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Article

Evidence of Holocene Sea-Level Rise from Buried Oyster Reef Terrain in a Land-Locked Insular Embayment in Greece

by
Evangelia Manoutsoglou
* and
Thomas Hasiotis
Department of Marine Sciences, University of the Aegean, 81100 Mytilene, Lesvos, Greece
*
Author to whom correspondence should be addressed.
Geosciences 2025, 15(3), 105; https://doi.org/10.3390/geosciences15030105
Submission received: 6 February 2025 / Revised: 7 March 2025 / Accepted: 12 March 2025 / Published: 16 March 2025
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Abstract

:
Gera Gulf, a relatively small embayment on the island of Lesvos, serves as a representative example of a semi-enclosed, shallow marine system in Greece. Previous studies revealed that the gulf seafloor is occupied by numerous small reefs that are evenly distributed. Recently, seismic surveys together with gravity coring have shown numerous relict reefs within a fine-grained matrix, hosted at different stratigraphic levels above the inferred Holocene/Pleistocene boundary and locally extending up to the present seabed. The reefs are primarily engineered by the bivalve Ostrea edulis, with additional colonization by other marine organisms such as the coral Cladocora caespitosa. Key features identified in the seismic profiles include the widespread distribution of buried reef structures, erosional surfaces and unconformities also related to a paleolake, extensive fluid concentrations, and a major fault system paralleling the northeastern coast. Seismic record analysis and sediment dating suggest that the flooding of Gera Gulf began approximately 7500 BP, with O. edulis colonizing the seabed shortly thereafter. Buried reef structures were identified within the transgressive and highstand system tracts, characterized by varying sedimentation rates. These variations reflect changing environmental conditions, probably linked to specific climatic events during the Holocene epoch, which contributed to the evolution and shaping of the oyster reef terrain. Given the limited studies on recent or buried oyster reefs in similar environments, this study provides critical insights into the Holocene evolution of oyster reef terrains and their response to climate changes.

1. Introduction

The study of Holocene sea-level records and interactions with the coastal landscape is fundamental to understanding the response of coastal environments to future climatic changes [1]. With regard to continental and marine environments, the climatic variability of the Quaternary period has given rise to a rich and complex record of landforms, sediments, and biogenic remains. These bio-geological records, among others, provide indirect information of former climates and environments [2]. Examples of Holocene sea-level proxies vary, including salt marshes, sediments (beach rocks), foraminifera, and pollen, which are among the most commonly employed indicators [3,4].
Oyster shell accumulations are commonly used in conjunction with other transgressive deposits, such as salt marsh pits, as indicators of sea level rise [5,6]. The calcium carbonate content of the shell makes them a significant part of the fossil record, and they are often used as paleontological indicators [7,8,9]. Oyster reefs are self-accreting structures formed through deposition mainly of shells and sediments and are either growing or degrading at varying rates, primarily influenced by changes in environmental conditions [10]. When developing undisturbed, they are likely to preserve a depth–age relationship, thus providing a chronological record of the depositional environment [11,12].
However, transgressive deposits are often poorly preserved in geological time due to the high-energy environment of the continental shelf [13]. After the Last Glacial Maximum, well-preserved transgressive carbonate deposits, including oyster reefs, are rarely observed [13]. Therefore, intact relict oyster reef structures are relatively uncommon. Rare examples of buried and well-preserved oyster reef structures have been detected in the seismic stratigraphic records of Apalachicola Bay and St. George Sound—Florida [14], Hudson River estuary—New York [15], Corpus Christi Bay—Texas [16], and Kalloni Gulf, which neighbors the study area [17], all of them indicating that the reef die-offs/burials are associated with environmental changes linked to plausible climatic changes.
Holocene climate variability in the Aegean has been documented by multiple studies, e.g., [18,19,20,21], which either complement each other or present differences. These differences are attributed to their distinct approaches but mainly to the geomorphological, tectonic, and therefore environmental complexity of the Aegean region. Given these challenges, semi-enclosed marine systems serve as optimal models of depositional environments that are ideal for studying climatic and environmental variability as their shallow and protected basins favor the preservation of the sedimentary sequences. This, in turn, provides an intact record of the major phases of marine transgression and regression during the Late Quaternary period [13].
A typical example of a semi-enclosed marine system is Gera Gulf, where numerous oyster reefs kept pace with the infilling sediments during the early stages of accumulation, occupying deeper stratigraphic levels up to the seafloor and having experienced different burial phases. This study presents the distribution of intact oyster reef structures within the stratigraphy of Gera Gulf as well as the possible pattern and timing of their deterioration, along with other sub-bottom features, which serve as indicators of the environmental activity and evolution of the embayment through recent geological times. This research highlights one of the rare cases of well-preserved modern and buried oyster terrains, providing important insights into the climatic and environmental variability reflected in the growth patterns of the reefs.

2. Study Area

Gera Gulf (Figure 1) is a relatively small (approximately 42.5 km2) semi-enclosed shallow embayment, located in Lesvos Island in the North Aegean (Greece). The bathymetry is developed concentrically, with the maximum depth of 19 m found in the center. The gulf is connected to the open sea to the south through a narrow, long channel (6.5 km) with a depth range of 10 to 30 m. An important morphological feature is a sill, about 10 m deep, which separates the channel from the inner part of the gulf.
The drainage basin is characterized by various rock formations, including Carboniferous to Triassic metamorphic, early Paleozoic to Pliocene igneous, and Neogene sedimentary rocks, as well as alluvial deposits, along with fault systems, some of which are still active [22]. A thermal spring is located on the NNE coast (Figure 1), emerging from a fault in Triassic marbles and phyllites [23].
The surrounding drainage system is rich, with the main sediment inputs consisting of fine material [24,25]. These inputs derive from seasonal streams, with the Evergetoulas River being the most significant source. As a result, Gera Gulf serves as a sink for fine-grained (muddy) sediments. The circulation pattern and, consequently, the transport of sediments in the gulf are controlled by numerous factors and processes. Hydrodynamic modeling for southeast blowing winds has shown a cyclonic movement of seawater masses with relatively low current velocities during the winter period and a reverse pattern occurring during the summer season [26].
Benthic communities have been described by Zenetos and Papathanasiou [27], whereas Tsatiris et al. [28] depicted among others the habitat distribution. Manoutsoglou et al. [25] and Lioupa et al. [29] described in detail the low but complex morphology of the gulf deeper than 11 m water depth, built by thousands of small reefs, and also presented initial information of buried structures. Other preliminary studies that focused on the sub-bottom structure have also identified wavy reflectors in seismic profiles resembling buried relief and have been interpreted as buried reefs [25,30].

