Alfred Vespremeanu-Stroe

Young Danube delta documents stable Black Sea level since the middle Holocene: Morphodynamic, paleogeographic, and archaeological implications

New radiocarbon and optical dates show that the Holocene Danube delta started to build out of a Black Sea embayment 5200 yr ago. Delta lobe development phases differ by as much as 5 k.y. from previously proposed ages. The new chronology allows for a better understanding of the Danube delta paleogeography, includ- ing the demise of Istria, the main ancient Greek-Roman city in the region. Prior reconstructions of sea level in the Black Sea inferred fluctuations to 15 m in range; however, stratigraphy of beach ridg- es in the delta shows that the relative Black Sea level for the past 5 k.y. was stable in the Danube delta region within 2 m and 1.5 of the current level. Hydroisostatic effects related to a proposed catastrophic reconnection of the Black Sea to the World Ocean in the early Holocene may have been responsible for the sea level reaching the highstand earlier than estimated by models. The new sea-level data suggest that submergence at several ancient settle- ments around the Black Sea may be better explained by local fac- tors such as subsidence rather than by basin-wide sea-level fluctuations.

Alongshore variations in beach-dune system response to major storm events on the Danube Delta coast

ABSTRACT Tătui, F., Vespremeanu-Stroe, A, Preoteasa, L., 2014. Alongshore variations in beach-dune system response to major storm events on the Danube Delta coast. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 693–699, ISSN 0749-0208. Deltaic beach-dune systems are extremely dynamic, responding to processes operating on scales from short-term variations related to storm and floods to long-term evolution driven by large-scale sediment dynamics (including lobe switching). On Danube Delta beaches, coastal storms and associated processes lead to a wide range of morphological impacts from moderate deposition to significant erosion. Coastal processes develop with marked temporal differences as a result of variations in storminess related to changes in climatic systems (North Atlantic Oscillation). In order to assess the variations in vulnerability to extreme storms, different sectors along the study site were examined using two storm impact indexes: Storm Impact Categories of Sallenger, 2000 and Dune Stability Factor of Armaroli et al., 2012, based on specific storm thresholds. There is a very good correspondence between the effects of the December 1997 – January 1998 extreme storm cluster and the vulnerability of the beach-dune system predicted with both indicators, with significant alongshore variations of storm impact. The driving factors imposing this variability at different time scales are also discussed, ranging from nearshore slope (which imposes different wave heights and storm induced water level increase) and sediment availability (in direct connection with the evolution of different deltaic lobes, position into the littoral cell and the distance to the Sf. Gheorghe arm mouth, river discharge variability and human interventions) to beach-dune morphology (accommodation space and pre-existing coastal morphology).

Coastal changes from open coast to present lagoon system in Histria region (Danube delta)

ABSTRACT Preoteasa, L., Vespremeanu-Stroe, A., Hanganu, D, Katona, O.,Timar-Gabor, A., 2013. Histria (Istros) is a Milesian colony founded on the Black Sea coast during the 7th c.BC. Nowadays, the remains of the ancient city are located 8 km inland form the Black Sea coast, on the continental edge of the Razelm-Sinoe lagoon system which forms the southern part of the Danube Delta. Significant environmental changes occurred during and after the ancient citys lifetime, particularly related to dramatic shoreline displacement. The present study deals with the Late-Holocene coastal landscape transformation from the open-coast stage to the present-day lagoon system. Our research includes stratigraphic records by Ground Penetrating Radar scanning of the beach ridges, cores in lagoons and beach ridges and absolute age determination of the paleoshorelines. The study shows that the ancient city decoupling from the sea was due to the beach ridge plain development southward and eastward of the acropolis. The decline of the Histria city, documented during the 7th c. AD, temporally coincides with the decoupling of the city from the open coast as a consequence of the shoreline progradation. Discontinuous chronology and discordant stratigraphy obtained by OSL dating and GPR scanning of successive ridges document intense neotectonic movements which affected Saele-Chituc beach ridge plain. The seismic activity led to the recent drowning of its central part and the formation of the Sinoe lagoon; the same processes acted at the downdrift part of the Dunavat lobe and also in the areas presently occupied by Histria and Nuntasi Lakes. Several areas containing archeological remains are currently below sea-level due to local neotectonics.

