Late Glacial Marine Transgression and Ecosystem Response in the Landlocked Elefsis Bay (Northern Saronikos Gulf, Greece)

: Coastal landscapes are sensitive to changes due to the interplay between surface and submarine geological processes, climate variability, and relative sea level fluctuations. The sedi ‐ mentary archives of such marginal areas record in detail the complex evolution of the paleoenvi ‐ ronment and the diachronic biota response. The Elefsis Bay is nowadays a landlocked shallow marine basin with restricted communication to the open Saronikos Gulf. A multi ‐ proxy investiga ‐ tion of a high ‐ resolution sediment core recovered from the deepest part of the basin offered a unique opportunity to record the paleoenvironmental and aquatic ecosystem response to climate and glacioeustatic sea level changes since the Late Glacial marine transgression. The retrieved sedimentary deposits, subjected to thorough palynological (pollen, non ‐ pollen palynomorphs, dinoflagellates), micropaleontological (benthic foraminifera, calcareous nannoplankton, ostra ‐ cods), and mollusc analyses, indicates isolation of the Elefsis Bay from the Saronikos Gulf and the occurrence of a shallow freshwater paleolake since at least 13,500 cal BP, while after 11,3500 cal BP the transition towards lagoon conditions is evidenced. The marine transgression in the Elefsis Bay is dated at 7500 cal BP, marking the establishment of the modern marine realm.


Introduction
The complex Mediterranean coastal landscape includes rocky shorelines, bays, river deltas, coastal marshes, or lagoons, continuously shaped by geological processes, climate, and increasing human activity. Coastal areas, marginally located between land and sea, comprise a wide range of habitats and natural resources, thus, being continuously exploited by human populations since prehistoric times. The coastal zone is a dynamic environment, which is, however, particularly vulnerable in the context of global change [1] that has been dramatically transformed by the relative sea level changes in response to the Quaternary glacial/interglacial cycles (e.g., [2,3]).
Sedimentary records retrieved from coastal areas provide valuable evidence concerning past environmental changes, such as climate variability or sea level fluctuations, and ecosystem response. The study of such archives is critical for in-depth understand-playing modest activity [55]. Several volcanoes and volcanic outcrops of Plio-Quaternary age, being part of the western active Aegean Volcanic Arc, are situated in the gulf [56].
The Elefsis Bay lies in the northern part of Saronikos Gulf, covers an area of ~68 km 2 and forms a natural embayment between western Attiki and Salamis Island (Figure 1). The bay is relatively shallow with a maximum depth of 33 m and communicates with the Saronikos Gulf through two straits, a western and an eastern one, having depths of 8 m and 12 m, respectively [43]. The water circulation is from west to east during summer, influenced by temperature difference between the water in the bay and the Saronic Gulf, while a reversed attenuated surface flow is recorded during winter, caused by differences in salinity [57].The torrents of Sarantapotamos and Katsimidis, which drain the mountains of Pastra and Pateras, respectively, are the main sources of sediment and freshwater inputs to the basin [58]. Limestones and clastic sediments are found on land around the bay, associated with the Pliocene/Pleistocene substrate, while the Elefsis Plain is covered by recent post-alpine sediments, mostly talus, cones and scree [59]. The physical parameters of the water column in Elefsis Bay show a typical seasonal variation. The annual maximum temperature (25 °C) occurs during summer, while the minimum one (10 °C) is recorded in February/March. Salinity varies from 37.5 in March to 39 in summer (e.g., [60,61]). The reduced freshwater input and the restricted water mass exchange between the Elefsis Bay and Saronikos Gulf during summer leads to the development of a persistent thermocline down to a depth of 15 m. The dissolved oxygen stratification along the thermocline is also pronounced, featured by an upper layer (<15 m) of oxygen supersaturation and consequent bottom water anoxia. During winter, an intense vertical mixing of the water masses occurs and the entire water column is enriched with oxygen (e.g., [51,62]). Finally, the trophic status of the Elefsis Bay is characterized as eutrophic [50,51], even though a general environmental improvement has been recorded over the last 30 years [63].

Materials and Methods
The gravity core S2P was recovered from the deepest part of Elefsis Bay (38°00.50′ N, 23°27.48′ E; see Figure 1) at a depth of 35 m during an expedition with the R/V Aegaeo of the Hellenic Centre for Marine Research. The lithology and physical characteristics of the 342-cm-long core have already been presented by Petropoulos et al. [43]. Five major sedimentary units have been identified, based on their sand and biogenic debris content as well as their color ( Figure 2): (i) the Unit A (0-192 cm), consists of light olive grey to olive grey mud with shell fragments; (ii) the Unit B (192-231 cm), mainly comprising grey clay; (iii) the Unit C (231-297 cm), demonstrates a variety of colors (yellowish, light grey, grey, dark grey, olive grey) and alternations of mud and sandy mud layers, while biogenic fragments occur within the upper portion of the bed; (iv) the Unit D (297-300 cm), including an organic black sandy mud; and, finally, (v) the Unit E (300-342 cm), is composed of grey sandy mud with biogenic debris.
In addition to the previous sedimentological analysis, the relative proportion of sand and mud throughout the length of the core S2P was determined in detail by dry sieving of two hundred and thirty-one samples, while the total inorganic carbon (TIC) profile was derived by applying the equation of Jiang et al. [64].

