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International audience; Constructional sedimentary features can provide insight into past changes in relative sea-level (RSL) in regions where traditional bio-stratigraphic markers are absent. The paraglacial beach-ridge plain at Miquelon-Langlade, located 50 km south of Newfoundland, is an example of a well-preserved regressive barrier. Initiation of this plain correlates with a decrease in the rate of RSL rise (from + 4.4 mm/yr to ~ 1.3 mm/yr) at around 3000 years ago. It developed under conditions of normal regression during a period of slow RSL rise (< 1.3 mm/yr). The barrier is composed of two oppositely prograding mixed sand-and-gravel beach-ridge systems, which evolved contemporaneously along two open coasts. The growth of these features reflects high rates of sediment influx that was sourced from the erosion of proximal glacigenic sediment (moraines) and reworked alongshore and across-shore by wave action. The combination of stratigraphic (ground-penetrating radar and sediment cores), topographic (RTK-GPS) and chronologic (optically stimulated luminescence, OSL) data provide a detailed understanding of the constructional history of the plain. The well-defined contact between coarse-grained, wave-built facies and overlying aeolian deposits is used to demonstrate the dominant influences of RSL change in the development of the barrier system and produce a RSL curve over the period of its formation. A net increase of 2.4 m in the surface elevation of wave-built facies is observed across the plain, corresponding to the increase in mean sea-level during its formation. Coupled with OSL dates, trends in elevation of the wave-built facies across the plain are used to reconstruct the relative sea-level history during this period. Acknowledging the uncertainties inherent in the method applied in this study, three distinct periods of sea-level rise can be distinguished: (1) an increase from 2.4 to 1 m below modern MSL between 2400 and 1500 years (average rate of + 1.3 mm/yr); (2) relatively stable or slowly rising RSL (

International audience; This study shows the short-term sediment dynamics (2010) and the long-term morphological evolution (1905 to 2008) of a tidal sand bar emplaced in the Gironde Estuary, the Plassac Tidal Bar. This tidal sand bar was selected because it corresponds to a morphological category (lobate shape) that was not previously studied in the setting of estuarine bay-head deltas. A recent high-resolution bathymetric survey (2010) evi- dences the sediment transport pattern, inferred from the lee face orientation of subaqueous dunes. This sediment transport pattern, together with results from previous studies, explains the five main mechanisms for the tidal bar evolution identified from 29 bathymetric maps since 1905: namely, flood ramp infill, partial ebb shield breaching, lateral accretion of the ebb spits and ebb shield lengthening, generated by the merging of mini-flood lobes on the outer sides of the ebb spits. Lateral accretion seems to be a key-process of sediment accretion for lobate tidal sand bars. Most of the evolutions are explained by tidal processes, but a fluvial influence is evidenced by correlating the presence of mini-flood lobes (migrating seaward from the upper reaches of the bay-head delta) and the lengthening of the tidal bar with periods of high fluvial discharge. A ca. 25-year periodicity in both the fluvial discharge and the tidal bar width, length and volume variations is evidenced and suggests a climate control on the tidal bar evolution. A classification of lobate tidal sand bars is proposed in order to distinguish tide-dominated and wave-influenced sand bars emplaced in large estuary mouth or tidal inlets, from tide-dominated and fluvial-influenced sand bars emplaced in bay-head deltas.