History and influence of the Danube delta lobes on the evolution of the ancient harbour of Orgame (Dobrogea, Romania)

Abstract

International audienceOn the coast of Northern Dobrogea, south of the Danube delta, the Greek settlement of Orgame was founded in the mid 7th c. BC, probably by Milesian colonists. The ancient city was located on the Cape Dolojman which today overlooks a large lagoon complex. We undertook a chronostratigraphic study to:(i) understand coastal changes around Cape Dolojman since ca. 5000 years BP in connection with the construction of the Danube delta lobes, and (ii) identify potential sediment impacts related to human occupation of the site. Three cores were extracted from the lagoon area. Sedimentological and biological analyses were undertaken to reconstruct the evolution of the coastal palaeoenvironments. The results show a closure of the marine bay around 3500 cal. BP and its transformation into a lagoon environment. The first major environmental change was due to the construction of the lobe St. George I and the formation of the barrier Lupilor. Around 2000 cal. BP, the formation of an intra-lagoonal lobe, the Dunavatz, led to the gradual transformation of the lagoon into a fluvial-dominated system. Paradoxically, lagoon waters today still wash the ancient Greek harbour environment, which has not been totally infilled by alluvial sediments. To understand this paradox, in a context of coastal progradation, we compared and contrasted the geomorphological data with the nearby city of Istros/Histria, which was already landlocked at this time. The location of these two Greek colonies relative to the coastal sediment cell and barriers partly explains their contrasting palaeoenvironmental evolution. Until 2650 cal. BP, the increase in charcoal and organic matter in sedimentary archives is interpreted as an anthropogenic signal for a more extensive use of the vegetation cover following the foundation of the city of Orgame (e.g. for domestic use and funeral rites)

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“…The data were Hellingertransformed, i.e. square-rooting the relative abundances of count data, and standardised by subtracting the mean and dividing by the standard deviation (Legendre and Gallagher, 2001;Borcard et al, 2011). This procedure has been recommended for clustering of species abundance data because it limits the influence of high abundance values (Rao, 1995;Legendre and Gallagher, 2001).…”

“…This procedure has been recommended for clustering of species abundance data because it limits the influence of high abundance values (Rao, 1995;Legendre and Gallagher, 2001). We used the 20 most abundant species to avoid a bias from rare species (Borcard et al, 2011). We chose k to range between 2 and 10 in order to find appropriate groups representing different environmental types.…”

“…To find significant species associations, we calculated Kendall's W coefficient of concordance for each of the partitioned data sets (Legendre, 2005). This method tests for the most encompassing assemblages, hence the smallest number of clusters with the largest number of positively and significantly associated species (Borcard et al, 2011). For each k, a global Kendall's W test was run to estimate if the identified species groups were significantly associated.…”

“…For each k, a global Kendall's W test was run to estimate if the identified species groups were significantly associated. An a posteriori test was conducted to evaluate the contribution of each species to the groups (Legendre, 2005;Borcard et al, 2011). Species groups that were not globally significant, as well as species that were negatively or not significantly associated, were excluded.…”

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