Late Holocene erosion of the Canopic promontory (Nile Delta, Egypt)

The mouths of the Nile Delta are sensitive coastal areas, their geomorphology primarily being mediated by relative sea-level rise and sediment supply. To further document the Holocene evolution of the Nile's Canopic mouth, a core was taken from the southern shores of Abu Qir Bay, close to the ancient Canopic channel. Core bio-sedimentology and chronostratigraphy highlight four stages of marine incursion which are juxtaposed upon the general progradation trend of the Nile coast in this area. Compiled age-depth points from sediment cores taken in Abu Qir Bay underscore two phases of negative sediment budget at the Canopic mouth: (1) a first period, between 3.5 and 2 ka cal. yr BC, probably in relation to the well-documented mid-to-late Holocene decline in Nile flow; and (2) a second phase, after 0.5 ka cal. AD, linked to a decline in Canopic sediment supply to the coastal area, and concomitant with the development of the Rashid branch. The erosion and reworking of material flattened and lowered the promontory surface by up to 4 m. The submersion of the Canopic promontory was completed by relative sea-level rise, primarily controlled by the compaction and liquefaction of unconsolidated lagoonal muds. The lowering of the Canopic mouth led to the submersion of two ancient estuarine-harbor cities, known as East-Canopus and Herakleion, whose remains lie 4-7 m below present mean sea level. It is argued that the subsidence of the two cities cannot explain their abandonment during the late 7th-early 8th century AD, taking into account the regional occupation pattern during Antiquity. Rather, the longevity of the two cities, spanning more than 13 centuries, shows that adaptation to coastal risks including erosion, subsidence and high-energy events like storms or tsunamis, was the rule.