Microfossils of Polynesian cultigens in lake sediment ... - Springer Link

J Paleolimnol (2012) 47:185–204 DOI 10.1007/s10933-011-9570-5

ORIGINAL PAPER

Microfossils of Polynesian cultigens in lake sediment cores from Rano Kau, Easter Island M. Horrocks • W. T. Baisden • M. K. Nieuwoudt J. Flenley • D. Feek • L. Gonza´lez Nualart • S. Haoa-Cardinali • T. Edmunds Gorman

•

Received: 18 February 2011 / Accepted: 22 November 2011 / Published online: 6 December 2011 Ó Springer Science+Business Media B.V. 2011

Abstract Previous wetland vegetation records from Easter Island showing deforestation and Polynesian agriculture are limited to cores that rely on pollen, with a single cultigen pollen type identified: Urticaceae/ Moraceae, possibly Broussonetia payrifera (paper mulberry). Here we redress this by also using phytolith and starch analyses on four lake sediment cores on a *350-m transect along the southwest edge of Rano Kau, focusing on in-washed basal clayey layers. We also use a new method, Fourier transform infrared spectroscopy, to positively identify degraded starch collected from sedimentary deposits. The cores are the first samples recovered from an area in the lake that (a) lies below the relict village of Orongo, (b) is near a

section of the crater believed to be most accessible from the Pacific coast, and (c) is far from the northern crater rim and receives high solar radiation, a likely benefit for crops of tropical origin. Pollen and phytoliths are abundant in the clayey layers and sparse in overlying layers of organic lake detritus and living rhizomes. Mixing of core deposits as a result of human activity has disordered the radiocarbon sequence, precluding development of an reliable chronology. Containing microfossils of several introduced cultigens, the clayey layers represent gardened terraces that have slumped into the lake. The data indicate largescale deforestation and a mixed-crop production system including Broussonetia papyrifera, Colocasia esculenta (taro), Dioscorea alata (greater yam),

M. Horrocks (&) Microfossil Research Ltd, 31 Mont Le Grand Rd., Mt Eden, Auckland 1024, New Zealand e-mail: [email protected]

J. Flenley D. Feek Institute of Natural Resources, Massey University, Private Bag 11222, Palmerston North 4442, New Zealand e-mail: [email protected]

M. Horrocks School of Environment, University of Auckland, Auckland, New Zealand W. T. Baisden National Isotope Centre, GNS Science, PO Box 31312, Lower Hutt 5040, New Zealand e-mail: [email protected] M. K. Nieuwoudt Department of Chemistry, University of Auckland, Auckland 1142, New Zealand e-mail: [email protected]

D. Feek e-mail: [email protected] L. Gonza´lez Nualart S. Haoa-Cardinali T. Edmunds Gorman Hanga Roa, Easter Island, Chile e-mail: [email protected] S. Haoa-Cardinali e-mail: [email protected] T. Edmunds Gorman e-mail: [email protected]

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Ipomoea batatas (sweet potato), Lagenaria siceraria (bottle gourd) and Musa (banana) sp. The data show (a) the potential for using the combined analyses to provide direct evidence of Polynesian horticulture on Easter Island and (b) that the island’s wetlands potentially hold extensive horticultural records. The study highlights the concept of ‘transported landscapes,’ whereby colonising people replace indigenous forests with artificial, imported agricultural landscapes.

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During Polynesian settlement of the Pacific, agricultural practices combining Asian-Pacific and American crops were adapted to a diversity of island environments (Kirch 2000). This relied on non-cereal starch staples produced through a variety of ‘wet’ and ‘dry’ field-cropping methods and on tree cropping. Sedimentological and archaeological studies have provided evidence relating to early Polynesian deforestation and agriculture on Easter Island (Rapa Nui). Pollen analyses of wetland cores from Rano

Aroi, Rano Kau and Rano Raraku (Fig. 1) have shown that the island was formerly forested (Flenley et al. 1991; Peteet et al. 2003; Mann et al. 2008), with the main trees including Sophora toromiro, Triumfetta semitriloba and an Arecaceae (palm) species, identified as Paschalococos disperta by Dransfield et al. (1984) and as a close relative of the endemic Chilean Jubaea chilensis by Flenley et al. (1991). A major decline in trees and shrubs, including extinction of the Arecaceae and other taxa, coincident with an increase in Poaceae (grasses) and ferns in the cores, indicated deforestation by people. Archaeological studies of early Rapa Nui agriculture have shown abundant evidence for the spread of intensive, rainfall-supported horticulture. This was achieved by the selective use of swales, providing deeper, damper, more fertile soil, and the construction of features such as rock-walled gardens, stone circledefined planting pits and lithic mulch to provide wind protection, moisture and heat retention and wind erosion protection (Stevenson and Haoa 1998; Stevenson 1999; Wozniak 1999; Stevenson et al. 2005, 2006; Louwagie et al. 2006; Baer et al. 2008). As well as wetland proxy and archaeological evidence of past horticulture on Rapa Nui, direct evidence in the form of actual remains of introduced cultigens has also been found, almost entirely at dryland sites. Orliac and Orliac (1998) and Orliac (2000) identified tuberous roots of Ipomoea batatas and charcoal of Broussonetia papyrifera and Syzygium

Fig. 1 a Rano Kau, showing core site locations (inset: Rapa Nui). Previous cores: Flenley et al. (1991), Butler et al. (2004), Gossen (2007), Butler and Flenley (2010). b West side of Rano Kau 2009, looking north. The cores were taken 13–24 m from

the lake edge within an area encompassing both sides of large water body, centre. For approx. scale, the lake is *1.25 km wide and the narrow, dark green belt of Scirpus fringing the lake is up to 2? m high

Keywords Pollen Phytoliths Starch Fourier transform infrared spectroscopy Agriculture Easter Island