Core Chronology
The chronostratigraphic framework of the core was defined using accelerator mass spectrometry (AMS) radiocarbon ( 14 C) dating, carried out in the Beta Analytic laboratory. The AMS 14 C dates were obtained from seven marine mollusc shells and one terrestrial plant material ( Table 1). Calibration of the conventional 14 C ages to calendar years was made using the Marine13 calibration curve, by applying a ΔR correction of 73 ± 61 years for the marine reservoir effect in Piraeus (Saronikos Gulf), and the terrestrial IntCal13 calibration curve, respectively [65,66].
Finally, the age-depth model of the core was derived from the package rbacon [67] in the R console (v. 3.5.0).

Foraminifera and Ostracod Analysis
One hundred and fifty sediment samples (with the dry weight of each sample being 10 g) were treated with hydrogen peroxide (H2O2) solution 10%to remove the organic matter, washed through 63 μm and 125 μm stainless steel sieves and dried at 70 °C.
Seventy-two of the samples were analyzed for their benthic foraminiferal content in the >63 μm fraction. A subset containing at least 200 individuals per sample was obtained using an Otto microsplitter. The individuals were identified under a Zeiss Stemi 305 stereoscope, following the generic classification of Loeblich and Tappan [68,69], and based on the studies of Cimerman and Langer [70], Hottinger et al. [71], Sgarrella and Moncharmont-Zei [72], and Dimiza et al. [73]. The total foraminiferal density (FD; number of specimens/g) and the relative abundances in the benthic foraminiferal assemblages were calculated. For samples containing less than 25 individuals, the countings were excluded from the quantitative analysis. Further, the Shannon-Wiener diversity index (Η′) was calculated, using the Past.exe 1.23 software package [74]. Additionally, all broken-reworked specimens were selected, without, however, being included in the counting.
Eventually, the >125 μm fraction in each of the one hundred and fifty samples was examined for the qualitative determination of its ostracod content. Ostracod species were identified under a Leica stereo microscope, based on the studies of Bonaduce et al. [75], Stambolidis [76], Tsourou [77], and Tsourou et al. [78].

Calcareous Nannoplankton Analysis
Seventy-five samples have been prepared for Calcareous Nannoplankton (CN), following the standard smear slide techniques. Out of them, only forty samples in the last 186 cm of the core, were bearing CN assemblages. Detailed descriptions of the quantitative counting methods and taxonomy of CN analysis are presented in the studies of Triantaphyllou et al. and Triantaphyllou [25,26,79]. The results of the analysis are expressed as percentages in order to avoid any dilution effects, e.g., lithogenic input [80]. The variation in the joint abundance of Helicosphaera carteri and Syracosphaera spp. is used as a paleotracer of fresher upper water layer (e.g., [80][81][82]). Helicospahera carteri has been reported exhibiting high frequencies in modern regions influenced by riverine discharge [83], as a coastal water taxon (e.g., [84]). Hints for an opportunistic behavior in estuarine environments, have been already given by Cachão et al. [10,85,86]. The abundances of reworked taxa from older sedimentary units are used as a lithogenic input proxy.

Palynological Analysis
One hundred and four samples were investigated via a meticulous palynological analysis. The samples were weighted, spiked, and chemically treated, following the standard palynological protocol including sieving through a 10 μm sieve (e.g., [87]. During the microscopic analysis, pollen, dinoflagellate cysts, green algae coenobia, and spores, as well as non-pollen palynomorphs (NPPs), were identified and counted. The six bottom samples of the S2P sedimentary sequence, corresponding to the 295-334 cm core-depth interval, were excluded from the analysis due to their low and poorly-preserved palynomorph content. An average of 300 pollen grains, excluding pollen from aquatics and spores, was counted in each of the ninety-eight examined samples [87], while dinoflagellate cysts were counted in approximately every second sample. An average of 120 dinoflagellate cysts was counted in each examined sample from the upper 247 cm of the core, while below that core depth only pollen, green algae, and NPPs were identified. The dinoflagellate cyst identification was based on the studies of Mudie et al. and Van Nieuwenhove et al. [88][89][90]. Cysts of the Lingulodinium machaerophorum with processes length shorter than 10 μm (L. machaerophorum sh.p.) were separately counted, based on the work of Mertens et al. [91,92]. Finally, the results derived from the analysis of pollen, dinoflagellate and NPPs are presented in both percentage and concentration diagrams.

Mollusc Analysis
The invertebrate specimens of the S2P core were retrieved from one hundred and forty-three samples, spaced at 1-3 cm core-depth intervals. Following the standard wet-sieving procedure, a maximum of 10 g of dry sediment from each sample was chemically treated with water-diluted Η2Ο2 (Perhydrol, 30% v/v Η2Ο2) and sieved through a 63 μm sieve. Treated samples were inspected under a Leitz Wetzlar stereo microscope and identified based on the WoRMs database [93], the MSIP database [94] and the study of Sakellariou [95]. The shell selection was based on the minimum number of individuals (MNI) criterion, while rounded/reworked specimens from specific core depths were also examined. Moreover, the collected specimens were distinguished to fresh or etched, depending on the shell conservation, which was considered as a paleoenvironmental condition parameter.

