The ‘Ancient Harbour Parasequence’: Anthropogenic forcing of the stratigraphic highstand record

The ‘Ancient Harbour Parasequence’: Anthropogenic forcing of the stratigraphic highstand record

Sedimentary Geology 186 (2006) 13 – 17 www.elsevier.com/locate/sedgeo The dAncient Harbour ParasequenceT: Anthropogenic forcing of the stratigraphic ...

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Sedimentary Geology 186 (2006) 13 – 17 www.elsevier.com/locate/sedgeo

The dAncient Harbour ParasequenceT: Anthropogenic forcing of the stratigraphic highstand record N. Marriner *, C. Morhange CNRS CEREGE UMR 6635, Universite´ Aix-Marseille, Europoˆle de l’Arbois, BP 80, 13545 Aix-en-Provence, France Received 15 November 2005; received in revised form 27 November 2005; accepted 7 December 2005

Abstract Unique chronostratigraphic similarities are observed in recent geoscience literature on ancient Mediterranean harbours. From these empirical port data a clear pattern of millennial anthropogenic forcing, marked by a fining-up sequence, can be discerned in coastal stratigraphy. The stratigraphic peculiarity of this artificial, or human-modified, parasequence leads us to define a new working model: the Ancient Harbour Parasequence. D 2005 Elsevier B.V. All rights reserved. Keywords: Sequence stratigraphy; Geoarchaeology; Holocene; Ancient harbour

1. Introduction For decades, fine-grained sediments uncovered during archaeological excavation of ancient Mediterranean ports have intrigued researchers (e.g. Gouvernet, 1948 for the Greek harbour of Marseilles). Unfortunately, technical difficulties, namely presence of the water table and sediment instability, long limited research possibilities in these basins. It is geoscience which has remedied many of these problems, as attested to by an emergent corpus of geological literature focussing on the biosedimentology of ancient harbours (Reinhardt and Raban, 1999; Kraft et al., 2003). The ability of geoscience techniques, notably bio-, chrono- and lithostratigraphy, to solve important archaeological questions has been demonstrated at numerous sites throughout the Mediterranean. Ancient port basins are rich geological

* Corresponding author. E-mail address: [email protected] (N. Marriner). 0037-0738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2005.12.001

archives replete with information on human impacts, occupation histories, Holocene coastal evolution and natural catastrophes. Furthermore, the presence of enveloping marine muds has anoxically fossilised otherwise perishable artefacts, yielding a multiplicity of archaeological possibilities. The aim of this paper is to expound, using these unique base-level archives, the increasing intensity and role of anthropogenic forcing on the Mediterranean’s sedimentary record during the Holocene. From the diverse datasets, a number of general stratigraphic anomalies can be teased out, leading us to posit a working model, the dAncient Harbour ParasequenceT (AHP). We elucidate the best known and most rigorously studied examples, before moving on to a definition of the model and its geo-archaeological implications. 2. Coastal Progradational Parasequence (CPP) vs. Ancient Harbour Parasequence (AHP) Research shows eustatic variations to have been modest on stable Mediterranean coasts over the past

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6000 years (Laborel et al., 1994; Lambeck and Purcell, 2005; Pirazzoli, 2005). During this sea-level still stand, sediment supply has outstripped accommodation space leading to significant shoreline displacements. The rate of this advance is very much a function of sediment supply and has, in its most extreme examples, led to kilometre-scale coastline advances that completely landlocked many ancient maritime settlements (Bru¨ckner et al., 2002). After the initial transgression, or Maximum Flooding Surface (MFS), natural open coasts classically manifest a coarsening-upward sequence (progradational parasequence set sensu Van Wagoner, 1995; Coe, 2003), whereby younger sediments are deposited under increasingly high-energy shoreface conditions (the CPP, Fig. 1A). The stratigraphic premise is simple. At base level sediment first fills up the most proximal areas. Once these have infilled, sediment is transported to more distal zones causing the shoreline to prograde gradually seawards.

