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Fig. 8. Morphometric characteristics: A) Density and total length of the beach ridges computed for each ridgeset within the barrier-marsh plain; B) Mean orientation of the beach ridges; C Beach ridge rose for the updrift wing of the lobe: the mean orientation and length of the Saraturile beach ridges are marked with red lines (solid black line for the print version) and those o the Imputita ridges with blue lines (dashed black line for the print version); the middle of each segment is the origin of the rose; (D) Beach ridge rose for the downdrift wing of the lobe: th mean orientation and length of beach ridges are marked with red (light gray for the print version) lines and those of the truncation ridges from each ridgeset with blue lines (dark gray fo the print version); the middle of each segment is the origin of the rose.

Figure 8 Morphometric characteristics: A) Density and total length of the beach ridges computed for each ridgeset within the barrier-marsh plain; B) Mean orientation of the beach ridges; C Beach ridge rose for the updrift wing of the lobe: the mean orientation and length of the Saraturile beach ridges are marked with red lines (solid black line for the print version) and those o the Imputita ridges with blue lines (dashed black line for the print version); the middle of each segment is the origin of the rose; (D) Beach ridge rose for the downdrift wing of the lobe: th mean orientation and length of beach ridges are marked with red (light gray for the print version) lines and those of the truncation ridges from each ridgeset with blue lines (dark gray fo the print version); the middle of each segment is the origin of the rose.

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Fig. 1. A) Map of the Danube delta showing the location of the modern Sf. Gheorghe lobe (black rectangle, detailed in subplot B on a 2010 orthophoto) and the extent of the old lobes (presently abandoned) Sulina, Dunavat (dashed line) and imputita (dotted line); (B) Prominent beach ridges of each ridgeset and corresponding OSL (white circles; present study) and '4C (white rhombus; from Giosan et al., 2006) ages. The core positions are marked with white squares; the extent of the river mouth survey grid is represented by a black rectangle
Fig. 1. A) Map of the Danube delta showing the location of the modern Sf. Gheorghe lobe (black rectangle, detailed in subplot B on a 2010 orthophoto) and the extent of the old lobes (presently abandoned) Sulina, Dunavat (dashed line) and imputita (dotted line); (B) Prominent beach ridges of each ridgeset and corresponding OSL (white circles; present study) and '4C (white rhombus; from Giosan et al., 2006) ages. The core positions are marked with white squares; the extent of the river mouth survey grid is represented by a black rectangle
Summary of luminescence age results. Age estimations are expressed relative to 2013, the year of measurement. Uncertainties in age calculation are based on analytical errors and reflect combined systematic and experimental variability. Quoted errors represent 10.
Summary of luminescence age results. Age estimations are expressed relative to 2013, the year of measurement. Uncertainties in age calculation are based on analytical errors and reflect combined systematic and experimental variability. Quoted errors represent 10.
Fig. 2. Sf. Gheorghe (Danube) sediment discharge evolution during the period 1840 to the present. Trendlines are added to mark sediment discharge decline triggered by (I) Sulina channel regulation works (1860-1902) and (II) dams and riverbank consolidation works mainly undertaken during the communist era in lower watershed (Romania, Bul- garia, Serbia) along the Danube and its tributaries.
Fig. 2. Sf. Gheorghe (Danube) sediment discharge evolution during the period 1840 to the present. Trendlines are added to mark sediment discharge decline triggered by (I) Sulina channel regulation works (1860-1902) and (II) dams and riverbank consolidation works mainly undertaken during the communist era in lower watershed (Romania, Bul- garia, Serbia) along the Danube and its tributaries.
Fig. 4. Map of beach ridges and of ridge sets (S1-S10) within the barrier-marsh and beach ridge plains of Sf. Gheorghe lobe.