Thermal weathering of granite spheroidal boulders in a dry-temperate climate, Northern Dobrogea, Romania

ABSTRACT: Weathering microforms associated with exfoliation were investigated on 40 granitic spheroidal boulders identified on Pricopan Ridge (Măcin Mountains) in order to establish a spatial distribution pattern. Continuous thermal monitoring allowed the frequency and intensity distribution assessment of short‐term temperature changes triggered by summer storms, of intense day–night amplitudes and frost cycles across a uniform rounded boulder. Rock strength estimated by Schmidt hammer tests differentiates a significantly weaker resistance on the southern face of the boulders (rebound values of 27 to 33) in comparison with the northern face (43–50). The lowest resistance of the north–south cross‐boulder profile corresponds to the southern gentle slopes (0°–45°) thus defining the most susceptible area to exfoliation and other weathering processes. It is argued that this low‐resistant sector fits well with the maximum frequency and intensity of thermal processes recorded on the low and mid slopes (0°–45°) of the boulders south side, with small differences from one process to another, whilst the sector of 20° to 30° south corresponds to the peak activity of all. In accordance, the overlay map of exfoliated surfaces places the high frequency area on a spherical cap developed similarly (between 5° north and 45° south). The smallest exfoliated surfaces normally appear around 30° south and are inferred to extend in time both to the boulder top and downslope. The correlations between the frequency/intensity maps of thermal processes and the frequency map of exfoliated surfaces point to a complementary action in the exfoliated surfaces development of the short‐term temperature changes and diurnal cooling and heating due to the directional insolation effect, as similarly inferred in the development of meridional cracks. Copyright

Morphology and the Cyclic Evolution of Danube Delta Spits

This chapter is about the sand spits formation in relation with the evolution and morphology of the Danube mouths and deltaic lobes, developed within the general framework of the virtually tideless Black Sea basin in a medium-wave energy environment. It deals with both barrier spits and islands, in respect to their processual development as most of the Danube delta spits derive from former barrier islands. The morphodynamics of the modern spit barriers associated to Danube mouths is discussed in relation with human induced fluvial discharge decline and long-term storminess variability. Regarding the Danube delta evolution, based on the morphology, internal structure and revised chronologies, the authors argue for the first time that all open-coast deltaic lobes had or have an evolution marked by the cyclic development of sandy barrier spits and islands in front of their downdrift units.

The correlated behavior of sandbars and foredunes on a nontidal coast (Danube Delta, Romania)

ABSTRACT Tătui, F., Vespremeanu-Stroe, A. and Preoteasa, L., 2013. The correlated behavior of sandbars and foredunes on a nontidal coast (Danube Delta, Romania) Coastal foredunes, shoreline and nearshore sandbars compose one large-scale interactive morphological system. Although the variability of these coastal features and their connection to environmental factors have been the focus of many analyses, the direct interactions between them are yet poorly identified, especially for natural nontidal coasts. This paper presents the preliminary results regarding the correlated behavior of the foredunes, shoreline and sandbars in a multi-bar, low-lying deltaic coastal zone, based on six years of seasonal bathymetric and topographic surveys. The analysis shows that, at the multiannual scale, there is a good correlation between the morphometric parameters (volumes and widths) and behaviour (spatial and temporal) of sandbars and foredunes, especially along the accumulative and stable sectors of the study area. Good correlation of these sectors is expressed by the fact that the two entities are interdependent and they present similar variability. The weak correlation identified along the erosive sector is probably due to the significantly lower variability of the foredunes when compared to that of the sandbars. This could reflect the strong influence of the Sf. Gheorghe mouth bar and the pattern of sediment availability, wave climate and longshore sediment transport characteristics along the three sectors of the study area with different shoreline dynamics.

