Quaternary Evolutionary Stages of Selinitsa Cave (SW Peloponnese, Greece) Reveal Sea-Level Changes Based on 3D Scanning, Geomorphological, Biological, and Sedimentological Indicators

: Signiﬁcant evolutionary stages of Selinitsa Cave (SW Peloponnese, Greece) were revealed by 3D mapping, as well as geomorphological study of the cave and the nearby landscape. Four marine terraces were identiﬁed in the area of the coastal cave at 6, 10.7, 16.6, and 30–32 m above sea level (asl), with the terrace at 16.6 m representing Marine Isotope Stage (MIS) 5. The widest karstiﬁed space of Selinitsa Cave clusters between 15.73 and 18.05 m above sea level (asl), with the peak lying at 16.4 m asl, corresponding to the level where the phreatic/epiphreatic zone was stable for a sufﬁcient period of time. A tidal notch at 16.4 m asl at the cave entrance is correlated to the marine terrace at 16.6 m. Both features correspond to the sea-level stand at which intense karstiﬁcation occurred. The tidal notch bears a horizontal arrangement of Lithophaga borings at the vertex. Sedimentological investigation of the Selinitsa ﬁne-grained deposit revealed the paleohydrologic regime of the cave. It is characterized by “slack-water” facies, indicating very low water ﬂow speeds, whereas the thickness of the deposit points to stable hydrological conditions for prolonged periods. The cave sediment height of 18.8 m asl indicates a ﬂooding level higher than sea level. The overlying Plattenkalk ﬂysch is most probably the major source of detritus, and the predominance of authigenic dolomite (>98% modal in the carbonate fraction) indicates a hyposaline environment related to mixing of sea water with percolating fresh water. The approach of this study shows the signiﬁcance of 3D mapping, bio-geo-Relative Sea Level (RSL) indicators, and sedimentology in deciphering the paleogeographic evolution of coastal karstic systems and subsequently deﬁning the paleoclimate regime of coastal areas in Greece and the eastern Mediterranean during the Late Quaternary.


Introduction
Caves are considered valuable paleogeographic [1] and paleoclimatic archives [2,3]. They are shaped by karstification resulting from underground flowing water and are influenced by tectonic activity, as well as regional and global climate change, thus recording the evolution of the surficial environment [1]. The imprints of these processes can be traced to contribute to the reconstruction of the paleogeography of an area by interpreting the preserved landforms. Sometimes the only remaining features can be found in caves, as exogenic processes entirely erase such data. Paleoclimatic studies in caves take advantage of the climate encoded in speleothems [4][5][6], but limited attention has been given, so far, to the potential of cave morphology and surrounding karst geomorphology in revealing encrypted paleoclimatic signals.
The expansion and retreat of Late Quaternary global ice masses directly influenced the global sea level [7], inducing the latter in a 125 m fluctuating range. The landscape evolution triggered by these changes can be reconstructed from exogenic landforms, as well as the underground settings of coastal caves.
The peri-Mediterranean region hosts a large part of the world's coastal karst. Numerous studies focusing on paleoclimate [8][9][10][11], as well as sea-level changes [12][13][14][15], have taken advantage of this unique setting and shed light on the Mediterranean configuration during the Late Pleistocene and Holocene periods. Such studies use carbonate sea-level markers (speleothems, phreatic overgrowths on speleothems (POS), tidal notches, and marine terraces) in order to reconstruct the evolution of coastal Mediterranean paleogeography.
Besides these landforms, the integration of other cave features (e.g., cave deposits) may also help to identify the paleohydrological regime shifts a karstic system has undergone through time. For instance, the importance of deposits and their successions found in caves has not been adequately taken into account [16], particularly when combined with other sea-level indicators in coastal cave systems, such as marine terraces and tidal notches.
