Vegetation Composition in a Typical Mediterranean Setting (Gulf of Corinth, Greece) during Successive Quaternary Climatic Cycles

: The Gulf of Corinth is a semi-isolated basin in central Greece interrupting the Pindus Mountain Range, which nowadays is a biodiversity hotspot. Considering its key location, deep drilling was carried out within the International Ocean Discovery Program (IODP; Expedition 381: Corinth Active Rift Development) aiming to improve our understanding of climatic and environmental evolution in the region. Here, we present a new long pollen record from a Mediterranean setting in the southernmost tip of the Balkan Peninsula recording the vegetation succession within the Quaternary. The Corinth pollen record shows no major shifts in arboreal pollen between glacial and interglacial intervals, while Mediterranean and mesophilous taxa remain abundant throughout the study interval. During interglacials, the most frequent reconstructed biomes are cool mixed evergreen needleleaf (CMIX) and deciduous broadleaf forests (DBWB), while graminoid with forb (GRAM) and xerophytic shrubs (XSHB) dominate within glacials. Our ﬁndings support the hypothesis that the study area was a signiﬁcant refugium, providing suitable habitats for Mediterranean, mesophilous and montane trees during successive Quaternary climate cycles.


Introduction
The study of pollen records from southern Europe has long been acknowledged as fundamental for the reconstruction of past terrestrial environmental changes, being the only quantitative proxy that provides continuous and accurate records of vegetation changes over the Quaternary glacial-interglacial cycles (e.g., [1][2][3][4]). There is consensus that the Quaternary vegetation dynamics of the north-eastern Mediterranean region were alternations between steppe/grassland and deciduous/evergreen forests occurring as a response to glacial-interglacial climatic cycles (e.g., [5][6][7][8][9]). However, climate-driven shifts in vegetation composition and distribution may vary significantly between sites due to the diverse local topography, which is especially pronounced in the Balkan Peninsula. For instance, in the Lake Ohrid pollen record, several arboreal taxa are present, even during glacial intervals (e.g., [10]), while at Tenaghi Philippon, they often disappear and only spread again within interglacials [6,11]. This spatial heterogeneity and proximity to other biogeographical regions (i.e., Asia Minor) produced a mosaic of habitats suitable for a wide range of plants and resulted in a remarkable high floristic diversity (e.g., [12][13][14][15]).
High-resolution pollen records from the southern Balkans suggest that the amplitude and timing of millennial-scale oscillations in mesophilous tree abundances depend on the latitude and altitude of the study area as well as its proximity to mesophilous tree refugia.  [25,26]). In the inset map, the location of regional pollen records is also represented wit Ioannina (I), Lake Ohrid (LO), Lake Prespa (LP), Tenaghi Philippon (TP) and Kopais (K

Chronology
The chronological framework of the deposits is based on magnetostratig on stratigraphic correlation with absolute dated deposits of the neighbouring site M0079. The Brunhes/Matuyama boundary (MBB, 773 ka [27]) of M0078 ha orded at the depth of 386 mbsf, close to the boundary between LU1 and LU addition, the proposed occurrence of the Jaramillo Subchron (1000-1080 k 519.32 and 574.72 mbsf provides further independent chronostratigraphic poin ther age control points using coccolithophore assemblages are limited only to (interglacial) intervals when the basin was connected to the Mediterranean Sea fore, the last occurrence of Pseudoemiliania lacunosa recorded at 298.6 mbsf is not the true last appearance (LAD) at 430 ka [28] as it is recorded during the low 12, when the Gulf of Corinth was isolated from the Mediterranean Sea [23]. age constraints are obtained through stratigraphic correlation with the neigh M0079, while the age-depth model is based on magnetostratigraphy and pala data [29], as well as several Th/U absolute datings [26]. Gawthorpe et al. [26] three types of stratal packages (bioturbated, laminated and bedded), which allo detailed stratigraphic subdivision of lithostratigraphic unit 1, leading to a new lution correlation of stratal packages between sites M0078 and M0079, both from the central of the Gulf of Corinth, from MIS 1-13 ( Figure 2).  [25,26]). In the inset map, the location of regional pollen records is also represented with black dots: Ioannina (I), Lake Ohrid (LO), Lake Prespa (LP), Tenaghi Philippon (TP) and Kopais (K).