Evolution and Morphodynamics of Danube Delta Shoreface

The present Danube Delta shoreface morphology (in terms of longshore variability of cross-shore profile shape and slope) and behaviour (sediment distribution and transport, depth changes) reside from the co-existence of different types of accretionary, stable and erosive sectors. This configuration is the result of various controlling factors: the long-term evolution of the delta and of each deltaic lobe, the up/downdrift distance to the river mouths, the position of each sector into a specific littoral cell (expressed in sediment availability), the angle made by shoreline with the incident waves and the presence of engineering structures. Shoreface evolution in the past 150 years is highly influenced by the Danube river sediment supply changes, as a consequence of human pressure, and, secondary, is a function of the climatic forcing (storminess) variability, while the inter-annual shoreface morphodynamics is mainly linked to wave energy fluctuations and river floods. The upper shoreface variability is controlled by the multi-annual nearshore sandbars net offshore migration. Intra-site differences in the cross-shore bar behaviour characteristics (both spatially and temporally), expressed by different bar behaviour on the sediment-rich accretionary sectors in comparison with the erosive ones, are the result of the coast(line) evolution and of the morphodynamic state of the beach. Their variability in terms of morphometric (sandbar volumes), geometric (time-averaged widths and depths of the bar zone) and morphodynamic parameters (offshore migration rates and cycle return periods) is related to the complex feed-backs between three key environmental factors: (i) shoreface morphology (i.e. nearshore slope), (ii) alongshore sediment availability and (iii) surf-zone hydrodynamics.

Danube Delta Coastline Evolution (1856–2010)

Danube Delta coastline evolution showed a significant variability in the past 150 years related to different driving forces which change the leading role between them depending on the temporal and spatial scales taken into consideration. At long time scales (centuries), coastline dynamics is mainly driven by the dramatic decrease of Danube sediment discharge after 1950. This is pointed out by the significantly higher shoreline migration rates and area changes between 1856 and 1961/1979 in comparison with the subsequent period, especially along the accumulative sectors. As a consequence, since mid-twentieth century, Chilia lobe started the transition from fluvial-dominated morphology to wave-influenced aspect and behaviour. At multi-decadal scale, shoreline dynamics is ultimately driven by climate variability, related to North Atlantic Oscillation (NAO), which controls the storminess variations along the Danube Delta coast. From this point of view, there is a marked difference between the 1961–1979 time interval, characterised by dominantly negative NAO phase, which determined active storminess, inducing high shoreline mobility, and the 1979–2006 period, which showed less dynamic coastlines (on both prograding and erosive sectors) as a result of the lower storminess imposed by the dominance of positive NAO phase. At inter-annual scale, waterline morphodynamics is influenced by storm regime and river floods.

Long-term variability of coastal profile in different geomorphological conditions (the case of black sea sandy coasts)

Analysis of the dynamics of coastal relief during long time scales allows us to define differences and general trends in the evolution of different coastal types. Long-term dynamics of various coastal profiles was explored on the basis of cartographic data and individual field measurements for different types of sandy beaches located in the northern and northwestern coasts of the Black Sea. The variability observed in the coastal underwater profile dynamics can be associated with various factors as: long-term variability of sediment budgets, bed dynamics due to storm intensity and relative sea level changes as a result of tectonic conditions. The results suggest a relative stability of the outer border of the dynamic zone defined by wave regime during the analyzed period.

The Evolution of Danube Delta After Black Sea Reconnection to World Ocean

This chapter presents synthetically the latest progresses made on Danube Delta evolution based on new cores, sedimentological and morphological analyses which together with the newly obtained absolute ages (AMS 14C and OSL) shed a new light upon the delta formation in both its evolutionary phases (chronology) and the growth patterns. It is the first proposed reconstruction of the fluvial delta which succeeds to date delta front advancement (Old Danube lobe: 8/7.5–5.5 ka) into Danube Bay and the formation of the initial spit. Contrary to the former views, for the first time, it is proven that the early stage of delta plain formation preceded with more than a millennium both the inception of the initial spit and the relative stabilization of the sea level. Moreover, the fluvial delta morphology is reinterpreted to show that most of the present landscape is the recent result of fluvial aggradations which followed after the initial topography (former delta plain) was drowned through the concurrent action of subsidence and sea level rise. With regard to the maritime delta, we bring new arguments into the debate concerning the southern delta (composed by lagoons and sandy barriers built by longshore circulation versus deltaic lobes construction and reworking) which demonstrate that a southern distributary (Dunavăţ, derived from Sf. Gheorghe) had an intense activity and formed open-coast lobes during 2.6–1.3 ka. Moreover, the evolution of each of the six open-coast lobes belonging to maritime delta is systematically presented in relation with Danube flow changes with a focus on their chronology, progradation rates and spatial extension. New evidences have been also produced to document the changes induced by the solid discharge reduction on the Danube since the mid-twentieth century, which recently fostered the shifting of the active lobes from asymmetric to deflected (Sf. Gheorghe) or from fluvial dominated to wave influenced (Chilia).