3. Materials and Methods

Seismic profiles were collected during two surveys. Very high-resolution sub-bottom information was collected in 2013 during testing of a parametric SES-2000 light plus sub-bottom profiler (SBP) of Innomar Technologie (Rostok, Germany) along six closely spaced lines (21.5 km) at the southeastern part of the gulf (Figure 1). This dataset was initially interpreted by [25], but it is re-evaluated in the present study. In 2017, about 150 km of seismic profiles were collected (Figure 1) using an Applied Acoustics Boomer SBP of Subsea Technologies (Houston, TX, USA), consisting of a CSP-D 700 Joules energy source, an AA251 Boomer plate mounded on a CAT-200 catamaran, and a 12-element hydrophone, achieving a maximum penetration of ~65 m under the seabed, although this was reduced significantly in areas shallower than 10–12 m. The research vessel of the University of the Aegean (R/V Amfitriti) was utilized for data acquisition during both surveys, maintaining a speed of ~4 knots. Positioning was provided by an RTK-DGPS (Topcon HiPer of Topcon Corporation, Tokyo, Japan). SBP data processing included band-pass filtering, swell filtering, bottom tracking, gain filters, and digitization of specific horizons and features. ArcGIS 10.2 was used for mapping purposes.
In order to ground-truth the seismic data and gain information regarding the features observed under the seafloor, 12 gravity cores were acquired during two campaigns (Figure 1). The first was in 2019 with R/V Amfitriti, when ten coring attempts were made (G1–G10) and finally eight sediment cores up to 76 cm long were retrieved using a small gravity corer (1 m long, 5 cm in diameter). Two coring attempts over relatively larger reefs near the entrance of the gulf failed (G9 and G10) due to the coarse texture of the seafloor. G1, collected before the gulf entrance, retrieved only a few traces of sand. During the second campaign in 2020, the R/V AEGAEO of H.C.M.R. used a longer gravity core (3 m) at two pre-selected locations (GL1 and GL2); however, it managed to collect a maximum length of only 153 cm of sediment. Laboratory analysis included (i) macroscopic description of texture and sedimentary unit separation, (ii) grain-size determination with the pipette method due to the fine-grained matrix, and (iii) radiocarbon dating.
Initially, the plastic cores were split lengthwise, and half of the cores were photographed and described in detail. The cores were sub-sampled every 5–10 cm for grain size analysis depending on the presence of thin layers of biogenic fragments and shells and visually observed lithological changes. The grain size analysis followed the standard method of Folk [31]. The fine-grained fraction (<63 µm) was separated by wet sieving, after which the pipette method was applied due to the prevailing fine-grained matrix. Particle dispersion was managed by adding a 5% sodium hexametaphosphate solution. The coarse sediment fractions were sorted through dry sieving at intervals of 1-Ø after drying at 60 °C. Statistical parameters were calculated using the GRADISTAT software [32].
Five samples, three Ostrea edulis shells and two samples consisting of skeletal fragments of the coral Cladocora caespitosa, from the two longer cores were selected for radiocarbon dating after careful mechanical and chemical treatment [33] using the 14C dating method in NCSR Demokritos. The chronological framework that was used is based on date calibration using Oxcalv.4.4.2 [34], considering the recent calibration curves of Reimer et al. [35]. The precise Reservoir Constant (ΔR) value for Gera Gulf is not available; thus, ΔR was set to zero. The impact of different carbon reservoirs was assessed by comparison with other neighboring regions with available ΔR values (e.g., Gioura: 149 ± 30 [36]), assuming that the results may shift by a few hundred years. However, this range of variation remains within an acceptable level of uncertainty given that the analysis is based on millennial-scale chronological framework.

4. Results

4.1. Stratigraphy

The general stratigraphy of Gera Gulf is represented by four main seismic units (SUs). SU1 is related to an acoustic semi-transparent layer with few faint internal reflectors, indicative of an almost homogeneous fine-grained layer with a few slightly coarser internal layers (Figure 2). The surficial echo appears mounded (up to 2.5 m in height) at depths greater than 11 m, indicating a dense microrelief. SU1 reaches a maximum thickness of ~5 m and becomes more indistinct off Evergetoulas River and at depths less than ~13 m, probably due to the presence of slightly coarser sediments in the shallower parts of the gulf [37]. SU1 corresponds to the Holocene highstand system tract (HST) and overlies SU2 unconformably, which shows wavy reflectors of low to medium intensity that reach a thickness of up to ~8 m (Figure 2). The main differences between the two units are that (i) the SU2 upper reflector is more wavy and locally of a higher intensity in relation to the undulating bottom-echo of SU1 and (ii) the slightly higher intensity of the SU2 internal reflectors, locally being intermittent. The combined thickness of the first two units is less than 10 m, with the maximum thickness observed at the center of the gulf. SU2 is interpreted to consist of sediments deposited during the Holocene transgression (TST). Manoutsoglou et al. [25] demonstrated that the wavy surficial echo of SU1 in the Innomar SBP profiles corresponds to numerous reefs of biogenic origin (mainly bivalves) that populate the seafloor, developed in a fine-grained matrix. Similarly, the buried wavy reflectors of SU2 that resemble the seabed echo are interpreted as buried reefs, which appear to have developed during the Holocene transgression.
SU3 comprises parallel to subparallel and locally discontinuous reflectors of medium to high intensity, suggesting consolidated and/or coarser stratified sedimentary layers (Figure 2). It has a maximum thickness exceeding 40 m that appears to reduce towards shallower waters. The unconformities within SU3 imply the presence of lowstand and older sedimentary units that may also correspond to terrestrial environments. The formation of a concave depositional sequence, with a maximum thickness of 5 m, at the top of SU3 in the eastern and deeper part of the Gulf probably indicates the existence of sedimentary layers of lacustrine origin. A large area within SU3 is characterized by acoustic anomalies that display a range of distinct acoustic signatures, suggesting the presence of fluids in the sediment pores.
SU4 is interpreted as the acoustic basement and appears as an irregular high-amplitude reflector with a scattered and chaotic internal acoustic pattern and with rare reflections. It has been detected in a small number of seismic profiles, always near the shoreline, typically being buried. The irregular surface of SU4 resembles an erosional surface, where SU3 usually terminates with an onlap relation.

4.2. Sub-Bottom Features and Structure

The similarity in acoustic characterization and the presence of the wavy reflector at the top of SU1 and SU2 indicate that SU2 is a depositional system of an earlier reef phase that occurred at lower sea levels. Yet, the number and vertical relief of the mounds at the top of SU2 in comparison to the modern seabed, along all the seismic profiles examined (Figure 2 and Figure 3), indicate that the reefs probably thrived more abundantly in the past. Also, the Boomer profiles revealed that the bases of the reefs are located even deeper, locally close to the boundary between SU2 and SU3. The buried reef heights appear to reach 4 m, about 60% higher than the recently exposed relief [25]. The unconformity between SU2 and SU3 most likely corresponds to a paleosurface along which the development of the initial reefs began. The higher resolution of the SES-2000 light plus seismic profiles, which covered only a small part on the eastern side of the gulf, revealed another wavy reflector within SU2 and close to the SU2/SU3 boundary, verifying gradual reef development in different phases during transgression (Figure 3). However, due to the lower resolution of Boomer SBP profiles, which have covered almost the entire gulf, it was not possible to distinguish/map additional deeper reef phases.
In depths shallower than 11 m, closer to the coastline and especially near the Evergetoulas River, SU1 and SU2 appear to merge in a single unit with stronger, discontinuous, and locally chaotic reflections, indicative of non-stratified sediments deposited under higher sedimentation rates in relation to the deeper parts of the gulf. The composition of the sediments in these areas significantly limits the penetration of sound waves in Boomer seismic profiles. As a result, it was difficult to distinguish lower stratigraphic levels. The absence of a distinct prograding seismic unit or evidence of clinoforms and erosional features and/or buried river channels confirm the reduced river sediment discharge through time. On the contrary, the Innomar profiles closer to the shoreline showed the existence of relatively small buried channels and valleys at different stratigraphic levels, demonstrating the occurrence of a potential small hydrographic network associated with steeper onshore relief.
One of the most important features observed in the seismic profiles is an extensive zone of buried faults in the northeastern part of the gulf. Along this zone, the seismic reflectors terminate abruptly, and they are displaced, or they present abrupt changes in slope, all indicative of faulting (Figure 4). Short-length, nearly parallel profiles, which locally intersect with the main lines that cover the entire gulf, show an overall elongated zone of numerous closely spaced (up to 30–40 m apart) faults. These faults are synthetic and antithetic to the onshore fault systems, forming a complex pattern. Locally small horst and graben structures are detected. The observed buried fault system develops parallel to the shoreline and is probably related to the onshore faults appearing at the north and eastern sides of the gulf, probably representing a complex and extensive fault zone. This zone appears within SU3 and locally affects SU2 (wherever visible shallower than ~15 m) (Figure 4b). On the contrary, no signs of faulting were observed to disturb SU1. The fact that some faults affect the SU2 deposits denotes activity during the Holocene.
A plethora of acoustic anomalies have also been detected in the seismic profiles, occurring in a large part of the gulf, indicating the presence of fluids within the sediment pores (Figure 5). Anomalies have the form of acoustic turbidity, enhanced reflectors, acoustic masking, acoustic heterogeneity, and velocity pull-down (terminology from [38,39,40]). While strong reflectors (prolonged or enhanced) are frequently linked to layers of coarser or more compacted sediments, the acoustic pattern and the distribution of acoustic anomalies across various stratigraphic levels more convincingly suggest the presence of fluids within the sediment pores. The acoustic anomalies were grouped into three areas (I-III) (Figure 6a), each one characterized by specific acoustic signatures, depth of occurrence, and related to particular stratigraphic and structural features.
Area I expands over a region of 7.3 km2, located in the eastern and deeper part of the gulf (Figure 6a), and it represents an extensive zone of acoustic masking. The boundary between SU2 and SU3 and the concave base of the lake sediments often appears as a prolonged reflector or an acoustic turbidity zone, representing fluid accumulation horizons that induce acoustic blanking of the underlying layers (Figure 5a). Locally, small turbid pockets/plumes are observed within the lake sediments and occasionally within SU2, suggesting fluid leakage and trapping in shallower stratigraphic levels. Velocity pull-downs are rarely detected at the edges of enhanced reflectors, indicating increased fluid concentration.
Area II is linked to various acoustic anomalies, recorded in the seismic profiles, primarily observed in the deeper stratigraphic layers (SU3), and occupies 7.5 km2 at the north (near Evergetoulas River) and northwestern parts of the gulf (Figure 6a). The acoustic anomalies consist of acoustic heterogeneity, enhanced reflectors, and turbid zones, all of a small aerial extent, observed at various stratigraphic levels within SU3 and extending up to the base of SU2 or locally leaking within the TST deposits (in general, developing up to ~35 m below the seabed) (Figure 5b).
Area III is situated in the NE part of the gulf, developing parallel to the coastline, and it occupies a zone of 2.3 km2 (Figure 6a) and is related to the presence of fluids within the fault zone. In this region, fluids concentrate either in between closely spaced faults or immediately over fault traces in the form of small plumes or short segments of strong (enhanced) reflectors (Figure 5c and Figure 6a). The manifestation of fluid seepage in this area is a submarine hydrothermal spring observed at ~7.5 m water depth within the fault zone [37].
A Boomer profile along the deeper part of the sill at the entrance of Gera Gulf, despite the limited penetration due to the coarse sediment nature [37], revealed a distinct reflector at a depth of about 12 m that, based on its acoustic return and morphology, as well as its lateral continuation, appears to be the SU2/SU3 unconformity (Figure 7). This paleosurface mimics the present seafloor sill and most probably represents a paleo-ridge that prevented gradual gulf transgression during the Holocene sea level rise. This surface was eventually exceeded in the last stages of the Holocene transgression, leading to marine flooding of the paleolandscape environments (land-locked valley and lake).