Introduction

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cf. S. malaccense (Malay apple). Cummings (1998) identified pollen of Cordyline fruticosa, Ipomoea batatas and Moraceae (possibly B. papyrifera) and a probable Musa phytolith. A subsequent study identified pollen of Lagenaria siceraria and starch grains of Colocasia esculenta, Dioscorea alata and I. batatas (Horrocks and Wozniak 2008). At wetland sites, however, possible direct evidence is limited to ‘Urticaceae/Moraceae’ pollen, which shows peaks in surficial sediments from Rano Kau and Rano Raraku (Flenley et al. 1991). Pollen of these two families is difficult to differentiate. Four new sediment cores recently taken from the crater lake of Rano Kau present an opportunity to shed further light on the history of deforestation and agriculture on Rapa Nui. We were particularly interested in horticultural activity in the crater and its effects on the indigenous vegetation. We considered that clayey layers eroded from the inner crater slopes, identified in previous sediment core studies from this site (Flenley et al. 1991; Gossen 2007), could contain remains of cultivated plants. The cores were taken from along the near shoreline to enhance the likelihood of this. The aims of the study were to: a) identify human impacts on the local environment and provide direct evidence of introduced crops using combined microfossil analyses of pollen, phytoliths and starch and b) apply a new method (Fourier transform infrared spectroscopy) of positively identifying degraded starch, often uncertain due to loss of distinguishing features, collected from sedimentary deposits. The study area and sampling site Rapa Nui is situated in an isolated position in southeast Polynesia (27°090 S, 109°260 W). The island is basaltic and triangular in shape, with an inactive volcano at each corner (Maunga Terevaka, Poike and Rano Kau) (Fig. 1). The climate of the island is subtropical, with a mean annual temperature of *22°C and rainfall of *1,100 mm (Heyerdahl 1961). The latter however varies considerably (500–2,000 mm), with decadelong periods of droughty or wet weather. The timing of Polynesian settlement of Rapa Nui is uncertain. Based mainly on radiocarbon evidence from archaeological excavations, many researchers agree on a little before 1,100 BP (Steadman 1995; Green 2000; Martinsson-Wallin and Crockford 2001).

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This has recently been challenged, however. Based on an archaeological excavation and reassessment of previous radiocarbon dates from the island, Hunt and Lipo (2006) suggested a late colonisation of 800 BP. Wetland sedimentary chronologies show unconformities, inversions, long age plateaux, hiatuses and older than expected dates (Flenley et al. 1991; Peteet et al. 2003; Butler et al. 2004; Mann et al. 2008; Butler and Flenley 2010). Our coring site is the circular crater lake (110 m altitude) of Rano Kau (324 m altitude) (Fig. 1). The site has been described previously (Ferdon 1961a; Flenley et al. 1991). The crater is *2 km wide and forms the southwest corner of Rapa Nui. The inner walls are steep,[30°. The lake is *1.25 km wide and deposits are deep, [20 m. The lake surface is mostly covered by *1–3-m-deep floating mats of vegetation, with pockets of open water. The water gaps between the floating mats and the underlying sediment surface are up to 8 m. The upper third of the interior walls of the crater are typically basalt cliffs, with the lower two-thirds comprising loose boulder talus. The latter comes to the lake edge, with no evidence of beach formation. There is also no evidence of natural terracing due to water level fluctuations, or seasonal range in the water level. Rock types at Rano Kau, supplying clastic sediments to the basin, comprise benmoreite lava flows and tuff/agglomerate containing obsidian fragments (Gonzalez-Ferran and Baker 1974). Soils of Rapa Nui are typically fine sandy to silty clay loams, high in organic matter and neutral to moderately acidic (Benedetti 1991). Several sediment core studies from Rano Kau have previously been carried out. Pollen cores collected 27 m and *300 m from the north-northwest edge of the lake showed initial dominance of Arecaceae, Sophora and Triumfetta forest, then decline of this vegetation as a result of human activity (Fig. 1) (Flenley et al. 1991; Butler and Flenley 2010). Gossen (2007) carried out dating on another core, taken *400 m from the northwest edge. Ferdon (1961b, c) and McCoy (1976) recorded a heavy concentration of archaeological sites in and around Rano Kau. The commonest feature within the crater was terracing, for gardens and dwellings. The relict village of Orongo, on the western crater rim, comprises *50 stone houses (Fig. 1). A recent study of radiocarbon age determinations of charcoal in soils

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around Orongo showed age ranges up to c. 730 cal BP (Mann et al. 2008). The soils on the upper slopes of Rano Kau and those comprising slips support mostly Poaceae, especially Melinis minutiflora. Stunted, shrub-sized Broussonetia papyrifera trees are common growing in the loose boulder talus. Moistened soil around the lake edge supports a narrow broken line of vegetation including mainly exotic trees. Vegetation on the southwest part of the floating mat surfaces, our immediate sampling area, is limited to two Cyperaceae species: Cyperus polystachyus and Scirpus californicus; three ferns: Asplenium sp., Thelypteris gongyloides and Vittaria elongate; and Polygonum acuminatum, a dicotyledonous herb. The latter is largely confined to the edge. Occasional very small patches of an unidentified Poaceae species were also noted. Scirpus is taller and denser closer to the lake edge, presumably reflecting availability of nutrient run-off. Our immediate coring site in the crater is in the southwest sector of the lake, 600? m from the aforementioned previous coring sites in the northwest part (Fig. 1). The cores are the first samples recovered from an area in the lake that (a) lies below the relict village of Orongo, (b) is near a section of the crater believed to be most accessible from the Pacific coast, and (c) is far from the northern crater rim and receives high solar radiation, a likely benefit for crops of tropical origin. We therefore investigated cores likely to provide improved evidence of agricultural development on Rapa Nui.

