of the first Easter Island settlers. Several lines of evi-dence (cultural, archaeological and genetic) suggest that themodern islanders are descended from Polynesians (Flenley et al.,1991; Hagelberg et al., 1994; van Tilburg, 1994; Flenley and Bahn,2003). They probably came from some French Polynesian archi-pelago situated >2000 km to the west and northwest of EasterIsland (Flenley and Bahn, 2003) (Fig. 8). Nevertheless, the Americanorigin of V. litoralis and its continued presence during the lastmillennia revives the idea of a possible role of American connectionin the history of Easter Island. Heyerdahl and Ferdon (1961) werethe first to formally suggest that the island was colonised initiallyby Amerindians. Their hypothesis was founded on the possibility

Fig. 8. Possible migration routes to Easter Island. Easter Island (EI) is indicated by acircle. A) Directly from French Polynesia. B) On a return voyage of French Polynesiansfrom American. C) From America. Map modified from Storey et al. (2007).

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that travel from South America to Easter Island was favoured bydominant currents and winds, and on the similarity of somearchaeological elements. On other hand, Gill (1998) proposed thatthe settlement of Easter Islandwas the result of a two-stage processof colonisation from East Polynesia to America and then to EasterIsland. These hypotheses have been ruled out for lack of conclusiveevidence (Flenley and Bahn, 2003). However, evidence does exist insupport of a prehistoric contact between South American andPolynesian peoples, although when and where this occurred isunclear. Evidence for this contact includes the prehistoric intro-duction of South American crops such as sweet potato (Ipomoeabatatas (L.) Lamarck) and the bottle gourd (Lagenaria siceraria(Molina) Standley) to Polynesia (Green, 2005; Clarke et al., 2006),and the pre-Columbian introduction of Polynesian chickens to Chile(Storey et al., 2007). The most accepted view is that Polynesianswould have reached the American continent and later come back toPolynesia. However, some archaeological studies have emphasisedthe capacity of Amerindian societies for long distance oceanictravelling (Montenegro et al., 2008) and potential long voyages ofAmerindians have been also proposed (Dumont et al., 1998;Montenegro et al., 2008).

The Polynesian origin of the modern Easter Islanders is notincompatible with a short-lived American influence. In this regard,recent studies in molecular genetics have shown a prehistoricAmerindian contribution to the human gene pool of Easter Is-landers although after Polynesian settlement (Thorsby, 2012).Possible hypotheses to be tested by future studies in the light of ourresults include: 1) the eventual presence on the island of a smallAmerindian population that disappeared after the Polynesianarrival or was assimilated into the subsequent settlers, and 2) thepossible arrival of a Polynesian group from America (Fig. 8). Alter-natively, as stated before, the V. litoralis could have reached theisland through natural vectors, as is the case of other plants ofAmerican origin (e.g. S. californicus) (Zizka, 1991). Our study showsthat, despite the absence of conclusive evidence, further research iswarranted to support or challenge these possibilities.

6. Conclusions and final remarks

This multiproxy study of a Lake Raraku core (Easter Island)shows that climate and, especially, human activities have broughtabout significant environmental and vegetation changes during thelast millennia. From c. 1790 to c. 450 BC, a mire covered the bottomof the Raraku crater, which was surrounded by awooded landscapedominated by palms. The first landscape opening occurred c. 450BC, as evidenced by the palm decline and the increase in herbs(mainly Poaceae and the weed V. litoralis). This change coincidedwith a slight charcoal increase, and may indicate human arrival inthe island. The presence of a sedimentary gap (c. AD 500 to c. AD1165) would indicate drought phases that could be related to somedry phases of the Medieval Climatic Anomaly. The bottom of theRaraku crater was flooded again c. AD 1200, which resulted in ashallow lake. This suggests a higher precipitation/evaporation ratiothat could match the AD 1300 event, a pan-Pacific phase of rapidcooling and wetter conditions. At c. AD 1200, a second step ofvegetation clearance took place, and herbs became progressivelydominant, coinciding with further increases in fire incidence,which suggests an intensification of human impact. From c. AD1475 to c. AD 1875, the landscape was probably almost entirelydominated by herb communities, with abundant V. litoralis, and afew scattered palms. The highest frequency of burning is observedduring this period. A short hiatus (c. AD 1570ec. AD 1720) inter-rupted sedimentation, indicating a new drought phase. This couldbe related to drought events during the Little Ice Age. Since c. AD1875, grass communities have proliferated and several

allochthonous plants have been introduced and have spread at theexpense of native ones.

Shorter sedimentary gaps in our study than those obtained inearlier works provide a more comprehensive record of ecologicaland sedimentary changes. Our results show that the replacement ofthe palm-dominated vegetation by herb-dominant communitieswas a long, gradual and stepped process. This is in sharp contrast tothe commonly accepted scenario of a catastrophic vegetationchange, resulting in sudden and rapid deforestation. According toour data, the vegetation change started c. 450 yr BC, some 1500 yrearlier than the accepted date. On the other hand, the expansion ofthe weed V. litoralis, which is linked to the palm demise, suggests arelationship between human activities and early landscape modi-fication. This hypothesis is also supported by notable increases incharcoal. Moreover, the significant correlation of herbs with char-coal values suggests that burning played a prominent role inshaping the landscape. Further studies on sedimentary cores fromother sites of the island are warranted to ascertain whether theobserved changes are local or more widespread on the Island. Ourdata suggest that human presence on Easter Island may be signif-icantly older than commonly accepted, which would have far-reaching implications for the Pacific colonisation chronologies.The American origin of V. litoralis precludes the rejection of thepossibility of an early American influence on Easter Island history.In light of the results obtained, several hypotheses to be tested withfuture studies are erected.

Acknowledgements

This work was supported by funding from the Spanish Ministryof Science and Education through the projects LAVOLTER(CGL2004-00683/BTE), GEOBILA (CGL2007-60932/BTE) and CON-SOLIDER GRACCIE (CSD2007-00067) and through an undergradu-ate grant (BES-2008-002938 to N. Cañellas-Boltà). We gratefullyacknowledge CONAF (Chile) and the Riroroko family for the facil-ities provided on Easter Island. We also thank Josep Vigo for theidentification of Verbena litoralis and Arturo Morales for his assis-tance in the identification of the small Pseudostaurosira species.Grateful thanks are also due to Raymond Bradley, José Carrión,William Gosling and Peter Kershaw for their critical revision of anearlier version of the manuscript and to Vera Markgraf for hersuggestions and revision of the final version.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.quascirev.2013.04.004.

Uncited references Q3

Christen and Pérez, 2009; Hua and Barbetti, 2004; McCormacet al., 2004.

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Please cite this article in press as: Cañellas-Boltà, N., et al., Vegetation changes and human settlement of Easter Island during the lastmillennia: amultiproxy study of the Lake Raraku sediments, Quaternary Science Reviews (2013), http://dx.doi.org/10.1016/j.quascirev.2013.04.004