Organic Geochemistry
Selected lipid biomarkers, i.e., aliphatic hydrocarbons, long-chain alkenones and aliphatic alcohols, were determined in thirty-eight samples of the sedimentary Unit A as indices of paleoenvironmental conditions and land-sea interactions. Lipids were extracted from freeze-dried sediments by ultrasonication with a mixture of dichloromethane/methanol (4:1, v/v) and individual compounds were identified and quantified by means of gas chromatography, following Gogou et al. [96]. Estimates of past sea surface temperatures (SSTs) were made using the unsaturation ratios of alkenones (U k' 37) [97]. The U k' 37 index was converted into SST using the global calibration of Conte et al. [98]. High molecular weight odd-carbon number n-alkanes (n-C27, n-C29, n-C31 and n-C33) and even-carbon number n-alkanols (n-C24, n-C26, n-C28 and n-C30), major components of epicuticular higher plant waxes, are used as proxies of allochthonous natural (terrestrial) inputs [99,100]. The sums of their concentrations are defined as ΣTerNA and ΣTerN-OH, respectively. Variations of the average chain length of terrestrial n-alkanes, i.e., ACL index [101], are commonly related to changes in the temperature and humidity/aridity in the growing environment, since plants tend to synthesize longer chain length waxes in response to elevated temperatures. The proportion of ΣTerN-OH in the sum of ΣTerNA and ΣTerN-OH (HPA index [101]) can be used to evaluate the proportions of labile and refractory organic matter delivered in the marine environment as well as in situ preservation vs degradation.

Sedimentology/Age Depth Model
The mud fractions prevail in the S2P sedimentary record; however, increased sand content is observed within the 0-100 cm and 250-342 cm core-depth intervals ( Figure 2).
In the bottom Unit E, calcite layering is an outstanding feature, possibly indicating the substrate of the area that is most probably associated with the "Marls of Piraeus" considered of Pliocene age [102].
Upwards in Unit D, the occurring 3-cm-thick layer shows increased TOC values (~1.78%) despite its high sand content ( Figure 2). This in combination with the black color of the sediment suggests a peat material, formed by incomplete decomposition of organic matter due to waterlogging and the subsequent anoxic condition. In addition, it is remarkable that Unit D disconformably overlies the eroded upper limit of Unit E (see photograph in Figure 4 of Petropoulos et al. [43]), thus, revealing a hiatus within the S2P depositional sequence. For this reason, a valid age-depth model for the S2P sequence could only be derived for the 0-300 cm core-depth interval, with the indicated ages ranging from 13,500 to 107 cal BP (see Figure 2).

Figure 2.
Physical characteristics of the core S2P, as have been already determined by Petropoulos et al. [43], and a Bayesian age-depth model (for the 0-297 cm core-depth interval) derived from the R package rbacon (see Blaauw and Christen [67]). TOC/Ntotal ratios of pure marine organic matter are usually less than 8, while those between 8 and 12 indicate a mixed source of organic matter from land and sea [103]. Therefore, considering the relevant profile in Figure 2, Unit D and most of Unit C (248-297 cm) can be interpreted as terrestrial environments (C/N: 16-21.5), while the signal in Unit A reveals a marine environment, which, however, is affected by terrestrial processes (C/N: 10.8-13). Eventually, Unit B and the rest of Unit C display C/N values (13-14.6) that imply a transitional depositional system, e.g., estuarine or lagoon.
Finally, in Unit A, the age model indicates a higher sedimentation rate at ~4000 cal BP ( Figure 2). This could imply the occurrence of an upper shoreface depositional environment at that time.

Foraminiferal and Ostracod Analysis
The benthic foraminiferal assemblages in the S2P deposit are generally well preserved and a total of 67 species, belonging to 29 genera, were identified. The FD values demonstrate a high variability, ranging from 5 to 3908 specimens/g, while the H' index displays great fluctuations as well (0-2.76). The assemblages (see Figure 3) are mostly composed of Ammonia beccarii, A. tepida, Aubignyna perlucida, other small rotaliids (mainly Nonionella turgida, Rosalina brady, and Planorbulina mediterranensis), bolivininds (mainly Bolivina elongata), elphidiids (mainly Elphidium translucens and E. gunteri), miliolids (mainly Triloculina trigonula and T. tricarinata), and agglutinant species (mainly Textularia bocki and T. conica). The interval from the bottom to 297 cm is characterized by the presence of calcified benthic foraminiferal specimens, mostly of A. tepida. The ostracod assemblage is characterized by the scarce presence of Cyprideis torosa calcified valves, which just below the peat material of Unit D (300-301 cm) occur in high numbers, exceeding 100 individuals/g. Other ostracod taxa, such as Xestoleberis and Hiltermannicythere, are also present, but are considered to be reworked.
Between 297 and 248 cm (13,500-11,350 cal BP), scarce E. gunteri specimens constitute the assemblage, being actually the only species observed in most samples ( Figure 3). In this interval, only the upper two samples fulfilled the criterion to be included in the quantitative analysis, with E. gunteri appearing to be dominant and A. tepida showing a low abundance (11.6%). The FD parameter varies from 118 to 316 specimens/g, even though the H' index exhibits very low values (max 0.36). The ostracod assemblage is marked by the high occurrence of C. torosa, with many noded specimens (mainly juveniles), forming monospecific assemblages; in addition, charophytes were encountered.
Within 248-187 cm (11,350-7500 cal BP), an abrupt increase in A. tepida values is evidenced, exceeding 90% in the assemblage for the majority of the samples. Elphidium gunteri shows a lower abundance, while A. perlucida and other small rotaliids, miliolids, and agglutinated taxa participate in the assemblage composition with minor values, mainly occurring within the upper part of the interval. The FD parameter demonstrates the highest values in the S2P sequence (max 977 specimens/g), while the H' index still presents a low level (max 1.75). In the same interval the ostracod assemblage are more diversified but they still remain oligospecific. Cyprideis torosa is the most abundant species, mainly accompanied by Xestoleberis sp., Aurila woodwardii, Leptocythere lagunae, and Cytherois fischeri. From