4.3. Gravity Cores

From the macroscopic inspection of the cores, four different sedimentary units (SedUs) were distinguished (Figure 8), often alternating at short distances. SedU1 consists of homogeneous, gray, and water-rich fine-grained sediments, with small sparsely distributed biogenic fragments, few shells, and scattered black spots. SedU2 consists of thin layers that are rich in shells and/or biogenic fragments of various sizes, which are either part of the benthic fauna (i.e., Turritella communis) or belong to reef community organisms (Mytilus galloprovincialis, Vermetus semisurrectus, Ostrea edulis, Echinocardium sp., and Holothuria sp.). The thin layers of biogenic material are embedded in a fine-grained matrix. These two sedimentary units alternate with each other, although SedU2 is dominant in most cores, in correspondence with the presence of reefs in deeper layers. SedU3 contains fine-grained sediments that are rich in skeletal fragments of the coral Cladocora caespitosa, along with bivalve shells of Ostrea edulis and Arca noae. SedU4 consists of brownish fine-grained and slightly compacted sediments, and it is characterized by the absence of biogenic fragments and shells, indicative of terrestrial deposits.
The shorter gravity cores (G1–G8) contained only SedU1 and SedU2 (Figure 9a), whereas the longer cores also revealed SedU3 and SedU4. Core GL1 was collected from the central and deepest part of the gulf, in a location where the boundary between SU1 and SU2 was about 1 m below the seafloor, aiming to ground-truth these units. Indeed, the uppermost part of the core (112 cm) consists of fine-grained sediments (SedU1) (Figure 9b), whilst, at 112 cm, SedU2 appears with a thickness of 10 cm and overlies SedU3, which extends down to 153 cm. According to the findings in the seismic profiles, it seems that the coarse textures of SedU2 and SedU3 are related to the top of SU2, the uppermost part of which was interpreted to constitute a buried reef.
GL2 was collected near the entrance of the gulf, in an area of highly undulating relief, where G9 and G10 failed to retrieve a sample. The surficial part of the core comprises SedU2 (0–2 cm) and SedU3 (3–58 cm), whilst, down to 92 cm, alternations of SedU1 and SedU2 are detected (Figure 9b). The deepest part of the core (92–130 cm) is occupied by SedU4.

4.4. Grain Size Analysis and Radiocarbon Dating

A grain size analysis of selected samples from all the gravity cores revealed the dominance of fine-grained sediments with slight alterations between the fractions of silt and clay, thus confirming the almost homogenous fine-grained sediment matrix of the surficial layer (SU1) in the seismic data. Gravity cores also manifested the biogenic origin of the reefs due to the abundant presence of shells and biogenic assemblages of SedU2 and SedU3 in certain stratigraphic levels, matching with the observations of the SU1 and SU2 characteristics.
In general, SedU1 and the soft matrix of SedU2, where biogenic fragments and shells are embedded, have a mean size of 7 to 8.6 Ø. Only in core G3, in the southeastern part of the gulf, does the mean size slightly increase with depth (up to 5.8 Ø at 65 cm), whilst the short core G2 near the gulf entrance shows values up to 5.3 Ø at 11 cm under the seabed. SedU3 shows higher mean size values (2.4–3.7 Ø) since small biogenic fragments constitute a major part of the sediment matrix, whereas SedU4 has mean size values of 4.2–4.9 Ø.
The sand concentration appears to be higher along GL2. This can be attributed to the widespread distribution of biogenic assemblages, the presence of SedU4, and the proximity of the GL2 sampling location to the gulf entrance.
Small variations in sediment fractions may reflect minor environmental changes during the HST, possibly linked to shifts in river discharge or alterations in the circulation and intensity of currents. These changes in the environmental processes probably affected both the development and growth rate of reefs during the earlier Holocene stages.
The radiocarbon dating (Table 1) of the GL1 samples, from the central and deepest part of the gulf, offered calibrated dates of 3450–3790 BP at 130 cm and 5587–5889 BP at 150 cm below the seabed, representing average sedimentation rates of 0.38 mm/y for the surficial layer (top 130 cm) and 0.09 mm/y for the layer between 130 and 150 cm. These results confirmed that the unconformity between SU1 and SU2 represents a paleo-reef surface and that there was a die-off episode at ~3000 BP.
The dating of the GL2 samples revealed calibrated dates of 6424–6754 BP, 7043–7382 BP, and 7310–7572 BP at 52 cm, 79 cm, and 90 cm below the seabed, respectively, with an average sedimentation rate of 0.08 mm/y for the upper 52 cm and a significant increase to about 0.4 mm/y towards the lower parts of the core. The analysis of GL2 verifies that SedU4 was colonized by the Ostrea edulis species almost immediately after its flooding at about 7500 BP, although increased sedimentation rates at that time prevented reef growth.