Materials and methods In this work, we report four sediment cores (2–5) taken through floating mat vegetation from Rano Kau, using D-section and Livingstone corers (Fig. 1). The latter was used for some of the softer sediments. The cores were taken along a *350-m transect. Respective UTM easting/northing GPS coordinates are 654770 6991443, 654647 6991517, 654500 6991672, 654528 6991641. Distances of cores from the lake edge and between each other are shown in Fig. 2. For cores 2, 3 and 5, the full depth to basal rock was cored. One of our D-section corer heads was lost in core hole 4, however the similarity of core 4’s recovered basal deposit with those of the others almost certainly

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indicates near complete recovery to basal rock for this core also. Magnetic susceptibility was recorded to identify mineral layers using cross-calibrated Geoinstruments JH-8 and Bartington MS2E1 devices. Values are plotted in SI units on log10 scale, after adding 1.5 (approximating the detection limit) to avoid plotting the logarithm of near-zero values. The cores were analysed for pollen, phytoliths and starch. The first core analysed was core 2, which showed abundant pollen in the clayey layer and very little in the overlying layers, with all except one sample in the latter having insufficient pollen for meaningful analysis. For these samples, from 0 to 0.9, 1.7, 1.9 and 2.8 to 3.4 m depths, relative amounts (albeit small) of pollen types not included in the pollen sum are shown in the pollen diagram, while types comprising the pollen sum, usually only one or two instances per type per sample, are shown as present/ absent. Similarly, phytoliths were found in only three samples above the clayey layer in core 2, at 1.6, 1.7 and 3.3 m depths. For these reasons, analysis of cores 3 and 4 was confined to depths encompassing the basal clayey layers and some of the immediately overlying deposits. All the recovered deposits of core 5, comprising the clayey basal layer, were analysed; recovery of its upper deposits failed. Gaps in the microfossil diagrams thus reflect samples with insufficient microfossils for analysis and also water gaps in the cores. Regarding the starch analysis, it became apparent during analysis of the first batch of samples from the detritus layer in core 2 that, unlike the clayey layers, starch separations of the detritus deposits yielded very high concentrations of swamp plant material and apparently no starch material. For this reason, the remaining starch preparations for all of the cores were confined to the clayey layers and a few samples from the non-clayey deposits immediately above. Samples were prepared for pollen analysis by the standard acetylation method (Moore et al. 1991). The pollen sum for most samples was at least 200, excluding ‘Wetland taxa’ and ‘Ferns and others.’ Tablets containing a known quantity of exotic Lycopodium spores were added to pollen samples to allow concentrations of charcoal fragments to be calculated. As a 130-lm sieve was used for pollen preparation, fragments over this size were excluded. Samples were prepared for phytolith analysis by density separation

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Fig. 2 Lithostratigraphy and dating (cal BP, 2r) of core profiles from Rano Kau. See Table 1 for age determination details and Fig. 3 for further lithographic detail

(Horrocks 2005; Piperno 2006). The phytolith sum was at least 200, excluding Cyperaceae and other biosilicate types, in this case rare diatom tissue. As well as starch grains, starch analysis includes other plant material such as calcium oxalate crystals (Torrence and Barton 2006). Starch and other remains were prepared for analysis by density separation (Horrocks 2005). Some previous fossil studies have reported uncertainty in identifying degraded, putative starch material lacking clear diagnostic characteristics, for example the Maltese cross, which is visible in cross-polarised light in fresh and well preserved fossil starch grains (Horrocks and Wozniak 2008). In this study, Fourier Transform Infrared (FTIR) microscopy was used to positively identify such material as starch. The FTIR spectra of all starches display characteristic bands between 1,300 and 900 cm-1 due to C–O and C–C vibrational modes of the amylase and amylopectin polymers, which form the two main components of starch. The bands are broad and consist of many

overlapping components due to coupling of these modes, with the result that assignment of the individual components has been difficult to establish. However, the shape and intensities of these bands are sensitive to the polymer conformation, making FTIR a useful technique to distinguish between different types of starches (Higgins et al. 1961) and to examine structural changes in starch (Goodfellow and Wilson 1990). The FTIR spectrum of a degraded putative starch grain of Ipomoea batatas from core 4 (at 4.7 m depth) was recorded using the Continuum FTIR microscope attachment of the Thermo Electron Nicolet 8700 FTIR spectrometer with Atlus spectroscopic software. The spectrum was recorded in transmission mode with a 159 Reflachromat objective using a 50 9 50 lm aperture and at 4 cm-1 resolution. The spectrum of the grain was compared with a spectrum of reference I. batatas starch powder and with that of a clast (0.7 cm dm) from 18.0 m depth in a clayey layer in a core taken *50 m from the southwest edge of Rano

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Kau lake, not included in this study. The latter comparison was necessary to distinguish between the starch grain and similar size granules of local basalt silicates, which also present a strong, broad band due to Si–O stretch modes at around the same frequency as the strongest band for starch centered at *1,000 cm-1. The spectra of the reference I. batatas starch and clast were recorded in ATR mode using a diamond crystal of the Smart Orbit accessory of the Thermo Electron Nicolet 8700 FTIR spectrometer and OMNIC spectroscopic software. We also tried Raman microscopic analysis of putative starch grains, using the Renishaw System 1000 Raman Microprobe with the 785 nm excitation line of a solid state diode. However, the spectra did not yield useful information because they were masked by fluorescence. Radiocarbon age determinations were carried out by Rafter Radiocarbon Laboratory, Lower Hutt using the EN-Tandem AMS and National Electrostatics Corporation Extended Compact AMS (Table 1). With a few noted exceptions, we used tiny (B1.5 mm) fruits/seeds for dating, many of which were degraded/ fragmented. All calibrated ages referred to throughout are 2r.

Results Lithostratigraphy, magnetic susceptibility and radiocarbon The four Rano Kau cores all have observed clayey basal layers confirmed by magnetic susceptibility (Figs. 2, 3). Core 2’s basal layer, from 5.8 to 4.4 m depth, comprises clay and detritus. The latter, throughout this paper, refers to organic material originating from wetland/lake vegetation. The gap in this layer, immediately below 5 m depth, was formed while the core was being transported. Several thin organic bands are within the clayey layer, between 4.8 and 4.6 m depths. Water gaps are above this to 0.9 m depth, with two layers of coarse detritus at 3.4–2.8 m and 1.9–1.7 m depths. Living rhizomes comprise the surface layer. Core 3’s clayey layer, from 5.2 to 4.9 m depth, also contains gravel and minor detritus (Figs. 2, 3; Table 1). Two layers of coarse detritus, separated by a water gap, overlie this at depths of 4.9–4.45 m and 3.75–3.1 m. There is also clay and gravel, with minor detritus, at the base of core 4 from 12.0 to 11.5 m depth (Figs. 2, 3).