Fig. 4. Map of beach ridges and of ridge sets (S1-S10) within the barrier-marsh and beach ridge plains of Sf. Gheorghe lobe.
wet sieving. The quartz and plagioclase grains were separated by centri- fugation in a 2.63 g/cm? heavy liquid solution from the lighter minerals such as potassium and sodium feldspars. The next step employed a 2.75 g/cm? heavy liquid solution, in which the quartz and plagioclase red light. The first step in isolating pure quartz consisted in treating the samples with hydrochloric acid (HCl) for calcium carbonate removal followed by a hydrogen peroxide (H202) treatment in order to remove organic matter. The sand grain fractions (>63 tum) were separated by
wet sieving. The quartz and plagioclase grains were separated by centri- fugation in a 2.63 g/cm? heavy liquid solution from the lighter minerals such as potassium and sodium feldspars. The next step employed a 2.75 g/cm? heavy liquid solution, in which the quartz and plagioclase red light. The first step in isolating pure quartz consisted in treating the samples with hydrochloric acid (HCl) for calcium carbonate removal followed by a hydrogen peroxide (H202) treatment in order to remove organic matter. The sand grain fractions (>63 tum) were separated by
ig. 6. Stratigraphy of the main units of the wave-asymmetric lobe: A) Saraturile beach ridge plain (CV, after Giosan et al., 2011) and B) barrier-marsh plain (F6-C, F11-T). Core positions are 1own in Fig. 1B.
ig. 6. Stratigraphy of the main units of the wave-asymmetric lobe: A) Saraturile beach ridge plain (CV, after Giosan et al., 2011) and B) barrier-marsh plain (F6-C, F11-T). Core positions are 1own in Fig. 1B.
Numerical data on the main controlling sedimentary fluxes (LST, Qs), resulting mean shoreline exposure and corresponding ridgeset chronology at key moments in deltaic lobe evolution. different rates, leading to different time return periods for each cycle of ridgeset building. Table 3
Numerical data on the main controlling sedimentary fluxes (LST, Qs), resulting mean shoreline exposure and corresponding ridgeset chronology at key moments in deltaic lobe evolution. different rates, leading to different time return periods for each cycle of ridgeset building. Table 3
Fig. 7. A) Schematic illustration of the modern Sf. Gheorghe lobe morphology under alter- natively changing longshore and fluvial sediment transport; B) Si (LST/Qs) evolution; the stages depicted with numbers (1-4) are described in Table 3. Solid line describe the cyclic evolution of the lobe (S2-S10 stages), whereas the dashed line corresponds to the early stage S1.
Fig. 7. A) Schematic illustration of the modern Sf. Gheorghe lobe morphology under alter- natively changing longshore and fluvial sediment transport; B) Si (LST/Qs) evolution; the stages depicted with numbers (1-4) are described in Table 3. Solid line describe the cyclic evolution of the lobe (S2-S10 stages), whereas the dashed line corresponds to the early stage S1.
Fig. 9. A) Surface, B) Volume and C) Lifespan of the ridgesets (equivalent to an evolutionary cycle) composing the barrier-marsh plain. In each subplot, the isolated rhombus marks the currently evolving ridgeset (S10). The red lines (dashed lines for the greyscale version) represent the exponential trendline.
Fig. 9. A) Surface, B) Volume and C) Lifespan of the ridgesets (equivalent to an evolutionary cycle) composing the barrier-marsh plain. In each subplot, the isolated rhombus marks the currently evolving ridgeset (S10). The red lines (dashed lines for the greyscale version) represent the exponential trendline.
Fig. 10. Evolution of Sf. Gheorghe mouth between 1856 and 1927. Note the mouth-bar morphology transformations and the subaqueous platform growth preparing the emergence of the Sacalin barrier island (1897). Legend: 1 — subaqueous deltaic platform; 2 — barrier island; 3 — beach ridge plain; 4 — barrier-marsh plain; 5 — fluvial islands.