5. Discussion

5.1. The Buried Reef Terrain

The analysis of the seismic profiles and gravity cores in Gera Gulf revealed a buried relief consisting of relict oyster reefs within a fine-grained matrix. The reefs thrived on the seafloor soon after the seawater overtopped the sill in the gulf entrance and flooded the paleolake environment at the center of the gulf, along with transgressive sediment deposition. Consequently, the reef landscape initially occupied the central part of the gulf and spread evenly with the sea level rise, especially during the highstand period (Figure 10). Also, it appears that the reefs grew at different stratigraphic levels during transgression; some developed almost constantly throughout the Holocene, whilst others ceased.
It seems that the HST/TST unconformity hosts an extensive terrain of buried reefs, occupying a large part at the center of the gulf. Still, the modern reefs on the seafloor extend into a wider area (Figure 6b), towards the shallower part of the gulf, highlighting the relationship between sea level rise and reef growth during the Holocene.
Buried oyster reefs of similar architectures have been reported from various shallow-water environments, such as the Hudson River estuary [15], Apalachicola Bay, St. George Sound, Florida [14], Corpus Christi Bay [16], and Kalloni Gulf [17]. Their burial is directly linked to climatic variability during the Holocene, which caused alterations in environmental parameters (e.g., salinity) and ultimately led to reef die-offs. However, some individual reefs have survived until today, as observed in the case of the neighboring Kalloni Gulf [17] and as detected along the study area.
Core GL1 from the center of Gera Gulf confirmed (i) the presence of a buried oyster reef lying along the TST/HST boundary, (ii) that, for the first couple thousand years of the HST period, the reef vertical growth rate decelerated, (iii) an adverse change in environmental conditions at ~3000 BP, when the average rate of sedimentation increased and overcame the decelerated rate of reef growth, resulting in the cessation of reef evolution and their burial, and (iv) that different development/growth reef stages probably prevailed during the middle and upper Holocene since, locally, reefs in deeper stratigraphic levels managed to develop vertically up to the seafloor, whilst others formed and grew gradually only during the upper Holocene.
The base of GL2 revealed the terrestrial substrate (SedU4) that was colonized by the Ostrea edulis species soon after the area was inundated by the sea, approximately 7500 BP (Figure 10). During the transgression, the sedimentation rates were higher (up to 0.44 mm/y) but gradually decreased to 0.08 mm/y, contributing to the establishment of favorable conditions for reef growth. The oyster reefs in this area, close to the entrance of the gulf, have continued to develop within the HST and up to the present day as the stronger currents near the entrance prevented rapid sedimentation and enhanced nutrient enrichment, which benefits the filter-feeding organisms of the reefs.

5.2. Reefs and Fluids

The acoustic anomalies observed in the seismic profiles along Gera Gulf are typical of fluid presence in sediment interstices. Fluids can be of biogenic (methane) or thermogenic origin, or even concentrations of groundwater or hydrothermal fluids. Gera Gulf represents an ideal semi-enclosed environment for the accumulation and bacterial degradation of organic material, leading to methane gas production, a process thought to have persisted during periods of lower sea level since fluids were detected at various stratigraphic levels in the seismic records. Also, In Lesvos, the presence of thermal springs are manifestations of former volcanic activity on the island and are mainly associated with modern active tectonic movements [41]. In the northeastern part of Gera Gulf, a submarine hot spring associated with few small craters was detected in side-scan sonar records by Manoutsoglou [37], neighboring an onshore thermal spring [23]. During this survey, it became evident that the submarine spring is located within the observed fault zone, which serves as a migration conduit for hydrothermal fluids.
In many studies, the presence and growth of biogenic reefs or the occurrence of extensive biogenic assemblages on the seafloor are directly related to the presence of fluids (i.e., [38,42]). Examples include deep sea environments where benthic organisms have developed different survival mechanisms [43], unverified mounded structures that were inferred to be of biogenic origin [44], and submarine groundwater discharge through fault systems in lakes that build calcareous mounded deposits [45]. However, these environments have different physiographic and habitat characteristics in relation to Gera Gulf. Also, although the presence of fluids as well as pockmarks in shallow and especially in semi-enclosed marine environments is common in Greece (i.e., [46,47,48,49]), biogenic structures similar to those found in Gera Gulf have not been reported elsewhere.
The effect of fluid seepage in shallow environments where oyster reefs occur has divided the scientific community. Some argue that seepage through the seafloor constitutes a drawback to the recruitment of young oysters, resulting in the delay of reef growth [50], while others hold that submarine fluid leakage contributes positively to nutrient balance and therefore to the acceleration of reef development [51]. The submarine spring in the NE part of the gulf is located in shallow waters (~7.5 m), less than the limit where reefs start to appear (>10–11 m) [29]. Also important is the absence of faults or acoustic indications of fluids within SU1, as well as the lack of fluid seepage in the water column (apart from the location of the submarine spring), evidence that does not support the association between fluid leakage and the development of modern reefs. Regarding the spatial distribution of fluids (Figure 6b), although it appears to be related to the distribution of buried reefs, it is not easy to assess their potential influence on reef growth during periods of lower sea level. On the other hand, it could also be speculated that, before the stabilization of the sea level rise in the middle Holocene, seepage along the fault zone might have been more intense and actively contributed to salinity changes and/or nutrient distribution, and therefore to oyster growth and reef evolution.

5.3. Reef Growth and Die-Offs

The Holocene epoch was once thought to be climatically stable; however, studies during recent decades (i.e., [52,53,54,55,56,57,58,59,60]) have revealed a few millennium-scale climatic events associated with abrupt climatic changes combined with sea-level fluctuations that caused massive oyster reef die-offs during the Holocene. Examples of such events are described by Goff et al. [16] in Corpus Christi Bay (Texas), Jing et al. [61] in Bohai Bay (Yellow Sea), and Dickson et al. [62] in Los Peñasquitos lagoon (Southern California).
The Holocene Climatic Optimum (10−6 ka) in the Mediterranean region is characterized by intense rainfall [63] and increased temperatures [49,50], which led to a positive freshwater balance in the marine environment and rising sedimentation rates due to increased river run-off [53,55,56]. These climatic changes can affect salinity and sedimentation rates in semi-enclosed marine systems like Gera Gulf, leading to eutrophication events or even altering the circulation pattern, factors that affect the growth rates of oyster reefs. Katsouras et al. [57] observed changes during the deposition of the S1 sapropel in the Aegean Sea, between ~8.6 and 7.6 kyr BP, and variations in sea surface temperature between 4.9 and 4.1 kyr BP. These observations are consistent with those of Bini et al. [59], who reported climate and environmental changes between 4.3 and 3.8 ka, primarily related to temperature increase.
According to the abovementioned factors, it becomes obvious that climatic changes and the alterations in environmental factors and sedimentation rates within the Holocene in the Aegean Sea are very likely to have influenced the evolution of the reefs during the last stages of transgression and throughout the highstand and might have led to the decline or even ceasing of their development for specific periods. This is reflected in the stratigraphy of Gera Gulf, where a buried undulating surface corresponding to an older reef terrain was identified within the transgressive deposits of SU2. Also, according to the 14C dating and the calculated sedimentation rates for core GL1, the boundary between the HST and TST is estimated at ~5600 years BP. Thus, the reef die-off observed within SU2 could likely be related to climatic events that occurred in the “aftermath” and as a late consequence of the climatic disturbance at 8.2 ka [54]. These reefs must have been in decline very soon after their initial formation, as evidenced by their low relief (Figure 3). The abrupt changes in environmental conditions during this period could have caused massive stress on the reef ecosystem and hindered its recovery. In addition, climatic changes between 4.3 and 3.8 ka [59] may have locally affected and gradually interrupted reef growth, eventually causing the deterioration and gradual burial of the reefs, as observed in core GL1. Yet, other biological factors, such as competition with other species, predation, and diseases, may have periodically wiped out active oyster communities [64].
In contrast, during intermediate periods, when favorable environmental conditions dominate, the oyster reefs thrived. The abundance of the observed reefs along the SU1/SU2 boundary indicates that the reefs probably prospered immediately after the end of the transgressive system tract deposition in relation to the modern more stressed environment due to anthropogenic activities [25,30].
In the GL2 location, the Ostrea edulis species seems to have settled in between ~7300 and 6500 BP, immediately after the Gera Gulf flooding, a period during which no significant climatic changes are mentioned in the literature. However, the sedimentation rates were relatively high and prevented the development of reef structures. From ~6500 BP and during the HST, the sedimentation rates decreased, creating favorable conditions for reef development, which continues up to the present day. The stronger currents near the gulf’s entrance promoted reef growth by reducing the sedimentation rates and enhancing the dispersion of nutrients for filter-feeding organisms. Environmental changes that might have impacted the reef evolution in the central part of the gulf between 4.3 and 3.8 ka do not appear to have negatively affected the development of the reefs at this specific location, likely because the intensity of the currents did not change significantly. For these reasons, the oyster reefs near the entrance of the gulf have been continuously colonized after 6500 BP, resulting in the development of larger oyster reef structures [25].