Calcareous Nannoplankton Analysis
All samples taken from Units B-E were barren of nannofossils. CN specimens begin to sporadically appear in the S2P record at 186 cm core depth, since ~7500 cal BP ( Figure  4) and their assemblage is marked by a low number of species. Emiliania huxleyi is the dominant species throughout the studied interval (Figure 4), contributing to the total assemblage with frequencies up to 85%. Rhabdosphaera clavigera is the second most abundant species, representing 15%, on average, of the total coccolithophore assemblage. Its relative abundance varies between 5% and 18% during 7500-4000 cal BP (Figure 4), while afterwards shows an increase with a distinct peak (30%) centered at ~1300 cal BP and a relatively constant decreasing trend from 20% to less than 2% between 1000 and 100 cal BP. The distribution pattern of the species group Helicosphaera carteri + Syracosphaera spp. demonstrates relatively high values at ~7000 cal BP and between 3500 and 2000 cal BP ( Figure 4). Finally, reworked taxa are present at low frequencies since 3800 cal BP, reaching their maximum abundances after 2000 cal BP with a prominent peak (up to 5%) at ~600 cal BP (Figure 4).

Palynological Analysis
The 0-300 cm interval of the S2P deposit yielded abundant pollen, dinoflagellate cysts, green algae, and other palynomorphs with significant temporal variability (see Figure 5). During the last 13,300 cal years BP the mean pollen concentration was 14,000 g/g (grains/g), while maximum values of 46,000 g/g were recorded at ~12,000 cal BP. A detailed description of the pollen assemblages and inferred vegetation development has already been presented in the study of Kyrikou et al. [87]. Dinoflagellate cysts are continuously evidenced since 11,350 cal BP with a mean concentration of 6250 c/g (cysts/g), while green algae, mainly represented by Botryococcus coenobia, are present throughout the studied interval. Based on the aquatic palynomorphs encountered, three zones, i.e., PZ-I to PZ-III, may be determined, each one corresponding to a distinct palynomorph assemblage:

Mollusc Analysis
A total of 10,500 mollusc specimens were collected, which resulted (MNI) in 7710 individuals (7251 fresh and 459 etched shells) belonging to 45 genera and 47 bivalve and gastropod species (see Figure 6).
From the bottom to 297 cm the fauna includes exclusively rounded and possibly "fossilized" marine mollusc fragments, belonging mainly to bivalves but also to gastropods. Although the taxonomic attribution is not possible in a large amount of the specimens, some was possible to identify and they belong to Cerastoderma, Veneridae, Loripes, small Hydrobiidae, and few internal casts of hydrobiid gastropods. However, a few core-depth intervals showed a lack of any molluscan fauna.

Alkenone-Based SSTs and Paleoenvironmental Indices
The alkenone-derived SST measurements provide an average of 24.6 ± 0.8 °C, ranging from 22.3 to 25.7 °C. SSTs in Elefsis Bay increase (by ~1° C) from 7500 to 7100 cal BP (Figure 7), reaching 25.2 °C at ~5500 cal BP and 25.7 °C at ~1600 cal BP, which is the highest value of the record. Afterwards, SST values decrease from 1300 to 600 cal BP (average of 24.7 ± 0.3 °C), while a distinct decrease in SST is evident after 250 cal BP, reaching values as low as 22.3 °C (Figure 7). ΣTerNA and ΣTerN-OH concentrations present minor fluctuations from 7700 to 250 cal BP, with negative shifts at ~7500 and 4000 cal BP. Finally, an increasing trend is evident from ~1400 cal BP up to the top of the sequence. Τhe HPA index sharply increases to 0.76 at 4200 cal BP, while an increasing trend, up to a 0.71, is also observed at ~1200 cal BP. Further, minor fluctuations around a value of 0.65 are noticed up to the top of the record. The ACL profile demonstrates an increasing trend from 7700 to 7100 cal BP and afterwards to ~5500 cal BP. Then, after a sharp reduction at ~2400 cal BP, ACL rises to 31.1 at ~1800 cal BP, displaying a secondary peak at ~1200 cal BP; the previous maxima are the highest of the record. Finally, ACL displays a distinct decline from ~700 cal BP towards the top of the sequence (Figure 7).

