Geomorphology of the Coastal Sand Dune Fields and Their Association with the Palaeolandscape Evolution of Akrotiri Peninsula, Lemesos, Cyprus

Two well-developed late Pleistocene dune fields have been identified on the western and eastern side of Akrotiri promontory (Lemesos, Cyprus). The dune fields extend immediately from the low level of their source beaches onto higher ground (>48 m amsl). Geomorphic observations supported by OSL dating and sedimentological data provided evidence of the dune development and for the palaeogeographic reconstruction of the area. Relative sea level changes and wave action during the upper Pleistocene and Holocene played an important role into the development of the palaeolandscape and affected the formation of the dunes. From the collected data the development of the western dune field started at 56.2 ± 5.5 ka when the relative sea level was at approximately −60 m and contributed to the development of the western tombolo of the area whereas the eastern dune field developed in the late Holocene, after the formation of the eastern spit that resulted in the formation of the Akrotiri Salt lake.


Introduction
Dune systems are important geomorphological features found along a significant part of the world's coastline. An interaction of natural factors such as sediment supply, flat topography, strong sand-moving winds and vegetation cover are responsible for the accumulation and formation of sand dunes [1][2][3][4][5][6][7][8]. Storms and sea level fluctuations can also impact dune system development and mobility, through changes in sediment supply and vegetation [9,10].
Sediment supply from the coast is the source for the formation and growth of coastal sand dunes [11]. Change in sediment supply can modify dune mobility. Specifically, high sand supply can bury vegetation, resulting in drifting sand and mobile sand dunes, while low sand input can encourage vegetation survival, leading to an increase in dune stability [12].
At the most basic level, sand dunes can be categorized in those that form from the direct supply of sediment from the beach face (primary dunes), and those that form from the subsequent modification of primary dunes (secondary dunes). Primary dunes are composed of sand, blown directly from the beach face (active beach), whereas secondary dunes develop following the subsequent modification of primary dunes. Primary dunes are those closest to the shoreline, dynamically linked to beach processes and significantly influenced by wave action [13][14][15].
Vegetation is a crucial element in the evolution of dune landscapes. It is necessary to trap the sand for dunes to grow but also to stabilize the ground. The pioneer species, by this action, will facilitate the establishment of other species, increasing biodiversity (flora and fauna) [10]. The main driver for coastal sand dune erosion is the near-surface wind vector (speed, direction) consistent with standard formulations of aeolian sediment transport models [16,17]. Wind velocity is generally reduced by plant cover, encouraging deposition and trapping of wind-borne sand. However, wind velocity may also accelerate locally in gaps between plants, especially those having a clumpy form.
Reconstructing coastal evolution is typically based on geological evidence derived from field observations and sedimentary or paleontological laboratory analyses, but receives critical support from geochronology information obtained by numeric dating methods [18,19]. OSL dating of quartz using the single-aliquot regenerative-dose protocol (SAR) [20] was successfully applied in many of studies to Holocene coastal sand dunes [21,22] and inland dunes [23][24][25].
Very few studies have focused on geomorphological evolution and palaeogeography of coastal dunes in Cyprus. The majority of the studies were focused on the flora and fauna development on the dune formations without emphasizing to the geomorphological and palaeogeographical evolution [26][27][28]. In this context, the aim of this study is to investigate the dunes of the southern coast of Cyprus and furthermore to evaluate the usability of sand dune systems as a tool for the palaeogeographic reconstruction of similar settings in the area of the Eastern Mediterranean.

Study Area
Akrotiri Peninsula is the southernmost part of the island of Cyprus. It is located approximately 5 km to the southwest of the port of Lemesos. 90% of the peninsula is situated within the British Sovereign Base Area. Akrotiri Peninsula covers roughly 60 km 2 ( Figure 1). It includes a southern plateau formed by uplifted marine terraces [29][30][31] with a maximum elevation of over 60 m amsl. The northern part lies in general below 10 m elevation, covered by alluvial fan deposits which are the result of the continuous supply of material by the Kouris river. The Akrotiri aquifer consist the most important porous aquifer of Southern Cyprus. The southern boundary of the aquifer runs along the edge of the Akrotiri salt lake. The Akrotiri aquifer has an approximate surface area of 45 km 2 and consists of deltaic sediments, deposited in two big fan deltas, coming from the Kouris River in the west. The Kouris River is the largest river in Cyprus and drains a catchment area of 300 km 2 , extending far up into the Troodos mountains [32]. Its middle is occupied by the Akrotiri Salt Lake which is covering an area of approximately 20 km 2 . Today, the maximum depth of the Salt Lake reaches 2.8 m below mean sea level during the winter period [30,31,33]. The northern and southern part of the peninsula is connected by two tombolos. The western tombolo consists mainly of sands and gravels and its formation is connected with the material discharged by Kouris river [30,31]. The eastern tombolo consists mainly of sand and it is formed by the eroded material (western dune field sand and exposed sandy marls) of the western part of the peninsula, which follows the anticlockwise long shore drift [34] and the western prevailing winds of the study area [30,31].

