«Palaeoenvironmental changes of the Lefkada Sound (NW Greece) and their archaeological relevance Svenja Brockmüller, Andreas Vött, Simon Matthias ...»
G. Gönnert, B. Pflüger & J.-A. Bremer
Von der Geoarchäologie über die Küstendynamik zum Küstenzonenmanagement
Coastline Reports 9 (2007), ISSN 0928-2734, ISBN 978-3-9811839-1-7
S. 127 - 138
Palaeoenvironmental changes of the Lefkada Sound (NW Greece)
and their archaeological relevance
Svenja Brockmüller, Andreas Vött, Simon Matthias May & Helmut Brückner
Faculty of Geography, Philipps-Universität Marburg
Lefkada Island is separated from the Greek mainland by the Lefkada Sound, a shallow lagoonal environment. Ancient accounts report that the Corinthians cut a navigable canal across a former isthmus in the 7th century BC. A nowadays submerged Corinthian mole at the southern end of the sound closed off a protected harbour area.
The main objectives of our studies were to detect environmental changes throughout the Holocene and to reconstruct the palaeogeographical evolution of the Lefkada Sound for different points in time based on the analysis of the sedimentary record encountered in various geoarchives.
Geomorphological, geochemical, microfaunal, archaeobotanical and geochronological methods as well as earth resistivity measurements were applied.
From the northern Lefkada Sound and adjacent regions, severe tsunamigenic impact is known.
Vibracore data shows considerable influence of extreme events on the development of the northern subarea in historical times.
In the central part of the sound, our results revealed a terrestrial connection of Lefkada Island with the Greek mainland, which is gradually displaced by a limnic and later lagoonal environment. At least twofold high energy impact disturbed the quiescent lagoonal conditions at or after Hellenistic times. The marginal parts of the lagoon have been affected by siltation due to alluvial or anthropogenic deposition.
In the area of the presumed harbour of ancient Lefkada, our results indicate an early marine transgression sometime before ~2800-2700 cal BC. Subsequently, open marine conditions prevailed for a long time, however showing temporary influence of high energy events. Layers with numerous artefacts probably reflect the period during which the harbour was in use and are subsequently covered by alluvial sediments.
Several relative sea level indicators document a remarkably rapid relative sea level rise in the inner sound, partly due to tectonic subsidence. In more lateral parts, tectonic uplift is observed. Thus, distinct local variations in tectonic movements have to be considered.
Introduction Postglacial sea level rise and delta formation since the mid-Holocene have caused enormous landscape changes and thus have strongly influenced human activity and settlements in coastal areas.
Additionally, catastrophic events like seismic movements, storms or tsunami waves are assumed to have considerably affected the coastal evolution. In the eastern Mediterranean, coastline changes and their implications for archaeological sites are well studied in various regions. For the Lefkada Sound, detailed palaeogeographical analyses did not exist so far. This paper presents first results of our research on palaeoenvironmental changes of the area.
2 Topographic and tectonic setting Lefkada Island is separated from the Greek mainland – the Plaghia Peninsula – by the Lefkada Sound, a shallow lagoonal environment (Fig. 1). Towards the north, the lagoon is cut off from the open Ionian Sea by an extended barrier beach system. Its partly destroyed and drowned section is called Plaka. To the south, the sound is bordered by a submerged mole, the so-called “Mole of the Corinthians”. Across 128 Brockmüller et al.
the shallow lagoon runs an artificial (navigable) canal. Lefkada city lies at the north-eastern corner of Lefkada Island at around 1-5 m above present mean sea level (m a.s.l.), while the archaeological remains of ancient Lefkada are located in a higher position further south. A bridge, today completely ruined and covered by sediments, connected Lefkada Island and the Plaghia Peninsula during Hellenistic to Roman times.
Figure 1: Overview of the study area and location of the presented sedimentary and archaeological archives Tectonically, the area of the Lefkada Sound is part of the Ionian Zone of the outer Hellenides (Monopolis & Bruneton 1982) and is located close to the triple junction of the African, the Aegean and the Adriatic microplates (Haslinger et al. 1999). The study area belongs to the tectonically and seismically most active regions of the Mediterranean. Especially the Cefalonia Fault, situated directly to the west of Lefkada Island, is responsible for severe earthquakes (Galanopoulos 1952, 1954, Hatzfeld et al. 1995). The last strong earthquake (MW 6.2) occurred on August 14, 2003, causing mass movements and extensive damages especially along the western coast of the island (Benetatos et al.
2005, Parcharidis et al. 2006). The high seismic and tectonic activity indicates a considerable tsunami hazard for the region.
The sound lies in a tectonic depression and is flanked by tectonic horsts. Severe differences in the vertical tectonic movement are represented by findings of Eemian sea level indicators in different elevations, ranging between around 14 m below present mean sea level (m b.s.l.; PER 5, Fig. 3) and up to 8.65 m a.s.l. at the northern coast of Lefkada Island (IOA 1, Fig. 1). Here, ESR-dating of in-situ branches of corals provided an age of 130 +/- 15 ka BP.
Historical background The palaeogeographical setting in the area of the Lefkada Sound was discussed in a controversial way already at the end of the 19th century. Was Lefkada always an island or was it connected to the Akarnanian mainland in the past? The ancient historians Livius, Strabo and Plinius report that an isthmus existed east of the ancient town of Lefkada in Archaic-Classic times, just where an ancient bridge crossed the sound during the Hellenistic to Roman epoch (Négris 1904, Fig. 1). However, Partsch (1889, 1907), von Marées (1907), Lehman-Hartleben (1923) and von Seidlitz (1927), for instance, suggest that the described isthmus was located further northward, either close to the fortification of Aghia Mavra or at the so-called “Canali Stretti”. Anyway, historical tradition says that an isthmus was cut through by the Corinthians in order to get a navigable canal. The nature of Palaeoenvironmental changes of the Lefkada Sound (NW Greece) 129 Lefkada, island or peninsula, was of particular interest for the archaeological research trying to solve the question whether Lefkada possibly was the Homeric Ithaka, the homeland of Odysseus (Goessler 1904, Lang 1905, Dörpfeld 1927).