Conversely, ancient harbour facies are characterised by a completely paradoxical stratigraphy; the AHP. Although the artificial parasequence is still progradational, low energy conditions are translated stratigraphically into an anomalous fining-upwards sequence (Fig. 1B). The best examples include Punic Carthage (Gifford et al., 1992), Roman Caesarea (Reinhardt and Raban, 1999), Kition-Bamboula (Morhange et al., 2000), Hellenistic Alexandria (Goiran, 2001), GrecoRoman Marseilles (Morhange et al., 2003a), Sidon (Morhange et al., 2003b) and Tyre (Marriner et al., 2005, 2006; see Fig. 2). A typical vertical succession (Fig. 1B) through the AHP comprises three components: (1) the transgressive contact or Maximum Flooding Surface (MFS), dated ca. 6000 BP in most ancient harbours; (2) wave and current rippled lower/middle shoreface facies constituting medium grained sands and shell debris; (3) finingupward harbour silts representing an anthropogenic, artificially protected, basin; and (4) harbour abandon-

Fig. 1. Lithostratigrahy of the (A) Coastal Progradational Parasequence and (B) Ancient Harbour Parasequence.

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Fig. 2. Lithostratigraphy of the Mediterranean’s best studied ancient harbours. This highstand anthropogenic facies is characterised by an anomalous over-representation of fine-grained material.

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ment expressed by cross-bedded medium to coarse sands coeval with the upper shoreface zone. These key facies and surfaces can be traced laterally throughout the artificial basin. Set against the classic CPP, the AHP displays a number of stratigraphic aberrations:

nostic molluscan and microfaunal biostratigraphies (e.g. Parvicardium exiguum, Cerastoderma glaucum, Cyprideis torosa, Loxoconcha elliptica, Ammonia spp.). The lower surface of this facies (Harbour Foundation Surface, Fig. 1B) lies in chrono-spatial discontinuity with the classic CPP.

(1) A fining-upward granulometry, marked by coarser grained transgressive deposits at the base, overlain by increasingly fine-grained deposits up-sequence. For example, the Roman harbour facies of Marseilles, Sidon and Alexandria comprise plastic marine clays consisting of N90% silt. (2) Accelerated sedimentation rates are observed in these artificial depocentres, at least ~ 10 times greater than nearby naturally prograding coasts. Enveloping port infrastructure reached its apex with Roman pozzolan concrete, that hardens under water (Oleson et al., 2004), and engendered rapid silting up of the basins. This phenomenon is explained by a sharp fall in the competence of sediment transport due to perturbation and attenuation of marine currents. Rate of sediment supply is also critical to understanding sedimentation patterns. This supply term is a function of a number of variables, namely climatic fluctuations and anthropogenic changes in the catchments of supplying rivers, modifications in the pattern of currents, erosion of adobe constructions and finally, use of the basins as ad hoc waste dumps. High sediment supply causes the harbour bottoms to aggrade before the system starts to prograde, a problematical dilemma for ancient societies (see below). We term this rapid artificial process: anthropogenic progradation. (3) Frequent chrono-stratigraphic inversions within this facies are evidence of the human response, namely dredging, to rapid siltation (Marriner et al., 2005; Marriner and Morhange, 2006). This unique change in stratal geometry is akin to anthropogenic overdeepening and can create significant breaks in the sediment archive. In the case of Tyre and Sidon dredging is the origin of more gaps than record for the Phoenician and Persian periods. Not readily assimilated in the literature until now, we expect more examples of these artificial sediment gaps to be elucidated in the future. Constraining these apparent dhiatusesT is important in estimating the archaeological scope of the archive in question. (4) Chrono-spatial discontinuity. Ancient harbour sediments are artificial lagoonal strata, with diag-