Table 1 AMS radiocarbon data from Rano Kau (McCormac et al. 2004; Reimer et al. 2004) Core 2

Depth (m) 1.7

Material (*fruits/seeds)

NZA-

14

Cal BP (2r)

r13C (%)

2 Scirpus, 7 undif*

37242

-646 ± 30 (mod)

N/A

-25.6 -25.8

C BP

2

2.8

1 Scirpus, 11 undif*

37243

389 ± 30

490–323

2

3.4

I Scirpus, 11 undif*

37214

188 ± 25

282–0

-27.5

2

4.3


37248

377 ± 30

485–319

-26.8

2

5.0

25 undif*

37247

1,025 ± 30

943–799

-28.1

2 2

5.3 5.8

2 Scirpus, 25 undif* 1 Scirpus*

37369 32806

1,971 ± 40 1,067 ± 50

1,965–1,729 1,051–799

-25.9 -25.4

3

3.6

1 Scirpus, 3 undif*, 1 undif. shoot

37251

572 ± 30

622–506

-28.0

3

4.8

9 Scirpus, 9 undif*, 1 undif. bract

37249

414 ± 30

500–325

-26.3

3

5.1

4

11.1

1 Scirpus*

32813

416 ± 35

3 Scirpus, 2 undif*, 2 undif. shoot

37244

1,685 ± 30

502–324

-27.0

1,605–1,414

-28.6

4

11.3


37246

1,971 ± 30

1,945–1,739

-28.1

4

11.5

1 Scirpus, 29 undif*, 1 undif. bract

37245

1,684 ± 30

1,605–1,413

-28.5 -24.2

4

11.8

54 undif*

37370

1,994 ± 40

1,992–1,741

4

12.0

3 Scirpus, 5 undif*

33513

3,177 ± 90

3,629–3,164

-27.4

511–316

-27.7

5,984–5,713

-25.7

5

5.95

1 undif. Cyperaceae shoot pith

33973

386 ± 40

5

7.25


37372

5,155 ± 50

5

8.55

1 Scirpus shoot

32815

-297 ± 35 (mod)

N/A

-26.7

5

8.55

I undif. Cyperaceae, 71 undif*

37371

2,579 ± 45

2,748–2,365

-26.9

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Fig. 3 Pollen percentage diagrams of cores 2–5, Rano Kau. ? = rare

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A layer of clay, detritus and humus overlies this to 11.25 m depth. Above this to 10.5 m depth is mediumtextured detritus. The core 5 profile comprises a layer of clay and detritus from 8.85 to 5.95 m depth (Figs. 2, 3). The lower portion of this, up to 7.65 m depth, is firmer and more clayey. The upper portion has abundant humus. Radiocarbon age determinations are shown in Table 1. Many of the dates are much older than expected. For example, the basal deposits of cores 4 and 5 contain microfossil evidence of human activity, but are dated to 3,629–3,164 cal BP and 2,748– 2,365 cal BP, respectively, obviously well before settlement of East Polynesia (Kirch et al. 2010). This effect and accompanying inversions show lack of a coherent temporal sequence within or among cores, hindering any sequential interpretation of the pollen/ phytolith/starch diagrams in terms of time (Fig. 2). We therefore put the interpretation into a very general temporal framework, and focus on the novel microfossil applications. Pollen The pollen sum of the basal clay and detritus layer of core 2 is dominated by Sophora, Triumfetta and especially Arecaceae pollen (Fig. 3). The latter however is largely replaced by Poaceae in the upper part of this layer. Also present are small amounts of spores of the Anthocerotaceae, a family of hornworts, which are low-stature, inconspicuous plants. To our knowledge, spores of the Anthocerotophyta, the hornwort division, have not previously been reported in the Rapa Nui pollen record. The fossil spores are 50–58 lm in diameter, with reticulate surface marking, a reticulatefoveolate triradiate face and numerous simple or occasionally twinned spines 2.5–3.5 lm high on ridges of the spherical face (Campbell 1982). Fern spores peak sharply in the upper part of the clay/ detritus. Also, Ipomoea batatas pollen (Fig. 5a) was found in the upper portion of this layer, in the starch preparations. I. batatas may not survive the pollen treatment well, unlike the indigenous (Skottsberg 1956) I. pes-caprae, and may also degrade faster than other pollen types (Haberle and Atkin 2005). The pollen of these two Ipomoea species can be differentiated (Cummings 1998). Arecaceae, Sophora and Triumfetta pollen virtually disappears immediately above the clay/detritus, in the coarse detritus. In the

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coarse detritus sample with sufficient pollen sum grains for counting, from *1.75 m depth, Poaceae pollen dominates. The first possible European-introduced pollen type, cf. Psidium, appears at 3 m depth. More exotic types, namely Pinus, undifferentiated Myrtaceae and Plantago lanceolata, were found in the surface layer of living rhizomes. A small amount of cf. Musa pollen (Fig. 5b) was also found in this layer, reflecting the presence of this taxon at the site to the present day. Occasional relict plants are scattered around the lake edge. The apparent absence of Musa pollen in deeper deposits, where Musa phytoliths were found in numerous samples (see below), suggests that it is under-represented in pollen spectra and that its pollen is more susceptible to decay than many other pollen types. It has a very thin wall, suggesting delicacy. This and its few obvious distinguishing features and often collapsed morphology make it difficult to distinguish from detritus on slides. Fragments of charcoal are present throughout the core 2 profile and are most highly concentrated in the upper portion of the clay/detritus layer and in the overlying coarse detritus. The pollen sum of the lowermost sample of core 3, at 5 m depth in the basal clay and gravel layer, is dominated by Sophora (Fig. 3). This declines sharply and Poaceae pollen peaks broadly around the transition from clay/gravel to coarse detritus, and dominates the uppermost sample of the profile, in the coarse detritus layer at 3.6 m depth. Small amounts of Arecaceae, Urticaceae/Moraceae (Fig. 5d) and Anthocerotaceae spores were identified in the lower part of the profile. Arecaceae pollen was not found in the uppermost sample. Although no extant indigenous Rapa Nui species of the Urticaceae or Moraceae have been recorded to date, there almost certainly was at least one of these since this pollen type has been found in pre-settlement deposits at Rano Raraku, up to c. 25,000 BP (Flenley et al. 1991). This is presumably one of the several Rapa Nui plant taxa now extinct as a result of human activity. Identification of Polynesianintroduced Broussonetia papyrifera, of the Moraceae, is therefore complicated in the Rapa Nui pollen record. However, larger amounts of this pollen type occur in post-settlement deposits (Flenley et al. 1991), suggesting B. papyrifera cultivation. Charcoal is present in all pollen samples from core 3, with highest concentrations in the clay/gravel and lowermost sample of the overlying coarse detritus.