Chronology
The chronological framework of the deposits is based on magnetostratigraphy and on stratigraphic correlation with absolute dated deposits of the neighbouring IODP 381 site M0079. The Brunhes/Matuyama boundary (MBB, 773 ka [27]) of M0078 has been recorded at the depth of 386 mbsf, close to the boundary between LU1 and LU2 ( [23]). In addition, the proposed occurrence of the Jaramillo Subchron (1000-1080 ka) between 519.32 and 574.72 mbsf provides further independent chronostratigraphic points [23]. Further age control points using coccolithophore assemblages are limited only to high-stand (interglacial) intervals when the basin was connected to the Mediterranean Sea [23]. Therefore, the last occurrence of Pseudoemiliania lacunosa recorded at 298.6 mbsf is most likely not the true last appearance (LAD) at 430 ka [28] as it is recorded during the lowstand MIS 12, when the Gulf of Corinth was isolated from the Mediterranean Sea [23]. Additional age constraints are obtained through stratigraphic correlation with the neighboring site M0079, while the age-depth model is based on magnetostratigraphy and palaeointensity data [29], as well as several Th/U absolute datings [26]. Gawthorpe et al. [26] recognized three types of stratal packages (bioturbated, laminated and bedded), which allowed a new detailed stratigraphic subdivision of lithostratigraphic unit 1, leading to a new high-resolution correlation of stratal packages between sites M0078 and M0079, both originating from the central of the Gulf of Corinth, from MIS 1-13 ( Figure 2).  [23]. Total samples analyzed (in red) and samples included in the analysis (i black). Stratal packages and correlation with MIS after [26]. FA1: homogenous mud, FA2: greenis gray mud with dark gray to black silty-to-sandy beds (cm-scale), FA3: light gray to white sub-mm laminations (cc or aragonite) alternating with mud-silt beds, FA4: laminated greenish gray to gra mud with muddy beds, FA5: greenish gray mud with homogeneous cm thick gray mud beds, FA green bedded partly bioturbated mud, silt and sand, FA10: interbedded mud/silt and dm thick san beds, FA11: interbedded mud/silt and cm thick sand beds, FA12: light gray to buff, homogenous t weakly stratified mud. Stratal packages: bedded: deposition during glacial periods; biotu bated: deposition during interglacial periods and laminated: deposition during the transitions b tween glacial and interglacial periods.

Palynology and Biome Reconstruction
Sampling of the M0078 core for palynological analyses was carried out at the Breme Core Repository at the University of Bremen, Bremen, Germany. For the present study, total of one hundred seventy-four (174) samples ( Figure 2) were chemically treated. Th upper (LU1; 384.3-0 mbsf) and the lower part (LU2; 610-385 mbsf) of the sequence com prise 112 and 62 samples, respectively. For palynological analysis, approximately thre grams of dry sediment were chemically treated using the laboratory protocol develope during IODP Exp. 381 [23]. Preparation of the samples included the use of hot hydrochlo ric acid (HCl 37%), cold hydrofluoric acid (HF 38%) and sieving through a 10 µm siev To calculate the concentration of palynomorphs in each sample, one Lycopodium tablet wa  [23]. Total samples analyzed (in red) and samples included in the analysis (in black). Stratal packages and correlation with MIS after [26]. FA1: homogenous mud, FA2: greenish gray mud with dark gray to black silty-to-sandy beds (cm-scale), FA3: light gray to white sub-mm laminations (cc or aragonite) alternating with mud-silt beds, FA4: laminated greenish gray to gray mud with muddy beds, FA5: greenish gray mud with homogeneous cm thick gray mud beds, FA6: green bedded partly bioturbated mud, silt and sand, FA10: interbedded mud/silt and dm thick sand beds, FA11: interbedded mud/silt and cm thick sand beds, FA12: light gray to buff, homogenous to weakly stratified mud. Stratal packages: bedded: deposition during glacial periods; bioturbated: deposition during interglacial periods and laminated: deposition during the transitions between glacial and interglacial periods.

Palynology and Biome Reconstruction
Sampling of the M0078 core for palynological analyses was carried out at the Bremen Core Repository at the University of Bremen, Bremen, Germany. For the present study, a total of one hundred seventy-four (174) samples ( Figure 2) were chemically treated. The upper (LU1; 384.3-0 mbsf) and the lower part (LU2; 610-385 mbsf) of the sequence comprise 112 and 62 samples, respectively. For palynological analysis, approximately three grams of dry sediment were chemically treated using the laboratory protocol developed during IODP Exp. 381 [23]. Preparation of the samples included the use of hot hydrochloric acid (HCl 37%), cold hydrofluoric acid (HF 38%) and sieving through a 10 µm sieve. To calculate the concentration of palynomorphs in each sample, one Lycopodium tablet was added at the beginning of the treatment. Residues were mounted in glycerine, while microscopic analysis was conducted under a transmitted light Zeiss Axiolab 5 microscope at magnifications of ×400 and ×1000. Out of the 174 samples analyzed, only 141 are included in the present study as they yielded sufficient pollen content. In these samples, a mean of 400 terrestrial pollen grains were counted. Samples containing fewer than 100 terrestrial pollen grains, excluding Pinus, were omitted from the present study. Terrestrial pollen percentages were calculated based on a pollen sum of all terrestrial pollen excluding Pinus as a result of its overrepresentation in a large number of samples. Oak pollens were classified into three types according to their morphological characteristics, following [30]: Quercus robur-type (deciduous oaks), Quercus ilex-type (evergreen oaks) and Quercus cerristype (semi-deciduous oaks). Further identifications follow [31][32][33][34][35]. Asteraceae comprise Cirsiumand Senecio-types. Pollen taxa were classified into ecogroups following [13].
