Sedimentation on intertidal mudflats in the lower part of macrotidal estuaries: Sedimentation rhythms and their preservation

International audienceThe objective of this study is to propose an original approach to the analysis of the formation of intertidal rhythmites, their preservation, and the evaluation of sedimentation rates on estuarine mudflats. Three mudflats, one from each of three estuaries, were analysed using a combination of long-term (> a year), high-frequency (1 burst/10 or 20 min), and high-resolution (0.2 cm) altimeter datasets and X-ray images of sediment cores collected during topographic surveys. The results highlight the roles played by sediment supply, hydrodynamics, and morphology of the lower estuaries on the sedimentation rhythms. While the sediment-starved Medway estuary (Kent, UK) remains stable at different time-scales, the annual sedimentation rates on the sheltered Authie mudflat (Pas-de-Calais, France) and the open Seine mudflat (Normandy, France) are relatively high at 18 and 15 cm yr− 1, respectively. On the Authie mudflat, sedimentation rhythms correspond to the semi-lunar cycle, with a good correlation between tidal range and deposit thickness. Sedimentation occurs at the beginning of the recovery (mean value of 0.25 cm per semi-diurnal cycle), and is not disturbed by wind-induced waves. In the lower Seine estuary, semi-diurnal mechanisms of deposition occur mainly when the mudflat is covered by a minimal water height (tidal range threshold value = 7.1 m). Sedimentation rhythms are discontinuous and deposition occurs only during the highest spring tides. Mean deposit thickness is 0.6 cm per semi-diurnal cycle, controlled by the turbidity maximum and the long high tide slack (2–3 h). The fluid mud is sensitive to wind-waves in this open funnel-shaped estuary, which undergoes wind-induced erosion (0.2 to 2 cm) about 10 times per year.Comparison of altimeter datasets and lithology of the sediments cored at the same points provided improved understanding of the sediment record rhythms and the sedimentation events. On the sediment-starved Medway mudflat, the result of sediment processes is a single superficial lamina. The elementary deposit in the Authie bay corresponds to a semi-lunar-linked layer, typical of sheltered environments. On the lower Seine mudflat, sedimentation rhythms are linked to the highest spring tides (i.e., the lunar cycle), resulting from increased sediment availability related to the high suspended matter concentration in the turbidity maximum. These results underline the complex response of intertidal mudflats to hydrodynamics and sediment supply conditions, from the semi-diurnal to the annual scale