6. Conclusions

Numerous modern and buried oyster reefs, mainly engineered by the Ostrea edulis species, populate the seafloor and the lower stratigraphic levels of Gera Gulf, documenting their continuous presence after the gulf flooding during the Holocene epoch. The reefs thrive within a fine-grained matrix, a pattern that has seemingly persisted throughout the Holocene epoch.
The sill at the Gera Gulf entrance was overtopped around 7500 BP, leading to progressive flooding and the development of the first oyster reefs, relatively soon after, at the center of the gulf. Sample dating provides evidence of potential climatic changes that altered the environmental conditions and, consequently, affected the growth rates of the reefs. The climatic event between 4300 and 3800 BP described for the Mediterranean region probably manifested with a small delay, leading to reef die-offs at least in the center of the gulf. On the contrary, the environmental conditions at the gulf entrance have favored continuous reef flowering to the present day.
The stratigraphy of Gera Gulf reveals the complex interactions between oyster reefs and various environmental factors affected by Holocene climatic variability, as well as interactions with other features, such as the fluids and faults that might have also influenced reef growth. To fully understand the processes driving reef evolution and draw more robust conclusions, further surveys are needed. Gera Gulf offers a unique natural laboratory for investigating the key environmental factors and their influence on reef development. This setting also allows for high-resolution studies of climatic changes, which are meticulously recorded and preserved in this protected semi-enclosed environment, thereby contributing valuable data to the global climate archive. To this end, the use of sea level index points for the reconstruction of the local Holocene sea level curve, through more targeted core sampling and analysis, would provide further insights into sea level fluctuations and reef evolution [4,65]. This approach will enhance the understanding of sea level variations and potential tectonic influences in the area, ultimately improving the ability to assess the long-term interplay between climate change and reef development.

Author Contributions

Conceptualization, E.M. and T.H.; methodology, E.M.; validation, E.M. and T.H.; formal analysis, E.M.; resources, T.H.; data curation, T.H.; writing—original draft preparation, E.M.; writing—review and editing, T.H.; visualization, E.M. and T.H.; supervision, T.H.; project administration, T.H.; funding acquisition, E.M. All authors have read and agreed to the published version of the manuscript.

Funding

E.M. received funding from the project “Strengthening Human Resources Research Potential via Doctorate Research” (MIS-5000432), implemented by the State Scholarships Foundation (IΚΥ), co-financed by Greece and the European Union (European Social Fund- ESF) through the Operational Program «Human Resources Development, Education and Lifelong Learning».

Data Availability Statement

Data will be made available on request.