Selinitsa Cave (SW Peloponnese, Greece) comprises a coastal cave, with the largest part situated above the present sea level, owing to the active tectonic regime of the area. Certain morphological features of the cave environment indicate the previous successive positions of the cave in the phreatic and epiphreatic zones. The objective of the present study is to present hypotheses concerning the paleogeographic evolution of Selinitsa Cave, as well as its hydrologic changes, during the Quaternary. This study employs a combination of 3D cave mapping and a survey of geomorphological and biological indicators, as well as the texture, mineralogy, and trace element geochemistry of cave deposits. The results of this study are contextualized within a suggested climate regime and in extension to certain paleoclimatic periods during which coastal caves recorded past high sea-level stands, particularly in Greece.

Geotectonic Context
Selinitsa Cave (36 • 48 10.48 B, 22 • 17 51.37 E, floor entrance at 18 m above sea level; henceforth asl) is located 46 km south of Kalamata City on the western flank of Taygetos Mountain ( Figure 1). It is part of a 3.5 km composite karst system located on the western shores of Messiniakos Gulf, comprising Selinitsa and Drakos underground river caves [17]. Messiniakos Gulf is a NNW-SSE asymmetric graben of the Late Miocene age [18] formed as a result of active tectonic processes in the western Hellenic Arc, an area with an intense geodynamic regime, i.e., seismicity, normal faulting, and crustal uplift [19]. It borders the Mani Peninsula to the west and is 50 to 80 km east of the Hellenic subduction zone. Neotectonic activity is prominent along the NNW-SSE-oriented faults and, to a lesser extent, along the E-W-oriented faults [20].
Selinitsa Cave and its surroundings have been strongly influenced by both regional and local tectonic activity affecting the geomorphological evolution of Mani Peninsula. The lithology of the studied region comprises the Plattenkalk unit (PLK), mainly composed of pelagic, medium-to-thin-bedded, semi-crystalline, Late Senonian-Late Eocene gray to whitish limestones [21] corresponding to the deepest structural (autochthonous) unit of the Peloponnese [22,23]. The cave developed exclusively in the aforementioned lithology. In the surrounding area, there are also outcrops of flysch and Vigla limestones of the PLK unit ( Figure 1). The interplay between tectonic and climatic conditions throughout Quaternary has left its imprint on the well-preserved marine terraces in the wider area. Quaternary 2022, 5, x FOR PEER REVIEW 3 of 15 Selinitsa Cave and its surroundings have been strongly influenced by both regional and local tectonic activity affecting the geomorphological evolution of Mani Peninsula. The lithology of the studied region comprises the Plattenkalk unit (PLK), mainly composed of pelagic, medium-to-thin-bedded, semi-crystalline, Late Senonian-Late Eocene gray to whitish limestones [21] corresponding to the deepest structural (autochthonous) unit of the Peloponnese [22,23]. The cave developed exclusively in the aforementioned lithology. In the surrounding area, there are also outcrops of flysch and Vigla limestones of the PLK unit ( Figure 1). The interplay between tectonic and climatic conditions throughout Quaternary has left its imprint on the well-preserved marine terraces in the wider area.

Materials and Methods
We performed a 3D laser mapping of Selinitsa Cave, as well as the tidal notches across the entrance and the cave deposits inside the cave, in order to document its geomorphology. The 3D point cloud comprises 385 million points and exhibits speleomorphology with an accuracy of 2-3 cm. Laser mapping was conducted with a GeoSLAM ZEB-REVO portable laser scanner with simultaneous localization and mapping (SLAM) technology. The uncertainty of elevation measurements is ±0.03 m. The 3D data were assessed using CloudCompare version 2.10.2. The 3D point cloud of the cave, as well as its peculiar geomorphological and biological landforms, were georeferenced with a Javad Triumph-2 real-time kinematic global navigation satellite system (RTK-GNSS) based on five fixed geographical points measured in the outer environment. The elevation of all geomorphological and biological landforms reported in this study was measured with

Materials and Methods
We performed a 3D laser mapping of Selinitsa Cave, as well as the tidal notches across the entrance and the cave deposits inside the cave, in order to document its geomorphology. The 3D point cloud comprises 385 million points and exhibits speleomorphology with an accuracy of 2-3 cm. Laser mapping was conducted with a GeoSLAM ZEB-REVO portable laser scanner with simultaneous localization and mapping (SLAM) technology. The uncertainty of elevation measurements is ±0.03 m. The 3D data were assessed using CloudCompare version 2.10.2. The 3D point cloud of the cave, as well as its peculiar geomorphological and biological landforms, were georeferenced with a Javad Triumph-2 real-time kinematic global navigation satellite system (RTK-GNSS) based on five fixed geographical points measured in the outer environment. The elevation of all geomorphological and biological landforms reported in this study was measured with the aforementioned RTK-GNSS device, with an accuracy of ±0.03 m on both the horizontal and vertical axes. The precise morphology (profile) of the upper and lower notches was reconstructed by means of photogrammetry, allowing for the generation of two 3D models with Agisoft Photoscan software [24].
