The Lateglacial and Holocene environmental history of the Ioannina basin, north-west Greece

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The Lateglacial and Holocene environmental history of the Ioannina basin, north-west Greece Ian Lawsona,*, Mick Frogleyb, Charlotte Bryantc, Richard Preeced, Polychronis Tzedakise b

a Department of Geography and Environment, University of Aberdeen, St Mary’s, Elphinstone Road, Aberdeen AB24 3UF, UK Centre for Environmental Research, School of Chemistry, Physics and Environmental Science, University of Sussex, Falmer, Brighton BN1 9QJ, UK c NERC Radiocarbon Laboratory, Scottish Enterprise Technology Park, Rankine Avenue, East Kilbride G75 0QF, UK d Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK e School of Geography, University of Leeds, Leeds LS2 9JT, UK

Received 1 May 2003; accepted 7 February 2004

Abstract This study centres on palaeoenvironmental analysis of the upper part of a long core from the partially drained, tectonic Ioannina basin, north-west Greece. The data span the Lateglacial and Holocene and comprise pollen, mollusc, magnetic susceptibility, losson-ignition and particle size analyses. These augment previously published ostracod and stable isotope data from the same sediment sequence. The age model for the sequence is based on 19 AMS dates, five of which were obtained using a novel procedure for the separation of microcharcoal from sediment. The pollen data suggest that although temperate woodland expanded at the beginning of the Lateglacial, only limited woodland contraction occurred during the Younger Dryas chronozone, despite strong evidence for a sharp drop in sea surface temperature in the neighbouring Adriatic at this time. In contrast, large-scale vegetational and sedimentological changes occur in concert during the Holocene, some of which may be ascribed to human impact. Comparison with other records from the Ioannina basin suggests that previous, more littoral pollen sequences did not constitute a wholly representative record of Lateglacial and Holocene vegetational change. r 2004 Elsevier Ltd. All rights reserved.

1. Introduction In most of Europe the broad course of climate change during the Lateglacial and early Holocene is well established (Mangerud et al., 1974). From the extreme cold and aridity of the Last Glacial Maximum (LGM), climate began to ameliorate after about 13,000 radiocarbon years before present (14C BP) during the Lateglacial (Aller^d/B^lling) Interstadial. A reversal occurred during the Lateglacial Stadial beginning around 11,000 14C BP (the Younger Dryas cold event), followed by rapid warming at the beginning of the Holocene, ca 10,000 14C BP. Since climate is the dominant control on many aspects of the natural environment (Zolitschka and Negendank, 1999), not least vegetation (Woodward, 1987), this progression is reflected throughout most of Europe by changes from fossil pollen assemblages typical of cold and/or arid *Corresponding author. E-mail address: [email protected] (I. Lawson). 0277-3791/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2004.02.003

climates during glacials and stadials, to assemblages typical of warm and/or moist climates during interglacials and interstadials. This general pattern, or a modification of it, is seen at most sites throughout north-west, northern and central Europe (e.g., Walker et al., 1994; Moscariello et al., 1998; Leroy et al., 2000; Svobodova et al., 2001), and across much of Mediterranean Europe including Iberia (e.g., Jalut et al., 1992; ! and van Geel, 1999; Garcia et al., 2002), France Carrion (e.g., Reille and Andrieu, 1995; Nicol-Pichard and Dubar, 1998) and some sites in Italy (e.g., Lowe and Watson, 1993; Ramrath et al., 2000). One region where the Lateglacial fossil record appears to depart from this pattern is Greece. Here, data from several sites point to the conclusion that there was neither a substantial expansion in the range or abundance of plants typical of warm and moist environments during the time of the Lateglacial Interstadial, nor any obvious retraction of these taxa during the time of the Lateglacial Stadial (Bottema, 1991, 1995a, b; Willis, 1994, 1997). However, this conclusion is

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based on data from a handful of sites—no more than nine existing sites encompass the Lateglacial in Greece. Of these sites, only Kopais in south-east Greece (Greig and Turner, 1974; Turner and Greig, 1975; Allen, 1986, 1990; Okuda et al., 1999, 2001; Tzedakis, 1999) has a comprehensive and uncontested radiocarbon chronology, and here the pollen record suggests that temporary re-afforestation may have occurred during the Lateglacial. The majority of the other sequences suffer from problems including possible hiatuses, low sampling resolution, and difficulty in determining the pollen source area. This study attempts to test the hypothesis that the Lateglacial environment of Greece followed a pattern that was substantially different from that of the rest of Europe, using a new palaeoenvironmental record from Ioannina. By focusing on a relatively well-understood basin, on a sequence dominated by moderately deepwater facies, and by producing a high-resolution record of several proxies supported by the largest number of AMS radiocarbon determinations yet carried out for a Greek sequence, many of the usual problems afflicting work in this region have been avoided. Palaeoenvironmental work at Ioannina began with Bottema (1974), who analysed two pollen sequences as part of the first archaeological investigation of northwest Greece by Higgs et al. (1967). This work is regularly cited in the literature and Ioannina has become an important reference point for the region. Subsequently, an extensive survey of the basin was undertaken by the Greek Institute of Geology and Mineral Exploration (IGME) in the 1980s, prospecting for lignite deposits. Tzedakis (1991, 1993, 1994) studied one of the long cores taken as part of the IGME survey, I-249, looking at pollen, magnetic susceptibility and loss-on-ignition through the last four glacial/interglacial cycles. More detailed work has since been undertaken on the palynology, malacology and sedimentology of another long core, I-284, some of which has been published previously (Frogley, 1997; Frogley et al., 1999, 2001; Galanidou et al., 2000; Tzedakis et al., 2002, 2003) and some of which is presented here for the first time. I-284 is, in many respects, a more suitable sequence from a palaeoecological point of view than the littoral sequences to which Bottema had access: the distal location reduces the risk of hiatuses in the sequence and simplifies the interpretation of the pollen data (see below), while fast sedimentation has permitted relatively high-resolution analyses. We have also taken advantage of modern AMS 14C dating to produce a robust age model. Other relevant research at Ioannina has encompassed beach and cave sediments found around the lake (e.g., Higgs and Vita-Finzi, 1966; Higgs et al., 1967; Higgs, 1978; Prentice et al., 1992; Galanidou et al., 2000), the hydrology, sedimentology and pollution of Lake Ioan-

nina (e.g., Anagnostidis and Economou-Amilli, 1980; Albanis et al., 1986; Conispoliatis et al., 1986; Kalogeropoulos et al., 1994) and the geomorphology, bedrock geology and tectonic evolution of the Ioannina basin (e.g., King and Bailey, 1985; Katsikis, 1992; King et al., 1993), making this one of the best-understood lake basins in south-east Europe.

2. Site description The Ioannina basin is situated at ca 470 m above sea level in north-west Greece (Fig. 1). The total length of the basin along its NNW–SSE axis is 35 km, while its width varies from 3 to 10 km. Except in the south, the basin is surrounded by high land. To the west the land rises gently to the Tomarochoria mountains, up to 1173 m at the summit of Megali Tsouka, while to the north-east is the much steeper flank of Mitsikeli mountain (1810 m), an NNW–SSE trending limestone ridge on the western margin of the Pindus mountain range. The basin floor is largely flat with only modest (o10 m) variations in altitude, with the exception of a small number of limestone ‘islands’. Lake Ioannina (also known as Lake Pamvotis), the remnant of the lake which once filled the basin, lies close to the side of Mitsikeli mountain towards the southern end of the basin; the modern lake is 11 km long and 5 km wide at its maximum extent. The underlying bedrock consists of Mesozoic limestones, sometimes with interbedded cherts, and flysch derived ultimately from erosion of the limestone (King et al., 1994). Mitsikeli to the east, the hills to the west, and most of the isolated hills on the basin floor are Alpine anticlinal structures (Katsikis, 1992). The subsidence which has allowed Pliocene and Pleistocene sedimentation is a result of late Tertiary inner arc extension related to the subduction zone of the Hellenic arc to the west (Clews, 1989). Karst solution was partly responsible for the initial growth of the Ioannina basin until the floor of the basin was sealed by fluvial sediments in the early Pleistocene. Pliocene and Pleistocene limnic sediments comprise the superficial deposits throughout the basin. A surface sediment study by Conispoliatis et al. (1986) found that most of the bed of the modern lake is covered with silts or silty clays, the finer sediments occurring towards the centre of the lake, and sand-sized sediments restricted to the margins. The sediments are composed of quartz and feldspar, clay minerals and carbonates (mostly calcite with small amounts of high magnesian calcite, aragonite and dolomite in some samples). The major source of the silicates that make up the bulk of the lake sediments is probably the easily eroded and largely insoluble flysch, which has an extensive outcrop in the south-east corner of the basin close to the sites of I-249

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1601

1587

1596 Asfaka
1614

Ioannina-I Ioannina


1596

LI KE SI IT M T. M

G R E E C E


1810

Ioannina-II IOANNINA LAKE IOANNINA

I-284 I-249


2185

Kastritsa cave

Legend for main figure

2048
alt. (m)

Permanent stream

2000 1500

Summit (altitude in m)

O AR

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1974

S

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(a)


1816

M


1810

500

TO T.