Climate
The Eastern Mediterranean basin is located on the leeward side of the Asia Minor peninsula, where a cyclonic atmospheric circulation predominates [35][36][37]. This cyclogenetic activity generates a counter-clockwise circulation, initially passing through the Aegean Sea and then blowing over the Levantine Basin [38,39]. Winds are predominantly from the southwest-to-north and occasionally from the north-to-east during winter. During spring, winds are from the west-to-north, whereas they are from northwest to west during the summer and autumn. Crucially, north-westerly to southwesterly wind flows are particularly felt during storm events. Similar westerly and southwesterly cyclonic winter storm winds in Israel enable sand-transport [40][41][42]. These storms, which provide most of the annual rainfall, are usually not a constraint on sand transport, as rapid surface evaporation by the winds makes the upper sand surface erodible in minutes following rainfall [43].
Climatic data were obtained for the period 1982 to 2018 (Cyprus Department of Meteorology). The annual main wind direction is West 22% to West South West 16%. Main wind directions for a period of one year are presented in Figure 2 which include mainly W to SSW winds for the major part of the year (approx. 80%) and periodically E to ENE winds (approx. 20%). The annual mean precipitation of the study area is 377 mm. The average wind velocity for the study area is 14.05 km/h for a period of 36 years. The maximum annual average wind speed was 15.3 km/h in 2016 and the minimum 11.5 km/h in 2007. July has the highest average wind velocity with 15.8 km/h, while October has the lowest average wind velocity with 10.6 km/h. The palaeoclimatic conditions according to Schilman et al., (2001) [44], seem to follow the general aridification trend which has begun about 7000 years ago in the eastern Mediterranean region [44][45][46].

Material and Methods
For the evaluation of the geomorphic evolution of the study area, we conducted geomorphological mapping, remote sensing analysis, field survey and sampling ( Figure 1). Fourteen sand samples were extracted from the base of dunes, both at the west and east part of the study area. Ten of the fourteen samples were qualified for further analysis (Table 1). Sampling conducted using 2 inch diameter plastic PVC tubes, which were placed horizontally against the section of the sand dune and hammered into the dune. The tubes were sealed, and the collected material remained protected from exposure to the sunlight. The samples were transported to the laboratory of "Engineering Geology and Industrial Minerals" of the Cyprus Geological Survey Department.

Geochronology
Geochronological studies with optically stimulated luminescence (OSL) dating method was applied to four samples. Three samples retrieved from the western dunes and one sample from the eastern dune field. In-situ measurements for natural radioactivity of the predominating sand formations were taken by handheld scintillometers (Saphymo-Stell, model spp-2). The samples were processed and measured by the Laboratory of Palaeoenvironment and Ancient Metals Studies (PAMS lab), National Center for Scientific Research «DEMOKRITOS», Greece. All laboratory procedures were performed under controlled illumination conditions (subdued~580 nm light) in order to extract samples' light-safe interior. The calculations of the external dose rates (U, Th, K) were based on analytical data obtained by Inductively Coupled Plasma Mass Spectrometry (ICP-MS; ACME laboratories, Canada). Water content (%) was based on modern values with an error of ±15% and considered to remain constant during burial. Chemical treatment was conducted following well established laboratory protocols and standardized procedures [20,47]. Luminescence measurements were carried out using a RISØ-TL/OSL-15 reader.

Grain Size Analysis
Sieving analysis performed to the collected samples. For the determination of grain size, 8 samples (Table 1) were analyzed and classified based on Folk's, 1954 nomenclature [48]. Samples weighting between 200 and 300 g were dried for over 24 h at temperatures 100-110 • C. They were then weighed and sieved through a set of sieves (diameter of the sieves from 10 mm to 0.053 mm, according to CYSEN 933-1:2012). The statistical parameters, used for sedimentological interpretations such as mean, sorting, skewness, and kurtosis were calculated using Gradistat V.4 software [49].

Western Sand Dune Fields
The western dune fields are extending for approximately 6 km (north to south) and 1.6 km inland (west to east) (Figure 1). The primary dunes are located near the coastline, extending approximately until 200 m inland. They are mostly composed of loose sand, reaching heights up to a few meters and they are bare of vegetation (Figure 3). Based on the grain size analysis the western primary dunes (AK9/SD) are characterized as trimodal, very poorly sorted fine sand (Figure 4).  Semi-lithified dunes are lying behind the primary dunes. They extend approximately 470 m inland and they are covered by vegetation, contrary to the completely bare sand of the primary dunes. Their height is reaching approximately 10 m (Figure 3). After the grain size analysis, the dunes (AK5/SD and AK10/SD) are characterized as unimodal, moderately well sorted, fine sand ( Figure 5).
Lithified sand dunes are following the semi-lithified dune and they are extending approximately 1.5 km inland from the coastline. They are composed of layers of sediments with plane-parallel bedding and their maximum height reaches approximately 48 m. The vegetation is denser with a variety of plant species and even small trees (Figure 3). On top of the lithified sand dunes there is very little exposed sand. After the grain size analysis, the western dunes (AK1/SD and AK11/SD) are characterized as Unimodal, Poorly Sorted, fine sand ( Figure 6).