After antiquity, the relative decline of harbour works entrained a reopening of the artificial trap, fine-grained sedimentation being superseded by coarse clastics. Following thousands of years of accelerated anthropogenic confinement, (re)conversion to a natural coastal parasequence is typified by high-energy upper shoreface sands. A change in geometry is also observed, with transition from aggradational to progradational strata. This progradation significantly reduced the size of the ports, burying the heart of the ancient harbour basins beneath thick tracts of marine or deltaic sediments. 3. Timing of the AHP anomaly Within stage three of the AHP model, discrete phases of anthropogenic forcing are evidenced in the Mediterranean’s coastal chronostratigraphy. (1) The first, transitional, period corresponds to the Bronze Age and is best represented in the eastern Mediterranean. Around this time, ca. 4000 years ago, maritime technology was still very much in its infancy and proto-harbours were founded with modest stratigraphic impacts. (2) The second phase corresponds to the Iron Age. In the eastern Mediterranean, evidence from Atlit, present-day Israel, corroborates the presence of an artificial breakwater dated to the late 9th century BC and attributed to the Phoenicians (Haggai, in press). For the central and western Mediterranean, construction of artificial breakwaters did not occur before the Archaic period, for example the famous breakwater of Samos is attributed to the 6th century BC. (3) Finally, the Romans’ discovery of hydraulic pozzolan concrete, during the 2nd century BC, spawned a technological revolution in coastal management, epitomised by the construction of totally artificial all-weather harbours. For example, the ports of Caesarea, Pozzuoli and Portus Cladius in Ostia were enveloped by imposing concrete moles. 4. Conclusion Since 6000 BP, the dynamics of Mediterranean shoreline displacement have been a function of sediment influx, coastal physiography and the spatial distribution of energy. The dnaturalT CPP, characterised by gradual coarsening upwards of the sediments, is contrasted with

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the AHP comprising low energy silts and lagoon fauna. Preservation of thick transgressive sequences in (semi-) artificial depocentres renders the AHP a rich geoarchaeological archive. Acknowledgments We thank G. Clauzon and K. Crook for kindly reviewing earlier versions of this paper. N. Marriner benefited from a Leverhulme Study Abroad Studentship. References Bru¨ckner, H., Mu¨llenhoff, M., Handl, M., van der Borg, K., 2002. Holocene landscape evolution of the Bu¨yu¨k Menderes alluvial plain in the environs of Myous and Priene (Western Anatolia Turkey). Zeitschrift fu¨r Geomorphologie N.F. 127, 47 – 65. Coe, A.L. (Ed.), 2003. The Sedimentary Record of Sea-Level Change. Cambridge University Press, Cambridge, p. 286. Gifford, J., Rapp, G., Vitali, V., 1992. Paleogeography of Carthage (Tunisia): coastal change during the first millennium BC. Journal of Archaeological Science 19, 575 – 596. Goiran, J.-P., 2001. Recherches ge´omorphologiques dans la re´gion littorale d’Alexandrie, Egypte. Physical Geography PhD, Universite´ de Provence, p. 240. Gouvernet, Cl., 1948. Une plage ancienne dans le Lacydon a` Marseille. Bulletin de la Socie´te´ Linne´enne de Provence 16, 13 – 19. Haggai, A., in press. Excavation of the Phoenician harbour at Atlit: findings and reassessment of its foundation and purpose. Journal of the Tel Aviv Institute of Archaeology. Kraft, J.C., Rapp, G.R., Kayan, I., Luce, J.V., 2003. Harbor areas at ancient Troy: sedimentology and geomorphology complement Homer’s Iliad. Geology 31, 163 – 166. Laborel, J., Morhange, C., Lafont, R., Le Campion, J., LaborelDeguen, F., Sartoretto, S., 1994. Biological evidence of sealevel rise during the last 4500 years on the rocky coasts of continental southwestern France and Corsica. Marine Geology 120, 203 – 223.

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