Paleoenvironmental Interpretation
The multi-proxy record derived from the S2P core sheds light on the temporal environmental variability in Elefsis Bay since the Late Glacial.
The investigated sequence starts with the sedimentary Unit E, which is featured by calcite layering. The most common process for the development of this feature is the one driven by microorganisms, e.g., blue-green algae, which grow in a tidal environment. These organisms flourish as filaments and form mats by trapping and binding microcrystalline carbonates during the incoming tides that sweep over the sand. This leads to the formation of calcite laminations that consist of organic tissue interbedded with mud [104]. Therefore, Unit E could reflect a tidal depositional system. This is further supported by the presence of heavily reworked and rounded mollusc fragments. These macrofossils rather represent material, which was most probably derived from the post-alpine sediments of the broader study area and, subsequently, reworked during its transport to the Elefsis Bay through the local torrents that drain the catchment area, while later, after its deposition in the tidal environment, it was subjected to further reworking and alteration. The timing of the deposition of Unit E was not feasible to be determined due to the lack of autochthonous biogenic material. However, the striking erosion surface on top of Unit E, revealing a well-defined disconformity with the overlying Unit D, implies that the depositional environment of Unit E could be a paleobeach, being significantly older than the rest of the S2P sequence.
Based on the results of the multidisciplinary analysis of the 0-300 cm interval of the S2P deposit, the paleoenvironmental evolution of the Elefsis Bay may be interpreted as follows.

Stratigraphic Interval of 300-248 cm: The Elefsis Paleolake (13,500-11,350 cal BP)
The postglacial sedimentation in Elefsis bay actually begins with the formation of the 3-cm-thick peat horizon (Unit D) at 13,500 cal BP.
Above this level, the occurring aragonite-rich sediment is dominated by the presence of the colonial freshwater green alga Botryococcus (see PZ-1 in Figure 5). Botryococcus coenobia are commonly found in freshwater lakes, even though they can tolerate salinities up to 8 [105,106]. The high concentrations of Botryococcus in this stratigraphic interval can be related with green algae blooms, suggesting the existence of a freshwater to oligohaline lacustrine depositional environment, probably associated with the paleolake of Kyrheia [107]. In addition, the high relative and absolute abundances of pollen originating from terrestrial aquatic plants (see Figure 5) support the interpretation for the establishment of wetland conditions in the bottom part of the suggested paleolake environment, which, consequently, explains the peat formation. According to the glacio-hydro-isostatic model of Lambeck [3], the sea level stand was −80 m at 13,000 yr BP and −57 m at 11,000 yr BP, apparently favoring the formation of a paleolake within the enclosed depression of Elefsis.
In the current interval, the high TOC and C/N values (see Figures 2 and 8) are indicative of a terrestrial origin of the organic matter, while the recorded aragonite enrichment [4] suggests a shallow freshwater environment of high productivity and sulphate reduction [108]. Also, a typical inorganic precipitation of calcium carbonate in the form of aragonite frequently occurs in freshwater bodies affected by salinity increments. Hence, all previous interpretations as well as the pronounced rise in the carbonate contents (see Figure 2) further support the perception of a freshwater to oligohaline depositional environment for this sediment.
The gastropod Theodoxus sp., often associated with "hard" water [109], is the main mollusc species encountered within the present interval (see Figure 6). Thus, this is an extra indication of low salinity to freshwater conditions. Such conditions are also concluded from the coeval presence of perilithosis elements, etched specimens, and charo-phytes that never occur in marine habitats or under long-term conditions of high salinity, but they are usually found in freshwater and brackish water bodies [110].
Only the scarce occurrence of the foraminifer Elphidium gunteri was recorded during most of this stratigraphic interval (Figure 3). However, after 11,580 cal BP an evident increase of the FD values is observed, characterized by the concomitant presence of E. gunteri and Ammonia tepida. Elphidium gunteri is a common and, sometimes, dominant species in marshes and brackish lagoons and waters [23,[111][112][113], which is, however, capable not only surviving but even thriving in lacustrine environments of extremely low salinities (even <2) [114]. Also, it is generally recorded in intertidal environments along with A. tepida, which is an euryhaline opportunistic taxon exceptionally tolerant in salinity and temperature fluctuations [115,116]. Therefore, the simultaneous increased occurrence of the previous species in the current stratigraphic interval should be related to a high abundance of blue-green algae since the aforementioned species feed on these algae [117]. This actually reflects a very low salinity paleoenvironment [118].
More evidence that an oligohaline and shallow depositional environment had developed during this interval is further provided by the identified abundant monospecific assemblage of the ostracod species C. torosa with many noded individuals (see Section 4.2), which is a reliable indicator of such an environment. In detail, C. torosa is highly euryhaline and common in brackish waters showing an adaptability ranging from oligohaline to hypersaline waters [119], while dense assemblage of the species occur when salinity fluctuates between 2 and 17 [120]. Noded valves of C. torosa represent an ecologically-driven phenotypic variability developing under very low salinities [121]. In particular, nodes are usually present when salinity is less than 14, while more than 20% of noded valves point to a salinity of less than 5 [122].
The development of the Elefsis paleolake is further validated by the Late Glacial terrestrial vegetation succession in the area. The presence of well-developed open thermophilous deciduous and Mediterranean evergreen woodlands, implies relatively wet and mild climatic conditions in the Elefsis region during c.a. 13,500-11,500 cal BP [87]. Such relatively humid conditions are also supported by the occurrence of the continuous and extensive aquatic vegetation around the paleolake (see Figure 5) as well as the high abundances of the hydrophilous big Poaceae, recorded by Kyrikou et al. [87]. Interestingly, the Younger Dryas climatic event interval (12,900-11,700 cal BP; [123]), documented as a cold and arid period in the Aegean records [124,125] and terrestrial sites from northern Greece [126,127], coincides with the Elefsis paleolake phase. However, there is no signs of increasing aridity in the high-resolution Elefsis paleovegetation record [87], indicating the occurrence of milder conditions in south Greece. Increased humidity in southern sites across the Hellenic peninsula during the Younger Dryas is also supported by the glacial record of Chelmos Mountain (in Peloponnese) [128] compared to that of Pindus Mountain Range [129][130][131].