Pollen diagrams were plotted using the Tilia program [36]. Pollen assemblage zones were determined with the help of constrained incremental sums of squares (CONISS) cluster analysis for all terrestrial pollen taxa with percentages above 2% [36]. Given the millennial temporal resolution of the diagram and the forthcoming high-resolution studies, we assigned Corinth vegetation superzones (CVSZ 1-9 sensu [5]) that roughly correspond to major vegetation shifts. This approach allows for the definition of new pollen zones and subzones within these superzones once the high-resolution data from the Gulf of Corinth archive become available. The biomization technique developed by Prentice et al. [37] was applied to reconstruct the biome distribution in the Gulf of Corinth record. Pollen taxa were arranged into plant functional types (PFTs) and biomes that represent major vegetation types [37,38]. The biome-taxa matrix of Marinova et al. [39] that was developed for the Eastern Mediterranean-Black Sea-Caspian corridor (EMBSeC) has been used. Identified pollen taxa were assigned to twenty-six plant functional types and to thirteen biomes as defined for the region [39]. We ran the analysis in two different matrices including all terrestrial pollen, one with and one without pines due to their overrepresentation in the pollen record. In the analysis including Pinus, although the data matrix was square-rooted in order to increase the method sensitivity and stability, Pinus appeared to mask other vegetation types [39]. Hence, here we present the reconstructed biomes based on the data of the matrix excluding Pinus.

Results
The pollen assemblages are characterized by high concentrations of terrestrial pollen during interglacial intervals with concentrations exceeding 51,370 grain g −1 and a minimum of 127 grains g −1 , while during glacial intervals concentrations are significantly low (max: 5221 grain g −1 ; min: 143 grain g −1 ). The pollen diversity is generally high, and almost 100 different pollen taxa were identified (Table S1). Mesophilous and Mediterranean taxa dominate the pollen assemblages, reaching up to 55% and 48%, respectively. Relatively poor preservation and/or low pollen concentration were recorded in the two intervals 370-580 mbsf and 207-237 mbsf. Therefore, several samples were excluded from this study. Selected pollen taxa are presented in the two main palynological diagrams (Figures 3 and 4), and the main vegetation features are summarized in Table 1.

Vegetation and Climatic Inferences
In the pollen record from the Gulf of Corinth, Mediterranean and mesophilous taxa percentages co-dominate the interglacial pollen assemblages, whereas herbaceous (steppic) percentages dominate within glacial intervals (Figures 3 and 4). High arboreal pollen percentages and concentrations, more evident in the Mediterranean trees, characterize the pollen assemblages in CVSZ-7, CVSZ-5 and CVSZ-3, suggesting forested intervals. These superzones generally represent warmer and wetter climatic conditions corresponding to interglacial intervals.
Intervals characterized by higher non-arboreal pollen (NAP) percentages, when Poaceae and steppic taxa dominate the assemblage, suggest the occurrence of rather open vegetation, such as grasslands or steppe communities, in the surroundings of the gulf. Within these intervals, the persistence of Mediterranean taxa implies that this group remains a significant component of the vegetation. Similarly, the continuous presence of mesophilous and montane tree populations (e.g., CVSZ-6 and CVSZ-4) at favorable locations within the catchment is suggested by the related taxa abundances. These findings can be tentatively associated with rather cold and dry climatic conditions corresponding to glacial intervals and vertical zonation of vegetation surrounding the gulf.

CVSZ-8
Pollen assemblages in CVSZ-8 present significant AP fluctuations (23-67%), expressing the alternation of periods characterized by the dominance of Mediterranean and mesophilous taxa and intervals when steppic elements reach their maximum percentages. Intervals of maximum percentages of Chenopodiaceae (26%), Artemisia (8%), Cichorieae (18%), Asteraceae (14%) and Poaceae (19%) are interrupted by the abrupt increase in trees (e.g., Q. robur-type max: 40%, Q. ilex-type max: 30%). These vegetation shifts are also traced in the reconstructed biomes that show alternations between steppe (GRAM and XSHB) and forested (CMIX and DBWD) communities. Out of 40 samples analyzed in total within this superzone, only 15 samples contained a sufficient number of pollen grains and were, therefore, included in the pollen diagrams. Despite poor pollen preservation and relatively low sampling resolution, these vegetation changes are most likely associated with shifts occurring in the depositional environment during successive glacial-interglacial intervals.
