The evolution of Sf. Gheorghe (Danube) asymmetric deltaic lobe

The wave asymmetric Sf. Gheorghe lobe is the only active in the Danube delta where river mouth bar (and the associated barrier islands and spits) continuously deployed a cyclic development for almost two millennia. During first stage, the Sf. Gheorghe distributary had a small discharge (with an order of magnitude lower than present) which after that experienced a rapid increase in consequence of the successive avulsions of Împuţita (southern distributary of Sulina arm) and Dunavăţ distributaries. Our morphological analyses together with the newly obtained chronology, revealed the multiple ridgesets structure of Sf. Gheorghe deltaic lobe. In fact, all ridgesets (10) follow a common morphodynamic pattern characterized by the cyclic succession of three stages: i) subaqueous mouth bar development, ii) barrier island emergence, iii) barrier spit phase with several secondary spits derived from an updrift trunk ridge. The size of each ridgeset increased exponentially with every new cycle due to the constant lengthening of the coastline as the downdrift side of the lobe advances seaward through a series of progressively larger similar quadrilaterals, yielding to a constant enlargment of the delta front size. </p

The impact of the Late Holocene coastal changes on the rise and decay of the ancient city of Histria (southern Danube delta)

Histria (Istros) is one of the earliest Milesian colonies founded on the Black Sea coast during the archaic period (7th c. BC). Its remains are located within what is today the Razelm–Sinoe lagoon system which forms the southern part of the Danube Delta. During its 13 centuries of existence, Histria experienced an environment favourable for a prosperous economy (e.g. fishery, farming, grazing, pottery, marine trades) as indicated by numerous archeological finds from both in situ and elsewhere in the Black Sea and Mediterranean Sea basins. Moreover, it benefited from a strategic position on an open-coast, which was easy to defend. Important environmental changes occurred both during and after the existence of the ancient city, and the most significant are related to major shoreline displacement. This study reports Late Holocene coastal landscape changes, from the open-coast stage to the present-day lagoon system, based on numerical age determination of the paleoshorelines, stratigraphic records by cores in lagoons and beach ridges and topographic surveys. A new chronological framework is established which reports younger ages than previously suggested for the coastal morphosedimentary units of the Histria region. Further, the findings shed new light upon the evolution of the southern part of the Danube delta. The results point to the development of an ancient deltaic lobe (Dunavăţ lobe, 2000–1300 BP, associated with the southernmost Danube distributary) which was situated 10–20 km north of the settlement. The lobe was subsequently reworked by waves, providing sediments which were accumulated downdrift (southward) as a continuous beach ridge plain (Saele–Chituc) in front of the city, leading to its isolation from the sea. The abandonment of Histria (7th c. AD) coincides with the decoupling of the city from the open coast as a consequence of the shoreline progradation. The new beach ridge plain (Saele–Chituc) is affected by intense neotectonic movements which led to the recent drowning of its central part and the formation of Sinoe lagoon; the same processes affected the downdrift part of the Dunavăţ lobe and Histria and Nuntaşi Lakes. The new sea-level curve obtained for the Histria region shows relative stability within the last four millennia, with oscillations within 0 to −2 m of the current level. This contradicts the concept of a marine regression (Phanagorian) which is suggested to have occurred during the 1st millennium BC. However, a few areas containing archeological remains are currently below sea-level owing to local neotectonics