Moreover, in order to further define the prevailing paleohydrologic conditions within Selinitsa Cave during its evolution, we also sampled infill identified during field work and mapping. These deposits include fine-grained material identified in a large chamber approximately 565 m from the entrance of the cave, as well as clogging material filling cracks, voids, joints, and faults in the same area ( Figure 2). Samples of the fine-grained deposits were collected from several depths, whereas samples of the clogging material were hand-picked from several locations in the same area. Mineralogy and texture of the collected samples was determined by combination of transmitted-light optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy (details on the employed techniques are available in the Supplementary Materials). Besides textural and mineralogical analyses of the fine-grained deposits, we also performed geochemical analyses of the material in order to define its origin and correlate it with the paleogeographic evolution of the Selinitsa Cave. The cave deposits were commercially analyzed for major and trace elements at ALS Laboratories (Ireland) (see Supplementary Material for details). Geochemical analyses and the calculated standard deviation (S.D.) for the whole suite of elements are presented in Table S1. models with Agisoft Photoscan software [24].
Moreover, in order to further define the prevailing paleohydrologic conditions within Selinitsa Cave during its evolution, we also sampled infill identified during field work and mapping. These deposits include fine-grained material identified in a large chamber approximately 565 m from the entrance of the cave, as well as clogging material filling cracks, voids, joints, and faults in the same area ( Figure 2). Samples of the fine-grained deposits were collected from several depths, whereas samples of the clogging material were hand-picked from several locations in the same area. Mineralogy and texture of the collected samples was determined by combination of transmitted-light optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy (details on the employed techniques are available in the Supplementary Materials). Besides textural and mineralogical analyses of the fine-grained deposits, we also performed geochemical analyses of the material in order to define its origin and correlate it with the paleogeographic evolution of the Selinitsa Cave. The cave deposits were commercially analyzed for major and trace elements at ALS Laboratories (Ireland) (see Supplementary Material for details). Geochemical analyses and the calculated standard deviation (S.D.) for the whole suite of elements are presented in Table S1.

Three-Dimensional Mapping and Cave Morphology
The speleogenesis of the Selinitsa cave system is determined by the main tectonic structures of the area. The main cave passage ( Figure 2) follows the NW-SE Late Miocene tectonic discontinuities [17,25], whereas Drakos mainly follows the NE-SW Mid-Late Pleistocene tectonic direction. The entrance floor of Selinitsa Cave lies at 18 m asl, whereas Drakos is located 10 m underwater and 500 m south of Selinitsa [26]. Selinitsa is mainly a dry cave, partially extending underwater. The cave system extends 80 m in

Three-Dimensional Mapping and Cave Morphology
The speleogenesis of the Selinitsa cave system is determined by the main tectonic structures of the area. The main cave passage ( Figure 2) follows the NW-SE Late Miocene tectonic discontinuities [17,25], whereas Drakos mainly follows the NE-SW Mid-Late Pleistocene tectonic direction. The entrance floor of Selinitsa Cave lies at 18 m asl, whereas Drakos is located 10 m underwater and 500 m south of Selinitsa [26]. Selinitsa is mainly a dry cave, partially extending underwater. The cave system extends 80 m in depth, with a large underwater portion still unexplored. Thus, the karstification base of the cave system and, by extension, of the entire area, cannot be precisely determined.