Acknowledgments

The authors would like to thank the H.C.M.R. and G. Rousakis for GL1 and GL2 core sampling, the N.C.S.R. “Demokritos” and Y. Maniatis for the C-14 dating, and O. Andreadis and A. Oikonomou for their support during field work and laboratory analysis, respectively.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Williams, A.N.; Ulm, S.; Sapienza, T.; Lewis, S.; Turney, C.S.M. Sea-level change and demography during the last glacial termination and early Holocene across the Australian continent. Quat. Sci. Rev. 2018, 182, 144–154. [Google Scholar] [CrossRef]
  2. Gordillo, S.; Bayer, M.; Boretto, G.; Charo, M. Mollusk Shells as Bio-Geo-Archives: Evaluating Environmental Changes During the Quaternary; Springer: New York, NY, USA, 2014. [Google Scholar]
  3. Vacchi, M.; Marriner, N.; Morhange, C.; Spada, G.; Fontana, A.; Rovere, A. Multiproxy assessment of Holocene relative sea-level changes in the western Mediterranean: Sea-level variability and improvements in the definition of the isostatic signal. Earth-Sci. Rev. 2016, 155, 172–197. [Google Scholar] [CrossRef]
  4. Barnett, R.L.; Bernatchez, P.; Garneau, M.; Juneau, M.N. Reconstructing late Holocene relative sea-level changes at the Magdalen Islands (Gulf of St. Lawrence, Canada) using multi-proxy analyses. J. Quat. Sci. 2017, 32, 380–395. [Google Scholar] [CrossRef]
  5. Gutiérrez, J.L.; Jones, C.G.; Strayer, D.L.; Iribarne, O.O. Mollusks as ecosystem engineers: The role of shell production in aquatic habitats. Oikos 2003, 101, 79–90. [Google Scholar] [CrossRef]
  6. Oliver, G.J.H.; Terry, J.P. Relative sea-level highstands in Thailand since the Mid-Holocene based on 14C rock oyster chronology. Palaeogeogr. Palaeoclimatol. Palaeoecol. 2019, 517, 30–38. [Google Scholar] [CrossRef]
  7. Bougeois, L.; De Rafélis, M.; Reichart, G.J.; de Nooijer, L.J.; Dupont-Nivet, G. Mg/Ca in fossil oyster shells as palaeotemperature proxy, an example from the Palaeogene of Central Asia. Palaeogeogr. Palaeoclimatol. Palaeoecol. 2016, 441, 611–626. [Google Scholar] [CrossRef]
  8. Kusnerik, K.M.; Lockwood, R.; Grant, A.N. Using the fossil record to establish a baseline and recommendations for oyster mitigation in the Mid-Atlantic US. In Marine Conservation Paleobiology; Springer: Cham, Switzerland, 2018; pp. 75–103. [Google Scholar]
  9. Lockwood, R.; Mann, R. A conservation palaeobiological perspective on Chesapeake Bay oysters. Philos. Trans. R. Soc. B 2019, 374, 20190209. [Google Scholar] [CrossRef] [PubMed]
  10. De Alteris, J.T. The role of bottom current and estuarine geomorphology on the sedimentation processes and productivity of Wreck Shoal, an oyster reef of the James River, Virginia. In Estuarine Circulation; Humana Press: Totowa, NJ, USA, 1989; pp. 279–307. [Google Scholar]
  11. Nielsen, S.N. Mollusk Shells as Bio-Geo-Archives. Evaluating Environmental Changes During the Quaternary. Ameghiniana 2014, 51, 155. [Google Scholar] [CrossRef]
  12. Song, B.; Yi, S.; Yu, S.Y.; Nahm, W.H.; Lee, J.Y.; Lim, J.; Kim, J.-C.; Yang, Z.; Han, M.; Jo, K.-N.; et al. Holocene relative sea-level changes inferred from multiple proxies on the west coast of South Korea. Palaeogeogr. Palaeoclimatol. Palaeoecol. 2018, 496, 268–281. [Google Scholar] [CrossRef]
  13. Cattaneo, A.; Steel, R.J. Transgressive deposits: A review of their variability. Earth-Sci. Rev. 2003, 62, 187–228. [Google Scholar] [CrossRef]
  14. Twichell, D.; Edmiston, L.; Andrews, B.; Stevenson, W.; Donoghue, J.; Poore, R.; Osterman, L. Geologic controls on the recent evolution of oyster reefs in Apalachicola Bay and St. George Sound, Florida. Estuar. Coast. Shelf Sci. 2010, 88, 385–394. [Google Scholar] [CrossRef]
  15. Carbotte, S.M.; Bell, R.E.; Ryan, W.B.F.; McHugh, C.; Slagle, A.; Nitsche, F.; Rubenstone, J. Environmental change and oyster colonization within the Hudson River estuary linked to Holocene climate. Geo-Mar. Lett. 2004, 24, 212–224. [Google Scholar] [CrossRef]
  16. Goff, J.A.; Lugrin, L.; Gulick, S.P.; Thirumalai, K.; Okumura, Y. Oyster reef die-offs in stratigraphic record of Corpus Christi Bay, Texas, possibly caused by drought-driven extreme salinity changes. Holocene 2016, 26, 511–519. [Google Scholar] [CrossRef]
  17. Manoutsoglou, E.; Hasiotis, T.; Velegrakis, A.F. Holocene records of oyster reefs in a shallow semi-enclosed island embayment of the Aegean Sea. Estuar. Coast. Shelf Sci. 2024, 302, 108–781. [Google Scholar] [CrossRef]
  18. Marino, G.; Rohling, E.J.; Sangiorgi, F.; Hayes, A.; Casford, J.L.; Lotter, A.F.; Kucera, M.; Brinkhuis, H. Early and middle Holocene in the Aegean Sea: Interplay between high and low latitude climate variability. Quat. Sci. Rev. 2009, 28, 3246–3262. [Google Scholar] [CrossRef]
  19. Triantaphyllou, M.V.; Ziveri, P.; Gogou, A.; Marino, G.; Lykousis, V.; Bouloubassi, I.; Emeis, K.-C.; Kouli, K.; Dimiza, M.; Rosell-Melé, A.; et al. Late Glacial–Holocene climate variability at the south-eastern margin of the Aegean Sea. Mar. Geol. 2009, 266, 182–197. [Google Scholar] [CrossRef]
  20. Giamali, C.; Koskeridou, E.; Antonarakou, A.; Ioakim, C.; Kontakiotis, G.; Karageorgis, A.P.; Roussakis, G.; Karakitsios, V. Multiproxy ecosystem response of abrupt Holocene climatic changes in the northeastern Mediterranean sedimentary archive and hydrologic regime. Quat. Res. 2019, 92, 665–685. [Google Scholar] [CrossRef]
  21. Seeliger, M.; Pint, A.; Frenzel, P.; Marriner, N.; Spada, G.; Vacchi, M.; Başaran, S.; Dan, A.; Seeger, F.; Seeger, K.; et al. Mid-to late-Holocene sea-level evolution of the northeastern Aegean sea. Holocene 2021, 31, 1621–1634. [Google Scholar] [CrossRef]
  22. Pavlides, S.; Tsapanos, T.; Zouros, N.; Sboras, S.; Koravos, G.; Chatzipetros, A. Using active fault data for assessing seismic hazard: A case study from NE Aegean Sea, Greece. In Proceedings of the Earthquake Geotechnical Engineering Satellite Conference, XVIIth International Conference on Soil Mechanics & Geotechnical Engineering, Alexandria, Egypt, 2–3 October 2009; Volume 10. [Google Scholar]
  23. Lambrakis, N.J.; Stamatis, G.N. Contribution to the study of thermal waters in Greece: Chemical patterns and origin of thermal water in the thermal springs of Lesvos. Hydrol. Process. 2008, 22, 171–180. [Google Scholar] [CrossRef]
  24. Anagnostou, C.; Sioulas, A. A sedimentological study of the gulf of Gera (Lesvos) and the possible anthropogenic influence in the geochemical cycle of the region. Bull. Geol. Soc. Greece 1989, XXIII, 193–207. [Google Scholar]
  25. Manoutsoglou, E.; Hasiotis, T.; Kyriakoudi, D.; Velegrakis, A.; Lowag, J. Puzzling micro-relief (mounds) of a soft-bottomed, semi-enclosed shallow marine environment. Geo-Mar. Lett. 2018, 38, 359–370. [Google Scholar] [CrossRef]
  26. Arhonditsis, G.; Tsirtsis, G.; Karydis, M. The effects of episodic rainfall events to the dynamics of coastal marine ecosystems: Applications to a semi-enclosed gulf in the Mediterranean Sea. J. Mar. Syst. 2002, 35, 183–205. [Google Scholar] [CrossRef]
  27. Zenetos, A.; Papathanassiou, E. Community parameters and multivariate analysis as a means of assessing the effects of tannery effluents on macrobenthos. Mar. Pollut. Bull. 1989, 20, 176–181. [Google Scholar] [CrossRef]
  28. Tsatiris, A.; Papadopoulos, V.; Makri, D.; Topouzelis, K.; Manoutsoglou, E.; Hasiotis, T.; Katsanevakis, S. Spatial distribution, abundance, and habitat use of the endemic Mediterranean fan mussel Pinna nobilis in Gera Gulf, Lesvos (Greece): Comparison of design-based and model-based approaches. Mediterr. Mar. Sci. 2018, 19, 642–655. [Google Scholar] [CrossRef]
  29. Lioupa, V.; Karsiotis, P.; Arosio, R.; Hasiotis, T.; Wheeler, A.J. A Comparative Crash-Test of Manual and Semi-Automated Methods for Detecting Complex Submarine Morphologies. Remote Sens. 2024, 16, 4093. [Google Scholar] [CrossRef]
  30. Manoutsoglou, E.; Hasiotis, T.; Andreadis, O.; Velegrakis, A. Seismic evidence of fluids in the sediments of the semi-enclosed Gulf of Gera (Lesvos, Greece). In Proceedings of the Hydromedit 2021 (4th International Congress on Applied Ichthyology, Oceanography & Aquatic Environment), Mytilene, Greece, 4–6 November 2021; pp. 249–253. [Google Scholar]
  31. Folk, R.L. Petrology of the Sedimentary Rocks, 2nd ed.; Hemphill Publishing Company: Austin, TX, USA, 1980. [Google Scholar]