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M

Ephemeral stream

1000

0

km

10

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1137


2143

Fig. 1. Maps showing (a) the location of Ioannina in western Greece and (b) the Ioannina basin and the location of sites mentioned in the text. The town of Ioannina lies on the western shore of the modern lake, which is a remnant of the larger fossil lake whose maximum extent coincides approximately with the 500 m contour on this map.

and I-284, although the impure limestones and Pliocene lake sediments may be secondary sources. Lake Ioannina formerly filled the floor of the basin, but its size has been dramatically reduced by drainage schemes which began around AD 1600 and culminated in the drilling of a 4 km long tunnel through the hills to the west in 1944 (Anon, 1944; Higgs et al., 1967). The present lake is nowhere more than 10 m deep and bathymetric gradients are gentle (Conispoliatis et al., 1986). There are no major fluvial inputs or outputs; only ephemeral streams exist on the surrounding hills, with most precipitation percolating directly into the bedrock. The present lake surface coincides with that of the water table, and springs occur on the eastern side of the lake. It is probable that some limited and/or intermittent connection with the surrounding karstic system has occurred in the past (e.g., Fels, 1957; Higgs et al., 1967), although from the point of view of the isotopic composition of the water, Ioannina effectively behaves as a closed basin (Frogley, 1997). Ioannina receives an average of 1200 mm of precipitation each year (Anon, 1944; Tzedakis, 1991), more than eastern or southern parts of Greece, due to orographic uplift of moist warm air from the Adriatic. Although there is a pronounced rainfall minimum in the summer

months and the winters are generally mild—the definitive attributes of a ‘Mediterranean’ climate (Barry and Chorley, 1998)—the absence of extreme drought and the relative frequency of frosts, which can persist as late as May in exceptional years, means that the semi-natural vegetation is lacking in some ‘typical’ Mediterranean elements such as Olea. Crops and pastures occupy most of the basin floor, while the vegetation of the surrounding slopes is heavily grazed semi-natural garigue and maquis. Forestry, mainly involving Pinus nigra, occurs on some hills. A zone of wetland vegetation, including Phragmites, Cyperus longus, Scirpus spp. and Iris pseudacorus (Higgs et al., 1967) borders the lake, and formed extensive fens in the northern part of the basin prior to the 1944 drainage scheme (Bottema, 1974).

3. Material and methods Core I-284 (39
450 N, 20
510 E, 473 m above sea level; Fig. 1) was recovered mechanically by IGME (the Greek Institute for Geology and Mineral Exploration) in 1989. The 319 m sediment sequence consists almost entirely of homogeneous olive-grey silts. The upper 8.44 m show signs of relatively shallow-water deposition including

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abundant shell fragments and fine organic material, while the top 1.36 m consist of limestone gravel in a silt matrix (Frogley, 1997). All changes in sedimentology are gradual and apparently conformable, and apart from an unrecovered section of core between 164 and 164.50 m, the sequence appears to be continuous. The material was allowed to dry out and was stored for some years by IGME before being brought to the UK. 3.1. Palaeontological analyses 3.1.1. Pollen analysis Pollen preparation followed standard procedures (e.g., Berglund and Ralska-Jasiewiczowa, 1986): 0.5 cm3 samples were treated with 7% HCl, 10% NaOH, 60% HF and acetolysed. An exotic spike was added to each sample in the form of tablets of spores of Lycopodium (Stockmarr, 1971, 1973). At least 300 pollen grains were counted from each sample, following the recommendations of Maher (1972) and Rull (1987). Taxonomic nomenclature follows the currently incomplete Flora Hellenica (Strid and Tan, 1997) where applicable, and Flora Europaea (Tutin et al., 1964– 1983) otherwise. 3.1.2. Faunal analyses Slices of sediment 4 cm thick were disaggregated mechanically by soaking in deionised water and subjecting the samples to repeated gentle freeze–thaw cycles, before drying and sieving. Mollusc and ostracod remains were separated from the sediment and identified under a low-power (100  ) stereomicroscope. Where the remains were fragmentary, only the following were counted: intact apices, in the case of gastropods; complete valves or hinge fragments, in the case of bivalves; complete valves, in the case of ostracods. Except where ostracod remains were especially abundant (more than 350 valves), the whole sample was analysed. 3.2. Sedimentological analyses 3.2.1. Particle size Particle size analysis was carried out on 125 samples plus 15 replicates. The Ioannina sediments often break down initially to coarse sand-sized aggregations, which are probably cemented by diagenetic carbonate and silica (cf. Frogley, 1997). In order to break up these aggregates and reconstruct the pre-deposition particle size distribution, the samples were thoroughly soaked in 4% sodium pyrophosphate (Na4O7P2) solution in an ultrasonic bath and stirred before analysis using both a plastic rod and a whirlimixer. Microscope examination of the samples suggested that the treatment had succeeded in breaking up most of these aggregations. However, it was difficult to break apart the diagenetic

aggregates adequately without crushing primary sedimentary particles such as shell fragments and diatom frustules. The measurement of particle size distribution was carried out using a Malvern Mastersizer-X laser granulometer. Data are quantified by the granulometer as relative volumetric proportions in half-f bands (Inman, 1952; Lindholm, 1987), from which further measures of particle size distribution such as the proportion of clay, skewness, and sorting coefficient have been derived. Each sample was analysed three times: first, after disaggregation (‘‘bulk data’’); then following further treatment with 10% HCl to remove carbonates (‘‘post-HCl data’’); and, finally, following further treatment with 30% H2O2 to remove organic matter (‘‘post-H2O2 data’’). Comparison of the resulting data sets helps clarify the cause of changing particle size distributions through the sequence. Visual examination of samples using a high-power microscope was also carried out for the same reason. 3.2.2. Loss-on-ignition The loss-on-ignition methodology was based on Dean (1974) and Bengtsson and Enell (1986). Weighed sediment samples of approximately 1 g were taken at 10 cm intervals through the sequence. After drying at 105
C to constant weight, the samples were heated to 550
C for 7 h to estimate organic content. A second heating phase, to 950
C for a further 7 h, was undertaken to assess the proportion of carbonate in the sediment. 3.2.3. Magnetic susceptibility Magnetic susceptibility measurements were made at a resolution of approximately 10 cm between 10.00 and 19.90 m, and approximately 20 cm between 1.40 and 10.00 m. Volumetric measurements were taken at both low and high frequency (0.465 and 4.65 Hz, respectively) using a Bartington Instruments MS2 meter and MS2B sensor. A data set from a pilot study was used as a replicate set to assess analytical errors. 3.2.4. Stable isotopes The stable isotope data and the methods used to generate them have previously been described by Frogley et al. (2001). Briefly, d13C and d18O measurements were undertaken on samples of 8–10 valves of the ostracod Candona cf. permanenta and corrected to the V-PDB scale. Analytical precision was better than 0.1%. 3.3. Radiocarbon dating I-284 consists almost entirely of distal lake sediments with very little terrestrially derived organic material, hindering radiocarbon dating of the Holocene and Last Glacial parts of the sequence. Three different approaches

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Table 1 Procedure for isolating microcharcoal for radiocarbon dating from bulk sediment Process

Notes

Sampling

Samples of at least 40 g were removed from the core, taking large pieces where possible. Each sample was cleaned by shaving off the outer part with a razor blade, and by washing with 7% HCl. Treated with 7% HCl to completion and washed to remove carbonates. Treated with hot 10% NaOH for 10 min and washed to deflocculate clays and remove humic acids. Acidified with 7% HCl and treated overnight with hot 60% HF to remove silicates. After HF the samples were washed with HCl twice to remove fluorosilicates. 30% H2O2 was found to be very effective in removing most of the unwanted organic material, while leaving charcoal and some pollen grains relatively untouched. Treatment with H2O2 was continued with heating until no further reaction was visible. Clumping of some samples occurred during the oxidation process; the samples were disaggregated by treatment with 10% NaOH in an ultrasonic bath. A solution of sodium polytungstate was prepared to a density of 1.35 g cm3. The density was checked by weighing accurately measured 5 ml aliquots on a microbalance. For each density separation, 5 ml of the dense liquid was placed in a centrifuge tube, and a few drops of the sample suspended in water was mixed in with the dense liquid. The tube was then centrifuged at 3000 rpm for 20 min. The composition of the light and heavy fractions was checked under a light microscope at 400  magnification. The dense liquid was diluted by mixing in 1 ml of water and repeating the centrifugation, until the heavy fraction contained most of the charcoal. The rest of the sample was then carefully pipetted onto the surface of the dense liquid (to avoid modifying its density) and centrifuged as before. The heavy fraction was washed three times in 7% HCl, transferred to a quartz ignition tube and dried at 70
C.

7% HCl 10% NaOH HF Oxidation

Density separation

Final preparation

have been taken to provide an age model for this part of the sequence. Initially, 12 AMS radiocarbon dates were obtained from molluscan material from I-284. However, these dates were likely to have been affected by the so-called hard water effect due to uptake by the molluscs of unquantifiable amounts of carbon derived from the Mesozoic limestones in the lake catchment. The effect of this can be in the order of thousands of years (Day, 1996; Newnham et al., 1998). Consequently, further dates were obtained from charcoal, which occurs at low concentrations in the sediments. In two cases, it proved possible to pick sufficient material by hand under a stereomicroscope, taking care to avoid contamination (cf. Wohlfarth et al., 1998). In the absence of other dateable macroscopic terrestrial material, attempts were made to isolate pure samples of microscopic charcoal for dating, following and adapting the pollen concentration methodologies of Brown et al. (1989), Regne! ll (1992), Richardson and Hall (1994), Regne! ll and Everitt (1996), and Nakagawa et al. (1998). A series of experiments (Lawson, 2001) led to the development of a methodology allowing the extraction of reasonably pure samples of charcoal with a total mass between 0.63 and 3.77 mg from dry sediment samples in an excess of 40 g. The method is described in Table 1. In order to quantify the background 14C and assess whether the method added significant modern carbon contamination, the following material was processed via all stages of the separation method: 1. three samples of bituminous coal, which is used routinely by the NERC Radiocarbon Laboratory for background 14C quantification; and 2. five ‘true backgrounds’—microcharcoal extracted from around 270 m depth in the I-284 sequence, with

an approximate age of 400 kyr (i.e., beyond radiocarbon detection limits). The radiocarbon dates available for the I-284 sequence and the estimated magnitude of the errors associated with each are reported as conventional 14C ages in years BP (Table 3). Samples with publication codes prefixed AA- were prepared to CO2 (publication codes AA-17655 to AA-17662, prepared to graphite at University of Arizona) or graphite (all other AApublication codes) at the NERC Radiocarbon Lab. Mollusc material was cleaned with distilled water, dried and weighed. There was insufficient material for removal of the outer layers by acid-etching but the samples were ground and hydrolysed to CO2 using 100% ortho-phosphoric acid (AnalaR) at 25
C. Microcharcoal samples were combusted to CO2 in sealed quartz combustion tubes (Boutton et al., 1983). CO2 was cryogenically separated from other hydrolysis products and an aliquot of CO2 converted to graphite by iron/ zinc reduction (Slota et al., 1987). Graphite was analysed for 14C at the NSF-AMS Facility, University of Arizona (Donahue, 1990). The two samples of macroscopic charcoal with Beta- prefixes were analysed by Beta Analytic, Inc., Miami, following pre-treatment with dilute HCl. The small sample size meant that pretreatment with NaOH could not be carried out.