The first expansion of arboreal vegetation (mostly sclerophyllous species) within this interval (LU1.15) suggests interglacial climate conditions, and can be tentatively correlated with MIS 17 or MIS 15, considering the occurrence of the Brunhes/Matuyama boundary at the depth of 386 mbsf (LU1.16 [23]). A similar pattern is observed in the upper part of CVSZ-8 within LU1.13 and LU1.11. The expansion of Mediterranean and mesophilous vegetation suggests the occurrence of warmer and wetter conditions. Based on the lithostratigraphic correlation with the site M0079 [26], these intervals correspond to MIS 13 and MIS 11, respectively.
In LUs 1.14, 1.12 and 1.10, which represent lowstand intervals during which the Gulf of Corinth was isolated from the Mediterranean [23], the AP concentration is significantly low, while several samples were barren. Prominent peaks in steppic taxa as well as lower AP concentrations and percentages within these glacial intervals suggest colder and drier climatic conditions. The most well-defined of all, recorded in six samples, is the AP minimum within LU1.12, which is suggested to correlate with the MIS 12 [26]. This interval presents minima in AP concentration (83 grains g −1 ), but moderate AP percentages (42%) associated mainly with the maxima of montane taxa (e.g., Abies 18%). This expansion is most probably the result of a migration of the montane vegetation zone to lower altitude areas, suggesting the occurrence of suitable habitats for their survival during MIS 12.
Similarly, pollen assemblages from Lake Ohrid during this interval demonstrate a reduction in AP percentages in two stages ( Figure 5), where forest contractions and expansions were recorded [41], suggesting that the MIS 12 glacial phase was cold but wet [7]. These events are similar to the short-term forest expansions recorded in southern Europe during the Last Glacial, both in composition and in duration [42]. In the Balkan Peninsula, for example, the Tenaghi Philippon pollen record shows that the most extreme tree population declines occurred during the glacial maxima of MIS 12, where the lowest AP percentages of all glacial intervals were recorded [11]. A cold but somehow wet MIS 12 is also suggested by the expansion of the mountain glaciers in Greece during MIS 12 and MIS 6, the most extensive glacial phase in the Greek mountains since the Middle Pleistocene [20,43,44].

CVSZ-7
CVSZ-7 is characterized by higher AP percentages compared to the previous interval, reaching a maximum of 70%. The Mediterranean and mesophilous taxa co-dominate and present two distinct peaks throughout the superzone. The first tree expansion (258.17 mbsf) comprises the maximum Mediterranean (36%) and mesophilous (22%) percentages. In the second AP peak (237.75 mbsf), mesophilous and Mediterranean taxa (30% and 26%, respectively) co-dominate, suggesting higher and/or more even moisture in the catchment. This is also reflected in the reconstructed biomes, which show a shift from Mediterranean shrubland (WTSHB and GRAM) to cool mixed forests (CMIX). According to the stratigraphical correlation, LU1.9 corresponds to MIS 9 [26]. It should be noted that despite a total of 18 samples analyzed, only 4 yielded sufficient pollen content. This poor palynomorph preservation could be associated with a concurrent shift in the depositional environment in the basin. Considering the low resolution of the pollen record, the double AP peak pattern of this interval might be tentatively correlated with other regional pollen records (e.g., Ioannina [45] and Tenaghi Philippon [46]) and Turkey (lake Van [47]), where a three-phase mesophilous tree expansion is interrupted by two marked peaks of steppe communities. Therefore, the two distinct AP peaks in this low-resolution interval might be a partial expression of the recorded climatic variability within MIS 9.

CVSZ-6
In CVSZ-6, NAP percentages increase in comparison to the previous interval, reaching a maximum of 72%. This NAP peak comprises mostly grasses (Poaceae 28%) and steppic taxa such as Chenopodiaceae (18%), Cichorieae (14%) and Artemisia (10%). The significant drop in AP concentration (mean: 211 grains g −1 ) in comparison to CVSZ-7, the very low evergreen oak percentages (4%) and the maximum of steppic percentages (33%) point to colder and drier conditions within this interval. The graminoid with forb (GRAM) and xeric shrubland (XSHB) biomes reconstructed during this period also suggest the occurrence of glacial climatic conditions. Based on the stratigraphic correlation with site M0079, sediments within LU1.8 were deposited within MIS 8 [26]. During this period, the Gulf of Corinth appears to be isolated from the Mediterranean Sea based on sedimentological, geochemical and dinoflagellate results [23,40].