Eastern Sand Dune Fields
The eastern dune fields are extending for approximately 4.8 km and they are lying from the coastline up to 1.4 km inland (Figure 1). Primary dunes are located in close proximity to the coastline. Their height reaches up to 2 m and they consist of loose sand. There is no evidence of vegetation on top of the dunes (Figure 7). After the grain size analysis, the eastern primary dunes (AK12/SD) are characterized as trimodal, very poorly sorted, fine sand ( Figure 8).

OSL Measurements
Quartz grains in the 200-250 µm fraction provided appropriate OSL signals that could be used for estimating reliable palaeodoses and thus the age of the samples. Dose rates were calculated using the software tool "DRc" [51]. The age estimates are given with 1σ in Table 2. OSL ages were calculated based on the simplified equation shown below and using the Central Age model proposed by Galbraith et al., (1999) [52].

Discussion
Climate conditions, especially wind regime seems to have a significant role for the activation, mobilization and elongation of dunes during the cold and dry periods of the late Quaternary [53][54][55][56]. When global wind power decreased during the Holocene, vegetated dune fields usually stabilized [57].
Pleistocene, high-energy fluvial material from the Kouris river catchment area, north of the western dune field, transported and deposited alluvium over the south-western part of Akrotiri peninsula which resulted the development of the western tombolo. These material is known as Fanglomerate formation [58]. These alluvial surfaces serve as a substrate over which most of the western dune field developed. Similar type of development is also know from the area of Negev in Israel, which resulted the vegetated linear dunes encroached on top of alluvial surfaces during Pleistocene [59]. According to Roskin et al., 2017 [60], It seems that during the late Pleistocene when climate fluctuations (changes in wind power, precipitation) had larger amplitudes [57,61], aeolian-fluvial interactions were more common in semi-arid environments and desert margins [62][63][64].
The Western dune field provided an age of 56.2 ± 5.5 ka, based on the oldest OSL sample extracted from the base of the aeolianite formation (Table 2). This age is in agreement with previous studies at the SE Cyprus, which provided ages of aeolianites approximately 65 ka [65], providing additional evidence for aeolian depositional events in SE and S Cyprus mainly after the MIS 5a. OSL ages in eastern Mediterranean aeolianites and littoral deposits have been dated as late Pleistocene, indicating that the formation of sand dunes mainly occurred in episodes in the time interval of the MIS 2, 3, 4 and 5 [65][66][67], showing a common late Pleistocene framework of deposition.
The material from the erosion of the western sand dunes moved along with the anticlockwise longshore drift [34] accumulated to the east side of the peninsula, in front of the already formed spit/beach ridge during Middle to Upper Holocene [31]. Beach ridges composed of shallow marine sediments downdip of lagoonal deposits are barriers. These are often associated with relative sea-level rise, thus representing a transgressive depositional complex [68]. Aeolian beach ridges are often linked to sea-level highstand [69] representing transgressive [70] or highstand deposits [71], or they are aggradational deposits formed by aeolian reworking of the transgressive beach ridge during times of relative sea-level stability [72]. On top of the flat terrain area of eastern side of Akrotiri peninsula, sand dunes were formed during the upper Holocene. The East dune field is considered to be directly connected with the regional sea level fluctuations [31] and tectonics [73] which formed the coastal landscape and with the development of the beach ridge provided the substrate for the formation of the dunes [74].
Regarding the above data, it is suggested that the area between the mainland and the former Akrotiri island was open before 56 ka (Figure 10a). The western side tombolo of Akrotiri peninsula was already formed before 56.2 ± 5.5 ka creating a bay with an opening to the sea on the east (Figure 10b). On top of the tombolo the sand dunes started to develop and were slowly mobilized to the east. Through climatic changes and tectonic forces, the sand dunes passed through a series of erosion and accumulation phases and eventually with the loss of land due to sea level changes the dune field suffered significant erosion. The eastern dune field developed after the creation of the spit/beach ridge in Upper Holocene, due to the sea level fluctuation (Figure 10c). The formation of both dune fields assisted to the isolation of the salt lake at the center of the peninsula and played an important role to the paleogeographic development of the eastern area of the peninsula [31].

Conclusions
The development of the sand dune fields at Akrotiri peninsula is driven by the supply of material by Kouris river to their source beaches and the movement of the material by the prevailing winds. Also, the tectonic forces from the activation of Quaternary faults and the climatic changes, resulted in the erosion of the dunes through the sea level fluctuations. At the Western side of the peninsula, there is a long record of dune development which started at approximately 56 ka BP and continued with multiple layers of deposition and periods of significant erosion. Dune development and mobilization is still under process and the prevailing western winds have contributed to the enlargement of the dune field.
At the eastern side of the peninsula, the dune field developed in a much later stage in the Upper Holocene. The chronology of the dunes yields a very recent development, which is consistent with the unconsolidated state of the sediment, the low height of the dunes and the absence of vegetation.
Our study further highlights that the morphological and geochronological analysis of sand dune fields in the eastern Mediterranean along with other geomorphological features can provide a significant tool to the paleogeographic reconstruction of an area.