Stratigraphic Interval of 248-187 cm: The Isolated Brackish Environment (11,350-7500 cal BP)
After 11,350 cal BP, a shift in the aquatic biota assemblages is evidenced as new taxa are introduced and the concentrations of the green algae Botryococcus abruptly decline (see PZ-II in Figure 5). A distinct dinoflagellate assemblage, dominated by L. machaerophorum sh.p. and complemented by S. cruciformis and Pyxidiniopsis psilata, is recorded, indicating the establishment of brackish conditions in the area. These autotrophic species are known from brackish deposits in the Black and Caspian seas [30,88,132] and are common in low salinity environments [133]. Lingulodinium machaerophorum is known to have a widespread distribution in coastal environments with increased river discharges [88]. The L. machaerophorum sh.p, with length of processes <10 μm, is typical of waters with reduced see surface salinity (SSS) [88,92,134], associated with annual SSTs of ~17-20 C and SSS values of ~12.4-13.2 [88]. Spiniferites cruciformis is a cyst found in the Eastern Mediterranean, mainly restricted to areas of the Black and Caspian seas with reduced salinity affected by river discharges [30,88,133,134], but also recorded in lacustrine deposits [28]. Comparable low diversity assemblages of S. cruciformis, P. psilata and L. machaerophorum sh.p. have already been recorded in the isolated/semi-isolated intervals of the sedimentary record obtained from the Gulf of Corinth during the IODP Expedition 381 [39,135]. Finally, the typical brackish assemblage of this stratigraphic interval is complemented by some euryhaline species, such as S. bentorii and P. zoharyi, which thrive in coastal environments [133].
In accordance to the interpretation inferred from the dinoflagellate analysis, the relatively increased C/N values (13-14.6) in the current interval further support the establishment of a transitional environment [103]. In addition, eutrophic conditions associated with riverine inputs can be inferred from the enhanced TOC values as well as the peak dinocyst concentration, most likely related to bloom conditions of the L. machaeorphorum [88,136], known for its flourishing in nutrient-rich environments. Towards the top of PZ-II, the gradual introduction of S. membranaceus, S. delicatus, and O. centrocarpum (see Figure 5) indicates a deepening of the suggested lagoon environment. The previous interpretation is further strengthened by the retreat of aquatic vegetation as a result of the reduction of the shallow areas.
In this stratigraphic interval, the foraminiferal assemblage ( Figure 3) is highly dominated by the euryhaline species A. tepida, accompanied by A. perlucida and E. gunteri, displaying low diversities but high FD values. The dominance of A. tepida is consistent with its tolerance to decreased marine influence [137], while the occurrences of A. perlucida and E. gunteri, together with the presence of A. tepida, suggest an environment of low salinity and/or intense salinity fluctuation, thus, revealing typical conditions of an inner lagoon or an isolated brackish waterbody [22,23,32,[138][139][140]. As has been observed by Melis and Covelli [113], the muddy composition and high organic matter content could explain the high density and dominance of A. tepida in this stratigraphic interval, taking also into account that the organic matter is a vital food resource for A. tepida [141].
More evidence that lagoonal conditions had prevailed during the present interval is further provided by the identified ostracod assemblage (see Section 4.2). It is primarily composed of C. torosa, with rare presence of noded juvenile valves, accompanied by Xestoleberis sp. The Xestoleberis species are abundant in shallow marine environments and are highly associated with algae [142,143]; however, several Xestoleberis species have been recorded in brackish marine environments with submerged aquatic vegetation (e.g., [144]). Even though other ostracod species are scarcely present in the current stratigraphic interval, the identified individuals reflect brackish conditions. For example: A. woodwardii is a marine littoral epibenthic and epiphytal species [77,145], found also in brackish lagoonal environments [146]; L. lagunae is considered as a marine littoral species, being common in lagoonal systems (e.g., [147]); and C. fischeri is a brackish water marine littoral species, tolerating a wide range of salinities   [148,149].
The evolution of the Elefsis paleolake into an isolated brackish waterbody is the result of the Early Holocene gradual marine transgression that caused a relative sea level rise from −57 m at 11,000 yr BP to −13.5 m at 7500 yr BP, as predicted by the glacio-hydro-isostatic model of Lambeck [3]. However, the shallow depth at the sills occurring in the western and eastern strait, respectively, actually prevented the seawater masses to inundate the Elefsis embayment. That might be the reason that we have not found any calcareous nannoplankton representatives during this interval, although their progressive increase in abundance could be of importance when characterizing the establishment of lagoonal conditions (e.g., [8]). Hence, we believe that the prevailing brackish lagoonal environment was not established via a direct connection with the sea but due to a potential salinization caused by lateral seawater intrusion processes as the sea level was constantly rising (e.g., [161]). The beginning of this isolated brackish phase with evidences of increasing riverine input is also featured by short-lived fluctuation in the abundance of deciduous trees ( Figure 8) and decline of Mediterranean elements in Elefsis pollen record [87] associated to the Preboreal oscillation interval (PBO). Similar temporal setbacks of forest expansion evidenced in several Mediterranean sites [124,126,162,163] are likely driven by changes in temperature and seasonal precipitation [164,165].