  32. Blott, S.J.; Pye, K. GRADISTAT: A grain size distribution and statistics package for the analysis of unconsolidated sediments. Earth Surf. Process. Landf. 2001, 26, 1237–1248. [Google Scholar] [CrossRef]
  33. Maniatis, Y.; Oberlin, C.; Tsirtsoni, Z. “BALKANS 4000”: The Radiocarbon dates from archaeological contexts. In The Human Face of Radiocarbon; TMO 69; Tsirtsoni, Z., Ed.; Maison de l’Orient et de Mediterranne: Lyon, France, 2016; pp. 41–65. [Google Scholar]
  34. Bronk-Ramsey, C. OxCal Version 4.4.2; 2020. Available online: https://c14.arch.ox.ac.uk (accessed on 1 April 2022).
  35. Reimer, P.; Austin, W.; Bard, E.; Bayliss, A.; Blackwell, P.G.; Ramsey, C.B.; Butzin, M.; Cheng, H.; Edwards, R.L.; Friedrich, M.; et al. The IntCal20 Northern Hemisphere radiocarbon age calibration curve (0–55 cal kBP). Radiocarbon 2020, 62, 725–757. [Google Scholar] [CrossRef]
  36. Facorellis, Y.; Maniatis, Y.; Kromer, B. Apparent 14C ages of marine mollusk shells from a Greek island: Calculation of the marine reservoir effect in the Aegean Sea. Radiocarbon 1997, 40, 963–973. [Google Scholar] [CrossRef]
  37. Manoutsoglou, E. A Study of Biogenic Reefs in Semi-Enclosed Embayments: The Cases of Gera and Kalloni Gulf in Lesvos Island. Ph.D. Thesis, University of the Aegean, Mitilini, Greece, 2022; p. 164. [Google Scholar]
  38. Judd, A.; Hovland, M. Seabed Fluid Flow: The Impact on Geology, Biology and the Marine Environment; Cambridge University Press: Cambridge, UK, 2007. [Google Scholar]
  39. Toth, Z.; Spiev, V.; Jensen, J. Seismo-acoustic signatures of shallow free gas in the Bornholm Basin, Baltic Sea. Cont. Shelf Res. 2014, 88, 228–239. [Google Scholar] [CrossRef]
  40. Jasniewicz, D.; Klusek, Z.; Brodecka-Goluch, A.; Bolalek, J. Acoustic investigations of shallow gas in the southern Baltic Sea (Polish Exclusive Economic Zone): A review. Geo-Mar. Lett. 2019, 39, 1–17. [Google Scholar] [CrossRef]
  41. Kontis, E.E.; Gaganis, P. Hydrochemical characteristics and groundwater quality in the Island of Lesvos, Greece. J. Glob. Nest 2012, 14, 422–430. [Google Scholar]
  42. Hovland, M. Deep-Water Coral Reefs: Unique Biodiversity Hotspots; Praxis (Springer): Zürich, Switzerland, 2008; ISBN 1402084617. [Google Scholar]
  43. Petersen, J.M.; Dubilier, N. Methanotrophic symbioses in marine invertebrates. Environ. Microbiol. Rep. 2009, 1, 319–335. [Google Scholar] [CrossRef]
  44. Karnaukh, V.N.; Astakhov, A.S.; Vereshchagina, O.F.; Tsoy, I.B.; Kosmach, D.A.; Sagalaev, S.G.; Volkova, T.I.; Dubina, V.A.; Prushkovskaya, I.A. Formation of subsurface shallow gas accumulations in Amurskiy Bay (Peter the Great Bay, Sea of Japan) as a result of postglacial sea-level change, paleoceanographic conditions and hydrological activity. Mar. Geol. 2016, 372, 31–52. [Google Scholar] [CrossRef]
  45. Cukur, D.; Krastel, S.; Çağatay, M.N.; Damcı, E.; Meydan, A.F.; Kim, S.P. Evidence of extensive carbonate mounds and sublacustrine channels in shallow waters of Lake Van, eastern Turkey, based on high-resolution chirp subbottom profiler and multibeam echosounder data. Geo-Mar. Lett. 2015, 35, 329–340. [Google Scholar] [CrossRef]
  46. Papatheodorou, G.; Hasiotis, T.; Ferentinos, G. Gas-charged sediments in the Aegean and Ionian Seas, Greece. Mar. Geol. 1993, 112, 171–184. [Google Scholar] [CrossRef]
  47. Christodoulou, D.; Papatheodorou, G.; Ferentinos, G.; Masson, M. Active seepage in two contrasting pockmark fields in the Patras and Corinth gulfs, Greece. Geo-Mar. Lett. 2003, 23, 194–199. [Google Scholar] [CrossRef]
  48. Kordella, S.; Christodoulou, D.; Fakiris, E.; Geraga, M.; Kokkalas, S.; Marinaro, G.; Iatrou, M.; Ferentinos, G.; Papatheodorou, G. Gas Seepage-Induced Features in the Hypoxic/Anoxic, Shallow, Marine Environment of Amfilochia Bay, Amvrakikos Gulf (Western Greece). Geosciences 2021, 11, 27. [Google Scholar] [CrossRef]
  49. Christodoulou, D.; Papatheodorou, G.; Geraga, M.; Etiope, G.; Giannopoulos, N.; Kokkalas, S.; Dimas, X.; Fakiris, E.; Sergiou, S.; Georgiou, N.; et al. Geophysical and Geochemical Exploration of the Pockmark Field in the Gulf of Patras: New Insights on Formation, Growth and Activity. Appl. Sci. 2023, 13, 10449. [Google Scholar] [CrossRef]
  50. Carroll, J.M.; Dashiell, R.; Watts, J.C.; Hunter, E.A. Tidal level affects the prevalence and impacts of pests and parasites on oysters (Crassostrea virginica) on intertidal reefs in Georgia, USA. Mar. Biol. 2021, 168, 45. [Google Scholar] [CrossRef]
  51. Spalt, N.; Murgulet, D.; Abdulla, H. Spatial variation and availability of nutrients at an oyster reef in relation to submarine groundwater discharge. Sci. Total Environ. 2020, 710, 136283. [Google Scholar] [CrossRef]
  52. O’Brien, S.R.; Mayewski, P.A.; Meeker, L.D.; Meese, D.A.; Twickler, M.S.; Whitlow, S.I. Complexity of Holocene climate as reconstructed from a Greenland ice core. Science 1995, 270, 1962–1964. [Google Scholar] [CrossRef]
  53. Aksu, A.E.; Yaşar, D.; Mudie, P.J.; Gillespie, H. Late glacial-Holocene paleoclimatic and paleoceanographic evolution of the Aegean Sea: Micropaleontological and stable isotopic evidence. Mar. Micropaleontol. 1995, 25, 1–28. [Google Scholar] [CrossRef]
  54. Weninger, B.; Alram-Stern, E.; Bauer, E.; Clare, L.; Danzeglocke, U.; Jöris, O.; Kubatzki, C.; Rollefson, G.; Todorova, H.; van Andel, T. Climate forcing due to the 8200 cal yr BP event observed at Early Neolithic sites in the eastern Mediterranean. Quat. Res. 2006, 66, 401–420. [Google Scholar] [CrossRef]
  55. Gogou, A.; Bouloubassi, I.; Lykousis, V.; Arnaboldi, M.; Gaitani, P.; Meyers, P.A. Organic geochemical evidence of Late Glacial–Holocene climate instability in the North Aegean Sea. Palaeogeogr. Palaeoclimatol. Palaeoecol. 2007, 256, 1–20. [Google Scholar] [CrossRef]
  56. Poulos, S.; Kapsimalis, V.; Tziavos, C.; Pavlakis, P.; Leivaditis, G.; Collins, M. Sea-level stands and Holocene geomorphological evolution of the northern deltaic margin of Amvrakikos Gulf (western Greece). Z. Geomorphol. 2005, 137, 125–145. [Google Scholar]
  57. Katsouras, G.; Gogou, A.; Bouloubassi, I.; Emeis, K.C.; Triantaphyllou, M.; Roussakis, G.; Lykousis, V. Organic carbon distribution and isotopic composition in three records from the eastern Mediterranean Sea during the Holocene. Org. Geochem. 2010, 41, 935–939. [Google Scholar] [CrossRef]
  58. Triantaphyllou, M.V.; Gogou, A.; Bouloubassi, I.; Dimiza, M.; Kouli, K.; Rousakis, G.; Kotthoff, U.; Emeis, K.C.; Papanikolaou, M.; Athanasiou, M.; et al. Evidence for a warm and humid Mid-Holocene episode in the Aegean and northern Levantine Seas (Greece, NE Mediterranean). Reg. Environ. Change 2014, 14, 1697–1712. [Google Scholar] [CrossRef]
  59. Bini, M.; Zanchetta, G.; Perşoiu, A.; Cartier, R.; Català, A.; Cacho, I.; Dean, J.R.; Di Rita, F.; Drysdale, R.N.; Finne, M.; et al. The 4.2 ka BP Event in the Mediterranean region: An overview. Clim. Past 2019, 15, 555–577. [Google Scholar] [CrossRef]
  60. Ryan, E.J.; Morgan, K.M.; Kench, P.S.; Owen, S.D.; Carvajal, C.P.; Turner, T. Fossil reefs reveal temporally distinct late Holocene lagoonal reef shutdown episodes at Kiritimati Island, central Pacific. Geophys. Res. Lett. 2021, 48, e2020GL092113. [Google Scholar] [CrossRef]
  61. Jing, F.; Fu, W.; Liping, Z.; Shuo, Y.; Tianjiao, W.; Yufei, L.; Yang, L. Study on the buried oyster reefs and environmental changes with the underlying muddy sediments on the Northwest Coast of Bohai Bay. IOP Conf. Ser. Mater. Sci. Eng. 2020, 780, 042059. [Google Scholar] [CrossRef]
  62. Dickson, S.; Carlin, J.; Bonuso, N.; Kirby, M.E. Documenting the Evolution of a Southern California Coastal Lagoon during the Late Holocene. Coasts 2022, 2, 102–124. [Google Scholar] [CrossRef]
  63. Peyron, O.; Combourieu-Nebout, N.; Brayshaw, D.; Goring, S.; Andrieu-Ponel, V.; Desprat, S.; Magny, M. Precipitation changes in the Mediterranean basin during the Holocene from terrestrial and marine pollen records: A model–data comparison. Clim. Past 2017, 13, 249–265. [Google Scholar] [CrossRef]
  64. Woods, H.; Hargis, W.J.; Hershner, C.H.; Mason, P. Disappearance of the natural emergent 3-dimensional oyster reef system of the James River, Virginia, 1871–1948. J. Shellfish Res. 2005, 24, 139–142. [Google Scholar]
