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Holocene morphogenesis of Alexander the Great's isthmus at Tyre in Lebanon

Author:
Marriner, Nick, Morhange, Christophe, Meulé, Samuel
Source:
Proceedings of the National Academy of Sciences of the United States of America 2007 v.104 no.22 pp. 9218-9223
ISSN:
0027-8424
Subject:
engineering, metamorphosis, models, morphogenesis, natural history, reefs, sand, sandstone, sea level, seawater, surface water, Lebanon
Abstract:
In 332 B.C., Alexander the Great constructed an [almost equal to]1,000-m-long causeway to seize the offshore island of Tyre. The logistics behind this engineering feat have long troubled archaeologists. Using the Holocene sedimentary record, we demonstrate that Alexander's engineers cleverly exploited a shallow proto-tombolo, or sublittoral sand spit, to breach the offshore city's defensive impregnability. We elucidate a three-phase geomorphological model for the spit's evolution. Settled since the Bronze Age, the area's geological record manifests a long history of natural and anthropogenic forcings. (i) Leeward of the island breakwater, the maximum flooding surface (e.g., drowning of the subaerial land surfaces by seawater) is dated [almost equal to]8000 B.P. Fine-grained sediments and brackish and marine-lagoonal faunas translate shallow, low-energy water bodies at this time. Shelter was afforded by Tyre's elongated sandstone reefs, which acted as a 6-km natural breakwater. (ii) By 6000 B.P., sea-level rise had reduced the dimensions of the island from 6 to 4 km. The leeward wave shadow generated by this island, allied with high sediment supply after 3000 B.P., culminated in a natural wave-dominated proto-tombolo within 1-2 m of mean sea level by the time of Alexander the Great (4th century B.C.). (iii) After 332 B.C., construction of Alexander's causeway entrained a complete anthropogenic metamorphosis of the Tyrian coastal system.
Agid:
2352777