Sratigraphic Interval of 187 cm to the Top: The Elefsis Bay (7500 cal BP to Present)
The establishment of marine conditions in the Elefsis Bay is marked by the development of diverse dinoflagellate, benthic foraminiferal, and mollusc assemblages at 7500 cal BP as well as the introduction of coccolithophores in the assemblages (Figures 3-6). The dinoflagellate assemblage is characterized by a sharp decrease of the low salinity species and the development of a marine assemblage consisting mainly of L. machaerophorum, Sp. Bentorii, and P. zoharyi (see PZ-III in Figure 5). In particular, the high abundances of the long processes morphotype of L. machaerophorum and the co-occurrence of P. zoharyi, S. bertonii, and S. belerius at the bottom of this interval (see PZ-III Figure 5) are indicative of a coastal environment enriched in nutrients due to enhanced terrestrial discharges [88,136,166,167]. The dinocyst assemblage becomes gradually enriched with pelagic species, such as Sp. delicatus, S. membranaceus, S. mirabilis, and O. centrocarpum, evidencing the deepening of the marine environment (Figures 8). The contemporaneous occurrence of E. huxleyi in the sedimentary record ( Figure 4) implies the establishment of full marine conditions in the Elefsis Bay, with a sufficient water column depth, i.e., at least 20 m, to sustain the pelagic coccolithophore community (e.g., [168]), conditions that coincide with the increase of pelagic environment dinoflagellate cysts (see Figure 8). Likewise, in benthic communities the marine transgression is featured by a sudden increase in diversity and the appearance of several marine species in the foraminiferal, ostracod, and molluscan assemblages; the latter comprising several typical marine gastropod and bivalve species, such as B. sebetia, N. nitidosa, T. ovata, B. reticulatum, and T. communis. The benthic foraminiferal composition until 4000 cal BP (Figure 3) mainly includes typical marine species of the infralittoral zone, such as A. beccarii, small epiphytic rotaliids, E. translucens, miliolids, and agglutinated taxa [72,111,169], similar to modern coastal environments of the Aegean Sea [23,73]. The abundance of the euryhaline A. tepida dramatically decreases, while A. beccarii appears together with shallow-water epiphytic foraminifera, such as P. mediterranensis, R. spinulosa, and T. bocki, common in the infralittoral and circalittoral zones of the Mediterranean Sea [72,170,171]. Similarly, the ostracod fauna comprises the typical marine species of C. neapolitana, P. agilis, S. versicolor, and L. multipunctata (e.g., [75,77,145,172]). In particular, C. neapolitana being the most abundant in the ostracod assemblages, indicates water depths higher than 20 m, tolerating organic matter enrichment and oxygen depletion [173,174].
More evidence for the development of a marine basin after 7500 cal BP is provided by the relative and absolute abundances of Pinus pollen (Figure 8). Pinus pollen is mostly overrepresented in offshore deposits [175][176][177] and, therefore, is commonly excluded from the analyses of marine pollen records [79,124,165,[178][179][180]. Even in the large terrestrial water body of Lake Ohrid, pine is considered overrepresented (e.g., [181]), while its gradual increase over the earliest lake stages has been connected with the increase of the lake surface area [182]. Despite the fact that pine is a significant component of the Mediterranean vegetation, its marked occurrence in the marine stratigraphic interval of the S2P record is most probably attributed to depositional processes and not to an expansion of the species in the borderlands of Elefsis Bay [87]. Thus, the observed abrupt, pine increase can be correlated with the marine transgression in the bay at 7500 cal BP.
The change in the paleoenvironmental conditions is clearly displayed in the C/N values (see Figure 2), which diminish to 12.2-13 in the lower part of this stratigraphic interval, indicating the establishment of a marine environment significantly affected by the terrestrial processes that, however, appear less effective after 4000 cal BP [103]. In agreement with the previous interpretation, the increased concentrations of the erosion indicating Glomus and Pseudoschizaea signal constant terrestrial runoff [183] culminating between 5000 to 4000 cal BP. The terrestrial inputs are further documented by the relatively elevated ΣTerNA and ΣTerN-OH concentrations and the high HPA values in the same time interval; the biomarkers evidence low degradation of terrigenous organic matter likely associated with enhanced freshwater inputs/land runoff, delivering "fresh" land plant organic matter (e.g., [96,184]). The persisting abundances of the green alga Botryococcus and the occurrence of the low-salinity L. machaerophorum sh.p. until 4600 cal BP provide additional evidence for seasonal freshwater fluxes into the basin [88,136], during a period of ongoing warm and humid paleoclimatic conditions associated with the Mid-Holocene warm period (5400-4300 cal BP; [25,184]) in the Aegean Sea (Figure 8), as marked by the increment of the ACL index indicating vegetation shift towards "warmer species". This phase ends with the "4.2 ka" significant Northern Hemisphere rapid climate cooling [185,186] that is also evidenced as negative shift in the SST profile of S2P record (Figure 7).
The prominent HPA minimum at ca. 4000 cal BP in line with a decrease in ΣTerNA and ΣTerN-OH concentrations (Figure 7) and the decline of the aquatic vegetation (Figure 5) mark the onset of an arid period. The simultaneous increase in the abundance of S. membranaceus after 4000 cal BP verifies the changes in the marine environment. The species is known to be abundant in mesotrophic to oligotrophic environments [88,187], therefore its enhanced occurrence most probably implies a decline in the trophic status, also reflected in the lower TOC values during the same interval. The simultaneous scarcity of the erosion indicators Glomus and Pseudoschizaea suggests a reduction in the ter-restrial runoff, related with increasing aridity recorded in a plethora of regional terrestrial and marine archives (e.g., [25,[188][189][190]. At the uppermost part of the S2P sequence, the establishment of the present-day conditions, characterized by dysoxic/anoxic conditions, restricted circulation, and increased terrigenous runoff [183], is reflected in all applied proxies. In regard to the benthic foraminiferal communities, these are characterized after 1500 cal BP by the peak values of B. elongata and the mud-dwelling N. turgida [191]. Both species are known to thrive in silty to sandy substrates under the influence of ample inputs of fluvial organic matter and sediment [191][192][193], and even to survive in dysoxic habitats [111,194,195]. Furthermore, the organic enrichment of the environment is implied by the presence of the tolerant in dysoxic conditions bivalves C. gibba, M. spinifera, and T. ovata [196][197][198][199]. Hence, the contemporaneous increase of ΣTerNA and ΣTerN-OH contents, high HPA values and the minima in the ACL profile after 1500 cal BP (Figure 7) should be mostly linked to the enhanced freshwater supply and subsequent stratification/oxygen depletion in the marine water column that promotes in-situ preservation (e.g., [96,200]).
Finally, the gradual increase of the dinocysts L. machaerophorum, S. bentorii, and P. zoharyi as well as the maxima in the Pseudoschizaea and Glomus concentration profiles are most probably correlated to enhanced fluvial inputs to the basin [88,187], which are further supported by the enhanced abundance of CN reworked taxa (e.g., [25]).