4. Results 4.1. Palaeontological data 4.1.1. Pollen data The results of the pollen analyses are shown in Fig. 2, plotted as percentages, and in Fig. 3, where selected taxa

Depth m

0 0

0 0

0 0

0 0

0 20

0 40

20 0 0

0

0 0 20 0

20

40 0

er e La als bi M ata en e Pl tha und an t. iff . R tag an o Th unc al ula R ictr ce os u a m e Fi ace un lip a di e ff. R end un ub u d l i i R ac a ff. um ea e O ex xy U ria m t. b U ell rti ife c O a rae th e Pt r N er A Pt ops P er id o a M psid (m yr a on N ioph (tr ole ym y ile te l Sp ph lum te) ) un un d a ac di iff. O rga eae ff. th ni e Pe r A um di qu as a tru tic m

0

ie nu

s

t.

s

de

ci

du

ou

s

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N AP

Pi

in us or C a c ia n u s ar p Ti i n u lia s be Fr tu ax lu s Fa i n u gu s e Q s xc ue el si rc or O us le ev a er g re Ph en il C lyre as a Pl tan at ea J u anu gl s C ans or yl Ep us h av Er edr ella ic a na J u ac e t. n ae O ip e r th u e s AP r A P

ax

Fr

U lm O us st ry a

Q s ue rc u

Al

Ab

tu Pi la nu s

Be

1604

C

ar y C op he h no ylla p c C is odia eae ta C ce cea om a e p e C osit om a e (L Ar po ig te sit m ae uli f is ia (T lora ub e) ul ifl or C ru ae c )u C ifer nd yp a iff er e . a G ra cea m e in ea e ex cl .c er ea ls

C

Depth m

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4 I-284-1

5

6

7 I-284-2

8 I-284-3

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11

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13

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15

16 I-284-7 I-284-8 I-284-9

17 I-284-10

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% 20

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8 I-284-3

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11

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13

14

15 I-284-6

16 I-284-7 I-284-8 I-284-9

17

18

I-284-10

19

20

21

I-284-11

22

23

I-284-12

2

× 10 2

4

6

8 10

Fig. 2. Pollen percentage data for the uppermost 23.65 m of core I-284. The main sum includes all terrestrial trees, shrubs and herbs.

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no po Ar di ac te m ea is e G i a ra m in ea To e ta ex lm cl .c ai er n ea su ls m co nc en tra tio n

s C ar

C he

pi n Fr us b ax et in us ulus O o st ry rnu a s t. Ti lia co rd Pi at st ac a ia Fa gu s Ju ni pe AP rus

ou ci du de us Pi nu s Q ue rc

Ab ie s

1605

3 I-284-1 4 5 I-284-2

6 7

I-284-3

8 I-284-4

9 10

Depth m

11

I-284-5

12 13 I-284-6

14

16

I-284-7 I-284-8 I-284-9

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I-284-10

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20 21 22

I-284-12

23

0

0

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x 105 grains cm3

Fig. 3. Pollen concentration data for selected taxa from I-284.

are shown plotted as concentrations. To facilitate description of the data, 12 pollen zones have been defined with the aid of statistical analysis (binary splitting based on information content, Birks and Gordon, 1985; Bennett, 1993, 1998). Overall, the pollen data set is dominated by a few abundant taxa—deciduous Quercus, Ostrya-type, Artemisia, Gramineae and Chenopodiaceae—with a wide range of other taxa present in smaller quantities. Pollen concentrations in the I-284 sediments are sometimes very high, with an absolute maximum in this data set of 1 300 000 grains cm3, although they vary considerably. In general the pollen is well preserved and identification was straightforward.

At the base of the data set presented here, in zones I284-12 and I-284-11, arboreal pollen (AP) values fluctuate around ca 20%; the herbaceous taxa are dominated by Gramineae, Artemisia and Chenopodiaceae, while AP consists mostly of deciduous Quercus, Pinus, Abies and Juniperus. Low values of a number of other thermophilous tree taxa, including Carpinus betulus, Fraxinus ornus, Ostrya-type, Tilia cordata-type and Fagus, also occur almost continuously. Total pollen concentrations are low compared to other zones, averaging 150 000 grains cm3. Zones I-284-10 to I-284-8 record an increase in AP and pollen concentration, although the overall upwards trend is punctuated by small fluctuations, the most

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significant of which occurs in the upper part of I-284-8. The rise in AP is almost entirely attributable to deciduous Quercus—in fact, Pinus percentages decrease considerably. Other arboreal taxa show little change. The increase in AP is balanced by decreases in Artemisia and Chenopodiaceae, while Gramineae values also decrease. A long period of abundant tree pollen is recorded in zones I-284-7 to I-284-5. Deciduous Quercus achieves its acme in I-284-7, exceeding 50%, while values of other thermophilous taxa such as F. ornus, Ostrya-type, T. cordata-type and Pistacia increase rapidly at the beginning of the zone. In I-284-6 Pistacia values peak while deciduous Quercus declines in favour of Ostryatype; Gramineae values also increase. Pistacia and Gramineae values decline again in I-284-5 with Ostryatype remaining important and a general increase in C. betulus, and decrease in F. ornus, through the zone. Pollen concentrations are variable but generally high in I-284-7, decline considerably in I-284-6, and are high again in I-284-5. The upper four zones of the sequence show a series of oscillations in AP, beginning with a decline in I-284-4 to around 20%, a recovery to almost 80% in I-284-3, a second decline in I-284-2 to around 10% at the base of I-284-1, followed by a gradual partial recovery. This pattern is mostly attributable to shifts in the values of deciduous Quercus, Ostrya-type and Pinus; not all tree taxa follow their lead. The decreases in AP are balanced within the main sum by increases in a number of herb taxa including Gramineae, Chenopodiaceae, Compositae, Cereal-type, Plantago and Umbelliferae. Pollen concentrations are generally low in the upper four zones. 4.1.2. Ostracod and mollusc data Fig. 4 shows ostracod faunal data expressed as percentages. These data have previously been published (Frogley et al., 2001) and are reproduced here for ease of comparison with the rest of the I-284 data set. A total of 14 species was recovered. In general, faunal assemblages are dominated by Candona cf. permanenta, which probably prefers sublittoral to profundal habitats (Frogley, 1997; Frogley et al., 2001). Low concentrations mean that mollusc counts, which are not presented here, were too small to allow statistical treatment of the data (see, Frogley, 1997; Frogley and Preece, in press). Mollusc fragments were moderately common throughout the upper 25 m of the sequence, becoming more frequent towards the top, especially above 7.5 m. Eight taxa were identified that are generally representative of sub-littoral or profundal habitats of which the most common were Valvata piscinalis, Viviparus janinensis, Bithynia graeca and Dreissena cf. stankovici.

4.2. Sedimentological data 4.2.1. Particle size analysis The particle size data are shown in Fig. 5. Analysis of 15 replicate samples showed that analytical errors were small; the standard deviation of the difference between the value of median f of the original and replicate samples was less than 0.2f both before and after treatment with HCl (Lawson, 2001). (a) Bulk data (Fig. 5a): The lowermost particle size data show a broadly stable sedimentary regime throughout pollen zones I-284-11, -10, -9 and -8, with sediments dominated by silts (73.0%) and clays (24.3%) with very little sand (2.7%; see Fig. 5 for definitions of size classes). The first substantial change in the data occurs at the base of I-284-7, where the sediments contain ca 20% sand. Coarse sediments persist into I-284-6 but fine again back to clayey silt in the upper part of the zone. The beginning of zone I-284-5 coincides with a second coarsening of the sediments, rapid at first but continuing more slowly towards a peak of ca 35% sand around 10 m depth. The sediments fine again above 10 m and throughout zone I-284-4; at the base of I-284-3 the trend culminates in poorly sorted samples with a high proportion of clay. A third, rapid coarsening then occurs, with the upper part of I-284-3 and the lower part of I-284-2 featuring by coarse and poorly sorted sediments. Another, more rapid fining event occurs in the upper part of I-284-2, followed by a final, gradual coarsening to the top of the data set. (Above this, the sediments coarsen towards gravels at the top of the core.) (b) Post-HCl data (Fig. 5b): Generally, the post-HCl data are very similar to the bulk data. Overall, there is a slight difference in the proportions of clay, silt and sand compared with the bulk data, the sands and clays being rather less important in the post-HCl data. This suggests that the carbonate material removed by the HCl treatment was concentrated in the sand and clay size fractions, possibly as shell material and authigenic cements, respectively. Only one part of the sequence shows a substantial variation between the two data sets. The coarsening in zone I-284-7 is much more subdued in the post-HCl data, and the return to fine sediments begins at the start of I-284-6 rather than half way through. (c) Post-H2O2 data (Fig. 5c): Following removal of the organic (and carbonate) fractions of the sediment, this data set is very different from the previous two. The remaining sediments, mostly silicates, are substantially finer. Most of the difference is attributable to a decrease in sand content, which averages 5.5% compared with 14.3% in the bulk data. Overall, the sediments are better sorted and show much less variation from sample to sample in this data set. However, three periods of relatively coarse and poorly sorted sediments stand out:

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na cf Ily .p oc ar vu yp la ris de Pr c ip io ie no ns cy pr is Le sp pt . oc yt he re D ar sp w .A in ul a st Ph ev ys en oc so yp ni ria C kr yp ae ria pe op lin ht i C a lm an ic do a na sp C .A yp rid (ju op v. ) si s vi du a

do C an

C an

do

na

cf .p

er m an

en

ta

1607

2

Pollen zones

3 I-284-1

4 5

I-284-2

6 7

I-284-3

8 I-284-4

9 Depth m

10 11

I-284-5

12 13

16

I-284-6 I-284-7 I-284-8 I-284-9

17

I-284-10

14 15

18 19 I-284-11

20 21 22

I-284-12

23 24 25 0

5 10 15 20

0

5

0

5

0

5

0

5

0

5

0

5

0

5

0

5

0

5

valves g-1 dry sediment

Fig. 4. Sub-fossil Ostracoda from the upper 25 m of I-284, expressed as frequency of valves per gram of dry sediment. Some rare taxa have been omitted (see Frogley, 1997).

one from the base of I-284-7 to the middle of I-284-5, peaking in I-284-6; another straddling the boundary between I-284-4 and I-284-3; and a third episode at the base of I-284-1. Towards the top of the sequence the sediments coarsen as before. 4.2.2. Loss-on-ignition The loss-on-ignition data for the upper part of I-284 are presented in Fig. 6. The mass loss between 105
C and 550
C is a proxy for organic content (Dean, 1974). The curve for ‘‘calcite’’ has been calculated on the assumption that mass loss between 550
C and 950
C is

entirely due to the production of CO2 by thermal decomposition of CaCO3—a reasonable approximation in the case of I-284 as sedimentary carbonates have been found to be low in magnesium (Conispoliatis et al., 1986; Frogley, 1997). Nine replicate analyses made using a calcimeter were compared with the loss-onignition estimates of calcite content; the original and replicate data were found to have a significant covariance of 0.99. The ‘‘residue’’ curve is calculated by subtracting the organic and calcite values from 100%; this provides an estimate of the silicate content of the sediment (although the loss-on-ignition

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Median particle size (φ) 4

5

6

7

8

% silt (4-8 φ; 3.9-63 µm) 80

60

40

0 1 2

Particle size (upper φ limit of size class) 5 10 Pollen zones

3 I-284-1

4 5

I-284-2

6 7

I-284-3

Depth (m)

8 I-284-4

9 10 11

I-284-5

12 13 I-284-6

14

I-284-7

15

I-284-8 I-284-9

16

I-284-10

17 18

I-284-11

19 20 60 40 20 0 % sand (1-4 φ; 63-500 µm)

60 40 20 0 0 2 4 6 8 10 12 % clay (>8 φ; <3.9 µm) Frequency in half-φ size bracket (%)

(a)

Median particle size (φ) 4

5

6

7

8

% silt (4-8 φ; 3.9-63 µm) 80

1 2

60

40

Particle size (upper φ limit of size class) Pollen 5 zones

3 I-284-1

4 5

I-284-2

6 7

I-284-3

Depth (m)

8 I-284-4

9 10 11

I-284-5

12 13 I-284-6

14

I-284-7

15

I-284-8 I-284-9

16

I-284-10

17 18

I-284-11

19 20 60 40 20 0 % sand (1-4 φ; 63-500 µm) (b)

60 40 20 0 0 2 4 6 8 10 12 % clay (>8 φ; <3.9 µm) Frequency in half-φ size bracket (%)

Fig. 5. Particle size data for the upper 20 m of I-284. In all plots finer sediments tend towards the right. The primary data—relative volumes of sediment in half-f bands (f¼ log2 D; where D is the diameter of the particle in mm), as estimated by the laser particle size analyser—are shown in the contour plots on the right. The raw data are summarised using two derived parameters: median particle size, showing the broad trends towards coarser and finer sediments; and volume percentages of sand (mostly coarse shell debris), silt (largely biogenic silicates and carbonates) and clay (fine-grained silicates). The three sets of charts show the particle size data for (a) the bulk sediment, (b) following treatment with 7% HCl to remove carbonates and (c) following further treatment with 30% H2O2 to remove organic material.

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Median particle size (φ) 4

5

6

7

8

1609

% silt (4-8 φ; 3.9-63 µm) 80

60

40

0

Particle size (upper φ limit of size class) 5

1 2

Pollen zones

3 I-284-1

4 5

I-284-2

6 7

I-284-3

Depth (m)

8 I-284-4

9 10 11

I-284-5

12 13 I-284-6

14

I-284-7

15

I-284-8 I-284-9

16

I-284-10

17 18

I-284-11

19 20 60 40 20 0 % sand (1-4 φ; 63-500 µm) (c)

60 40 20 0 0 2 4 6 8 10 12 % clay (>8 φ; <3.9 µm) Frequency in half-φ size bracket (%)

Fig. 5 (continued).

residue also contains small amounts of non-silicate minerals such as metal oxides). Organic content is less than 20% throughout the sequence and broadly follows the pattern of the AP curve, being low through pollen zones I-284-11 to I-2848, high during I-284-7, lower during I-284-6, high again during I-284-5, then showing a low–high–low oscillation through the upper four zones. Calcite content varies between about 10% and 60% and has a more complex pattern than organic content, especially in the lower part of the sequence where quite high values are achieved in zones I-284-10 and I-284-8, followed by low values in I284-7, I-284-6 and the lower part of I-284-5. Calcite content follows the same general pattern as organic content in the upper four zones of the sequence, albeit with numerically larger swings. 4.2.3. Magnetic susceptibility The low-frequency XLF data are presented in Fig. 6. Analysis of the differences between original and replicate data suggest that analytical error is negligible (standard deviation of differences=0.008  106 m3 kg1, n ¼ 19; see Lawson, 2001 for details). XLF clearly tracks the loss-on-ignition residue curve to a large extent. This

is unsurprising given that most carriers of positive magnetic susceptibility, the most important of which are iron oxides, remain in the loss-on-ignition residue; the magnetic minerals in the sediment are diluted by varying amounts of organic material and calcite (in fact, the latter has a small negative magnetic susceptibility). Since the loss-on-ignition and magnetic susceptibility measurements were made on the same samples, it is possible to approximately compensate for this dilution effect using the following formula, based on those of Allen (1986) and Yu and Oldfield (1993):   100ðwtotal  ðwcalcite Mcalcite ÞÞ Morganicþsilicate wLFðsilicateÞ ¼ ; ð1Þ Msilicate where M is the mass proportion of the various sediment components estimated by loss-on-ignition, Xtotal is the measured magnetic susceptibility of the sample and Xcalcite is the magnetic susceptibility of a similar mass of pure calcite. In general, both XLF and XLFðsilicateÞ show moderate susceptibility through zone I-284-11 to I-284-8; where calcite values are high during this period, XLF drops due to the dilution effect referred to above. Both XLF and

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Loss-on-ignition Organic (solid line) and calcite (dashed line) (weight%) 0

50

Magnetic susceptibility: χLF(silicate) (solid line) and χLF(bulk) (dashed line) (S.I. units x 10-6 m3 kg-1)

Residue (weight%) 100

0

50

100

0.0

0.2

0.4

0 1 2 3 I-284-1 4 5 I-284-2

6 7

I-284-3

8 I-284-4

9

Depth m

10 11

I-284-5

12 13 I-284-6

14

16

I-284-7 I-284-8 I-284-9

17

I-284-10

15

18 19 I-284-11

20 21 22

I-284-12

23 24 25

Fig. 6. Loss-on-ignition and magnetic susceptibility data. The loss-on-ignition residue data correlate strongly with the bulk low-frequency magnetic susceptibility data where the two data sets overlap (r ¼ 0:83) because magnetic susceptibility is partly controlled by the proportion of non- or weakly magnetic endogenic sediments (carbonates and organic material). XLFðsilicateÞ is corrected for this dilution effect, showing only changes in the magnetic susceptibility of the silicate fraction of the sediment.

XLFðsilicateÞ values are low through I-284-7 and the first part of I-284-6, are high at the end of I-284-6, then decrease again through I-284-5. Magnetic susceptibility increases dramatically through the top four pollen zones, rising rapidly through I-284-4, stabilising through I-284-3 and I-284-2 (although XLF responds to the diluting effect of changing calcite values), then oscillating high-low-high within zone I-284-1. High-frequency magnetic susceptibility measurements were also made with the aim of determining the size of the magnetic particles present in the sediment. However, the generally small values of XLF meant that the relative magnitude of analytical error obscured any

frequency-dependent susceptibility signal (cf. Dearing et al., 1996).

5. Age model Radiocarbon analyses are available from three types of material from the upper 36.25 m of the I-284 sequence (Table 2): 1. Mollusc dates, which may be affected by hard water error and may therefore be erroneously old.

