Geological methods in Archeology of ancient Mediterranean harbors

Frank Scholze1

1Technische Universität Bergakademie Freiberg

Abstract. Ancient harbor sites in the Mediterranean have always been of special

interest for archeologist. In geoarcheology, the archeological and geologic meth-

ods are used for reconstructions of paleogeography and paleoenvironment in an-

cient harbor sites. Several types of ancient harbor preservation can be distin-

guished. The ancient Troia is an example for a buried land-locked Mediterranean

harbor site, where the accumulation of delta sediments by Karamenderes river

caused a progradating coast line. The usage of geological tools like core drilling

and biostratigraphy combined with the help of 14C-dating enable a reconstruction

of the evolution of the alluvial plain of Troia since the Holocene.

Introduction

There are several important archeological sites in the Mediterranean. But not only the cities and their relicts of ancient buildings like an antic theatre are of ar-cheological interest, also their ancient harbors have a high scientific relevance. Since centuries scientist were researching on these old human occupation sites and try to reconstruct them. Beside the classical archeological methods used for reconstruction of an archeological site, the usage of modern geological methods are necessary to integrate morphological and environmental aspects to understand the development of the landscape, the structure of social communities, and the economic relations. Coastal changes and ancient Mediterranean harbors

There are several reasons for the declining of an ancient Mediterranean harbor. Besides wars and diseases as anthropogenic reasons, also environmental changes like a progradating delta, coastal dynamics, tectonic subsidence, high sedimentary

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supply, tectonic uplift, floods, and a changing sea level could lead to the destruc-tion of the harbor sites.

Generally, the changes of the sea level needs to be stressed. The settlement phases in the Mediterranean coastal areas during Bronze Age are from great ar-cheological interest. From a geological point of view the Holocene is a time, which is characterized by the ending of Pleistocene glacial period and the melting of ice masses combined with a rising sea-level. The present Mediterranean coast line compared with the coast line during last glacial period is sown in figure 1.

Fig. 1. Last Glacial Maximum shoreline and transgression of Mediterranean’s coastal margins (modified after Marriner and Morhange 2007).

The rapid rising sea level in the early Holocene had a regional different influ-ence to the coast line depending on different geographical conditions and local tectonic activities. In the Aegean costal areas the present sea level was reached 6000 years ago (Kayan 1999; Vött and Brückner 2006). Especially for studying the history of an early settlement it is necessary to know when the present sea level in a certain coastal area has been reached: in case of a permanent rising sea level since the beginning of the Holocene until today, a settlement from Bronze Age will now lie in sediments below the present sea level. Otherwise, if after a fast transgression in early Holocene the present sea level was reached in Bronze Age, an ancient coastal settlement from Middle Age should be searched today by archeologists in sediments above the present sea level (Vött and Brückner 2006). Of cause, these two examples do not include tectonic shifts or further aspects of coastal dynamics like a progradating delta.

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Fig. 2. Ancient harbor classification based on proximity to coastline, position rela-tive to sea level, sedimentary environment, and taphonomy (modified after Mar-riner and Morhange 2007).

Changes in sea level affect the sedimentary processes on a coast. An increase in sediment supply is often connected with a rising distance between the former harbor and the coast and finally leads to the closure of the harbor. Of cause, the reasons therefore are different and enable a differentiation in (1) buried urban har-bors with high sediment supply linked to fluvial inputs and human activity, (2) buried land-locked harbors with rapid coastal progradation, (3) buried lagoonal harbors with gradually infilled low energy depocentres and (4) buried river har-bors with canalization of the river bed and vertical accretion of riverbanks by flooding (Marriner and Morhange 2006). Depending on the position relative to the sea level, proximity to coastline, sedimentary environments and the taphonomy of an ancient Mediterranean harbor, Marriner and Morhange (2006) distinguish seven preservation types of harbors (Fig. 2). Similar to the taphonomy of fossils, the kind of bedding into sediment is decisive for a good or poor preservation of ancient harbor sites. Beside the better preservational potential of buried harbors, the higher current energy in coastal areas or a lower sedimentation rate could cause a limited preservation potential of eroded harbors.