Fig. 10. Evolution of Sf. Gheorghe mouth between 1856 and 1927. Note the mouth-bar morphology transformations and the subaqueous platform growth preparing the emergence of the Sacalin barrier island (1897). Legend: 1 — subaqueous deltaic platform; 2 — barrier island; 3 — beach ridge plain; 4 — barrier-marsh plain; 5 — fluvial islands.
Fig. 11. Rates of sediment volume change (gray columns) computed at the river mouth for different intervals (for which bathymetric maps were available). The dashed line represents t Danube's sediment discharge variability since 1840. In the late 1930s, the third developing phase began as the updrift coast gradually ceased its progradation and became metastable, whereas the barrier island underwent accelerated landward migration and achieved a position aligned with the updrift shoreline (Fig. 12). The river mouth could no longer act as a depocentre as its net volume losses increased to the tune of — 1.1 x 10° m?/year (1935-2013) even though large var- iations occurred, as in the cases of the two intervals dominated by Dan- ube floods (1960-1975/2004-2014) when slight accumulation occurred (Figs. 11 and 12). Mostly during its last two phases (1897- 2013), the last cycle (S10) recorded an abrupt change in the sedimenta- ry balance between marine and river origin for the first time since the lobe started its rapid expansion (over the last 1500 years). This is best 4.3.1.1. Subaqueous deltaic platform building. The first phase (Fig. 12 A) tends to a river-dominated shoreline configuration with more irregular- ities (Fig. 10A) which result only when the river is able to build long, flat offshore profiles (Wright and Coleman, 1973). The ongoing interaction
Fig. 11. Rates of sediment volume change (gray columns) computed at the river mouth for different intervals (for which bathymetric maps were available). The dashed line represents t Danube's sediment discharge variability since 1840. In the late 1930s, the third developing phase began as the updrift coast gradually ceased its progradation and became metastable, whereas the barrier island underwent accelerated landward migration and achieved a position aligned with the updrift shoreline (Fig. 12). The river mouth could no longer act as a depocentre as its net volume losses increased to the tune of — 1.1 x 10° m?/year (1935-2013) even though large var- iations occurred, as in the cases of the two intervals dominated by Dan- ube floods (1960-1975/2004-2014) when slight accumulation occurred (Figs. 11 and 12). Mostly during its last two phases (1897- 2013), the last cycle (S10) recorded an abrupt change in the sedimenta- ry balance between marine and river origin for the first time since the lobe started its rapid expansion (over the last 1500 years). This is best 4.3.1.1. Subaqueous deltaic platform building. The first phase (Fig. 12 A) tends to a river-dominated shoreline configuration with more irregular- ities (Fig. 10A) which result only when the river is able to build long, flat offshore profiles (Wright and Coleman, 1973). The ongoing interaction
4.3.1.2. Barrier Island build-up. As a result of steady sediment deposition on the mouth-bar platform (2-10 cm/year), the latter becomes strongly offset seaward and progressively wave-exposed, enabling the accom- modation of a shallow bar at the outer edge (Fig. 10B, C), especially but not necessarily after a significant sedimentary input from river floods (Fig. 13B). The emergence of the bar and construction of a distinct barrier island are not an instantaneous event. This usually happens after a significant flood and it takes several years for storm waves to rework the mouth-bar crest sediments (which firstly emerge as sparse islets) and to merge them alongshore into a well-established barrier island. The newly-formed barrier island acts as a trap for fluvial and nearshore sourced sediments; it evolves through continuous downdrift elongation and landward migration, particularly when overwash occurs and washover fans develop. The barrier island dynamics are highly influ- enced by overwash processes which control the alongshore variability of its width. The erosional sectors characterized by more or less exten- sive washover fans due to long quiescent periods and to colonization The exposure to the prevalent and vigorous northern longshore cur- rents favors a skewed development of the mouth bar with a consider- able downdrift elongation of the platform (5-15 km relative to the river channel, whereas the bar crest is anchored on the southern cape Fig. 13. Evolutionary model for the Sf. Gheorghe deltaic lobe: A) subaqueous deltaic platform extension and fluvial island emergence, B) Barrier island build-up, and C) Barrier spit devel- opment. The relative size of the red arrows (white arrowhead for the greyscale version) indicate the LST strength. The blue arrows (black arrowhead for the greyscale version) indicate shoreline behavior: progradation (A and B) and stagnation (C).