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Table 2 Radiocarbon dates from I-284 Laboratory code

Level (m)

Minimum age (14C BP)

Conventional age (14C BP)

Maximum age (14C BP)

AA-17662 AA-17661 AA-23530

3.30 3.30 7.20

Depends on hard water error Depends on hard water error Depends on hard water error

1480745 1915745 2465745

1525 1960 2510

AA-38882 AA-23529

9.13 9.20

4845 Depends on hard water error

4900755 5370750

5885 5420

AA-23528

11.50

Depends on hard water error

7505760

7565

AA-38885 AA-38883 Beta-147745

11.60 14.55 14.90

6310 9380 8760

6365755 9450770 8800740

AA-23527

15.10

Depends on hard water error

10,080770

AA-17660 AA-38884 AA-17659 AA-38886 AA-17658 Beta-147746

17.17 17.34 23.18 23.34 26.20 29.38

Depends on hard water error 18 430 Depends on hard water error 24 480 Depends on hard water error 26 370

15,3307140 18,5607130 20,7607230 24,7107230 30,3407460 26,5107140

AA-17657 AA-17656 AA-17655

30.15 33.75 36.25

Depends on hard water error Depends on hard water error Depends on hard water error

38,60071000 40,10071300 44,20072000

2. Macrocharcoal dates, which, due to the necessary omission of an alkali wash in the pre-treatment, may be affected by migration of humic acids through the sequence and hence may be erroneously young. 3. Microcharcoal dates, which have relatively wide upper age limit estimates, based on maximum 14C measured in background material. Maximum and minimum ages have been estimated, where possible, using both the maximum background estimate derived as above and the analytical error, and are listed in Table 2. In the case of the microcharcoal dates, analysis of the standards showed 14C enrichment between 0.51% and 2.07% modern carbon equivalent. The higher figure was used to generate a conservative estimate of the analytical error, corrected for sample mass, for the five dates obtained from the upper part of the I-284 sequence. Interpretation of the three sets of dates is not straightforward, for different reasons. However, estimates of maximum and minimum age limits can provide an age-depth profile, albeit one with rather low precision. This degree of uncertainty in radiocarbon age models is, unfortunately, by no means uncommon (Lowe and Walker, 2000), especially for sites such as I-284 with limestone catchments and sediments

6735 10,765 Depends on humic acid mobilisation 10,150 15,470 22,250 20,990 34,050 30,800 Depends on humic acid mobilisation 39,600 41,400 46,200

Material Dreissena shell Viviparus shell Bithynia opercula Microcharcoal Bithynia opercula Bithynia opercula Microcharcoal Microcharcoal Macrocharcoal Bithynia opercula Viviparus shell Microcharcoal Viviparus shell Microcharcoal Dreissena shell Macrocharcoal Dreissena shell Dreissena shell Dreissena shell

containing limited terrestrial macrofossil material. Nonetheless, the dates through at least the upper 15 m are reasonably self-consistent and provide the basis for an independent age model. Fig. 7 shows these dates plotted against depth, with error estimates as listed in Table 2. Given that the maximum/minimum age limits are estimates, it would be misleading to try to fit a curve quantitatively through these points. Instead, a more qualitative, transparent approach is used based on linear interpolation between control points. The control points used are listed in Table 3. The top of the sequence is assumed to have an age of 0 14C BP, and a straight line from this point to the mollusc date at 7.20 m passes close to the two dates at 3.20 m. The next control point is the shell date at 9.20 m, which places the age model adequately through the error bars of the microcharcoal date at 9.13 m. The shell date at 11.50 m and the microcharcoal date at 11.60 m are averaged to give a control point of 6935 14C BP at 11.55 m. The age model fits through the cluster of dates around 15 m using a single control point based on the mollusc date at 15.10 m. Although the dating of this section of the sequence is very important for the interpretation of the Lateglacial and early Holocene record, the fact that these three dates are out of stratigraphic sequence makes

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0

5

15

10

age (14C BP x 103) 20 25 30

35

40

45

50

Table 3 Control points used in the age model shown in Fig. 8

0

1480 (shell) 1915 (shell)

2465 (shell)

Depth (m)

Age (14C BP)

0 7.20 9.165 11.55 15.10 17.17 23.34

0 2465 5135 6935 10 080 15 330 24 710

4900 (microcharcoal) 10

5370 (shell) 7505 (shell) 6365 (microcharcoal)

9450 (microcharcoal) (macrocharcoal) 8800 10 080 (shell) 15 330 (shell)

pro po da

20

se ge

depth (m)

18 560 (microcharcoal)

mo l de

20 760 (shell)

24 710 (microcharcoal)

30 340 (shell)

26 510 (macrocharcoal) 30

38 600 (shell)

40 100 (shell)

44 200 (shell)

40

Fig. 7. Radiocarbon dates and age model for the upper part of I-284. Bars indicate an estimate of the likely accuracy of the dates. In the case of the shell and macrocharcoal dates, solid error bars show calculated statistical errors at the 1  s level. In the case of the microcharcoal dates, a conservative allowance has also been made for increased uncertainty introduced by the complex preparation procedure, taking into account the mass of the sample (Lawson, 2001). Where the errors are less predictable, mostly due to the unknown magnitude of hard water error, a dashed error bar is shown with an arbitrary magnitude of 2 kyr. The proposed age model is based on linear interpolation between those dates considered to be the most reliable.

a more precise age model impossible. The age model is more difficult to construct below 15.10 m as the dates are more scattered. Only two control points are defined, both based on mollusc dates at 17.17 and 36.25 m, and

the resulting age model passes close to most of the mollusc dates. One approach to improving the age model could be to use pollen stratigraphy to cross-correlate the Ioannina sequence with other dated sequences from the region. However, this approach suffers from dating uncertainties in other sequences, difficulty in correlating with other data sets showing dissimilar palynological trends, and the strong probability of diachroneity in vegetation changes. There are few clear vegetation changes suitable for correlation, the most useful being found in the upper part of the sequence. The Fagus expansion in pollen zone I-284-4 could be correlated with similar expansions seen in Ioannina II (Bottema, 1974), Khimaditis I (Bottema, 1974), Tenaghi Philippon (Turner and Greig, 1975) and Maliq (Dene" fle et al., 2000), all of which have associated dates of between 3995 and 4535 BP. Platanus and Juglans tend to expand together and slightly later, between 3800 and 3200 14C BP (Bottema, 1980, 1982), but in I-284 they do not expand at the same time; Juglans expands in zone I-284-3 and Platanus in zone I284-2, which suggests that this vegetation event is poorly detected by the I-284 record. In general, these ages are in agreement with the radiocarbon age model, but the imprecision of the biostratigraphic approach does not allow us to constrain the age model further. In summary, the age model for the upper part of the core is sufficient to allow us to state that the top 23.65 m of the I-284 sequence covers part of the Last Glacial and most of the Holocene. The age of the base of the pollen diagram presented here is probably about 24 000 14C BP, but could be as young as 20 000 14C BP. The Holocene/ Lateglacial chronostratigraphic boundary at 10 000 14C BP falls at approximately 15 m.

6. Discussion This section begins with a synthetic interpretation of the various data sets, followed by a more general discussion of the implications of the data. The reader is referred to Fig. 8, a summary diagram showing key environmental parameters plotted against age.

no he C

Ar

te

m

is

po

ia

di

ac

ea

e

e) AP – (e (P xc . + l. G J.) ra m in ea

4

5

6

7

18

δ O(ostracod) (‰) trendline: 3-point running mean

Magnetic susceptibility

Bulk sediment median grain size (φ) trendline: 3-point running mean

-6

8

0.0

(S.I. units x 10 0.2

3

-1

m kg ) -3

0.4

-2

-1

0

1

2

3

4

0

Pollen zones

1000

I-284-1

2000

I-284-2

3000

I-284-3

4000

Solid line: χLF(silicate)

6000

Dashed line: χLF(bulk)

7000

I-284-5

8000

14

Age ( C BP)

I-284-8

11000 12000

I-284-9

13000 14000

I-284-10

15000 16000 17000 18000 19000

I-284-11

20000 21000

4

22000

5 6 7 8 Post-H2O2 median grain size (φ) trendline: 3-point running mean

23000 I-284-12

24000 25000

0

10 20 30 40 50 60 70 80 %

0

10 20 0

Dashed line: calcite Solid line: organic

0

40 Loss-on-ignition (%)

80

Fig. 8. Selected data from I-284 plotted against age. Radiocarbon ages have not been calibrated due to the difficulty in calibrating dates older than 20 kyr. ‘AP  ðP: þ J:Þ’ is a summary pollen curve for all tree taxa other than Pinus and Juniperus, calculated with Gramineae excluded from the pollen sum (since Gramineae can be a dominant taxon under both glacial and interglacial conditions). This parameter is considered to reflect those taxa that are most responsive to warm and/or moist conditions (cf. Bottema, 1974). Stable isotope data are redrawn from Frogley (1997) and Frogley et al. (2001).

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6.1. Synthetic interpretation For simplicity, the interpretation is divided into four parts using boundaries defined by the pollen zonation. 6.1.1. Full glacial (pollen zones I-284-12 to I-284-11; ca 25 100–15 600 14C BP) Pollen assemblages from zones I-284-12 and I-284-11 are dominated by herb taxa, notably Gramineae, Artemisia and Chenopodiaceae. Most authors agree that, by comparison with modern vegetation communities in the Near East (e.g., Zohary, 1973), these taxa represent arid conditions. Tarasov et al. (1998) suggest that warm and cool steppe biomes can be differentiated by looking for the presence in pollen records of indicator taxa such as Hippophae and Polygonaceae (cool steppe), and Cruciferae, Labiatae and Crassulaceae (warm steppe). The fact that all of these taxa are approximately equally well represented in the glacial Ioannina record suggests that a variety of temperature regimes existed within the Ioannina catchment. Given the topographic diversity of the modern Ioannina basin, it seems likely that a range of different arid vegetation communities could have existed at different elevations and on different soils, forming a patchwork of moister Gramineae-dominated steppe and shrubby Artemisia-Chenopodiaceae semi-desert. A notable feature of the glacial record is that, despite the dominance of herbs, there is a moderate abundance of tree taxa, especially deciduous Quercus and Pinus. In fact, the uninterrupted presence of trees persists throughout the Last Glacial (Tzedakis et al., 2002) and probably much before (Tzedakis, 1993). This has been attributed to two factors: firstly, the relatively high precipitation of the south-west Balkans, a result of orographic uplift of moist air from the Adriatic, which provided sufficient moisture for the persistence of thermophilous trees during the arid glacial stages of the Quaternary; and secondly, the topographic variability of the catchment, providing a range of habitats and shelter from incursions of cold polar air from the north (Tzedakis et al., 2002). Tree distribution within the catchment was probably determined largely by temperature rather than moisture availability, with trees concentrated in sheltered valleys and on the thickest, most stable soils. Certainly, the low percentages and concentrations of AP during the lowermost two zones of the data presented here do not suggest that the glacial landscape as a whole was thickly wooded. The glacial-stage sediments are amongst the most fine-grained of those studied here. At first this may seen counterintuitive; in the Mediterranean, as elsewhere in Europe, coarse lake sediments are often associated with cold stages (e.g., Digerfeldt et al., 2000), principally as a result of enhanced erosion and sediment transport due to bare slopes, freeze–thaw activity, seasonal snowmelt