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Fig. 4 Phytolith percentage and starch diagrams of cores 2–5, Rano Kau. Filled circle = present

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The pollen sum of the basal clay and gravel layer of core 4 is dominated by Arecaceae (Fig. 3). Sophora and Triumfetta pollen increases in the upper, more organic clay/detritus/humus part of the profile, at the expense of Arecaceae pollen. Small amounts of Anthocerotaceae spores were found throughout most of the profile. Cf. Lagenaria siceraria pollen (Fig. 5c) was found in the clay/gravel at 11.8 m depth, as well as Urticaceae/Moraceae pollen. Charcoal fragments are present throughout the core 4 profile, with the highest concentration in the uppermost sample, at 11.2 m depth in the medium-textured detritus. In core 5, pollen was not preserved in the lower half, between 8.85 and 7.25 m depth, nor between 7.0 and 6.6 m depth. Arecaceae, Sophora and Triumfetta dominate the pollen sum throughout the pollen profile, with Poaceae pollen increasing after 6.5 m depth (Fig. 3). Urticaceae/Moraceae pollen is present in small amounts throughout. Pollen of cf. Psidium was found at 6.25 m depth. Small amounts of Anthocerotaceae spores were found in several samples encompassing the full depth of the profile. Charcoal was present throughout the core 5 pollen profile, with highest concentrations in the uppermost four samples, above 6.5 m depth.

Phytoliths Most of the phytolith assemblages from the basal clay and detritus layer of core 2 are overwhelmingly dominated by Arecaceae, which declines near the top of this layer (Fig. 4). Musa phytoliths (Fig. 5e) were found in the upper four samples of the clay/detritus. Musa phytoliths are diagnostic (Mindzie et al. 2001) and therefore unequivocal. A small amount of phytoliths consistent with Broussonetia papyrifera, threadlike type and hooked type (Fig. 5f), were also found in this part of the layer, at 4.7 m depth. Phytoliths of the latter are thus useful in dealing with the difficulty of differentiating Broussonetia pollen from that of other Moraceae species and Urticaceae species. Scirpus phytoliths also appear around this depth. Possible European-introduced Melinis minutiflora hair phytoliths appear in a large amount in the overlying coarse detritus layer at 3.3 m depth and are also present in the uppermost two samples, above 2 m depth, the three samples in which phytoliths are in sufficient quantity

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J Paleolimnol (2012) 47:185–204 Fig. 5 Microfossils of Polynesian-introduced cultigens from c Rano Kau. a–d, l, n stained with basic fuchsin; e, f mounted in Caedex, remainder in glycerol jelly; transmitted or crosspolarised light (CPL), the latter with black background; 1009, 4009 or 6009; scale bars: 20 lm. a Ipomoea batatas pollen grain, unacetolysed (cf. Fig. 6a). b cf. Musa sp. pollen grain, acetolysed (cf. Fig. 6b). c degraded cf. Lagenaria siceraria pollen grain, acetolysed (cf. Fig. 6c). d Urticaceae/Moraceae pollen grain, acetolysed (cf. Fig. 6d). e Musa sp. leaf phytoliths (cf. Fig. 6e). f cf. Broussonetia papyrifera leaf hair phytoliths. Long example is *250 lm in length (cf. Fig. 6f). g–i Large, elongated ovate starch grains, with growth rings mostly visible, some also shown in CPL, with eccentric Maltese cross most visible in larger grain, (i) (cf. starch grains of D. alata tuber, Fig. 6 g–i). j, k Faceted starch grains with central vacuole, also shown in CPL with central Maltese cross (cf. starch grains of I. batatas root, Fig. 6j, k). Degraded, putative I. batatas starch grain with expanded vacuole shown far right. l High concentration of tiny starch grains (cf. starch of Colocasia esculenta corm, Fig. 6l). m Raphides (cf. raphides of C. esculenta corm, Fig 6m). n starch mass with druse inclusion, and o the same druse in CPL (cf. starch mass with druse inclusion of C. esculenta corm, Fig. 6n, o). p, q Druse, also shown in CPL (cf. druse of C. esculenta corm, Fig. 6p, q)

for counting in this layer. Small amounts of Arecaceae pollen are present in these samples also. Arecaceae phytoliths dominate the core 3 profile (Fig. 4). Musa phytoliths were found in all five samples; those of cf. Melinis were found in three. Arecaceae almost completely dominates the phytolith profile of core 4, through the basal clay/gravel layer and overlying clay/detritus/humus layer and into the detritus layer (Fig. 4). A phytolith consistent with Broussonetia papyrifera, thread-like type, was found in the upper part of the profile at 11.2 m depth. Arecaceae overwhelmingly dominates most of the phytolith profile of core 5, except for the two samples from immediately below 7 m depth, where festucoid phytoliths record high percentages (Fig. 4). These two samples also contained large amounts of phytoliths that were so degraded they could not be identified to type. Musa phytoliths were found in several samples in the profile: one from near the base at 8.65 m depth and three from the top at 6.45–5.95 m depth. Cf. Melinis phytoliths appear near the top of the profile, above 6.5 m depth. We expected to find that other biosilicate microfossils had been extracted along with phytoliths during preparation of samples from the Rano Kau cores. Nevertheless, these other biosilicate remains, in particular diatoms, were rare.