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Geologic methods

The making of archeological and paleoenvironmental reconstructions are major tasks of archeologists and geologists. Therefore several geological methods could be used. In general it can be distinguished between field work (e.g. geomor-phological methods, geophysical methods, coastal stratigraphy) and additional laboratory methods (e.g. sedimentology, biostratigraphy, geochemistry). The ap-plication of geological methods in archeological sites and the additional use of ar-cheological information sources (e.g. artifacts, old literature) are combined in geoarcheology.

Field work

For the fieldwork in coastal areas geoarcheologists get their information to re-construct the landscape complex from two sources: geomorphology and sediment archives (Marriner and Morhange 2007).

Geophysical methods

The sedimentary construct of a former harbor basin can be explored by geo-physical measurements of electric resistivity. A higher clay content causes a higher electric conductivity. A graphic evaluation of the geophysical data is shown in figure 3. In this example the transition from basement into the Holocene basin filling can be clearly distinguished. In geophysical data sets, lagoon sediments are very similar to lake sediments and therefore the integration of additional informa-tion earned by drilling and core analyses of ancient harbor sediments is needed. Also sandy accumulation in fluvial channels can be recognized in higher values of electrical resistivity (Vött and Brückner 2006).

Fig. 3. Profile of electric resistivity in ancient harbor of Oiniadai (modified after Vött and Brückner 2006).

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Laboratory work

Geological laboratory work on geologic methods in combination with different examination results led to more complex outcome on coastal stratigraphy and har-bor history. The connection of facies, sedimentary processes and anthropogenic influences are summarized in figure 4.

Fig. 4. Research tools (facies, sedimentary processes and anthropogenic impacts) in ancient harbor sequences (modified after Marriner and Morhange 2007)

Sedimentological methods

The application of sedimentological methods are used in laboratory work on samples from coring. The description of samples includes color, sedimentary structures and grain size. After Marriner and Morhange 2007 especially the gravel-fraction of an harbor sediment includes interesting material like molluscs, seeds, wood fragments and ceramic shards. As kind of base-level waste dump the harbor basin deliver important information of human society. Furthermore it is possible to distinguish silty and clayish harbor sediment from more coarse grained harbor beach sediment (Marriner and Morhange 2007).

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Fig. 5. Harbor and paleo-tsunami quartz grains with traces of high energy shock waves from Alexandria’s ancient harbor (modified after Marriner and Morhange 2007).

Exoscopic indicators also belong to sedimentological methods. These investiga-tions deal with the grain shape and traces of surface solutions and shocks (Mar-riner and Morhange 2007). Also sphericity and roundness deliver information of coastal processes and depositional environment. For quartz Marriner and Mor-hange 2007 distinguish two types of grains: quartz grains with a polished surface from low energy harbor muds and quartz grains from midlittoral beaches which are marked by shock traces from waves with high energy (fig. 5). As an exception high energy events like tsunamis have to be mentioned. Such single events may also lead to quartz grains with shock marks within layers of lower energy harbor muds.

Biostratigraphical methods

An overview on the usability of several fossil taxa is summarized in Marriner and Morhange 2007. Biostratigraphy can deliver indications of depositional envi-

Geological methods in Archeology of ancient Mediterranean harbors 7

ronments, evolution of biocenoses and human influences on coastal ecosystems. Working on biostratigraphy needs good knowledge on taxonomy of species and the understanding of ecological and geographical relations.

After Marriner and Morhange (2007) a special molluscan classification system the malacology allows a differentiation in in situ and extra situ assemblages. This differentiation is helpful for 14C-dating: samples of extra situ molluscs shells, which were reworked by wave action and bedded into younger or older sediment layers should be identified and better be avoided. Further the occurrences of cer-tain mollusc taxa in ancient harbor sediments can be seen as evidences for eco-logical stress and harbor pollution (Kraft et al. 2003). Figure 6 presents examples of typical taxa like Cerastoderma glaucum, Cerithium vulgatum and Ammonia tepida. They are indicators of protected harbor areas or lagoonal environment rich in fine-grained sediments (Marriner and Morehange 2007).

Fig. 6. Ecological indicators of harbor conditions (modified after Marriner and Morhange 2007).