feeding high fluvial discharge, and glacial erosion in the mountains. However, the coarser sediments further up the I-284 sequence are dominated by biogenic material, mostly shell fragments, diatoms and sponge spicules. The fine silts and clays of the glacial period are best interpreted as indicating relatively stable, distal sedimentation dominated by allogenic sediments, with only subdued biogenic sediment production reflected in generally low organic and calcite contents. As such, the relatively uniform character of the sediments of this period probably owes more to the relatively unchanging nature of lacustrine sediment transport processes than to any absence of environmental variability over time on land. This conclusion is broadly supported by the ostracod analyses. Although subdued changes in the species present may indicate a slight fluctuation of water depth at the site of I-284 over the course of this period, the persistent domination of assemblages by Candona cf. permanenta indicates that conditions were sub-littoral to profundal throughout (Frogley et al., 2001). 6.1.2. Lateglacial (pollen zones I-284-10 to I-284-8; ca 15 600–9900 14C BP) Given the contention over the nature of the Lateglacial environment of Greece, particular attention was paid to collecting data at high resolution through this part of the sequence, and it consequently deserves close analysis. In contrast to the relatively stable conditions of the lowermost two pollen zones, the Lateglacial part of the core records a series of major changes in the vegetation. These are reflected in the summary diagram (Fig. 8) by the curve for AP excluding Pinus and Juniperus, where percentages are calculated on the basis of a pollen sum that excludes Gramineae. This curve can be interpreted as an indication of the spatial extent of thermophilous, moisture-demanding woodland (relative to steppe vegetation, not including the climatically ambiguous Gramineae), and therefore as a proxy for warm, moist climatic conditions (cf. Tzedakis, 1993; Tzedakis et al., 2002). In zone I-284-10 the pattern of vegetation change takes the form of an expansion of deciduous Quercus at the expense of Pinus and an increase in Artemisia and Chenopodiaceae at the expense of Gramineae. The best explanation for this would be an increase in temperature, encouraging the colonisation of higher ground by deciduous Quercus. Thermophilous taxa continue to expand through I-284-9, with a sharp and short-lived spike at 15.99 m probably indicating a minor climatic amelioration. A general increase in the abundance through I-284-9 and I-284-8 of Rumex and Oxyria-type pollen, together with Corylus, may reflect the rapid colonisation of newly habitable terrain by ruderal and pioneer taxa in advance of the slower

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expansion of Quercus-dominated woodland. There is a notable reversal in the expansion of thermophilous species such as Ostrya-type and F. ornus in the middle of zone I-284-8. In general, then, the pattern of vegetation change during I-284-10, -9 and -8, dated to between 15 600 and 9900 14C BP, represents the transition between a glacial landscape dominated by steppe-grassland with limited tree presence, and the forests of the interglacial. This transition occurs relatively smoothly in comparison to the well demarcated Aller^d/B^lling–Younger Dryas oscillation of continental north-west Europe. The only reversals in the process of afforestation visible in the I-284 record are comparatively subdued, shortlived and restricted to the most sensitive taxa. During the most important Lateglacial oscillation in AP, in I-284-8, it appears from the pollen data that deciduous Quercus in fact continues to increase unabated while the more thermophilous taxa, such as Ostrya-type and F. ornus, diminish. Artemisia and Chenopodiaceae show a slight resurgence in this zone which points to increased aridity. Sedimentologically, this part of the sequence is rather complex and enigmatic. At the beginning of zone I-28410 the calcite content of the sediment increases from about 15% to a peak of about 45%. The particle size data suggest that the calcite is silt-sized, and therefore probably not diagenetic; it may represent an increase in clastic carbonate. The magnetic susceptibility data suggest no change in the nature of the silicate sedimentation; although XLF drops in response to the diluting effect of the increase in calcite, XLFðsilicateÞ does not. Calcite values drop in zone I-284-9 and recover in I284-8, and concentrations of the aquatic alga Pediastrum, which is present in small quantities almost continuously through the sequence, become extremely high in I-284-9. Stable XLFðsilicateÞ and particle size data suggest that the changes in productivity within the lake recorded by the loss-on-ignition data and Pediastrum values were not accompanied by major changes in allogenic sedimentation. Increases in ambient temperature, or the nutrient status of the lake water, could be responsible for these productivity changes. At face value, the stable isotope data through the Lateglacial part of the sequence suggest significant changes in the balance between precipitation and evaporation and, by extension, lake level. The stable isotope oscillations through the Lateglacial and Holocene show much greater amplitude and frequency than comparable data from the Eemian part of I-284 (Frogley et al., 1999), although this is probably a result of productivity variations, and/or inwash of carbonates, resulting from the long-term infilling of the basin. The evidence for substantial change from the stable isotope data contrasts with the particle size data, which indicate continuing distal sedimentation, and with ostracod

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faunal data which show no conclusive evidence for an appreciable drop in lake level during the Lateglacial. However, the particle size and ostracod faunal data show few changes at all during the glacial part of the record, suggesting that these parameters were insensitive to environmental change during this period. Their sensitivity seems to pick up during the Holocene, possibly due to some combination of the shallowing of the lake and a biological threshold response to increasing lake temperature. 6.1.3. Early Holocene (pollen zones I-284-7 to I-284-5; 9900–5500 14C BP) The first half of the Holocene sees gradual and longterm changes in forest composition. The early Holocene woodland of I-284-7, dominated by deciduous Quercus, gradually gives way to a more mixed woodland with considerable quantities of Ostrya carpinifolia/Carpinus orientalis, F. ornus and smaller amounts of C. betulus. There are at least three possible explanations for these long-term changes in woodland composition: climatic change, human disturbance, and internal, autogenic ecological changes. Climatic change probably does have some role to play in explaining observed pollen changes in the early Holocene. This is particularly the case during I-284-6, when AP values drop slightly and Pistacia values reach a peak. Pistacia is a poor pollen producer—even closed Pistacia woodland often produces a pollen rain with only 5% Pistacia pollen (Bottema, 1974; van Zeist and Bottema, 1977)—and hence the small percentages of I-284-6 are much more significant than they appear. Of the two Pistacia species present in Greece, given the absence of other maquis species such as Olea and Phillyrea the pollen in this zone probably derives from the deciduous P. terebinthus, which unlike its evergreen relative P. lentiscus is tolerant of winter frost. P. terebinthus is a sclerophyllous tree adapted to withstand summer drought. An expansion of Pistacia part-way through the early Holocene is almost ubiquitous in the eastern Mediterranean (Rossignol-Strick, 1995) and the most likely explanation is a climatic change in its favour. Throughout this apparent period of relative aridity, O. carpinifolia/C. orientalis gains at the expense of deciduous Quercus. This expansion of O. carpinifolia/C. orientalis is not limited to the acme of Pistacia in I-2846, but accelerates at the beginning of I-284-5, before reaching a plateau. The early Holocene sees complex changes in sedimentation at the site of I-284. The lake level was probably relatively low in I-284-7; high values of Nymphaea pollen, and an increase in the amount of large clastic silicates in the sediment, support this view. I-284-6 begins with a peak in coarse calcite, probably bioclastic; later in the zone the sediments revert almost

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to their glacial condition, with high magnetic susceptibility and a dominance of clay-sized silicates. This may reflect erosion and reworking of sediments and soils exposed at the lake edge by a fall in lake level, and/or an increase in erosion rates due to thinning of the woodland during this (inferred) arid event. Considerable changes in sediment properties also occur in I-284-5. Microscopic examination showed an increase in diatoms and sponge spicules, suggesting increased biogenic sediment production. This conclusion is supported by increases in organic and calcite content, together with coarsening particle size, to about 10 m depth. There are two likely causes: either an increase in lake productivity due, for example, to a gradual increase in the nutrient status and/or temperature of the lake water over time; or a drop in lake level such that the site of I-284 fell within the photic zone. These two explanations are not mutually exclusive. It should be emphasised that, although lake level may have dropped during I-284-5, comparison with sediments in the upper part of the sequence suggests that the depth of water at the site was still several metres. Ostracod assemblages continued to be dominated by Candona cf. permanenta throughout the early Holocene, with only occasional occurrences of other taxa. 6.1.4. Late Holocene (pollen zones I-284-4 to I-284-1; 5500 14C BP–present) Major changes occur in this uppermost part of the sequence, due at least in part to increasing human pressure on the landscape, and the termination of sedimentation at the site of I-284. Zone I-284-4 charts a rapid and profound decrease in AP, representing a substantial decrease in tree cover in the landscape. Various taxa that point to an expansion of pastoralism and agriculture increase through the zone: evergreen Quercus and Phillyrea are both typical of heavily grazed maquis vegetation; Castanea and Juglans, the latter almost certainly introduced (Bottema, 1980), are potential tree crops; and Plantago coronopustype, Rumex, Umbelliferae, Chenopodiaceae and Artemisia are, today, common weeds associated with cultivation. Some authors (Magri, 1995; Roberts et al., 2001) have suggested that a dry event around 3500 14C BP may have spurred the expansion of agriculture in the eastern Mediterranean by thinning the woodland, aiding its clearance by farmers. The fact that the woodland recovers almost completely in I-284-3 perhaps lends some support to this notion of a short-term climatic event. Very high magnetic susceptibility values, the dominance of clay-sized silicates and an abundance of physically robust taxa such as spores and Compositae Liguliflorae in the pollen record, all point towards a considerable influx of soil material; ostracods become