Although the vegetation composition suggests the prevalence of glacial conditions, significant mesophilous trees (mean: 16.2%) persist in the catchment throughout this interval. A moderate expansion of mesophilous and montane trees (max: 28 and 10%, respectively) at the upper part of this interval, combined with the decreasing steppic taxa percentages and the maximum values of Poaceae (28%), implies an increase in moisture. High Poaceae percentages are mainly ascribed to extensive grasslands that are typical for rather open landscapes occurring in glacial periods (e.g., [6,48]), whereas a concurrent Cyperaceae peak could also indicate an increase in run-off and the occurrence of wetlands in the borderlands of the gulf. These findings point to a rather open landscape and the colonization of newly exposed land by herbaceous species, such as Chenopods, following this marine regression phase.
This shift in the vegetation composition within MIS 8 is also demonstrated in the reconstructed biomes, which show a transition from herbaceous (meadow) vegetation (GRAM and XSHB) to a more meadow-forest vegetation type (CMIX and GRAMM). These findings point to a more humid late MIS 8, which is in agreement with other regional pollen records ( Figure 5). In the Lake Ohrid record, mesophilous percentages increase within MIS 8, although they do not exceed 10% [10], while the Tenaghi Philippon record shows a shift from steppic to forest-steppe communities [46]. The complex topography of the Gulf of Corinth catchment most likely allowed the vertical migration of species and the parallel occurrence of all major vegetation zones. The east-west direction of the gulf and the prevalence of moist westerly winds could also account for the persistence of a significant mesophilous vegetation component in the borderlands of the gulf.
The lowermost (213.55 mbsf) AP expansion (AP concentration 765 grains g −1 ) presents the highest values of Mediterranean trees (40%), the minimum mesophilous percentages (8%) and the absence of montane taxa, implying warmer climatic conditions, but increased summer aridity. Within the second phase (202.04 mbsf), maxima of AP concentration as well mesophilous and montane taxa (2018 grains g −1 , 20% and 8%, respectively) are evidenced, while Mediterranean trees remain abundant (35%), suggesting an increase in moisture availability. At the uppermost (186.95 mbsf) AP expansion, the AP concentration is again lower (1000 grain g −1 ), mesophilous elements remain stable (19%) and a minor increase in Mediterranean is observed (38%), while montane taxa are almost absent (<2%). This three-step tree expansion is disrupted by two intervals (205 and 197-190 mbsf) of profound steppic vegetation expansion, corresponding to periods with deteriorated climatic conditions (stadials). Within the lower interval (205 mbsf), the minimum values of Mediterranean trees (6%) combined with the maximum values of Cichorieae and, surprisingly, mesophilous taxa (19% and 35%, respectively) indicate a period with decreased temperatures but with relative humidity. The reduction in AP concentration (108 grains g −1 ) supports the notion of forest openness. A more steppe-like environment with maximum of Artemisia (14%), Chenopodiaceae (16%), Asteraceae (15%) and Poaceae (52%) is observed within the following interval (197-190 mbsf). The decrease in mesophilous (min: 6%) and the short-lived expansion of montane taxa (max: 17%), combined with the increasing trend of steppic taxa (max: 25%), attest to a drop in temperature and a decrease in moisture availability during this stadial interval. The recorded abrupt maxima of Poaceae (>50%) most probably feature the expansion of the Phragmites lacustrine vegetation belt around the Gulf of Corinth during the temporal sea lowstand recorded in LU1.6 [23,40].
The vegetation shifts observed within the MIS 7 interglacial complex are also evident in the reconstructed biomes. A transition from herbaceous vegetation (GRAM) to cool mixed evergreen and deciduous broadleaf woodland (CMIX and DBWD) indicates the climatic oscillations occurring within CVSZ-5. The response of vegetation during this interval differs from that of MIS 9 and MIS 11. In particular, evergreen oaks present higher abundances than deciduous oaks, suggesting the occurrence of warmer and increased seasonality than during previous interglacial intervals. In the Lake Ohrid pollen record, this interglacial/interstadial complex is characterized by three-step alternations of a cooccurrence of montane and mesophilous taxa with two steppe vegetation expansions [7]. In our record, the forest expansion phases are mainly represented by the Mediterranean trees; however, mesophilous trees appear to be less abundant in this southern Mediterranean site. However, similarly to other western Balkan sites, they persist during the open vegetation expansion intervals (Ioannina: [49]; Lake Ohrid: [7]), while at the eastern site of Tenaghi Philippon, mesophilous tree populations almost disappear during the stadial intervals [5]. Finally, in both Lake Ohrid and the Gulf of Corinth records, maximum values of Artemisia indicating arid conditions are observed during the later stadial of the MIS 7 complex [7].