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Starch and other plant material Starch was found in cores 2, 4 and 5, in the clayey layers. Three types of starch were identified. The following rationale for identification of the starch types to species is from previous studies of ancient

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starch (Horrocks and Wozniak 2008). It is based on the known starch taxonomy of economic plants and, except for Ipomoea batatas, absence of close relatives from the study area. Although the starch taxonomy of economic plants is well known and many species have distinctive starch grain morphology (Reichert 1913),

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Fig. 5 continued

the taxonomy of non-economic plants is not as well known. For this reason all local plants cannot unequivocally be ruled out as a possible source of the starch. The identifications to species are therefore made with caution. The first type of starch, found in cores 2, 4 and 5, comprises large, elongated ovate grains, without

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facets, consistent with the tuber of Dioscorea alata (Fig. 5g–i). These were found in small amounts, B5 per slide with a 20 9 40 mm cover slip, as individual grains or clumps of several grains. Identification relies on size and shape. We can probably rule out all other starch crops introduced to the Pacific except Dioscorea alata. Although the Dioscorea thought to have

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been introduced by Polynesians to Rapa Nui comprises only this species of the genus (Whistler 1991), the starch grains of each of the five Oceanic species can be differentiated (Loy et al. 1992). The natural distribution of Dioscorea does not include Polynesia (Whistler 1991). The second type of starch was found in the same cores as the first type. This comprises small amounts, B5 per slide with a 20 9 40 mm cover slip, of individual medium-sized grains, ovate to subtriangular, often bell-shaped, with an often fissured ‘vacuole’ (Loy et al. 1992) at the central hilum, and one domed surface and several flattened pressure facets. These features are consistent with the root of Ipomoea batatas (Fig. 5j, k). Identification is based on size, shape and features of the hilum. Of the Pacific starch crops, only Ipomoea batatas has grains with an obvious vacuole (Loy et al. 1992) (Fig. 6j, k). However, given that this feature is small and could be difficult to determine on some grains, two other starch crops with a similar grain size and shape, Alocasia macrorrhiza (elephant ear taro) and Tacca leontopetaloides (Polynesian arrowroot), cannot be ruled out. Ipomoea starch is also complicated by the presence of the indigenous I. pes-caprae, reported as a famine food by Me´traux (1940). However, Alocasia and Tacca could have been introduced after European contact (Me´traux 1940), I. pes-caprae does not have tuberous roots like I. batatas and in our case pollen of the latter has been found in the same deposits as the starch. A logical conclusion is that these starch grains found in the cultural deposits with I. batatas pollen and remains of other cultivated crops at Rano Kau are from I. batatas. In addition to the well preserved cf. Ipomoea batatas starch grains, degraded objects with similar morphology were found in much higher concentrations in many of the samples from the clayey layers. These have been found elsewhere on Rapa Nui and wider Polynesia and are thought to be I. batatas starch grains (Horrocks et al. 2007; Horrocks and Wozniak 2008). The objects show colour change to yellow/amber, expansion and distortion of the grain and vacuole, loss of the Maltese cross, pitting, cracking, fragmentation and disintegration (Fig. 5k, lower right). The FTIR spectrum of the putative I. batatas starch grain is given in Fig. 7a, along with spectra of reference I. batatas starch powder and the clast from the clay layer. All three spectra have a strong band

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centered around 1,000 cm-1. However, the relative intensity of this band in the clast is significantly greater and arises from the different Si–O–Si and Si–O stretch modes of the silicate groups. The relative intensities of the bands around 1,000 cm-1 are similar for the ancient grain and the reference I. batatas starch powder. The broad band at 3,200 cm-1 arises from the O–H stretch mode of water and the different intensities for these two spectra indicates a difference in water content between the two samples. Also common to the spectra of the ancient grain and the reference I. batatas starch powder are the C–H stretch modes of the –CH2 groups at 2,928 and 2,854 cm-1 and the bands between 1,500 and 1,200 cm-1 which are due to complex vibrations involving –CH2, C–O–H and C–C–H motions of the starch polymers (Cael et al. 1975). The complex bands between 800 and 1,800 cm-1 are of similar shape and relative intensity for the reference I. batatas starch powder and the ancient grain. The two spectra are shown expanded over this region in Fig. 7b for closer comparison. Common to both spectra are the C–H stretch modes of the -CH2 groups at 2,928 and 2,854 cm-1 and the bands between 1,500 and 1,200 cm-1 arising from complex vibrations involving –CH2, C–O–H and C– C–H motions of the starch polymers (Cael et al. 1975). The extensively overlapping bands between 900 and 1,300 cm-1 arise from contributions from C–C and C– O vibrations of the pyranose rings, from coupling between these vibrations, and from random interactions between the starch polymer chains (Higgins et al. 1961; Cael et al. 1975). The number and intensities of the band components are sensitive to the conformation of the starch polymer chains (Goodfellow and Wilson 1990); the intensities in particular are affected by the degree of lateral order in the polymer chains which is largely determined by hydrogen bonding (Higgins et al. 1961). The bands in the spectrum of the ancient grain are broader and more ill defined than those of the reference starch powder, and some differences can be observed in the relative intensities of the modes in the complex band between 1,100 and 1, 000 cm-1 which suggest degradation of the starch polymer in the ancient grain. The third type of starch, and associated material, was identified in small amounts only in the clayey layer of core 4 (Fig. 5l–q). It comprises tiny, spherical starch grains consistent with the corm of Colocasia esculenta, found densely packed in masses, B5 per

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slide with a 20 9 40 mm cover slip. Also found in this core were similarly small amounts of calcium oxalate crystals (raphides and druses), mostly fragmented, consistent with C. esculenta. Perhaps unexpectedly, this starch and associated material were found in the pollen, not the starch preparations. This is presumably because the cf. Colocasia starch masses are heavier than the 1.7 specific gravity of the sodium polytungstate solution used for the starch separation. This could be a result of calcium oxalate crystals, which have a specific gravity of 2.2, embedded in the masses or a degradation-induced change in starch specific gravity. Identification of this type of starch relies on size. On this basis all Polynesian starch crops (Whistler 1991) except C. esculenta can be ruled out. All the other crops have much larger starch grains (Loy et al. 1992; Horrocks and Wozniak 2008). Given the tiny size of C. esculenta grains and that starch extractions from sediments also contain similarly sized particles of other material, they are most easily identified in these deposits in large groups, namely masses or amyloplasts. The coincident presence of calcium oxalate crystals, not found in the other cores, strongly supports the case for C. esculenta starch grains. Although calcium oxalate crystals occur in many plant taxa, the particular raphide type (thickness) and the druses identified in this study occur in relatively extremely high concentrations in the corms and shoots of C. esculenta (Fig. 6m–q) (Sunell and Healey 1979; Loy et al. 1992). The natural distribution of the family to which Colocasia belongs, the Araceae, not to be confused with the Arecaceae, does not include eastern Polynesia (Whistler 1991).