extremely rare in the sediment, while remobilised opercula of the gastropod Bithynia become abundant. Despite the strong palynological evidence for increasing human activity at this time, it is not possible to determine whether this occurred in concert with any climatic event, nor indeed if it was caused by such an event. The recovery of the woodland in I-284-3 is surprisingly complete, given the strong evidence for considerable soil loss in the previous zone. By the top of the I284-3 a mixed deciduous woodland, broadly similar to that of I-284-5, is once again in place, showing a remarkable ability to recover from deforestation. However, Abies and Pinus do not recover completely, perhaps suggesting that high-altitude environments were more fragile, and evergreen Quercus expands at the expense of deciduous trees, possibly as a response to continued grazing pressure (tough, bitter, prickly evergreen oaks are less readily browsed by goats than other trees). The abundance of cereal-type grains suggests that farming was continuing in the Ioannina basin, so the reason for the re-afforestation is obscure. Although the recovery of the vegetation significantly pre-empts a decrease in soil influx (decrease in magnetic susceptibility), by the end of zone I-284-3 the sediments are once again mostly endogenic. However, the presence of large clastic silicates and high values of shallow-water aquatics such as Myriophyllum and Sparganium show that, by this time, the terminal shallowing of the site of I-284 was well under way. In I-284-2 a second and final deforestation occurs, reducing AP to levels last seen during the glacial. The only tree taxa to depart from this trend are Olea— possibly cultivated, given the very high values at the top of I-284-1—and Phillyrea, a maquis constituent. Cereals, and weeds such as Plantago and Umbelliferae, increase, indicating the further expansion of agriculture. The sampling resolution towards the top of the sequence is perhaps inadequate to capture fully the complex series of changes that occur in the sediments, as significant changes occur from sample to sample. The dominant features of the upper two pollen zones are evidence for shallow water (high biogenic sedimentation, coarse particle size) and considerable soil inwash (high magnetic susceptibility). Pollen preservation deteriorates as the sediments become coarser and more oxidised towards the top of the core. The shallowing of the lake is reflected both by an increased abundance of molluscs, and the decline of the sub-littoral ostracod Candona cf. permanenta and its partial replacement by littoral species, especially Cypria ophtalmica, whose modern association with the reed Phragmites suggests the source of much of the Gramineae pollen in zones I-284-2 and I-284-1. The upper 1.40 m are composed of gravels, the final sedimentation at the site following artificial drainage.

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6.2. Comparisons with previous pollen sequences from Ioannina The present work on I-284 is the fourth pollen sequence from the Ioannina basin. Bottema (1974) produced two pollen diagrams, with associated radiocarbon dates and sediment descriptions, from cores taken by hand. The first core, Ioannina-I (Fig. 9a), was taken towards the north-western extremity of the basin (see Fig. 1 for locations). Prior to the mid-20th century human intervention a sizeable lake, Lapsista, occupied the northern part of the basin. The northern and southern sub-basins are separated by a low ridge. Bottema’s second core, Ioannina-II (Fig. 9b), was taken on the southern side of this ridge in a marshy area at the edge of the present Lake Ioannina. It is not clear whether the two lakes would have been conjoined in the past (Higgs et al., 1967; Bottema, 1974; Tzedakis, 1991). More recently, Tzedakis (1991) worked on the palynology of the upper 162.5 m of I-249 (Fig. 10), a 230 m long core taken close to the site of I-284 on the south-eastern edge of the modern lake. In general, the pollen spectra across the basin would be expected to show time-parallel behaviour, within the temporal resolution of the data sets, since the basin integrates the pollen rain of a wide area. However, differences emerge between the various data sets. The pollen record from I-249 is the most comparable to I284, reflecting their close proximity and similar facies. The upper 10.20 m of the I-249 sequence is missing due to heavy sub-sampling by IGME; the top of the I-249 pollen data set correlates approximately with the upper boundary of pollen zone I-284-5. The sampling resolution of I-249 is not high enough to detect minor oscillations during the Lateglacial and early Holocene. Bottema’s short Ioannina-II core may contain the oldest pollen assemblages yet analysed from Ioannina. Between 4.20 and 4.60 m Ioannina-II contained no pollen; Bottema interpreted this as marking a hiatus in deposition, with oxidation of the exposed sediments. The pollen assemblages of the pre-hiatus sequence include up to 10% Parrotia, a genus of trees which at present occur in the Middle East (van Zeist and Bottema, 1977) and which has not been found in any part of the 423 kyr I-249 sequence, nor yet in the stratigraphically longer I-284 sequence. The lowest part of Ioannina-II was therefore probably uplifted from depth by tectonic action known to have occurred in this part of the basin (Brousoulis et al., 1999). The upper 4.20 m of Ioannina-II contain a sequence which biostratigraphic comparison with I-284, and a radiocarbon date of 4535 14C BP between 2.20 and 2.30 m, suggest is dominantly Holocene. Major patterns seen in both sequences include the deciduous Quercus– Pistacia phase and subsequent deciduous Quercus/ Ostrya-type phase; two major Gramineae peaks above

this in Ioannina-II probably represent the deforestation episodes of I-284-4 and I-284-2. Systematic differences in pollen percentages suggest that the shallow-water site of Ioannina-II is slightly biased towards lowland vegetation (Pistacia, Olea), possibly local to the site, while the more distal site of I-284 records more upland vegetation (Fagus, Abies, Pinus). Bottema’s core Ioannina-I (Fig. 9a) shows two periods of relatively high AP (mostly deciduous Quercus and Pinus), the first from the top down to about 4 m depth (zones X, Y and Z), and the second between 6.70 and the base of the sequence at 11.65 m (zones S, T and U). Tzedakis (1991) considered the lower period of high AP to be possibly correlative with an interstadial during the last (Weichselian) glacial, zone IN-38b in I-249, but noted that the proportions of the taxa were somewhat different and the correlation was not very strong. No better match for the assemblages could be found in the I-249 sequence. This suggests either strong differences between the source vegetation or taphonomy of the pollen assemblages at the two sites, or that a lengthy hiatus occurs somewhere in the Ioannina-I sequence. The upper 2.85 m of the Ioannina-I diagram, zones X, Y and Z, present a very similar record to the uppermost pollen records from I-284, I-249 and Ioannina-II, although approximately the last 4000 years of the Holocene, with the appearance of Juglans and major drops in AP, is missing; the top of Ioannina-I probably correlates with some point in zone I-284-5. The lowermost part of the continuous Pistacia curve coincides with Bottema’s date of 10 190790 14C BP, which he acknowledged could be subject to hard water error. By extrapolation downwards, and assuming there are no breaks in the sedimentation in either Ioannina-I, I284 or I-249, the pollen assemblages of zone W in Ioannina-I (2.85–3.85 m) contrast significantly with the corresponding parts of the I-284 and I-249 sequences. Many taxa, including deciduous Quercus, Artemisia and Chenopodiaceae occur at values corresponding to those in I-284 and I-249. However, Abies and Pinus are both high in subzones W1 and W3 in Ioannina-I, and lower in the intervening W2 where they are replaced by Gramineae. No corresponding increases are seen in I284 or I-249. This is important because zone W at Ioannina has long been taken as representative of the Lateglacial vegetation in the Balkans (Bottema, 1974, 1995a, b; Willis, 1994). The comparison with the data from I-284 and I-249 suggests that it is, in fact, atypical for the basin. In the absence of evidence for a sedimentary hiatus in any of the sequences, and given the more littoral depositional environment of Ioannina-I which would tend to preserve local, rather than regional pollen assemblages, the peaks in Abies and Pinus in zone W should perhaps be ascribed to local vegetation changes which were not representative of the wider catchment.

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Fig. 10. Pollen percentage data and loss-on-ignition data from I-249 (Tzedakis, 1991, 1993), together with selected data from I-284, and a suggested correlation between the two based primarily on comparison of the Quercus curves (grey bar).

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As with Ioannina-II, systematic differences between the pollen values of Ioannina-I and I-284, such as the considerably lower proportion of Ostrya-type relative to Pistacia in Ioannina-I, suggest that the catchment of Ioannina-I is biased towards local, basin-floor vegetation, while I-284 receives more pollen from the wider region. From the above discussion two important conclusions can be drawn. The first is that I-284 and I-249, composed of distal sediments and therefore situated well away from the shore for most of their history, record consistently greater values of montane taxa such as Abies, Pinus and Fagus than the shallow-water sequences of Ioannina-I and Ioannina-II (with the exception of the peaks of Abies and Pinus in zone W of Ioannina-I). Thus, the distal sites record regional vegetation change, while the littoral sites are biased towards local vegetation. This finding accords with the results of a study by Huntley et al. (1999) using multiple cores from Monticchio, Italy, a large lake with many similarities to Ioannina, where slight differences in the age-equivalent pollen spectra of different cores were attributed to variation in spatial dispersal of pollen. Secondly, while the Lateglacial sequences of I-284 and I-249 are compatible, the peaks of Pinus and Abies in zone W of Ioannina-I, previously held to represent the regional Lateglacial vegetation, are not seen in the deeper-water sequences. These features of the IoanninaI diagram may reflect local growth of conifers within the northern sub-basin during the glacial. An alternative explanation is that a substantial hiatus is present within the relatively compressed Ioannina-I sequence.

