Holocene evolution of the Danube delta: An integral reconstruction and a revised chronology
Résumé
The Danube delta is one pf the few large deltas in the world the evolution of which has involved numerous and varied episodes within a complex framework of interactivity between river sediment supply, allochthonous sediments supplied by longshore currents, marine dispersing forces, vertical movements (neotectonics, sediment compaction) and sea level. The resulting complex morphology comprises diversified landscapes varying from labyrinthic net of channels and lakes (fluvial delta) to massive tracts of monotonous reed marshes, large lagoons divided by barriers, or beach-ridge plains accommodating large transgressive dtmefields (maritime delta). Whilst preVious studies have focused on various sectors of the Danube delta, the current paper proposes for the first time an integral reconstruction of delta evolution based on existing and new sedimentological and morphological analyses and absolute ages (AMS 14C and OSL), enabling a comprehensive synthesis in terms of both evolutionary phases and growth patterns. A chronological framework was established for all the deltaic lobes and beach-ridge plains, highlighting the relationship between formation timespan, growth rates, and the resultant morphology. This work unveils the early stage of delta formation, including the reconstruction of delta front advancement into Danube Bay (Old Danube lobe: prior to 7.5-5.5 ka) and initial spit/barrier development (6.7/6.5-5.8 ka). Inception of the bayhead delta started > 1000 yr before the relative stabilization of sea level and of the initial spit formation. The original fluvial delta plain topography is now buried at a depth of 4-6 m depth below the present topography (representing the current stage of fluvial aggradation), as a result of subsidence and sea-level rise. Regarding the maritime delta, six large open-coast lobes developed in the last six millennia, of which four were formed by the Sf. Gheorghe branch, attesting the long uninterrupted activity of this branch, whereas the other two were created respectively by the Sulina and the Chilia branches. The evolution of each lobe is derived from successive (chronological) shoreline positions and discussed in relation with changes in Danube discharge. Special attention has been paid to their growth stages and progradation rates. For the southern delta, we bring in new arguments for an active southern distributary (the Dunavat, derived from the Sf. Gheorghe branch) that formed successive open-coast lobes between 2.6 and 1.3 ka. Additionally, we discuss the effects of modern anthropogenically-driven fluvial sediment reduction on the morphology and morphodynamics of the active lobes of the Danube.