3D mapping of Selinitsa revealed the development of the cave passages following a general NW-SE direction, in agreement with [25] (Figure 2). The entrance of the cave is located 47.6 m from the coastline. The bedding plane at the entrance is horizontal, dipping to the WSW (10 • /267 • ). The bedding plane changes direction to NE (19 • /029 • ) 90 m from the entrance, implying a structural fold. The limestone bedding plane seems to play a secondary role in the speleogenetic process, as some passage segments follow the geometry of the bedding plane. The main corridor has a maximum elevation of~32 m. The 'big hall', 8.5 m above the main conduit, represents a collapse chamber with an area of 252 × 154 m, featuring a triangular shape. A NW-SE fault (81 • /250 • ) delimits the western side of this chamber, comprised of breakdown deposits and boulders larger than 1 m 3 . The bedding plane at the chamber's roof ranges between 25 • /020 • and 30 • /050 • . The altitudinal range of the cave is between 18 and 86 m. The collapse chamber extends from 40.58 to 86 m (Figure 2), and the epikarst zone above it reaches a width of 134 m. This chamber has no connection to the surface. The largest space volume in Selinitsa Cave is found between 15.73 and 18.05 m (thick red line, Figure 2). The peak of the latter range is 16.4 m.

Geomorphological and Biological Sea-Level (SL) Indicators
Fieldwork revealed geomorphological and biological SL indicators in the vicinity of the Selinitsa Cave entrance. Four marine terraces are present at 6 m, 10.7 m, 16.6 m, and 30-32 m asl between Selinitsa and Trachila Peninsula (Figure 1). The lower terrace at 6 m asl is mainly visible at Trachila, whereas the most prominent terrace in the region is 16.6 m asl. The terrace between 30 and 32 m asl is an almost flat surface above the entrance of Selinitsa, along which the main road is constructed (Figure 3). This shoreline/terrace at 30-32 m asl is also documented by the presence of a notch horizon at 32.02 m asl in a small shelter. The shelter opens~10 m from the sea, approximately 1.2 km southeast of Selinitsa, and faces seawards; this could be a flank margin cave. Two other tidal notches are present: one below the cave entrance at +15.32 m asl and the second above the cave entrance at +16.4 m asl ( Figure 3).  The biological sea-level indicator used in this study is the numerous Lithophaga borings that occur between the present sea level and 45 m asl. These borings are hollows in limestone created by endolithic Lithophaga bivalves that live in the upper 30 m of the water column [27,28]. The highest frequency of Lithophaga populations usually coincides with the first few meters below sea level, with their upper limit at the mean sea level [29]. Consequently, when these mollusk burrows are well delineated and/or are correlated with another morphological indicator, such as a tidal notch, they constitute a biological sea-level proxy [27]. In this case, attention should be given to populations with a horizontal clearcut upper limit. The upper limit of Lithophaga boreholes has been used to date RSL changes [30][31][32] or as a sea-level indicator in submerged speleothems [33]. Other geomorphological landforms include two small caves and a partially preserved The biological sea-level indicator used in this study is the numerous Lithophaga borings that occur between the present sea level and 45 m asl. These borings are hollows in limestone created by endolithic Lithophaga bivalves that live in the upper 30 m of the water column [27,28]. The highest frequency of Lithophaga populations usually coincides with the first few meters below sea level, with their upper limit at the mean sea level [29]. Consequently, when these mollusk burrows are well delineated and/or are correlated with another morphological indicator, such as a tidal notch, they constitute a biological sea-level proxy [27]. In this case, attention should be given to populations with a horizontal clearcut upper limit. The upper limit of Lithophaga boreholes has been used to date RSL changes [30][31][32] or as a sea-level indicator in submerged speleothems [33]. Other geomorphological landforms include two small caves and a partially preserved coastline located at~41 m asl with Lithophaga biological perforations in a horizontal configuration.