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Also ostracods are used in biostratigraphical work on harbor sediments. The shells of ostracods have a high number of morphological characteristics which make them useable for taxonomic and phylogenetic studies. For several reasons they are good indicators of paleoenvironmental conditions: living in fresh and ma-rine waters, small size, the good preservation potential of their shells. Caused by strong dependence on environmental conditions (e.g. water temperature, salinity, water depth, grain size and anthropogenic influences) the occurrences of ostracods vary in faunal composition, population density and diversity (Marriner and Mor-hange 2007). Also the human activity can influence the occurrence of ostracods in three ways: individual density and diversity, abundance and distribution of spe-cies, chemistry of the shells. Finally, for the reason that certain taxa only live in fresh water, brackish lagoonal, marine lagoonal, coastal or marine environments, ostracods belong to that organisms best usable as biostratigraphic tools in archeo-logical harbors sites (Marriner and Morhange 2007).

Foraminifers along with molluscs and ostracods are used in biostratigraphy,

too. Generally, it is advantageous that foraminifers do not occur in fresh water. Additional advantages for the use of foraminifers in geoarcheology of ancient har-bors are: the population density of more than one million individuals per square meter in marine environments, the mineralized shell with high preservation poten-tial, the rapid reaction to environmental changes, the very specific occurrence of characteristic species in distinct environments (Marriner and Morhange 2007). Further, as a consequence of the high richness, even small sediment samplings contain enough foraminifers to reach statistical safety.

Finally there are a lot of other organisms which can be used for biostratigraphy

in archeogeology. This is also valid for diatoms living in lagoons and fluvial har-bors. If an easy identification of molluscs, ostracods or foraminifers is not possi-ble, information of regional vegetation and human live could be earned by paly-nology. The fine-grained harbor mud is well usable for preservation of pollen. However, it must be mentioned that a sediment could be contaminated by coastal transport and deposition processes with strange pollen (Marriner and Morhange 2007).

Example: Troia

The archeological site of Troia consists of ruins from ten main settlement phases (Troia I-X). Troia (today the Turkish city named Hissarjik) is located near the mouth of the Dardanelles between the Aegean Sea and the Sea of Marmara next to the alluvial plain with the rivers Karamenderes (former Scamander) and Dümrek (former Simois, see fig. 7). The strategic comfortable position on the an-cient trading route between Mediterranean Sea and Black Sea made the area of ancient Troia to a focal point of commercial and military activities through history (Kraft et al. 1980). For this reason also the ancient harbor is from great relevance for the archeological researches on Troia.

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Fig. 7. Geographic position of ancient Troia (modified after Göbel 2005).

For reconstruction this important historical site beside archeological tools also geologic methods were used to get more information of paleogeographical devel-opment of the alluvial plain next to Troia. A major task is the reconstruction of the former coast line in this area. Results based on investigation with geological tools are drillings, 14C-dating, sedimentary analyzes, geomorphology and their inter-pretation.

Figure 8 shows the sea level changes on the Aegean coasts during the Holo-cene. The data for the reconstruction of a changing sea level were delivered from a core-drilling program. Therefore alluvial sediments in Karamenderes flood plain and Dümrek plain have been drilled (Kraft et al. 1980, 2003; Kayan 1995, 1996, 1999; Göbel 2005). The investigations of drilled cores by methods of biostratigra-phy and coastal stratigraphy enabled the differentiation of units depending on their genesis. In detail is this the case for marine sediments from an early Holocene transgression, fluvial sediments and flood plain sediments. For identification of the character of depositions also fossils have been used. For example, the transi-tion from an unfossiliferous horizon into a horizon of the Karamenderes plain con-taining marine taxa (e.g. Cardium edule) can be understood as change from terres-trial into marine deposition. Furthermore, 14C-dating on fossil mollusc shells and wooden fragments allowed the reconstruction of the depositional character by tak-ing notice of their shifting through time (Kayan 1995, 1999; Kraft et al. 1980, 2003).

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Fig. 8. Paleogeographical evolution of the Aegean coastal plains of Anatolia (modified after Kayan 1999).