7. Discussion and conclusions The Ioannina record presented here represents a considerable increase in the quantity and range of data from what has become a reference site for the Balkan Peninsula. The upper part of I-284 now has the highest temporal resolution pollen record from the basin, supported by a large number of other proxy data which strengthen and inform the palaeoenvironmental interpretations based on the pollen data. This is now also the most comprehensively dated lake sequence in the region, albeit with limitations to the precision of the resulting age model. The deep-water sedimentation regime of the site suggests that the part of the sequence presented here is a reliable record of the regional-scale vegetation and environmental history of north-west Greece over the last 20 000 years. The Holocene part of the I-284 record extends farther towards the present than other Ioannina sequences and confirms the presence of two major deforestation events, accompanied by considerable soil inwash, after 5500

14

C BP, well after the Neolithic transition. However, the possible role of climate in causing the first deforestation cannot be determined from the data. A ‘‘Pistacia event’’ (see Rossignol-Strick, 1995) is present in the early Holocene, and at this site it appears to coincide with an erosional event. The Holocene also sees long-term changes in vegetation composition, such as the slow but steady rise of Ostrya-type, C. betulus and Fagus, the reason for which is unclear. Perhaps the greatest significance of this work, however, lies in the Lateglacial part of the record. The pollen data support the previous statement of other workers (Bottema, 1974, 1991, 1995a, b; Willis, 1994, 1997) that the Lateglacial vegetation history of at least north-west Greece differs substantially from that of most of Europe. However, the new data yield fresh information on the exact nature of regional vegetation change. Bottema (1974) argued that his pollen records from Ioannina showed that the Aller^d/B^lling was the most arid period of the Lateglacial, while a rise in conifer pollen during the period loosely dated to the Younger Dryas in Ioannina-I could represent an amelioration of climate. Later, however, Bottema (1995a, b) revised his views, stating that the pollen evidence from Ioannina-I did not suggest any significant change in vegetation at all during the Younger Dryas. Bottema’s original position was echoed by Willis (1994) who suggested that trees were only likely to be able to survive the arid phases of the Lateglacial at high altitudes where precipitation was not limiting, in which case those tree populations would consist mostly of conifers. She explained the absence of evidence for change in deciduous taxa as a result of the small size of their Lateglacial refugia. However, as discussed above, the new I-284 pollen record does not show significant changes in conifer pollen during the Lateglacial, while fluctuations in many thermophilous deciduous taxa are readily apparent. Deciduous woodland increases steadily in abundance from about 15 000 to 9000 14C BP, with some minor oscillations, one of which coincides approximately with the Younger Dryas chronozone (see Mangerud et al., 1974). Given that the I-284 Lateglacial record comes from a deep-water part of the basin at the top of a thick and apparently unbroken sequence of sediments, while Ioannina-I is a much more stratigraphically compressed sequence from a more littoral situation, it seems probable that the I-284 record is more representative of regional vegetation change. Furthermore, the smooth increase in deciduous Quercus through the course of the Lateglacial is very similar to that seen in data from several sites from Italy (see below). The absence of a major Lateglacial regression of temperate woodland does not necessarily imply that the overall pattern of climate change was substantially different in north-west Greece than in surrounding

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areas. In fact, a number of records from the neighbouring Adriatic Sea show strong evidence for a pronounced decline in sea surface temperature (SST) during the Younger Dryas (e.g., Asioli et al., 1999; CombourieuNebout et al., 1998). Oceanographic records from across the Mediterranean tell the same story, although it is possible that the duration of depressed SST was shorter in the Mediterranean than in the North Atlantic (Cacho et al., 2001). Furthermore, although several sedimentological features of I-284 show little change through the Lateglacial, the stable isotope data (despite the caveats mentioned above) do suggest significant changes in precipitation/evaporation ratio. Instead, it seems likely that the effect of climate change on the vegetation was mitigated by the topographic setting of Ioannina. One potential mechanism for this centres on orographic precipitation. Westerly winds bring air charged with moisture from evaporation taking place over the Tyrrhenian and Adriatic seas. Most of that moisture falls over the mountains of western Greece as the moist air rises, expands and cools. Lowland areas, and regions to the east of the Pindus, experience much less precipitation: for example, the Kopais basin in the lowlands of eastern Greece receives around 470 mm yr1 of rainfall (Tzedakis, 1999), compared with 1200 mm yr1 on the floor of the basin at Ioannina. During the Younger Dryas drop in SST, precipitation values on land would have fallen as evaporation from cooler seas was reduced. However, a reduction from modern precipitation values by half at Ioannina and Kopais would have left ample precipitation for the survival of most tree taxa at Ioannina, whilst severely reducing the potential for deciduous tree growth at more easterly sites such as Kopais. Recent modelled palaeoclimate data, produced using a regional model nested within a global circulation model (Barron and Pollard, 2002), support this view: predicted precipitation values at the LGM are 655 mm yr1 at Ioannina and 175 mm yr1 at Kopais (Tzedakis et al., 2002). This accords with data from a number of sequences from Kopais, most with well-constrained age models, that suggest a significant decrease in temperate tree abundance approximately coincident with the Younger Dryas chronozone (Allen, 1990; Okuda et al., 1999; Tzedakis, 1999). More equivocal, but still strong evidence for a clear vegetational expression of the Younger Dryas occurs at Tenaghi Philippon in northeast Greece (Wjimstra, 1969; Greig and Turner, 1974; Tzedakis et al., 2002) and at sites in Bulgaria (Bozilova and Tonkov, 2000) where the climatic regime is similarly continental. Hence part of the explanation for the muted vegetational response to cooling during the Lateglacial Stadial at Ioannina may be the preservation of high moisture availability through orographic precipitation and proximity to the western coast.

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The problem with this explanation for the lack of a climatically driven woodland recession during the Lateglacial in northern Greece is that, in Italy (for example), such an event is a feature of sites at all altitudes and therefore under a wide variety of climate regimes: e.g., Lagaccione (Magri, 1999), 355 m asl, precipitation 987 mm yr1; Prato Spilla sites (Lowe, 1992; Lowe and Watson, 1993), 1280–1550 m asl, precipitation data not given; Monticchio (Watts et al., 1996a, b; Allen et al., 1999), 656 m asl, precipitation 815 mm yr1; Lago di Vico (Magri and Sadori, 1999), 510 m asl, precipitation 1400–1600 mm yr1. However, although a clear palynological Younger Dryas event has been claimed for all of these sites, the degree of woodland recession varies considerably from site to site, with no clear relationship between the degree of expression of the event and altitude or climate. This suggests that further factors are at work. An alternative explanation stems from the diversity of microclimates present within the mountainous Ioannina catchment at any one time. Microclimate is affected by topographic factors such as slope angle and aspect, exposure to wind, orographic precipitation and rainshadows, and the decrease in temperature with altitude. Together with soil diversity and varied hydrology, these factors make the Pindus Mountains a patchwork of varied habitats able to support a number of distinct vegetation communities. Even within the Ioannina basin in modern times, habitats include the lake edge, extensive fens, sub-Mediterranean climates on moist clay-rich soils around the lake, and on the mountains beyond, climate and soil conditions able to support everything from maquis, through mixed deciduous woodland and boreal coniferous forest, to alpine shrub and herb communities. The effect of this rich variety of habitats is that, should a species be placed under stress by a change in climate, it has the strong possibility of being able to survive by migrating within the locality, rather than the population becoming extinct. A site with a topographically diverse pollen catchment will hence show insensitivity to climate change because the effect of a climatic change will be (at least partially) expressed as a spatial reorganisation of taxa, rather than changes in absolute abundance. A simple example might be that a slight decrease in temperature could cause stands of deciduous woodland to migrate a few hundred metres downhill, where conditions are warmer. In this example, only very large temperature changes would cause significant decreases in the available area for occupation by deciduous taxa. In general, the taxa most sensitive to climate change would be those existing in the extreme microclimates of the catchment—in the case of Greece, frost-intolerant maquis species on the one hand, and alpine floras on the other hand. Interestingly, both have low visibility in pollen data sets (because of low pollen

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productivity, together with dominantly insect pollination in the case of maquis vegetation). This implies that the vegetation of intermediate altitudes—deciduous and coniferous woodland—will dominate the pollen record, and that its absolute abundance will change little with climate change, although its spatial distribution within the Ioannina catchment may vary. Although there is no evidence for a major reversal in the expansion of woodland during the Lateglacial at Ioannina, there are a number of minor oscillations and plateaux, the largest of which occurs in the upper part of pollen zone I-284-8. However, these features are not as clearly defined as the oscillations in AP that are taken to indicate major vegetation changes at, for example, the Italian sites, or in Bulgaria. From the arguments put forward above, if the overall pattern of Lateglacial climate change in north-west and central northern Greece did not differ from that of other parts of the Mediterranean basin and north-west Europe, then the local peculiarities of climate and topography in the study area could account for the limited expression of these climate events in the pollen record. After all, these are precisely the reasons that refugial populations of deciduous trees have been able to survive in the Pindus Mountains throughout glacial periods (Tzedakis et al., 2002).

Acknowledgements The authors wish to acknowledge the Director of IGME for making the core material available for study, and the staff of IGME, in particular J. Brousoulis, for their advice and assistance. Radiocarbon dating was generously supported by NERC (allocations 596.1294, 770.1298, and 798.0599). Thanks to K. Roucoux, M. Walker, and an anonymous referee for helpful comments on a draft of this paper. ITL acknowledges a NERC studentship (GT04/ES/97) and financial help from the Department of Geography, University of Cambridge, and from Clare College, Cambridge. MRF acknowledges a NERC studentship (GT04/93/6/G), a Fellowship from St Johns College, Cambridge, and additional financial support from the Department of Zoology and Fitzwilliam College, Cambridge. PCT acknowledges a Fellowship from Robinson College, Cambridge.

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