Fine-Grained Cave Deposits, Clogging Material
The Selinitsa cave deposits show consistent mineralogy, geochemistry, and texture from top to bottom (Figure 4). The material is fine-grained, unsorted, and predominated by angular-subangular quartz fragments (detritus) and euhedral to subhedral dolomite crystals, which are occasionally pitted (Figures 4, 5, S2 and S3). Muscovite laths, albite, anorthite, angular to subangular calcite, Mg-calcite, and Fe-oxides/hydroxides are less abundant ( Figures S4-S7), the latter providing reddish color to the deposits. Moreover, traces of apatite, rutile, sphene, and barite are also present ( Table 1). The clogging material filling cracks, voids, and faults in Selinitsa Cave is predominated by quartz, with minor chlorite and muscovite, whereas no carbonates were identified (Table 1, Figure S1).   The cave deposits sampled at 565 m from the entrance show Si content ranging from 57.4 to 62.2 (as SiO 2 wt%) and Al and Fe content ranging from 6.03 to 7.69 and 2.33 to 2.71 (as Al 2 O 3 wt% and Fe 2 O 3 wt%), respectively. Ca content is high, largely attributed to the presence of carbonates, primarily dolomite and, to a lesser extent, calcite and Mg-calcite (7.02 to 9.61 as CaO wt%), whereas Mn, K, and P contents are lower (Table S1). Mg content ranges between 4.93 and 6.17 (as MgO wt%) and is closely related to the abundance of dolomite in the sediment material. Sr content in the cave sediment is low (ranging between 55.9 and 67.3 ppm), whereas total REE values (ΣREE) are between 105.79 and 125. 16

Discussion
Although Selinitsa Cave is located on the shore, it shows evidence of phreatic/epiphreatic origin (tubular galleries, upstream divergence of passages, and scallops). The cave was once flooded, as Drakos is now, and due to tectonic uplift affecting the area, the cave passages are now in the vadose zone. On the contrary, the overall morphology of Drakos lies underwater and comprises successive karstified base levels [25] most likely corresponding to sea-level stands postdating the Last Glacial Maximum (LGM).
The vertical movement of the land relative to the sea is also evidenced by geomorphological sea-level markers, including tidal notches. The latter accurately depict former sea-level stands, formed as a result of bioerosion processes in the intertidal zone [35]. The presence of tidal notches in uplifting coastlines is indicative of the vertical motion of the land. At Selinitsa Cave, the lower notch at 15.32 ± 0.03 m asl has a symmetric profile, with a height of 25 ± 0.03 cm, whereas the upper notch (height~35 ± 0.03 cm) has a more open profile (Figure 3). Both notches correlate well with a horizontal arrangement of Lithophaga borings, with higher perforation frequency at the maximum retreating point of the notches (vertex). Consequently, the notches and Lithophaga borings represent former sea-level stands. Furthermore, 3D scanning data provide evidence concerning the altitudinal range of intense karstification. The highest volume of the cave ranges between 15.73 and 18.05 m asl (thick red line, Figure 2), representing the space where the major karstification phase of Selinitsa's development took place. It also correlates well with the geomorphological features of the area, such as the upper notch (16.4 m) at the cave entrance (Figures 3 and 6) and the marine terrace at 16.6 m. The peak of this karstification range lies at 16.4 m asl and represents the level where the phreatic/epiphreatic zone was stable for a sufficient period of time, resulting in the widening of the cave passage (Figures 2 and 6). Additionally, the marine terrace at 16.6 m supports the hypothesis that the sea stood at the level of the widest space resulting from the chemical dissolution of the host carbonate rocks (Upper Senonian-Upper Eocene limestones, Figures 1B and 2). Similar morphological features (widened passages) indicating successive sea-level positions can also be seen in the submerged cave of Drakos to the south [25].