Conclusions
The multi-proxy analysis of the Elefsis Bay sedimentary record provides insights into the sensitive response of marginal aquatic ecosystems to the environmental variability linked to sea level changes and the Late Glacial marine transgression. Marked shifts in benthic and planktic biota assemblages, complemented by geochemical data, were identified, featuring the paleoenvironmental evolution of this shallow landlocked marine area: • The bottom part of the S2P sequence, most probably representing the Pliocene/Pleistocene substrate of the area, was rather deposited in a tidal system as implied by the presence of calcite layering and heavily reworked and rounded mollusc fragments. A well-defined disconformity with the rest of the sequence is recorded. • Between 13,500 and 11,350 cal BP, a freshwater to oligohaline lacustrine environment developed, i.e., the Elefsis paleolake, characterized by green algae blooms and monospecific ostracod and gastropod oligohaline faunas. • The occurrence of an isolated brackish environment is evidenced after 11,350 cal BP by a marked shift in the aquatic biota. Brackish dinoflagellate assemblages, mainly composed of low salinity morphotypes of L. machaerophorum, replace the green algae in the assemblages. In parallel, benthic communities are featured by high foraminiferal density values of the euryhaline species A. tepida, A. perlucida, and E. gunteri, while the prevailing ostracods C. torosa and Xestoleberis sp. together with the molluscs C. glaucum, M. marioni, and Hydrobia sp. imply an inner lagoon depositional system. As the Early Holocene relative sea level stand was not high enough to allow a direct communication of the Elefsis waterbody with the sea, the established brackish conditions were most probably the result of salinization caused by lateral encroachment from coastal waters. • The marine conditions in Elefsis Bay are established after 7500 cal BP, evidenced by the development of a diverse dinoflagellate assemblage, gradually enriched with pelagic species, and the contemporaneous occurrence of calcareous nannoplankton assemblages, dominated by E. huxleyi. In the benthic communities the marine transgression is reflected by a sudden increase in the foraminiferal diversity and the appearance of several marine species. • After 1500 cal BP, both benthic foraminiferal and bivalve communities are dominated by taxa tolerant in dysoxic conditions, suggesting organic matter preserva-tion. In accordance, the contemporaneous increase of ΣTerNA and ΣTerN-OH content, high HPA values as well as the minima in the ACL profile are linked to an enhanced freshwater supply and subsequent stratification/oxygen depletion in the marine water column, implying the earliest establishment of the present-day seasonal anoxia in the Elefsis Bay.