In the lower part (177.59-150.12 mbsf), maxima in the Poaceae (36%), Cyperaceae (13%) and riparian taxa (14%) suggest increased run-off. Mesophilous taxa percentages show a gradual decline upwards, and their values decrease from 30% to approximately 5% at a depth of 150.12 mbsf to stabilise around 14%. In the upper part of CVSZ-4, Mediterranean trees and Artemisia reach their maxima (39% and 10%, respectively), which, combined with decreasing mesophilous percentages, imply a decrease in moisture availability in the catchment. This vegetation shift is also evidenced in the reconstructed biomes, which show a transition from cool mixed forests (CMIX) to herbaceous vegetation (GRAM) upcore.
These findings point to a more humid early MIS 6, which is in agreement with other regional records. A cold but wet MIS 6 is also suggested by the expansion of the mountain glaciers in Greece, as during MIS 6 and MIS 12, the most extensive glacial phases in the Greek mountains since the Middle Pleistocene were recorded [20,43,44]. Pollen records from the northern Pindus Mountain Range record a shift from grasslands to steppic communities (Lake Ohrid [7]) and a decrease in mesophilous percentages during MIS 6 (Ioannina [17]). In contrast to these archives, AP values remain above 20% throughout this interval within the Gulf of Corinth. This suggests that the surroundings of the gulf provided suitable habitats for the survival of montane, mesophilous and Mediterranean tree species during MIS 6 4.1.7. CVSZ-3 CVSZ-3 is characterized by the highest, though fluctuating, AP abundances up to 85%, featuring an interval with ameliorated but unstable climatic conditions. At the lower part of the superzone, the pollen data disclose an interval marked by three Mediterranean tree expansions with maximum pollen concentrations alternated with an expansion of mesophilous tree taxa and a forest openness phase upcore, respectively. Based on the age constraints of the deposits and the sedimentological description of LU1.3, the lower part of CVSZ-3 could tentatively correspond to the Last Interglacial Complex [26].
The lower part (112.71-91.62 mbsf) of CVSZ-3 is characterized by the highest AP concentration (maxima of 26,835 grains g −1 ) and sharp peaks in Mediterranean (max 48%) and mesophilous (max 33%) tree abundances, implying the occurrence of interglacial conditions. Three minor retreats of Mediterranean taxa are evidenced at 108.75 mbsf, 101.86 mbsf and 97.45 mbsf: the two coinciding with a major increase in mesophilous and montane taxa (29%) and the intermediate one with the short expansion of steppe communities (28%) with Artemisia (19%). These sharp shifts represent expansions and retreats of Mediterranean and mesophilous vegetation during the Last Interglacial Complex, with the peaks of montane and steppic elements characterizing the intervals of climatic deterioration. These vegetation shifts are also evidenced in the reconstructed biomes, which show a transition from cool mixed forests (CMIX) to herbaceous vegetation (GRAM and XSHB). During the interstadials of the MIS 5 complex, the vegetation composition of regional pollen records from the southern Balkans (e.g., Tenaghi Philippon [50] and Lake Ohrid [7,51]) presents the dominance of mesophilous communities in the assemblage. In contrast to these archives, where Mediterranean taxa never exceed percentages of 15%, in the Gulf of Corinth pollen record, they prevail and remain abundant even during stadials as a result of the catchment area location and topography.
Upwards, within the lower part of LU1.2 (91.62-80.24 mbsf), an increase in NAP percentages, mainly recorded in the maximum values of Artemisia (24%), Chenopodiaceae (15%), Asteraceae (15%) and Poaceae (36%) and a minimum in AP concentration (690 grains g −1 ), is evidenced. In addition, this features periods with deteriorated climatic conditions during the Last Glacial interval. Mediterranean taxa, mainly Q. ilex-type, present significantly lower values, compared to the previous interval, featuring the occurrence of deteriorated climatic conditions during the Last Glacial interval. In parallel, the maximum Poaceae percentages (36%) might represent the colonization of emerging areas during this lowstand interval. Following this, an expansion of mesophilous tree percentages (55%), featuring a maximum in the Q. cerris-type, combined with the decreased steppic abundances suggest a temporal expansion of the forest cover.
The Despite the recorded vegetation fluctuations within this upper part of CVSZ-3, the resilience of mesophilous and Mediterranean trees in the borderlands of the Gulf of Corinth should be highlighted. This could be assigned to the southern location of the site and the topography, which allows the vertical migration of vegetation belts. The persistence of mesophilous taxa, even during glacial intervals, is also recorded in both Lake Ohrid [7,10] and Lake Ioannina pollen records [17], in contrast to the Tenaghi Philippon archive [11]. Finally, an interesting feature is the Cedrus fluctuations. This taxon appears in higher abundances (max:~4%) within this Last Glacial interval, compared to the Lake Ohrid pollen record [10], in which it presents extremely low values (~1%). Overall, in the catchment of the Gulf of Corinth, Cedrus presents its maxima within glacial intervals, while in other records from the northern Pindus Mountain Range, the maximum percentages are evidenced during interglacial phases. This could likely be assigned to the southern position and the more humid climatic regime of the site, which allowed this summer drought-tolerant species to expand in snow-fed favorable habitats of the montane zone during glacials.