Discussion
Although Selinitsa Cave is located on the shore, it shows evidence of phreatic/epiphreatic origin (tubular galleries, upstream divergence of passages, and scallops). The cave was once flooded, as Drakos is now, and due to tectonic uplift affecting the area, the cave passages are now in the vadose zone. On the contrary, the overall morphology of Drakos lies underwater and comprises successive karstified base levels [25] most likely corresponding to sea-level stands postdating the Last Glacial Maximum (LGM).
The vertical movement of the land relative to the sea is also evidenced by geomorphological sea-level markers, including tidal notches. The latter accurately depict former sea-level stands, formed as a result of bioerosion processes in the intertidal zone [35]. The presence of tidal notches in uplifting coastlines is indicative of the vertical motion of the land. At Selinitsa Cave, the lower notch at 15.32 ± 0.03 m asl has a symmetric profile, with a height of 25 ± 0.03 cm, whereas the upper notch (height ~35 ± 0.03 cm) has a more open profile (Figure 3). Both notches correlate well with a horizontal arrangement of Lithophaga borings, with higher perforation frequency at the maximum retreating point of the notches (vertex). Consequently, the notches and Lithophaga borings represent former sea-level stands. Furthermore, 3D scanning data provide evidence concerning the altitudinal range of intense karstification. The highest volume of the cave ranges between 15.73 and 18.05 m asl (thick red line, Figure 2), representing the space where the major karstification phase of Selinitsa's development took place. It also correlates well with the geomorphological features of the area, such as the upper notch (16.4 m) at the cave entrance (Figures 3 and 6) and the marine terrace at 16.6 m. The peak of this karstification range lies at 16.4 m asl and represents the level where the phreatic/epiphreatic zone was stable for a sufficient period of time, resulting in the widening of the cave passage (Figures 2 and 6). Additionally, the marine terrace at 16.6 m supports the hypothesis that the sea stood at the level of the widest space resulting from the chemical dissolution of the host carbonate rocks (Upper Senonian-Upper Eocene limestones, Figures 1B and 2). Similar morphological features (widened passages) indicating successive sea-level positions can also be seen in the submerged cave of Drakos to the south [25].  Moreover, the maximum elevation of the main passage of Selinitsa Cave at 32 m provides additional evidence of the phreatic/epiphreatic functioning of the cave when correlated to the tidal notch with a Lithophaga horizontal arrangement at 32.02 m (Figures 3  and 6). This notch constitutes a biological sea-level proxy and pinpoint to a sea-level stand at this elevation. The divergence and tubular cross-sectional shape of the cave corridors, as well as the presence of scallops on the walls, can be explained by a past sea level at 32.02 m.
Other evidence of partial submergence in the phreatic/epiphreatic zone includes the presence of successive layers of fine-grained deposits (silt-clay alternations) in the inner part of the cave, about 565 m from the entrance to the far end [36]. In most karstic systems containing clastic deposits [16,37], a fine-grained, thin layer is deposited every time the cave is filled with water, and such deposits are characterized as "slack-water facies" [38]. These facies are usually observed at the uppermost part of a sedimentary succession [37], with low thickness, indicating a gradual decrease in the flowing speed of water. In the case of Selinitsa Cave, field observations revealed increased thickness of the fine-grained sediment, exceeding 2 m in height (Figures 2 and 4). The morphology of Selinitsa's main conduit played a determinant role in the deposition of fine-grained material, particularly during periods when the sea level stood at higher elevations than at present. 3D mapping revealed that the cave becomes progressively tighter towards the entrance, with the narrowest passage~55 m from the entrance. During periods of higher sea levels and conditions of increased water discharge by the cave system, water was unable to be successfully drained due to the aforementioned narrowing, which caused back flooding of the cave. A few meters beyond the fine-grained deposits, roof collapse led to the accumulation of sizeable limestone boulders (~1 m 3 ), which significantly reduced the cave volume ( Figure 4). Collapse material is present only in this part of the cave, and these collapses probably contributed to back flooding, as documented in other locations [16]. This observation, combined with the maximum altitude of the fine-grained deposits at 18.8 m asl, clearly indicates that for prolonged periods, this part of the cave was flooded by water forming stagnant ponds and small lakes, with the water level higher than sea level. The maximum elevation of the cave deposits is higher than all markers depicting former sea-level stands (notches and marine terraces). The adjacent markers at 15.32 m, 16.4 m, and 16.6 m asl support the hypothesis of a back flooding higher than sea level. Consequently, the fine-grained material was deposited on the pond floor, as had enough time to settle. The presence of clogging material in cracks, faults, and joints in the same area of the cave further contributed to the formation of these ponds. Presently, there are no active streams or pools in Selinitsa; therefore, the fine-grained deposits indicate former phreatic/epiphreatic conditions that might have triggered back flooding of the upstream passages, especially in areas where clogging material is found ( Table 1). As water recessed, deposition occurred along the main cave passage in areas where water was ponding. Similar sedimentary features have been reported and interpreted as the result of back flooding due to downstream cave-roof-collapse breccia in the VeǏký Dóm segment of the Demänová cave system in Slovakia [16].