Ancient Lefkada was probably founded in the 7th century BC by Corinthian settlers. Négris (1904) suggests that the Mole of the Corinthians (Fig. 1) was constructed to protect the southern entrance of the canal. Murray (1982, 1988) found that the harbour mole of ancient Lefkada did not only serve as protection, but also as a loading pier. Murray (1988) found fragments of Corinthian amphora and concludes that the harbour was in use between the 5th/4th century BC and the 4th-6th centuries AD.
According to Murray (1988), the submersion of the mole started shortly after the 6th century AD. Its remains – in parts the original mole surface is preserved in a minimum depth of 1.40 m b.s.l. – represent a reliable relative sea level indicator. With an assumed original surface elevation of 0.5m a.s.l., a minimum relative sea level rise of ~2.4 meters is assumed. Murray (1982: 240) even gives an average value of 3.4 meters. Based on his investigation of the remains of the Hellenistic to Roman bridge, Négris (1904: 24) suggests a relative sea level rise of ~3.0 m in the course of the last 2000 years.
In antiquity, the navigable canal through the sound was a safe shortcut along the way between Greece and Italy and therefore the control over the strait was a both lucrative economic and strategic factor.
According to historical data, compiled e.g. by Murray (1982: 247f.), the navigability of the sound was interrupted at least three times during Archaic and Roman times. Murray (1982) suggests that the natural tendency of the sound is to silt up and that whenever possible, the strait was dredged and the canal reopened, because that was essential for the prosperity of the city. Vött et al. (2007c, d) propose that tsunami impact might also have influenced the navigability of the sound.
Objectives and methods The prior aim of our research is to reconstruct the palaeogeographical evolution of the sound for different points in time since the late Pleistocene/early Holocene based on the analysis of sedimentary and archaeological records encountered in vibracores and profiles.
Geomorphological, geochemical, microfaunal, archaeobotanical and geochronological methods as well as thin section analyses and earth resistivity measurements were applied. The results were compared to both historical and archaeological data. Our geoarchaeological approach is summarized e.g. by Brückner (2003), the benefits resulting from interdisciplinary landscape research are exemplary shown by Lang et al. (2007).
In the surroundings of the Lefkada Sound, more than 50 sediment cores were retrieved with core diameters of 6 cm and 5 cm and a maximum recovery depth of 15 m. On-site, the sedimentary facies was determined based on sedimentological features and macrofossil remains. Samples were taken for systematic micro- (foraminifers, ostracods) and macrofaunal studies, for pollen analyses and for detailed geochemical analyses of various parameters such as electrical conductivity, pH-value, loss on ignition, contents of carbonate, orthophosphate as well as several (earth-)alkaline and heavy metal ions. Moreover, artificial trenches given by earthen cisterns were studied.
The exact position and elevation of vibracoring sites and trenches were determined by means of a differential GPS. 14C-AMS dates and diagnostic ceramic fragments as well as ESR-dating of several pre-Holocene coral fragments were used for establishing a local geochronology.
In this paper, we present two vibracores from the northern and southern parts of the lagoon and a vibracore transect across the central part of the sound (Fig. 1).
Radiocarbon ages given in Table 1 are calibrated using the software “Calib” by Stuiver et al. (2006).
Marine samples are corrected for an average reservoir effect of 402 years (Reimer & McCormac 2002) because of the lack of detailed information on the palaeo-reservoir effects in the Lefkada Sound.
5 Results: Sedimentological archives revealing the landscape history
Tsunamigenic impact on the northern part of the sound Vibracore LEF 2 (Fig. 2) exemplifies tsunamigenic impact on the northern part of the Sound of Lefkada (see Vött et al. 2006, 2007a). Several meters of shallow marine to littoral sediments (sand and gravel rich in marine mollusc fragments) at the base of the core are covered by 3.5 meters of lagoonal deposits. The lagoonal mud, mostly silty and rich in marine fossils, indicates the existence of a seaward barrier from ~4750 cal BC to ~300 cal BC, as 14C-AMS datings suggest (see Table 1).
Figure 2: Stratigraphic sequence of vibracore LEF 2. Note: Legend is also applicable for Fig. 4.
Palaeoenvironmental changes of the Lefkada Sound (NW Greece) 131 On top of the lagoonal sediments, high energy impact is represented by alternating layers of sand and gravel, slightly weathered in the uppermost part of the sequence. The seaward barrier must have been overflowed by tsunami waves at or after ~300 cal BC. There are further geomorphologic and sedimentologic findings in the area that prove multiple tsunami impact to the Lefkada coastal zone (Vött et al. 2006, 2007a, b, c, May et al. 2007). It is suggested that tsunamis had a considerable influence on the development of the inner Lefkada Sound (Vött et al. 2007d).
Geoarchaeological aspects of the evolution of the central sound In the central part of the sound (Fig. 3), thick packages of alluvial deposits, partly covered by (ephemerally) limnic sediments prove a former terrestrial connection of Lefkada Island with the Greek mainland up to a depth of around ~3.55-3.00 m b.s.l., where first saltwater-borne sediments occur in LEF 24 and PER 5. In both cases, we found lagoonal sediments on top of the limnic sedimentary unit.