Discussion Microfossil deposition and preservation The paucity of pollen in the detritus and rhizome layers overlying the clayey layers of Rano Kau core 2 suggests either lack of preservation as a consequence of periodic desiccation and/or growth and deposition of other plant material, sufficiently rapid to reduce the concentration of pollen (Fig. 3). Flenley et al.’s (1991) record from Rano Kau, of the last c. 1,350 years, showed a similar result. Given that phytoliths (and other types of biosilicates such as diatoms) are inorganic and therefore might

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J Paleolimnol (2012) 47:185–204 Fig. 6 Microparts of cultigens from modern reference samples. c e, f mounted in Caedex, remainder in glycerol jelly; transmitted or cross-polarised light (CPL), the latter with black background; 1009, 4009 or 6009; scale bars: 20 lm). a Ipomoea batatas pollen grain, unacetolysed. Grains are spherical, periporate, spinulose and c. 180 lm diam. b Musa 9 paradisiaca pollen grain, acetolysed. Grains are spherical, collapsed, inaperturate, psilate to faintly rugulate, *75 lm dm, with a thin exine (\0.5 lm). c Lagenaria siceraria pollen grain, acetolysed. These are spherical, tricolporate with long pores, scabrate, often collapsed/folded with pores and furrows obscured, and *75 lm dm. d Broussonetia papyrifera pollen grains, acetolysed. Grains are oblate, di- or tri-porate, scabrate and *15 lm dm. e Musa 9 paradisiaca leaf phytoliths. These consist of a rectangular/squarish base with side protuberances and a raised crater. Two of the examples shown here are viewed looking more or less into the crater and one shows crater side-on. f Broussonetia papyrifera leaf hair phytoliths. These are nonsegmented, and transparent and/or grey to black. The thread-like type is up to *450 lm long, up to *30 lm thick at the base and tapers to a point. Inset: hooked type, which is much smaller. g– i Starch grains of Dioscorea alata tuber. Grains are elongated ovate, up to *70 lm long, without facets, flattened and with an eccentric Maltese cross. j, k Starch grains of Ipomoea batatas root. Grains are spherical to sub-spherical, often bell-shaped, up to *25 lm dm, with a vacuole appearing as a small dot at the central hilum and often fissured in larger grains, and nearly all have one domed surface and up to six flattened pressure facets. l–q Starch and associated material of Colocasia esculenta corm. l amyloplast, with high concentration of tiny grains. Abundant individual grains are scattered outside. Amyloplasts are ovate, up to *185 lm dm, with thin walls,\0.5 lm thick. Grains are mostly \4 lm dm and spherical angular. m Raphides and scattered starch grains. Raphides are up to *90 lm long and 1.0–2.5 lm dm, tapering to a point at both ends. Raphide with one blunt end, top panel, is probably a fragment. n, o starch mass with druse fragment inclusion. p, q large druse. In the reference material in this study, druses up to *350 lm dm were observed

not be expected to be as adversely affected by desiccation as pollen, the similar lack of these in the overlying deposits could be a result of dissolution as well as rapid deposition of other plant material (Fig. 4). Siliciclastic deposits such as the phytolithrich clays presumably elevate silica concentrations, which work against phytolith dissolution, particularly at low pH. Basal clayey layers bearing charcoal and cultigen remains in the Rano Kau cores indicate that the entire record lies wholly within the period of human occupation, with eroded clayey soils, often gravelly, reflecting disturbance of the crater slopes by people (Figs. 2, 3, and 4). Dates in these deposits that are older than expected were probably caused by incorporation of old terrestrial carbon into the sediment. This process likely occurred when soil from terraced

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gardens was transported to the lake during landslides. We thus interpret the cultigen remains in the cores as mixed with older deposits by remobilisation of old organics and mechanical and other disturbance by people, not unexpected given the proximity to a steep occupation site (Ferdon 1961b, c; McCoy 1976)

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(Figs. 2, 3, and 4). As the clayey layers are a mixture of dryland soils and organic lake detritus, the dated material could be from both sources. A plausible explanation is that cultivated soils containing plant material, including buried fruits/seeds, and clayey material older than the time of initial cultivation were

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Fig. 6 continued

eroded downslope into the lake. Upon reaching the lake, the landslides may have also disturbed the floating vegetation mats and sediment column. All material will settle together, including lake detritus older than the time of the erosion events.

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Direct horticultural evidence from slumped deposits The identification of cultigen microfossils in the inwashed clays in the Rano Kau cores indicates a

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Fig. 7 FTIR microscope spectra from Rano Kau. a Spectra of a degraded fossil starch grain from core 2, 4.7 m depth, overlaid with spectra of a clast from a Rano Kau core not included in this study and reference Ipomoea batatas starch. Inset: fossil grain mounted in glycerol jelly, transmitted light, 6009, scale bar: 20 lm. Note central vacuole of grain, arrow, typical of I. batatas. b Spectra of degraded fossil starch grain and I. batatas starch, those in (a), expanded over the fingerprint region between 1,800 and 800 cm-1

mixed-crop production system in the crater, including Broussonetia papyrifera, Colocasia esculenta, Dioscorea alata, Ipomoea batatas, Lagenaria siceraria and Musa sp. (Figs. 3, 4). According to de Langle, who first described the crater in 1786, the moistened strip around the lake edge was filled with ‘the finest plantations of bananas and mulberry trees’ (Ferdon 1961b). These six crops, part of the group of 72 species intentionally introduced to the Pacific by early Polynesians migrating from the western Pacific, almost all originated in the broad area from Africa to Melanesia (Whistler 1991). Two of our identified species however, Ipomoea batatas and Lagenaria siceraria, first arrived in Polynesia from South America as a result of Polynesian contact (Hather and Kirch 1991; Green 2000). The