The textural features of the fine-grained deposits (unsorted, angular-subangular fragments) clearly point to small-scale, terrestrial transportation, rejecting possible penetration of detritus by sea-water flooding ( Figure 5) and supporting the proposed explanation of back flooding. The trace element geochemistry of the fine-grained sediment suggests a continental source of detritus, pointing to the overlying Plattenkalk flysch (Figure 7). ment of the Demänová cave system in Slovakia [16].
The textural features of the fine-grained deposits (unsorted, angular-subangular fragments) clearly point to small-scale, terrestrial transportation, rejecting possible penetration of detritus by sea-water flooding ( Figure 5) and supporting the proposed explanation of back flooding. The trace element geochemistry of the fine-grained sediment suggests a continental source of detritus, pointing to the overlying Plattenkalk flysch (Figure 7). The predominance of dolomite in the cave sediment relative to calcite and Mg-calcite in the carbonate fraction (>98% modal), combined with the development of Selinitsa in Upper Senonian to Upper Eocene limestones ( Figure 1B), indicates that dolomite is authigenic and formed during settling of water in the cave ponds. Dolomite formation is favored in hyposaline environments (such as the Selinitsa cave system) [42] following mixing of seawater and percolating fresh water. The texture of dolomite crystals found in the cave sediment (euhedral, romb-shaped, Figure 5) supports the authigenic character of dolomite. It is not clear whether dolomite originates from aragonite and/or calcite replacement, although the absence of CaCO 3 polymorphs in the cave sediment (<1% modal) supports the direct precipitation of dolomite in the cave ponds contemporaneously with sediment (penecontemporaneous or syndepositional according to the terminology of [42]). The presence of authigenic dolomite in sediments of Selinitsa Cave indicates that for prolonged periods, dissolved Mg concentrations in the pond water remained both stable and high enough to favor its precipitation over calcite and/or aragonite, as in similar pCO 2 conditions, higher Mg activity leads to dolomite precipitation [43]. Higher Mg activity may also point to higher residence time of percolating (meteoric) water interacting with host lithologies (Figure 1B), thus increasing the dissolved Mg content of the settling water, eventually leading to primarily dolomite formation. Moreover, the predominance of dolomite through the entire sedimentary succession indicates a constant source of Mg in water in Selinitsa Cave, either as a result of mixing fresh water and seawater, dissolution of host lithologies, or both in a stable geochemical environment for long periods of time.