  65. Seeliger, M.; Pint, A.; Frenzel, P.; Feuser, S.; Pirson, F.; Riedesel, S.; Brückner, H. Foraminifera as markers of Holocene sea-level fluctuations and water depths of ancient harbours—A case study from the Bay of Elaia (W Turkey). Palaeogeogr. Palaeoclimatol. Palaeoecol. 2017, 482, 17–29. [Google Scholar] [CrossRef]
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Figure 1. (a) The locations of Lesvos Island and Gera Gulf. (b) Surrounding geology and map showing the seismic lines, the sediment core locations (G and GL), and the sections of the seismic profiles presented in the text (thicker, numbered red lines), along with main bathymetry contours.
Figure 1. (a) The locations of Lesvos Island and Gera Gulf. (b) Surrounding geology and map showing the seismic lines, the sediment core locations (G and GL), and the sections of the seismic profiles presented in the text (thicker, numbered red lines), along with main bathymetry contours.
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Figure 2. (a) Seismic units SU1, SU2, and SU3 in Gera Gulf and (b,c) Innomar and Boomer profiles, respectively, along the same transect showing the seismic units and the modern and buried (dashed black line) reef terrain (dashed purple line: inferred base of the lake sediments; red line: SU2/SU3 boundary; arrows: enhanced reflectors; G: gas plume; M: multiple).
Figure 2. (a) Seismic units SU1, SU2, and SU3 in Gera Gulf and (b,c) Innomar and Boomer profiles, respectively, along the same transect showing the seismic units and the modern and buried (dashed black line) reef terrain (dashed purple line: inferred base of the lake sediments; red line: SU2/SU3 boundary; arrows: enhanced reflectors; G: gas plume; M: multiple).
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Figure 3. (a) The seismic profile of SSE-2000 light plus and (b) the interpretation showing both the surficial and buried reef phases (black dashed line: SU1/SU2 boundary (buried reef terrain); white dashed line: a deeper reef phase within SU2; red line: SU2/SU3 boundary; arrows: enhanced reflectors; G: gas plume; M: multiple).
Figure 3. (a) The seismic profile of SSE-2000 light plus and (b) the interpretation showing both the surficial and buried reef phases (black dashed line: SU1/SU2 boundary (buried reef terrain); white dashed line: a deeper reef phase within SU2; red line: SU2/SU3 boundary; arrows: enhanced reflectors; G: gas plume; M: multiple).
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Figure 4. (a) The fault zone in the NE part of Gera Gulf and (b) faults (red arrows) affecting SU3 and SU2 (red arrows). Black dashed line: SU1/SU2 boundary (buried reef terrain); red line: SU2/SU3 boundary; vertical dashed lines: faults; black arrows: enhanced reflectors; M: multiple.
Figure 4. (a) The fault zone in the NE part of Gera Gulf and (b) faults (red arrows) affecting SU3 and SU2 (red arrows). Black dashed line: SU1/SU2 boundary (buried reef terrain); red line: SU2/SU3 boundary; vertical dashed lines: faults; black arrows: enhanced reflectors; M: multiple.
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Figure 5. Examples of fluid acoustic response (arrows), locally creating acoustic masking, as observed in the seismic profiles: (a) enhanced reflectors and small turbid pockets/plumes (dashed purple line: inferred base of the lake sediments), (b) enhanced reflectors and acoustic pockets, and (c) acoustic turbidity/pockets and enhanced reflectors interrupted by the presence of faults (Black dashed line: SU1/SU2 boundary (buried reef terrain); red line: SU2/SU3 boundary; F: fault; M: multiple).
Figure 5. Examples of fluid acoustic response (arrows), locally creating acoustic masking, as observed in the seismic profiles: (a) enhanced reflectors and small turbid pockets/plumes (dashed purple line: inferred base of the lake sediments), (b) enhanced reflectors and acoustic pockets, and (c) acoustic turbidity/pockets and enhanced reflectors interrupted by the presence of faults (Black dashed line: SU1/SU2 boundary (buried reef terrain); red line: SU2/SU3 boundary; F: fault; M: multiple).
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Figure 6. (a) Distribution of fluids in sediment pores, distinguished in three areas of different acoustic anomalies and depths of occurrence, overlain by the extent of the buried reef relief; (b) the extent of both buried and modern [25] reefs.
Figure 6. (a) Distribution of fluids in sediment pores, distinguished in three areas of different acoustic anomalies and depths of occurrence, overlain by the extent of the buried reef relief; (b) the extent of both buried and modern [25] reefs.
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Figure 7. Seismic profile showing the sill (arrow), which probably corresponds to the paleosurface overtopped at the last stages of Holocene transgression, leading to the subsequent marine flooding of the gulf (Black dashed line: SU1/SU2 boundary (buried reef terrain); red line: SU2/SU3 boundary M: multiple).
Figure 7. Seismic profile showing the sill (arrow), which probably corresponds to the paleosurface overtopped at the last stages of Holocene transgression, leading to the subsequent marine flooding of the gulf (Black dashed line: SU1/SU2 boundary (buried reef terrain); red line: SU2/SU3 boundary M: multiple).
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Figure 8. Stills showing the observed four sedimentary units.
Figure 8. Stills showing the observed four sedimentary units.
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Figure 9. (a) Photos of seven small cores (spanning from NW to SE), along with stills of selected sections highlighting the increased accumulation of biogenic fragments and shells. (b) Photos of the two longer cores, accompanied by detailed schematic illustrations (with 14C sampling points), along with corresponding sections of the Boomer profiles for each sampling location.
Figure 9. (a) Photos of seven small cores (spanning from NW to SE), along with stills of selected sections highlighting the increased accumulation of biogenic fragments and shells. (b) Photos of the two longer cores, accompanied by detailed schematic illustrations (with 14C sampling points), along with corresponding sections of the Boomer profiles for each sampling location.
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Figure 10. Sketch diagram showing the gradual inundation of the gulf after 8000 BP and the colonization of reefs initially at the center of the gulf and gradually to its perimeter, where reefs appear to be slightly larger.
Figure 10. Sketch diagram showing the gradual inundation of the gulf after 8000 BP and the colonization of reefs initially at the center of the gulf and gradually to its perimeter, where reefs appear to be slightly larger.
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Table 1. Results of the radiocarbon dating with 14C (95.4% probability).
Table 1. Results of the radiocarbon dating with 14C (95.4% probability).
Lab CodeCore SampleDepth (cm)SpeciesUncorrected 14C (yr BP)Calibrated (yr BP)
DEM-2739GL1-A130C. caespitosa3834 ± 303790–3450
DEM-2735GL1-B150O. edulis5548 ± 305889–5587
DEM-2740GL2-A52C. caespitosa6345 ± 306754–6424
DEM-2737GL2-B79O. edulis6898 ± 407382–7043
DEM-2736GL2-C90O. edulis7145 ± 307572–7310
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MDPI and ACS Style

Manoutsoglou, E.; Hasiotis, T. Evidence of Holocene Sea-Level Rise from Buried Oyster Reef Terrain in a Land-Locked Insular Embayment in Greece. Geosciences 2025, 15, 105. https://doi.org/10.3390/geosciences15030105

AMA Style

Manoutsoglou E, Hasiotis T. Evidence of Holocene Sea-Level Rise from Buried Oyster Reef Terrain in a Land-Locked Insular Embayment in Greece. Geosciences. 2025; 15(3):105. https://doi.org/10.3390/geosciences15030105

Chicago/Turabian Style

Manoutsoglou, Evangelia, and Thomas Hasiotis. 2025. "Evidence of Holocene Sea-Level Rise from Buried Oyster Reef Terrain in a Land-Locked Insular Embayment in Greece" Geosciences 15, no. 3: 105. https://doi.org/10.3390/geosciences15030105

APA Style

Manoutsoglou, E., & Hasiotis, T. (2025). Evidence of Holocene Sea-Level Rise from Buried Oyster Reef Terrain in a Land-Locked Insular Embayment in Greece. Geosciences, 15(3), 105. https://doi.org/10.3390/geosciences15030105

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