The synchronous major drop in Mediterranean vegetation (min: 3%) is also indicative of the unfavorable climatic conditions that are also demonstrated by the dominance of the xeric shrubland (XSHB) biome. In the lower part of this interval (62.67-50.44 mbsf), maxima in Artemisia (24%) and Ephedra spp. (6%) and a gradual decline in mesophilous taxa percentages from 45% to 13% at a depth of 50.44 mbsf are recorded, suggesting the apparent opening of the landscape and the predominance of dry conditions. The sharp decrease in AP percentages and concentration (min: 32% and 118 grains g −1 , respectively) observed at a depth of 50.44 mbsf most likely demonstrate the vegetation response to the Last Glacial maximum. Furthermore, a sharp increase in montane taxa (max: 33%) and a major drop in Mediterranean taxa (min: 3%) indicate the occurrence of cool climatic conditions that pushed the tree line, shifting the montane zone towards lower altitudes. In parallel, the maximum percentages of Salix (3%) could probably suggest increased run-off.
At the upper part of CVSZ-2, an expanding trend of mesophilous and Mediterranean trees, reaching a maximum of 49% and 26%, respectively, an increase in AP concentration (mean: 460 grain g −1 ) and increased abundances of herbaceous vegetation (max: 37%) are observed. The reconstructed biomes also feature the transition from steppe communities (GRAM) to cool mixed forests (CMIX) within this upper part of the Last Glacial interval. The gradual expansion of mesophilous taxa, reaching 49%, is an interesting characteristic of the Gulf of Corinth record in comparison to other regional archives. A similar vegetation signal has been recorded in Lake Kopais [9,19] and in the nearby Saronic Gulf, implying relatively wet and mild climatic conditions during the Late Glacial [52]. In contrast, the northern records, along the Pindus Mountains, present either lower mesophilous taxa percentages (e.g., Lake Ioannina [16] and Lake Ohrid [7]) or the spread of pines, in some higher-altitude catchments such as Xinias, Lakes Orestias and Prespa, Edessa [53][54][55][56]. These findings suggest that the surroundings of the gulf provided suitable habitats for the survival of mesophilous taxa within the Last Glacial interval, confirming the regional climatic/ecological gradients along the Balkan Peninsula [17].

CVSZ-1
CVSZ-1 is characterized by high AP percentages (max: 73%) at the base, which gradually decrease upwards, presenting a minimum of 33%. The lower part represents the expansion of deciduous oaks (47%) at the onset of the Holocene in response to the climatic ameliorations. Following this, the sharp increase in Mediterranean tree taxa (max: 38%) and a reduction in mesophilous trees attest to the temporal expansion of Mediterranean shrubland during warmer and drier climatic conditions. In the upper part, the retreat of both deciduous and evergreen woodland, along with the expansion of Cichorieae (max: 21%) and Chenopodiaceae (max: 10%), can be attributed to the increasing human activity in the borderlands of the Corinth Gulf since antiquity, as also reported in several onshore palaeovegetation studies (e.g., [57,58]) ( Figure 3). The low AP concentration (mean: 270 grains g −1 ) in the upper part could be assigned to the high sediment influx in the basin [23] during the Holocene. The regeneration of Mediterranean tree taxa, leading to the development of the present-day landscape dominated by maquis vegetation, is attested at the uppermost part of the studied sequence.