With the aim of constraining a chronological framework for these processes, we used the available data from Diros Cave, located approximately 20 km south of Selinitsa. In the area of Diros, geo-bio SL indicators occur in the range of 12-15 and 5-6 m asl, and according to [44], they are considered representative of MIS 5e and 5a sea-level stands, respectively. In the case of Selinitsa Cave, we interpreted the marine terrace at 16.6 m as an outcome of coastal erosion during MIS 5. Although the values are slightly dissimilar between the two caves, the discrepancy may be attributed to different uplift rates of the Mani Peninsula. Kleman et al. [45] attributed the observed dissimilarity in this area to an upward tilting of the Mani pediment (the latter being of pre-Middle Pleistocene age, e.g., [46]) to the north, whereas Kelletat et al. [47] interpreted the altitudinal decrease in the MIS 5 terraces as a southward tilting of the entire western coast of the Mani Peninsula. Based on the aforementioned hypothesis, the two independent sources of sea-level data (peak volume at 16.4 m derived from 3D scanning and the marine terrace at 16.6 m asl) validate the presence of a Late Quaternary sea-level stand in the surficial area, as well as in the cave environment. The fine-grained deposits indicate that during this period, cave flooding took place, with the water column preserved at a higher level relative to the MIS 5 sea stand.
In conclusion, the multidisciplinary approach of the present study highlights the significance of 3D mapping and the implementation of geo-bio-SL indicators and sedimentology to decipher the paleogeographic evolution of coastal caves. The formation and evolution of Selinitsa Cave make it ideal for identifying the paleoclimate regime of Greece and eastern Mediterranean regions, as its development was largely affected by sea-level changes during the Late Quaternary. Combined with radiochronological data, the aforementioned may also reveal crucial information that could help constrain the time frame of Late Quaternary paleoclimatic regimes and the corresponding sea-level fluctuations in eastern Mediterranean regions.

Conclusions
The outcome of this study regarding the evolution of Selinitsa Cave during the Quaternary may be summarized as follows:

•
In the area of Selinitsa Cave, there are four distinct marine terraces at 6, 10.7, 16.6, and 30-32 m asl, with the terrace at 16.6 m representing the MIS5 sea-level stand; • 3D mapping revealed that the major volume of the cave clusters at 16.4 m asl, indicating that karstification occurred along the phreatic/epiphreatic zone, at approximately the same level as the MIS 5 marine terrace at 16.6 m asl; • The fine-grained deposits found in the cave indicate cave flooding during MIS 5, and the increased sediment thickness suggests stable hydrologic conditions for prolonged time-periods (reduced speed of water flow), favoring deposition of fine-grained material; • The detrital component of the deposits suggests a continental source based on major and trace element geochemistry (Plattenkalk flysch); • The predominance of authigenic dolomite in the cave deposits indicates that the material was deposited in a setting where mixing of seawater with percolating water occurred and at a higher altitude relative to sea level; • The geochemical and hydrologic regime of the settling ponds remained stable for prolonged time-periods, as authigenic dolomite occurs through the whole sedimentary succession; • The multidisciplinary approach of this study may provide crucial information regarding the paleogeographic evolution of karstic systems, and such data may be employed to unravel the paleoclimate regime of the coastal zone of the eastern Mediterranean during the Late Quaternary.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/quat5020024/s1. Figure S1: Representative XRD pattern of the clogging material filling cracks, joints, voids and faults in Selinitsa Cave; Figure S2: Representative XRD pattern of the finegrained deposits found in Selinitsa Cave; Figure S3: Representative XRD pattern of the fine-grained deposits found in Selinitsa Cave; Figure S4: Representative SEM-EDS spectrum of detrital albite from the fine-grained deposits from Selinitsa Cave; Figure S5: Representative SEM-EDS spectra of authigenic dolomite from the fine-grained deposits from Selinitsa Cave; Figure S6: Representative SEM-EDS spectrum of detrital muscovite from the fine-grained deposits from Selinitsa Cave; Figure S7: Representative SEM-EDS spectrum of Mg-calcite from the fine-grained deposits from Selinitsa Cave; Table S1: Representative major (wt.%) and trace (ppm) element analyses of the fine-grained deposits in Selinitsa Cave.
Funding: This research is co-financed by Greece and the European Union (European Social Fund-ESF) through the Operational Programme «Human Resources Development, Education and Lifelong Learning 2014-2020» in the context of the project "Geochemical, mineralogical and petrographical investigation of crystalline carbonate deposits and speleothems of southwest Peloponnese and applications in paleo-environmental studies" (MIS 5049093).