After Kayan 1999 the Holocene transgressive deposits are characterized by a

sequence of coastal swamp and transgressive sediments with a coarse sand-gravel lag deposit and abundant marine mollusc shells. Proofed by 14C-datings on ma-rine mollusc shells, the present sea level in Aegean coastal areas was reached 6000 years ago (Kayan 1991). Afterwards the alluvial plain of Troia became stronger influenced by fluvial deposition of Karamenderes river. Since Middle Holocene the Karamenderes river dominated the depositional system with a deltaic envi-ronment. Thus alluvial deposition filled marine embayments at the mouth of the rivers, and delta coast generally reached open sea during the Late Holocene (Kayan 1999). As a consequence of the seaward progradation of Karamenderes and Dümrek river floodplain backswamps and separate channels occurred, which leaded to a complex sequence of sand, silt and clay (Kraft et al. 1980). Thereafter the deltaic progradation became slower and delta surfaces were covered by flood plain sediments (Kayan 1999). Finally, after Kraft et al. 2003 the modern flood plain includes multiple river channels plus the older abandoned channels that were reactivated during later floods (see fig. 9).

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Fig. 9. Paleogeographic reconstruction of the Karamenderes plain (from Kayan 1995)

Especially for archeological questions of ancient Troia the results earned by

geologic methods are from great relevance. The information about the distance be-tween ancient Troia and the coast and also the knowledge about environmental conditions in the shallow marine embayment gives indications of the possible po-sition of the Troia’s ancient harbor and his character. Additionally, these results of paleoenvironment and paleogeographic reconstructions of Karamenderes plain can be compared with information from archeological sources like the geographic fea-tures described in Homer’s Ilias (Kraft et al. 2003). Especially this is the case for the time of Troian War (Troia VI-VII; ca. 3200y BP.), where places and events were described by Homer and which can be correlated with geologic, geomorphic and fossil evidences (Kraft et al. 2003).

Summary

Climatic changes, tectonic and coastal dynamics, and sedimentary supply are the major driving mechanisms of fluxes in the sea level. During the Holocene these factors influences the human settlements in Mediterranean coastal areas and caused several declines of ancient harbors. Today with the help of geoscientist and

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their geological methods it is the task of geoarcheology to reconstruct the ancient coastal landscape and the changes of the environment during time. For this more complex aim it is necessary to combine geological field work (e.g. geophysics, coastal stratigraphy) and laboratory methods (e.g. sedimentology, biostratigraphy). For the surrounding area of ancient Troia, which is one of the most popular ar-cheological sites in the world, the case study shows the paleogeographical recon-struction as a result of geomorphological investigations and coastal stratigraphy with the help of a core drilling program and biostratigraphical work. After the fast transgression in early Holocene the sea level reached the present level 6000 years ago. Afterwards the Troia site was stronger influenced by Karamenderes river with the formation of an alluvial plain and a progradating delta. Finally, the results of the geological methods are supported by archeological knowledge from the old literature by Homer.

References Göbel, J. (2005) Landschaftsrekonstruktion in Siedlungsgebiet von Troia. Disser-tation Universität Tübingen, 360 p. Kayan, I. (1995) The Troia Bay and supposed harbor sites in the bronze age. Studia Troica 5: 211-235 Kayan, I. (1996) Holocene stratigraphy of the lower Karamenderes-Dümrek plain and archeological material in the alluvial sediments in the north of the Troia ridge. Studia Troica 6: 239-249 Kayan, I. (1999) Holocene stratigraphy and geomorphological evolution of the Aegean coastal plains of Anatolia. Quarternary Science Reviews 18: 541-548 Kraft, J.C.; Kayan, I.; Erol, O. (1980) Geomorphic Reconstructions in the Envi-ronments of Ancient Troy. SCIENCE 209: 776-782 Kraft, J.C.; Rapp, G.; Kayan, I.; Luce, J.V. (2003) Harbor areas at ancient Troy: Sedimentology and geomorphology complement Homer’s Iliad. Geology 31, no. 2: 163-166 Marriner, N.; Morhange, C. (2007) Geoscience of ancient Mediterranean harbours. Earth-Science Reviews 80: 137-194 Vött, A.; Brückner, H. (2006) Versunkene Häfen im Mittelmeerraum. Geographische Rundschau 58, Heft 4: 12-21