Comparison with Regional Pollen Records
Comparison of the Gulf of Corinth with other long continuous sequences from the southern Balkan Peninsula, such as Kopais, Ioannina, Tenaghi Philippon or Lake Ohrid [7][8][9]11,17,59,60], showed that, despite certain differences, a correlation scheme among the pollen records could be assumed. The main similarity with Lake Ohrid is that arboreal pollen is continuously present in both sites, even during glacial intervals, while in other records of the Balkan Peninsula, such as Tenaghi Philippon or Kopais [9,11], they often disappear and only occur during interglacials [17] (Figure 5). The vegetation at Tenaghi Philippon seems to have reacted in a more sensitive way to the climatic cycles, possibly due to the reduction in temperature and moisture [51]. Quite a different situation is found at Ioannina (western Greece), a site characterized as a refugium for mesophilous trees, featuring sub-Mediterranean vegetation and climate during the last ∼480 ka [17,59,60]. Similarly, the survival of several mesophilous trees is recorded within the catchment of Lake Prespa during the Last Glacial [18]. It has already been proposed that mid-altitude sites were more suitable for maintaining refugial populations of mesophilous trees because of orographic precipitation [61]. The study of pollen records from different bioclimatic areas in Greece presented in [17] exhibited the significance of local topography and ecological factors in controlling the vegetation response to climate variation. Topography controls the occurrence and the availability of local microenvironments, in variable altitudes, to which populations can accommodate in response to climate changes. Both temperature and CO 2 concentration are known to increase with decreasing altitude. Vertical migration allows plant populations, at least in part, to avoid extirpation, by providing sufficient topographic variability to supply a range of microhabitats suitable for survival [17]. On a regional scale, the Gulf of Corinth pollen record presents the greatest similarities to the Lake Ohrid pollen record [7,10]; however, some very intriguing differences are reported. The Lake Ohrid record shows the sensitive response of vegetation to Quaternary climate variation, while in the Corinth pollen record, no major vegetation shifts are observed between glacials/interglacials. Mesophilous trees are dominant in Lake Ohrid [7,10]; however, Mediterranean trees appear to be the main vegetation component in the Gulf of Corinth, in high percentages both in glacial and interglacial intervals. They are more resilient to the climate fluctuations, possibly due to their lowland distribution and their ability to withstand aridity. In addition, mesophilous trees were significant components both in the glacial and interglacial vegetations (20%).
In the Gulf of Corinth pollen record, the Mediterranean trees represent the vegetation composition during interglacials, while the steppic maxima represent the vegetation during glacials, in contrast to the typical AP/NAP glacial/interglacial pattern. The response of vegetation in the borderlands of the Corinth Gulf to climatic variability appears more complex due to the southern location of the site and the topography allowing the vertical migration of vegetation belts. Long pollen records located in mid-altitudes (between 400 and 850 m above sea level) across the Pindus Mountain Range, such as the Ioannina, Prespa and Ohrid records, suggest the survival of mesophilous trees during glacial intervals cycles [7,[16][17][18]. In contrast, the Gulf of Corinth basin is surrounded by an extensive range of altitudes (0-2000 masl), allowing a wide spectrum of shifts in vegetation, resulting in plant survival during glacials. Finally, the sharp fluctuations observed in the tree pollen percentages in the Gulf of Corinth record have also been reported for the last climatic cycle in the southern Kopais, and most likely relate to the quick expansion of proximal refugial populations [19].
Finally, the persistence of Cedrus in the Gulf of Corinth record is noteworthy. Previous palaeobotanical studies across the northern Mediterranean region report the gradual retreat and disappearance of this subtropical species during the Early Pleistocene (e.g., [62,63]), while in southern Italy, they most likely persisted till MIS 13 [64,65]. The gradual extinction of Cedrus across the main three northern Mediterranean peninsulas has been assigned mainly to climate oscillations, catchment properties and biogeographical and ecological traits of the plant itself (e.g., [66,67]). In the Balkan Peninsula, Cedrus becomes extinct in the Tenaghi Philippon pollen record in the Early-Middle Pleistocene Transition [11]. In Lake Ohrid, Cedrus appears in low abundances throughout MIS 8-5, while in Gulf of Corinth, it exhibits a nearly continuous curve up to the Last Glacial interval. In addition, the major peak of Cedrus (>40%) in Gulf of Corinth, recorded in CVSZ-9, is significantly higher than its abundances observed in Lake Ohrid during the same intervals, while percentages above 30% have only been recorded during MIS 37 and MIS 39 [10,13]. High abundances of Cedrus (>20%) are also recorded in the pollen spectra of Tsampika microbasin (Rhodes), within the MIS 17 and MIS 18 [68]. The persistence of Cedrus in the borderlands of the Gulf of Corinth could be attributed to the local topography and the occurrence of a milder microclimate favoring the plants' survival.

Conclusions
The sedimentary archive from the Gulf of Corinth is the first long palaeoenvironmental record retrieved from the southernmost tip of the Balkan Peninsula, and provides a unique opportunity to reconstruct the terrestrial biodiversity and its response to climatic variability throughout the Quaternary. In the pollen record from the Gulf of Corinth, Mediterranean trees represent the vegetation composition during interglacials, while the steppic maxima represent the vegetation during glacials, in contrast to the typical AP/NAP glacial/interglacial pattern. Mediterranean sclerophyllous vegetation is among the most striking features of the assemblages, while mesophilous taxa are represented in lower abundances and open vegetation is a significant component of the pollen assemblage throughout the entire record. The persistence of Mediterranean and mesophilous vegetation during glacials presents the refugial character of the Gulf of Corinth. The complex topography of the Gulf of Corinth, with a high range of altitudes, and the large size of the basin catchment provide space for the vertical migration of the vegetation zones in the Gulf of Corinth borderlands. Comparison with other long pollen records of the Balkan Peninsula presented some similarities; however, significant differences are also reported.