Growth of the sandy isthmus of tyre and ensuing relocation of its harbors

The renowned Phoenician city of Tyre was settled on a small coastal island from which it resisted invasions and sieges for centuries. In 332 BCE, the city was taken when Alexander-the-Great built a 750 m-long causeway to seize the island. The causeway interrupted longshore sediment transport, forcing sand to accumulate along the causeway, creating a isthmus that today still connects the island to the mainland. This isthmus is no less than 530 m wide and 13 m high. We studied the impact of isthmus growth on city development during Antiquity, by combining archeological data, core stratigraphy, and multibeam bathymetry to track the paleogeographic evolution of the land-facing coast of Tyre, from the Holocene marine transgression up to today. In Phoenician time (900-300 BCE), sea-level markers indicate that relative sea level lied more than 2.5 m below current sea level, defining a 1500 × 600 m rocky island, 450 m longer in the south than the modern rocky headland. Between the island and the mainland, diffracted/refracted waves built a submerged sand bank that rests on eroded transgressive lagoonal clays. Along the mainland-facing coast of Tyre Island, Phoenician walls were built on the emerged end of the sand bank. Middle infrared spectroscopy (MIRS) shows that the sand of the bank and its successor isthmus is made of carbonate clasts (90–100%). 36Sr/37Sr ratios further indicate that these are modern bioclastic sands. Coeval, heterogeneous clay-rich sediments were deposited between the rocky island, and the sand bank. We interpret these as harbor sediments, deposited behind buried and/or submerged Phoenician breakwaters. The sand bank inflated dramatically after the building of Alexander's causeway, rising 5 m during the Hellenistic and Roman periods. It rose another 4 m–8 m in successive stages during Byzantine and Medieval times, up to the 19th century. The rapid growth of the sandy isthmus after 332 BCE led to the demise and burial of the harbors that had been developed along the mainland-facing coast. The Phoenician southern harbor of Tyre appears to have been buried in Roman times during the repurposing of the SE corner of the island for monumental baths. We interpret this repurposing as a response to the growth of the sandy isthmus, and suggest that the northern Phoenician harbor was then relocated to its present position. Subsequent relative sea level rise led to the erosion of the landfill overlying the southern harbor, exposing the formerly buried port on the seafloor.