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former species originated in South America, while the latter ironically originated in Africa and arrived in the New World by uncertain means (Whistler 1991). We assume that the Rano Kau clayey layers primarily represent gardened terraces that have slumped into the lake. McCoy (1976) reported that the commonest archaeological feature within the crater was terracing. The layers could also in part represent gardens fringing the lake for increased moisture availability, possibly inundated by small but undocumented changes in lake level. Despite their steep, rocky nature, which presumably required great effort to work, the inner crater walls of Rano Kau could be expected to have been favored for gardening because of the close proximity to an irrigation source and shelter from wind (Fig. 1). The former could have been especially important given that there are no permanent streams and only two other reasonably sized wetlands on the island. Our preliminary FTIR analysis to positively identify degraded starch (in this case from Ipomoea batatas) in lake sediments is a useful advance in plant microfossil analysis. Identification of such starch is often uncertain due to loss of morphological features. In an ancient context, this analysis has been carried out on starch in parchments and mortar (Zeng et al. 2008), but to our knowledge there has been no previous application of FTIR to plant material extracted from soils or sediments. Forest clearance for horticulture The pollen evidence from the Rano Kau cores indicates that dryland vegetation around the crater at the start of our record comprised primarily Arecaceae, Sophora and Triumfetta forest (Fig. 3). The lowermost samples of all four cores show high pollen values of these taxa and coincident low values or absence of disturbance indicators, in particular Poaceae. This vegetation is consistent with previous Rano Kau pollen records (Flenley et al. 1991; Butler and Flenley 2010). In the upper part of the pollen profiles, largescale forest disturbance is clearly demonstrated. Major decline or disappearance of forest pollen, coincident with a major increase in Poaceae microfossils and charcoal, influx of Anthocerotaceae spores and continuing cultigen microfossils indicates that forest in the area was largely replaced by open grassland and gardens (Figs. 3, 4). In several European and Central

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American pollen records, Anthocerotophyta spores are associated with large-scale vegetation and soil disturbance by early people and horticultural activity (Koelbloed and Kroeze 1965; Anchukaitis and Horn 2005; Overland and Hjelle 2009). Elsewhere in Polynesia, in New Zealand, Anthocerotaceae spores only appear in the Holocene in pollen spectra associated with landscape disturbance by people (Wilmshurst et al. 1999). The Anthocerotaceae spores in our study are presumably from the single unidentified species of this family recently added to the Rapa Nui indigenous flora (Grolle 2002). During this period of major forest clearance at Rano Kau, the cores show a temporary increase in Sophora and Triumfetta pollen as Arecaceae pollen declines. This could reflect a seral response by Sophora and Triumfetta trees following forest disturbance, filling gaps faster than possibly slower-growing Arecaceae. Alternative explanations are selective logging of Arecaceae for straight logs and cultivation of Triumfetta from which rope was manufactured. Not surprisingly, wood and rope have both been suggested for, among other Rapa Nui activities, transporting the hundreds of giant stone statues for which the island is famous (Me´traux 1940). Phytolith evidence of forest clearance is largely restricted to core 2, with Arecaceae showing very high values that decline slightly in the upper two samples of the basal clayey layer and then negligible values in the few samples in which phytoliths are preserved in the overlying deposits (Fig. 4). However, vegetation trends are obscured by the clear over-representation of Arecaceae in Rapa Nui phytolith spectra and by the representation of much fewer taxa than in pollen spectra. Arecaceae, Poaceae and Cyperaceae completely dominate the phytolith record, with many other taxa, including woody plants and ferns, poorly represented. The lack of phytolith representation of two of the main forest tree taxa, Sophora and Triumfetta, results in very high Arecaceae phytolith values persisting with disturbed forest, shown in cores 3, 4 and 5, until the demise of Arecaceae, shown in core 2. Also, as phytoliths are inorganic, they could persist in soils longer than pollen, as previous Rapa Nui phytolith studies suggested (Cummings 1998; Horrocks and Wozniak 2008). Because the clay material in the cores is redeposited from upslope where Arecaceae grew, it could be expected to have large amounts of Arecaceae phytoliths.

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Sudden appearance of a Poaceae phytolith (festucoid) peak, with a decline in Arecaceae phytoliths at *7 m depth in the humus-containing clay/detritus layer in core 5, suggests a major vegetation disturbance event (Fig. 4). As the same deposits have cf. Dioscorea starch at this depth and other cultigen microfossils below and above, an alternative explanation is that it reflects the use of grass as mulch (Me´traux 1940). Grass was also burnt for soil fertilization. However, the relatively very old date at this depth, 5,984–5,713 cal BP, and the coincident large amount of degraded phytoliths and lack of Poaceae pollen (Fig. 3) suggest disturbance of a very old dryland deposit by people. This phytolith type is from the Pooideae sub-family, of which there are three indigenous Rapa Nui species listed by Skottsberg (1956): Calamagrostis retrofracta, Dichelachne sciurea and Stipa horridula. The high percentage of hair phytoliths at 3.3 m depth in core 2 suggests that they are from a hairy plant (cf. Melinis, Fig. 4). Broussonetia papyrifera has hairy leaves but its phytoliths are not the same as these. The only other hairy plant with phytoliths similar to this type that we can think of is Melinis minutiflora (Gardner 2007). This phytolith type was also found in cores 3 and 5. This grass, native to Africa and widespread in the Pacific, was introduced to Rapa Nui after European contact and grows extensively on the inner crater slopes today.

Conclusions Results from multiple lake cores taken near the shoreline of Rano Kau provide clear evidence of Polynesian horticulture. The location and mechanism of deposition of micro-remains of cultivated plants— slumping from terraces—provide direct evidence of a multi-cropping site. The combined pollen, phytolith and starch analyses show that Rapa Nui’s wetland sediments, in particular inwashed soils, potentially hold extensive horticultural records. These results highlight the concept of ‘transported landscapes,’ whereby colonising people replace indigenous forests with artificial, imported agricultural landscapes (Anderson 1952; Kirch 1984). Polynesian islands are potentially very useful in showing this because they represent a multitude of small, isolated and therefore formerly fragile pristine

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environments. In addition, relatively late colonisation of Polynesia means that much of the evidence has not been obliterated over long time spans. Acknowledgments This work was supported by the Marsden Fund of the Royal Society of New Zealand. We thank the Corporacio´n Nacional Forestal for site access, Zoro Babel for fieldwork assistance, Rhys Gardner for identifying plant specimens growing on the floating vegetation mats and Andy Tulloch and John Dando for loaning magnetic susceptibility equipment.

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