4.3.1.2. Barrier Island build-up. As a result of steady sediment deposition on the mouth-bar platform (2-10 cm/year), the latter becomes strongly offset seaward and progressively wave-exposed, enabling the accom- modation of a shallow bar at the outer edge (Fig. 10B, C), especially but not necessarily after a significant sedimentary input from river floods (Fig. 13B). The emergence of the bar and construction of a distinct barrier island are not an instantaneous event. This usually happens after a significant flood and it takes several years for storm waves to rework the mouth-bar crest sediments (which firstly emerge as sparse islets) and to merge them alongshore into a well-established barrier island. The newly-formed barrier island acts as a trap for fluvial and nearshore sourced sediments; it evolves through continuous downdrift elongation and landward migration, particularly when overwash occurs and washover fans develop. The barrier island dynamics are highly influ- enced by overwash processes which control the alongshore variability of its width. The erosional sectors characterized by more or less exten- sive washover fans due to long quiescent periods and to colonization The exposure to the prevalent and vigorous northern longshore cur- rents favors a skewed development of the mouth bar with a consider- able downdrift elongation of the platform (5-15 km relative to the river channel, whereas the bar crest is anchored on the southern cape Fig. 13. Evolutionary model for the Sf. Gheorghe deltaic lobe: A) subaqueous deltaic platform extension and fluvial island emergence, B) Barrier island build-up, and C) Barrier spit devel- opment. The relative size of the red arrows (white arrowhead for the greyscale version) indicate the LST strength. The blue arrows (black arrowhead for the greyscale version) indicate shoreline behavior: progradation (A and B) and stagnation (C).
Fig. 12. Spatial migration versus volume change of the river mouth bar (1856-present).
Fig. 12. Spatial migration versus volume change of the river mouth bar (1856-present).
Fig. 14. River mouth migration since 1700 showing the direction change during the last century. The numbers in bold illustrate river mouth migration (m/year: The three-fold reduction in river sediment supply initiated by the end of 19th century has influenced the mouth-bar morphology, which has become narrower and therefore less efficient in blocking LST, trig- gering in turn the cessation of the depositional processes on the updrift All these new developments prove that the currently developing ridgeset (S10) marks a major change relative to the old evolutionary pattern. One new pattern initiated by the mid-20th century (1930- 1940), is expressed by: i) the complete cessation of updrift coastal progradation, ii) a substantial decline in subaqueous depositional rates
Fig. 14. River mouth migration since 1700 showing the direction change during the last century. The numbers in bold illustrate river mouth migration (m/year: The three-fold reduction in river sediment supply initiated by the end of 19th century has influenced the mouth-bar morphology, which has become narrower and therefore less efficient in blocking LST, trig- gering in turn the cessation of the depositional processes on the updrift All these new developments prove that the currently developing ridgeset (S10) marks a major change relative to the old evolutionary pattern. One new pattern initiated by the mid-20th century (1930- 1940), is expressed by: i) the complete cessation of updrift coastal progradation, ii) a substantial decline in subaqueous depositional rates
Fig. 15. Volume changes of the river mouth bar during the last decade (2004-2013). Note the short time scale impact of the exceptionally big floods recorded during the last century (2005-2006, 2010 and 2014) — gray columns.
Fig. 15. Volume changes of the river mouth bar during the last decade (2004-2013). Note the short time scale impact of the exceptionally big floods recorded during the last century (2005-2006, 2010 and 2014) — gray columns.
Related topics:
SedimentologyCoastal ManagementCoastal ProcessesCoastal GeomorphologyHuman impacts on ancient environmentsHolocene Evolution of Deltas
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